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Famous mineral localities: the Erongo Mountains Namibia.

Minerals have been collected in the Erongo Mountains of Namibia for nearly 90 years, culminating in the 1999 discovery of some of the finest Namibian aquamarine, schorl and jeremejevite ever seen. Other aesthetic species have also been recovered, including fluorite, quartz, goethite pseudomorphs after siderite, and other important species such as cassiterite, ferberite, metanovacekite, uranophane and metazeunerite. Erongo is also the type locality for brabantite, described in 1980.

INTRODUCTION

The 21st century began very well for Namibian minerals. From 1999 to 2006, some of the finest Namibian aquamarine, schorl and jeremejevite ever seen were collected in the Erongo Mountains, together with aesthetic fluorite, quartz, goethite pseudomorphs after siderite, and other important species such as cassiterite, ferberite and metazeunerite. These minerals made major impacts in the southern African collecting scene and at international mineral shows (Johnston, 2002; Gentry et al., 2004). But this bounty was merely the culmination of almost 90 years of collecting in and around the Erongo Mountains; the first production of mineral specimens from Erongo dates back to the early 20th century. The mineralogical future looks bright for this region, because there are still vast tracts of mountainous terrain that have yet to be explored--though exploration may be difficult in some areas, as much of the land is privately owned.

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LOCATION AND ACCESS

The Erongo Mountains are a prominent semicircular terrain 30 kilometers in diameter, located in Damaraland, Erongo Province, Namibia, approximately 20 km north of Usakos and 25 km northwest of Karibib. The climate is semi-arid, with thornveld vegetation and an average annual rainfall of 100-200 mm per year (Mendelsohn et al., 2003). The humidity ranges between 40% and 50% in the rainy season and 10% to 20% during the winter months. Although the mountains are close to the Namib Desert, the average annual temperature is a relatively constant 20 to 22[degrees] C, with minimums of 8 to 10[degrees] C and maximums of 32 to 36[degrees] C (Mendelsohn et al., 2003). The highest peak, Hohenstein, is 2,319 meters above sea level and looms over the southwest section of the mountains, where most minerals are collected.

Road access to the peripheral parts of the Erongo Mountains is relatively easy. A dirt road runs around the entire circumference of the mountains, and specimens can be bought from the locals at Tubussis village in the northwest. Hiking trips into the mountains are more strenuous: it is advisable to bring water and some food, good hiking boots and a hat. However, much of the Erongo Mountains is either privately owned land or forms part of the Erongo Mountain Nature Conservancy. Mineral rights are state-owned in Namibia and prospecting permits must be obtained from the necessary authorities in Windhoek.

There is a detailed 1:250,000 topographic map (Omaruru sheet 2114) of the Erongo Mountains, and the southwestern portion shows the locations of the various farms, defunct mines, mountain peaks, rivers and other physiographic features mentioned in this article. On-line satellite imagery of the Erongo Mountains is also freely available; in addition to Google Earth, NASA has excellent quality downloadable images of the region.

HISTORY

Cloos (1911) provided one of the earliest descriptions of the physiography and geology of the Erongo Mountains, as well as detailed descriptions of the rocks, accompanied by numerous field sketches and by the first published black and white photographs of the mountains. Eight years later the same author published the region's first geological map (Cloos, 1919). Meanwhile, the economic geology of the surrounding pegmatites was described by Wagner (1916). Wagner's map already shows economic tin workings at Ameib 60 and Davib Ost 61, on the southwestern periphery of the mountains. By 1928, there were several more working tin mines in this area, as indicated on map sheet 79, Karibib, South West Africa (Frommurze et al., 1942).

A search of the mineral collection of the Natural History Museum in London revealed some Erongo Mountains specimens from the early 20th century. One is a cassiterite collected in 1915 and presented to the museum by Percy C. Tarbutt. The crudely formed specimen weighs 8.16 kg (18 pounds) and comes from the farm Davib (spelled "Dawib" on the label). It would be expected that similar historical specimens are in some museums in Germany, considering that the country was under German control at the time.

From the 1960's through the 1980's, a trickle of specimens came from Erongo. For example, the Desmond Sacco collection has a large half-"bowtie" schorl acquired in the late 1960's; the Natural History Museum in London has a plumbogummite from Krantzberg, purchased from well-known collector-dealer Charles Key in 1985; and one of us (BC) has a thumbnail schorl on granite matrix purchased in 1981 from Carlton Gems & Minerals in Johannesburg. Notwithstanding early discoveries like these, the heyday for Erongo came at the end of the 20th century. Gebhard (2002) provides an interesting overview for the period between late 1999 and 2001, and his records, together with information from our own collections, are briefly summarized in Table 1.

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One of Erongo's most important mineralogical events occurred in April 2000 with the discovery of the first major pocket of aquamarine on the farm Bergsig 167 (Jahn, 2000; Jahn and Bahmann, 2000; Cairncross, 2001; Johnston, 2002). As a result of this "Easter Pocket" event, there was an intense resurgence in informal diggers' exploiting the mountains for mineral specimens (see the "What's new in minerals" column of the Mineralogical Record, November-December 1999, p. 471; January-February 2000, p. 99; March-April 2000, p. 193-194). For several weeks thereafter, more aquamarine was collected; some of the crystals are associated with schorl, fluorescent lime-green hyalite and complexly twinned orthoclase crystals displaying Carlsbad, Baveno and Manebach twins. The schorl is highly lustrous and commonly shows complex habits and terminations.

Pseudomorphs of goethite after siderite appeared later in 2000, associated with small struvite and/or ilmenorutile and pyrolusite crystals. In November 2000, one pocket yielded an interlocking network of pale blue beryl crystals, and some matrix specimens measure over 1 meter. In 2001, yellow beryl, gemmy monazite and Japan-law twinned quartz were added to the list of desirable collector's items from Erongo. In April-May of that year, schorl crystals up to 20 cm long in groups to 50 cm, the crystals displaying perfect trigonal symmetry, were collected. In March 2001, jeremejevite crystals were collected in situ and from weathered alluvium. Since then, sporadic discoveries of more aquamarine, schorl, quartz, siderite and some micromount material have been made. During mid-2005, beautiful emerald-green fluorite on white orthoclase was collected, adding to the growing list of aesthetic specimens. In January 2006, stalactitic fluorite, vermiform schorl and stellate groups of opaque white beryl were collected for the first time. In April-May 2006, more jeremejevite was discovered.

GEOLOGY

Three large alkaline igneous provinces are recognized in Namibia, and the Erongo Mountains are part of one of these, namely the Mesozoic-age Damaraland Alkaline Province (Pirajno, 1994). The other two are the Luderitz Alkaline Province, of roughly the same age as the Damaraland Province, and the Kaboos-Bremen Province, approximately 550 million years old (Pirajno, 1990), located in southern Namibia and extending across the Orange River into South Africa. The rocks of the Mesozoic-age provinces contain economic deposits of hydrothermal tin-tungsten-fluorine and rare-earth element and niobium mineralization (Gevers and Frommurze, 1930; Haughton et al., 1939; Frommurze et al., 1942; Gevers, 1969; Pirajno and Jacob, 1987; Pirajno, 1994).

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Mineral collectors usually refer to the Erongo as a "granite" mountain, but there are, apart from granite, several different rock types. Geologists who have researched the region refer lithostratigraphically to the Erongo Volcanic Complex (Pirajno, 1990). Geological interest in the rocks and mineral deposits of the Erongo dates back to the time, over 90 years ago, when the territory was controlled by Germany. During this time, two comprehensive publications (Cloos, 1911 and 1919) provided descriptions, diagrams and maps of the Erongo Mountains and the surrounding geological terrain. A more recent geological investigation of the Erongo was published by Blumel et al. (1979), and the most comprehensive overview of the geology is that of Pirajno (1990).

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The Erongo Mountains are a mixture of extrusive volcanic rocks (mafic and felsic lavas) and intrusive plutonic rocks (granites). The rock sequence formed during several geological evens (Pirajno, 1990). An initial extrusive basalt phase was followed by an extrusion of felsic volcanic material (termed the Erongorus Event by Pirajno, 1990) in the form of ash flows interbedded with basalts. This sequence was intruded by granodiorite while at the same time rhyodacitic ignimbrites were emplaced from a centrally located volcanic vent (this is termed the Ombu Event). The third and final phase was characterized by further eruptions of pyroclastic material, forming layers which were later intruded by lamprophyric dikes and mafic plugs, and the emplacement of the Erongo Granite: an A-type (anhydrous, anorogenic, alkaline) granite (Collins et al., 1982), enriched in boron and fluorine (Pirajno and Schlogl, 1987). The emplacement of the Erongo Granite was an important metallogenic event. It provided the heat and chemical components for alkali and boron metasomatism, for greisenization (as at Krantzberg), and for mineralization in the roof of the granite intrusion and in the surrounding country rocks. Pirajno et al. (2000) refer to the resulting crystal pockets as "nests":

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  A prominent and important feature of the Erongo granite is the
  presence of quartz-tourmaline nests up to 30 cm diameter. They are
  disseminated throughout, but are locally extremely abundant and may
  coalesce, especially in the roof zones of the granite stocks. The
  nests have a leucocratic reaction halo containing quartz and
  K-feldspar with no biotite. In these nests, tourmaline and quartz have
  either replaced the feldspar and biotite of the original granitic
  mineral assemblage, or fill open spaces. In addition to tourmaline and
  quartz, the nests also contain accessory amounts of fluorite, apatite,
  topaz, and cassiterite, with relict feldspar and biotite. Tourmaline
  veins, breccias, and dyke-like bodies as well as more pervasive
  tourmaline replacements are widespread in all rocks around the Erongo
  granite up to several hundred meters from its contacts, both
  vertically and horizontally.


The Erongo Volcanic Complex consists mainly of a central caldera, 30 km in diameter, composed of ash-flow tuffs overlying mafic basalts. The entire Erongo Volcanic Complex, including the olivine gabbro cone-sheet of its outer rim, measures 48 km east to west. Following caldera subsidence, the volcano was intruded by several plutons, including the Erongo Granite, which forms a ring-shaped dike-like body. The intruding granite took advantage of structurally weak zones around the periphery of the caldera (Schlogl, 1984). Recent U-Pb dating of zircons and step-heating of [.sup.40.Ar]/[.sup.39. Ar] alkali feldspars indicate an eruption age of 132-135 Ma for the Complex (Pirajno et al., 2000).

It is the Erongo Granite per se that hosts tungsten, fluorine, tin and beryllium mineralization, some low-grade uranium mineralization, and gold (Hirsch and Genis, 1992; Roesener and Schreuder, 1992). Also, the granite hosts the miarolitic cavities that have produced the collectible aquamarine, schorl and other interesting minerals. However, there are satellite pegmatites on the fringe of the Erongo Granite to the west, southwest and southeast which are also mineralized (Frommurze et al., 1942)--see Table 2. These pegmatites are hosted by country-rock schist, not by the granite; some intrude into the metasedimentary rocks and granitoids. The pegmatites are stanniferous, and their genesis is related to the intrusion of the Erongo Granite. Pegmatites from which the cassiterite has been commercially exploited are located on the farms Davib Ost 61, Sandamap, Ameib 60, Onguati 52 and Brabant 68; the latter three are closely related spatially to the Erongo Granite (Diehl, 1992a). The Brabant pegmatites in particular intruded into a pre-Erongo inlier of granite and schist which was later engulfed by the Erongo Granite (the Brabant pegmatite was referred to as the Erongo Schlucht pegmatite by Frommurze et al., 1942 and as Ameiber Tal by Cloos, 1919). The Erongo Schlucht is the valley leading up from Ameib 60 and Davib Ost 61 into the Erongo Mountains. Cassiterite mined there was associated with quartz, K-feldspar, muscovite, schorl and fluorapatite. Ferrotantalite occurs in one of the Brabant pegmatites associated with Li-mica, fluorapatite, topaz and albite.

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Over 70 years ago, Gevers and Frommurze (1930) observed that black iron-rich tourmaline (schorl) "frequently in large, well-formed crystals" is characteristic only of the non-tin-bearing pegmatites, which explains why cassiterite has not typically been found associated with the minerals that have been collected over the past few years. The other suites of Li-Be pegmatites in the Karibib pegmatite area are genetically unrelated to the Erongo event and have been studied fairly extensively (Roering, 1961, 1963, 1964; Roering and Gevers, 1962; Steven, 1993; Keller et al., 1999); they are not discussed further here.

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KRANTZBERG MINE

One of the largest Erongo Mountains tungsten-tin mines was the Krantzberg mine, a major tungsten producer until it closed in 1979 (Schlogl, 1984; Pirajno and Schlogl, 1987; Diehl, 1992b). This was a polymetallic deposit with ferberite and minor cassiterite accompanied by fluorite, beryl and minor molybdenum, iron and copper sulfides. Some interesting and collectible minerals came from this mine. The defunct Krantzberg mine is one of the few mining operations in the region that has been well documented.

Location and History of Krantzberg

Krantzberg, an outlier of the Erongo Mountains, peaks at 1,714 meters above sea level. The hill is clearly visible from a distance, with its characteristic capping of vertical rock faces which give the appearance of a wreath atop the mountain, and hence its German name meaning "wreath mountain."

The Krantzberg tungsten mine is located on the northeastern flank of the Erongo Mountains, 18 km west-southwest of Omaruru. There are several ferberite, cassiterite, tantalite and beryl occurrences in the vicinity of the mine, but historical mining operations tended to focus on the tungsten deposits on Krantzberg hill (Schlogl, 1984).

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Cassiterite and ferberite were discovered in alluvial gravels during the late 1920's. Initially and until 1938, most mining took place in excavations in highly mineralized parts of surface outcrops of greisen zones. Schlogl (1984) reports that ore grades published for the period 1933 to 1938 reveal that the W[O.sub.3] content ranged from 67% to 73% and that the old waste dumps still carry about 1% W[O.sub.3]. During the 1950's, African Mining and Trust exploited the tungsten deposits (see sidebar). The company's operations produced approximately 1,000 tons of tungsten concentrate, making the deposit the richest in Namibia at the time. The mine fell dormant until 1968, when Nord Mining and Exploration, a subsidiary of Nord Resources of Albuquerque, New Mexico, acquired the mining rights for Krantzberg and adjacent areas. After extensive drilling and exploration, mining commenced in 1973; it continued until 1979, when the price of tungsten dropped and ore reserves had been seriously depleted. Nord then offered the property to Anglo-American Corporation of South Africa, which subsequently undertook an exhaustive exploration and drilling program without any major success. Since then, the mine has been idle.

Geology

The lower part of Krantzberg Mountain consists of late Proterozoic Kuiseb Formation schist and Salem (Damara) Granite, underlain by the Erongo Granite and overlain by younger Karoo-age sedimentary strata and basalts (Schlogl, 1984; Pirajno and Schlogl, 1987). The steep slopes and cliffs that form the upper parts of the mountain are sedimentary breccias, capped by Etendeka Formation tourmalinized basalt. The breccias are very immature clastic rocks containing clasts and fragments of schist, granite and pegmatite in a sugary quartz-feldspar matrix (Hegenberger, 1988). The breccia is extensively sericitized and tourmalinized, and the pebbles and boulders are cemented together by a hard groundmass of schorl. The pervasive and intense tourmalinization of the Erongo breccia, conglomerate and basalts is obvious and pervasive (Cloos, 1919); there is also secondary tourmalinization in the older Late Proterozoic schist and granite, extending stratigraphically up to the base of the basalts that cap the Krantzberg. Cloos (1919) and later workers (Haughton et al., 1939; Martin, 1965; Schlogl, 1984) have unanimously concurred that the source of the tourmalinization and mineralization was the underlying, intrusive Erongo Granite. Haughton et al. (1939) described amygdules and geodes in the capping basalts, containing axinite, calcite, danburite, datolite, fluorite, goethite, hematite, pyrite, quartz, schorl and specular hematite.

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The tungsten mineralization originated from greisenization events associated with the Erongo Granite emplacement (Schlogl, 1984). During these alteration events, albitization and hydrogen ion metasomatism caused not only tungsten greisenization at Krantzberg but also uranium-tungsten-tin mineralization at Etemba 135 and tin-tungsten mineralization at Anibib 136, approximately 20 km west of Krantzberg (Pirajno and Schlogl, 1987). Greisen veins tend to be concentrated in the older schist, with a few in the younger Erongo strata. The gray-white to brown greisenized rock is composed of quartz and topaz, with fluorite, schorl, and sericitic muscovite. Accessory minerals are biotite, calcite, cassiterite, chalcopyrite, chlorite, ferberite, goethite, titanite, zircon, bismuth, powellite, scheelite and minor sulfides such as arsenopyrite, pyrite and molybdenite (Schlogl, 1984). Alteration was concentrated along major lithological and structural breaks.

Two main tungsten ore zones were exploited: the Koppie Zone on the south side of the hill and the C-Zone on the northeast slope. Collectible aquamarine came from the C-zone. The greisen veins are quartz-topaz and quartz-tourmaline with minor accessory minerals. Ferberite, scheelite and powellite (Schlogl, 1984) are erratically dispersed in the veins. Apart from these larger deposits, smaller mineralized greisen veins up to 6 cm wide occur in the older biotite schist. These are dark gray and consist of beryl, calcite, fluorite, goethite, quartz, schorl and topaz, with accessory ferberite, fluorapatite, muscovite, scheelite and serpentine (Diehl, 1992b). A list of the Krantzberg minerals is given in Table 3 and a tentative paragenesis is shown in Table 4.

Apart from the tungsten-tin deposits at Krantzberg, fluorite is associated with acicular schorl in vugs in an intrusive porphyritic dike 3.5 km southeast of Krantzberg Mountain (Schneider and Seeger, 1992a). Tin mineralization is also associated with quartz-topaz greisens at Krantzberg (Haughton et al., 1939)--see under cassiterite below. Haughton et al. (1939) described samples of parts of the mineralized zones as follows:
  The material consists of cavity fillings, the cavities lined with
  beautifully formed minute crystals of quartz, tourmaline, fluorite and
  cassiterite ... The minerals in the cavities have crystallized in the
  following order: tourmaline, beryl, cassiterite, quartz, fluorite.


MINERALOGY

The minerals listed below are primarily from the Erongo Mountains miarolitic cavities. However, for completeness, interesting minerals from pegmatites are also mentioned, because some of these pegmatites, notably the ones located close to the south, southwest and western side of the Erongo Mountains, are thought to be geologically related to the Erongo geological "event" (Pirajno, 1990).

Albite NaAl[Si.sub.3][O.sub.8]

Two generations of albite have been described from the Erongo Mountains' miarolitic cavities (Jahn and Bahmann, 2000): "Albite 1" consists of rare, several-centimeter-long but millimeter-thick acicular crystals. These tend to be epitactically grown on orthoclase. "Albite 2" consists of smaller crystals, up to 5 mm long, white to colorless; these are later, second-generation forms on previously crystallized minerals such as "albite 1," orthoclase, schorl and muscovite.

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Albite has been found as cream-white, elongated crystals up to 2 cm on highly corroded epimorphic molds of orthoclase, associated with pale blue aquamarine and pale yellow, purple-tinged fluorite. The albite tends to be oriented along the orthoclase cleavage planes.

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Almandine [Fe.sub.3.sup.2+][Al.sub.2](Si[O.sub.4])[.sub.3]

Almandine is found rarely as aesthetic specimens associated with the other minerals at Erongo. However, garnet (exact species not named) is reported from the following farms' pegmatites in the Erongo region: Brabant 68, Davib Ost 61, Davib West 62, Erongorus 166, Goabeb 63, Tsawisis 16 and Ukuib 116 (Schneider, 1992a). A well-formed almandine crystal on schorl is in the Nagele collection. Almandine-spessartine enclosed in triplite was described from the old Elliot claims, north of Pietershill below Hohenstein (Gevers and Frommurze, 1930).

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Andradite [Ca.sub.3][Fe.sub.3.sup.2+](Si[O.sub.4])[.sub.3]

Andradite, like spessartine, is fairly common in many of the pegmatites in the Erongo-Omaruru area, particularly in the cassiterite pegmatites on the farms Brabant 68, Davib Ost 61, Davib West 62, Goabeb 63, Erongorus 166, and Erongo Schlucht (Schneider, 1992a; Schneider and Seeger, 1992b).

Green to tan-green andradite is mined from calcsilicate marble west of the Erongo Mountains at Tubussis 22 (Grolig, 2005). The garnets are embedded in the hard matrix. Faceting-grade gems have been mined, and specimen crystals measuring 1 to 2 cm have been found (Niedermayr, 2000).

An interesting green tin-bearing andradite was found in a calcsilicate layer hosted in schist on Davib Ost 61, approximately 1 km southwest of the main slopes of the Erongo Mountains (McIver and Mihalik, 1975); this "stannian garnet" was analyzed and found to contain up to 4.58% tin. The andradite was first noticed in the 1930's when attractive green single crystals and plates of crystals were collected from the weathered eluvium. In 1937, the pegmatite was trenched for 13 meters for specimen-grade and facetting-grade andradite (McIver and Mihalik, 1975). Specimens collected from this deposit 35 years later consist of 2 cm-thick encrustations of 5-mm dodecahedral crystals. These are concentrated along the contact zone between a layer of massive vesuvianite and cream-pink grossular. The stanniferous andradite lines drusy cavities in the vesuvianite layer and occurs as isolated crystals disseminated in cemented silcrete breccia associated with the pegmatite. Most of the Sn-andradite is green, but some crystals are green-brown and some are color-zoned; the greater the tin content, the greener the andradite (McIver and Mihalik, 1975). There has been some debate in local collecting circles regarding the exact locality for this stanniferous andradite, even though McIver and Mihalik (1975) state that it is found on Davib Ost 61. The reason for the debate is that the current production of green andradite from Tubussis 22 resembles the Davib Ost material from southeast of the Erongo Mountains. The pegmatites of the two localities are similar, and in both places the green garnet is found in calcsilicate rock.

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Arsenopyrite FeAsS

Sharp arsenopyrite crystals up to several millimeters were sporadically found at the Krantzberg mine. They occur in aggregates from thumbnail to cabinet size, associated with schorl.

Barite BaS[O.sub.4]

A single thumbnail specimen showing pale brown prismatic barite crystals is currently known from the Erongo Mountains.

Beryl [Be.sub.3][Al.sub.2][Si.sub.6][O.sub.18]

Beryl is well known from the Erongo Mountains (Jahn and Bahmann, 2000; Cairncross, 2001) and, together with schorl, has placed the locality firmly on the mineralogical map. Outstanding specimens of aquamarine beryl, yellow and colorless beryl have all been found.

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Beryl is one of the premier collectible minerals from the Erongo miarolitic cavities. Individual crystals typically have a variety of habits, from simple hexagonal prisms with basal pinacoids to complex combinations of pyramidal and pinacoidal faces. What makes the Erongo beryl interesting is the wide variety of colors and associated species. In fact, "common" opaque pale green and colorless beryl is very rare. Most crystals are translucent to transparent, although the degree of clarity in some cases varies within individual crystals; aquamarine beryl typifies this type. Some blue crystals are opaque at the bases and grade along the c-axes into translucent sections, to terminal zones which are transparent and facetable. This phenomenon typifies the April 2000 aquamarine discovery on the farm Bergsig 167 that made such a major impact on the collector market. Aquamarine specimens have since been collected from miarolitic cavities dug on Anibib 136, Bergsig 167, Davib West 62, Erongorus 166 and Tubussis 22.

Jahn and Bahmann (2000) describe three different forms of beryl. One type consists of translucent aquamarine, with only about 5 to 10% of the crystals having transparent sections, particularly in the terminations. Most of the crystals have opaque blue sections that may or may not have inclusions of other minerals such as orthoclase and schorl. The crystals are hexagonal prisms {1010} with basal pinacoids {0001}. Some of the crystals are vertically color zoned, parallel to the c-axis, from dark blue at the base to paler blue, transparent terminations. Color zonations also occur in cross section, perpendicular to the c-axis, with colorless cores surrounded by intense blue outer rims. The second variety of aquamarine consists of matrix specimens up to 20 X 20 cm, and the third variety consists of colorless, transparent beryl. Since Jahn and Bahmann's (2000) publication, several other types and associations of beryl have been found.

Some of the early discoveries in September 1999 were typified by transparent green, fluorescent crystals. Later pockets produced crystals showing various shades of blue. It is important to note that not all of the aquamarine crystals are color zoned; some are transparent throughout the length of the prisms while others are completely included and opaque. Aquamarine crystals up to 10 cm are common and larger crystals up to 30 cm have been reported (Gerd Bachran, personal communication, 2005).

The association of blue beryl with black, lustrous schorl and white orthoclase makes for stunning specimens. Other paragenetic associations run the gamut of the Erongo mineral assemblages--variously colored fluorites, orthoclase, smoky quartz, white and opaque quartz, topaz, and goethite pseudomorphs after siderite. Some aquamarine crystals show naturally etched prism faces, while others have smooth faces with faint vertical striations. Some specimens are doubly terminated, single floater crystals. Crystals in clusters may form stellate groups, radiating from a common center. Specimens of this last type, with Manebach-twinned orthoclase crystals randomly attached to some of the aquamarine crystals, are typical of one particular pocket.

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Erongo aquamarine frequently contains inclusions of other minerals. A discovery in July-August 2005 produced specimens showing small greenish yellow fluorite cubes included within gemmy aquamarine crystals. Finely disseminated orthoclase gives a milky white to milky blue appearance to some aquamarine, while acicular schorl imparts a dark gray to black overtone.

An unusual aquamarine habit is shown by crystals associated with schorl found in May 2001. The habit is informally called "cotton-reel," as both terminations of each crystal have narrow tabular rims of blue-white beryl projecting beyond the edges of the prisms. Most of these are loose crystals, but some were collected as matrix specimens on schorl. In a variant of this habit, natural etching removed the rims, not the centers, of the prisms, producing tapering cone-shaped terminations. Even more bizarre are aquamarine crystals showing two stages of growth, one preserved in a tapering prism, the other in overgrowths of deeply striated, complex hexagonal prisms. The terminations of still other crystals have scalloped surfaces resulting from renewed growth of tiny crystals on the terminal faces. In November 2000, one cavity yielded an interlocking network ("jackstraw" cluster) of opaque cornflower-blue aquamarine. These crystals are all of similar size, form and color--hexagonal prisms a few millimeters thick and up to several centimeters long, with some included orthoclase. Small schorl crystals are associated with these interlocking meshes of blue aquamarine crystals.

In May and June 2001 and August 2003, yellow beryl was collected. The crystals are yellow throughout, not merely coated on the outer surfaces by yellow goethite staining. The May 2001 pocket also produced intense green beryl. The yellow beryl sometimes caps blue aquamarine, in some instances following interrupted growth between the two types. This interrupted crystal growth is seen on specimens that have a narrow gap immediately below the attachment point of the capping yellow beryl (Fig. 78). In early 2006, distinctly bicolored beryl crystals were collected; these have lower yellow-green sections capped by unusual orange upper sections. There is a very sharp contact between the two differently colored zones, and the crystals tend to break very easily along this contact.

In August 2000, August 2005 and January 2006, colorless beryl crystals were collected on Erongorus 166. Most of these are simple doubly terminated water-clear crystals up to 10 cm, commonly included by other minerals such as orthoclase and yellow muscovite in their middle sections. A few of the crystals are parallel groups rather than simple individuals. The August 2005 find produced matrix specimens showing either single colorless crystals or clusters of crystals up to 2 cm partially embedded in muscovite-rich matrix. Some crystals have pristine glassy faces; others are naturally etched and highly corroded. The most recent 2006 specimens are stellate clusters of white, opaque, highly lustrous beryl crystals up to cabinet-size.

Some Erongo aquamarine specimens have been featured subjects of local Namibian artists (Robinson, 2004).

At the Krantzberg mine, common beryl and aquamarine are associated with some ferberite-cassiterite veins, as noted by Haughton et al. (1939):
  Small cavities of beryl and wolframite (ferberite) occur in drusy
  cavities in a vein 400 yards due north of the beacon on Krantzberg.
  Beryl crystals are very slender, have a maximum length of three
  inches, are light and faint green in colour, and are mounted on top of
  quartz crystals.


A greisenized albite-rich pegmatite located 2.2 km northwest of the Krantzberg Mountain beacon contains well-developed beryl crystals up to 10 cm long and 3.5 cm in diameter. There are also drusy vugs scattered along the contact between the surrounding Kuiseb Formation schist and the overlying sedimentary breccia of the Erongo Formation. Here, attractive pale green beryl crystals occur together with fluorite, quartz and ferberite. A thumbnail specimen of pale green aquamarine from the Koppie Zone at Krantzberg is in the collection of the Natural History Museum (London): it is specimen number 36116, donated by P.G. Linzell in 1978. A pair of similar small crystals are owned by Herbert Nagele in Windhoek.

Biotite K(Mg,[Fe.sup.2+])[.sub.3](Al,[Fe.sup.3+])[Si.sub.3][O.sub.10](OH,F)[.sub.2]

Biotite is a minor constituent in some of the miarolitic cavities (von Bezing, 2006).

Brabantite [Ca.sub.0.5][Th.sub.0.5](P[O.sub.4])

Brabantite is a member of the monazite group; the type locality is the van der Made-Brabant pegmatite in the Erongo Schlucht on the farm Brabant 68 (Rose, 1980). The species was named after the farm. It occurs as gray-brown to red-brown to pale yellow-gray, dull to greasy-lustered, elongated crystals, aggregates and fragments up to 1.5 cm. It is radioactive and is associated with accessory hematite, muscovite and uraninite.

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Calcite CaC[O.sub.3]

Calcite is exceedingly rare from Erongo, and only a few specimens are known. Small, complex, transparent and colorless crystals are associated with fluorite and muscovite.

Cassiterite Sn[O.sub.2]

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The first discoveries of cassiterite in Namibia (then South West Africa) were made in 1910 at Ameib 60 in the vicinity of Erongo (Wagner, 1916). Shortly thereafter, more tin deposits were found. All of these consisted of cassiterite mineralization dispersed in veins and lenticular pegmatites (Diehl, 1992a). The pegmatites are coarse-grained and the cassiterite, in most cases, is scattered throughout the pegmatite bodies as individual grains and crystalline masses. Wagner (1916) describes one such occurrence at Davib Ost that yielded a solid 227-kg (500-lb) mass; some cassiterite is described as "ruby tin" and "a beautiful brown, transparent" cassiterite.

Black, complex, highly lustrous cassiterite crystals came from a miarolitic cavity on the farm Ameib 60, and von Bezing (2006) described 1-cm cassiterite crystals on schorl from Erongorus-Bergsig. However, matrix specimens are rare. The complexly intergrown cassiterite crystals measure to 4 to 6 cm (Jahn, 2003). Specimens of orange ilmenite have small tabular cassiterite crystals scattered over their surfaces. A second pocket was discovered in July 2004, with cassiterite of a similarly complex habit. Good descriptions of the cassiterite pegmatites located west, southwest and south of the Erongo Mountains are given in Frommurze et al. (1942) and summarized in Diehl (1992a).

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Cassiterite crystals found in miarolitic cavities differ markedly in habit from other cassiterites found in the adjacent stanniferous, zoned pegmatites and in the Krantzberg mine (see Table 2). The latter types show more typical simple forms and are brown to black with variable luster. At Krantzberg, some large specimens were collected (see Fig. 97). Here, the tin mineralization occurs in greisen veins, concentrated on the northern slope of the mountain and in a volcanic vent on the southeast slope. Highly mineralized zones are scattered around the periphery of the pipe. Some crystals are "coarsely crystalline ruby-red" (Haughton et al., 1939). These authors give details on every cassiterite occurrence in and around Krantzberg Mountain. The main Erongo region cassiterite occurrences are listed in Table 2.

Clinozoisite [Ca.sub.2][Al.sub.3](Si[O.sub.4])[.sub.3](OH)

Pale pink clinozoisite crystals to 1 cm are found intergrown with prehnite at the andradite locality at Tubussis 22 (Niedermayr, 2000).

Collinsite [Ca.sub.2](Mg,[Fe.sup.2+])(P[O.sub.4]) x 2[H.sub.2]O

Hexagonal, orange-brown crystals of collinsite to 2 mm were quantitatively identified (Gebhard, 2002). The collinsite is scattered over the surfaces of foitite crystals, the latter showing strong pink-green dichroism.

Diopside CaMg[Si.sub.2][O.sub.6]

The pyroxene-group species diopside has been found at the Tubussis andradite locality as radiating aggregates of small columnar crystals associated with tan-colored garnet.

Dolomite CaMg(C[O.sub.3])[.sub.2]

Clusters of typically curved, saddle-shaped dolomite crystals attached to quartz were discovered in October 2000. Individual dolomite crystals are generally 1 to 2 cm and most are matte gray; some are stained by secondary iron oxides. The dolomite-studded quartz varies from single crystals to multiple crystal clusters. The largest single quartz crystal is 15 cm long. The dolomite is characteristically attached to the pyramidal quartz faces but some is attached to the prism faces. Minor white fluorite, blue tourmaline crystals to 1 mm, schorl and hyaline opal complete the mineral assemblage from this unique dolomite-bearing miarolitic cavity.

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Dumortierite [Al.sub.7](B[O.sub.3])(Si[O.sub.4])[O.sub.3]

Schneider and Seeger (1992b) report bright deep blue dumortierite from a pegmatite on the farm Etemba 135, close to the northern boundary of the Erongo Mountains. This deposit was mined for ornamental stone, and yielded seven tons of material in 1957. Of historical interest is a bowl and obelisk made from this Etemba 135 dumortierite which were on display in the German Geological Survey in Berlin prior to the outbreak of World War II (Silberstein, 1933; South African Mining & Engineering Journal, 1974).

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Ferberite [Fe.sup.2+]W[O.sub.4]

Ferberite was mined from the pegmatites on Kudubis 19, Goabeb 63, Davib Ost 61 and Ameib 60 (Diehl, 1992b). Attractive, euhedral, steel-gray crystals have been found on rare occasions. The main tungsten deposit in the region was at Krantzberg (Haughton et al., 1939). Quartz veins in the Kuiseb schist contain ferberite associated with drusy fluorite and accessory bismuth and chalcopyrite. Well-formed ferberite crystals to about 5 cm occur sparingly with white and purple fluorite in a mineralized ferberite-schorl-fluorite vein 200 meters due north of the Krantzberg Mountain beacon. Ferberite crystals of similar size were discovered in another quartz-topaz greisen vein 2 km northwest of the Krantzberg beacon.

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Windhoek collectors Herbert Nagele and Ernst Schnaitmann regularly make field trips to the Erongo Mountains for specimens and confirm that Erongo ferberites are rare. The ferberite that may be found by local diggers is usually misidentified as "tantalite" and is erroneously sold locally as an ore of tantalum.

Ferrocolumbite [Fe.sup.2+][Nb.sub.2][O.sub.6] and

Ferrotantalite [Fe.sup.2+][Ta.sub.2][O.sub.6]

"Columbite-tantalite" has been reported by Jahn and Bahmann (2000) as crystal cleavages up to 6 cm, but these have not been quantitatively analyzed to determine their specific species. A specimen of tantalite-columbite from the Van der Made (= Erongo Schlucht) pegmatite was acquired in 1977 by the Johannesburg Geological Museum collection. One of the earliest recorded "columbite" crystals is a 0.5 x 4 x 7-cm specimen from Ameib 60, Erongo Mountains. The Natural History Museum in London bought this crystal in 1923 from James R. Gregory & Co.

Florencite-(Ce) La[Al.sub.3](P[O.sub.4])[.sub.2](OH,[H.sub.2]O)[.sub.6]

Recently, florencite-(Ce) was positively identified from Erongo. It occurs as dark red microcrystals on Carlsbad-law twinned, corroded orthoclase, together with schorl and topaz. The minute crystals look rather nondescript and could easily be overlooked.

Fluorapatite [Ca.sub.5](P[O.sub.4])[.sub.3](F,OH,Cl)

Fluorapatite is rare from the Erongo miarolitic cavities; some specimens thought to be fluorapatite turned out to be beryl. However, Jahn and Bahmann (2000) describe pale blue-gray crystals to 2 cm, but usually less than 1 cm, on gray milky quartz and schorl. The fluorapatite is characteristically deeply etched. The Brabant pegmatites in Erongo Schlucht contain fluorapatite, and von Bezing (2006) describes its occurrence from Bergsig 167 and Erongorus 166.

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Fluorite Ca[F.sub.2]

The miarolitic cavities on Davib West 62, Davib Ost 61, Bergsig 167 and Erongorus 166 have produced fluorite crystals of various colors and habits. The largest crystals are 10-cm cubes. Some of the earlier discoveries on Tubussis 22 produced pale green crystals with a semi-rounded appearance caused by etching and resorption. Notable pockets were unearthed in October 2000 and particularly in July 2005. The latter discovery was on the farm Bergsig 167, where diggers excavated pockets on the precipitous mountain slopes. Most of these fluorite crystals are simple cubes of a vivid, bright emerald-green color on a matrix of white orthoclase. Some of the cubes have dark purple corners. Dodecahedral and cuboctahedral fluorite crystals have also been found; some are deeply etched and display curvilinear crystal edges. Rare crystals display elongated growth patterns and serrated, colorless, transparent edges surrounding a dark green "faden" core running parallel to the elongation. Some transparent white crystals have globular purple cores. In some specimens, complex, dark purple-green fluorite crystals are scattered on matrix of coarsely crystalline, yellow hexagonal muscovite rosettes.

Most specimens are miniature to small cabinet-size, but rare plates of apple-green fluorite to 20 cm across have been recovered. Fluorite is associated with orthoclase crystals (single and twinned), topaz, quartz, aquamarine and muscovite. In rare cases, quartz crystals enclose small (less than 1 cm) fluorite crystals. In July 2005, unusual thumbnail to miniature specimens were collected which show transparent aquamarine crystals containing inclusions of yellow-green fluorite. Some fluorite is also attached to the outside surfaces of the aquamarine. Unusual specimens of purple-blue fluorite display a curvilinear habit, with crystal surfaces made up of a myriad of small crystal domains. Rare black octahedral crystals were once found and these resemble the yttrium fluorite from neighboring Klein Spitzkoppe (Cairncross, 2005).

In January 2006, specimens of stalactitic fluorite were recovered from one pocket. The "stalactites," which taper to points, consist of drusy green and dark purple fluorite, with associated colorless beryl and minor schorl.

The Brabant pegmatite on the farm Brabant 168, Erongo Schlucht, contained vugs in pegmatitic quartz that were lined with purple miarolitic cavity fluids.

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Interesting green and pink fluorite cubes modified by {111} octahedral faces were collected in the past from the Krantzberg tungsten mine. Specimens of these are in the Nagele collection in Windhoek and the von Bezing collection in Kimberley; the latter collection contains an outstanding cabinet-size specimen of fluorite associated with quartz. Krantzberg fluorite is colorless and pale blue with purple patches. Fluorite also partially filled and lined cavities in the orebodies.

Foitite [[Fe.sub.2.sup.2+](Al, [Fe.sup.3+])][Al.sub.6][Si.sub.6][O.sub.18](BO)[.sub.3](OH)[.sub.4]

Tourmaline is classified, according to X-site chemistry, into three groups: alkali, calcic and X-site-vacant groups. The tourmaline species foitite falls into the last of these (MacDonald et al., 1993; Hawthorne and Henry, 1999). The classification is well summarized by Simmons (2002b). Foitite is now considered a "very common tourmaline species" (King, 2002, page 8) and has been identified from the Erongo Mountains (Gebhard, 2002; Simmons, 2002a). Foitite, together with schorl, is presently the subject of a separate paper and detailed chemical analyses of these species is beyond the scope of this article (3).

In May 1999 a pocket of black tourmaline with unusual crystal habits was discovered at Erongo. The crystals measure up to 10 cm and have distinctive hemimorphic habits, with one end of the hexagonal prism terminated by either {10-12} or {02-21} pyramidal faces, the other termination exhibiting bundles of elongated acicular crystals, resembling an extended trigonal "Mercedes Benz" emblem. This form is caused by the rapid growth of the {10-11} pyramidal faces relative to the slower growth of the adjacent {02-21} pyramidal faces (Rustemeyer and Deyer, 2003). In some instances, the growth of the {10-12} faces is extreme and these three faces project several centimeters out as trigonal "wings." Many of the crystals are partly or wholly coated by hyaline opal. One termination is frequently attached to orthoclase, quartz or beryl, while the opposite "rocket wing" projects outwards. Some of the small foitite crystals are greenish black (von Bezing, 2006).

Foitite appears to be rare from the Erongo Mountains: detailed analyses of Erongo tourmalines in the Harvard collection have shown that most are schorl and only a few are foitite (F. Hawthorne, personal communication, 2005).

Galena PbS

One specimen of galena was collected in January 2006. It is a crudely formed cuboctahedron encrusted by a mixture of drusy orthoclase and goethite. The underside of the crystal is fractured and well cleaved metallic galena surfaces are visible.

Goethite [alpha]-[Fe.sup.3+]O(OH)

Goethite occurs in a variety of guises in the Erongo cavities. In some cases it pseudomorphically replaces rhombohedral and semi-rounded, disc-shaped crystals (siderite?), octahedral and cuboctahedral crystals (magnetite?) and precursor forms that resemble trigonal ilmenite. A few rare hematite "iron roses" have also been altered to goethite. Determining the original mineral that was replaced by the goethite is difficult, as only the remnant crystal shape remains. Goethite also exists as a common oxidation product associated with most of the minerals found at Erongo.

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Gold Au

Gold is present in the Erongo caldera and in some of the adjacent rocks and alluvium (Hirsch and Genis, 1992; Steven, 1993). It is reported in small amounts from farms in the northern, western and southwestern Erongo Mountains, including Pistelwitz 128 (where the Krantzberg tungsten mine is located), Omaruru Townlands 85, Eleen 164, Ekuta 129, Hoogenoeg 170, Ombu 130, Koedoeberg 169, and Niewoudt 151 and 156.

Alluvial gold has been panned from Ameib 60 and Chatzputz on Davib Ost 61. South of the Krantzberg mine, gold was found in some of the alluvial tin deposits (Haughton et al., 1939). Wagner (1916) describes "small nuggets of gold ... in the stanniferous gravels and 'floats' of the Erongo tinfield."

Hedenbergite Ca[Fe.sup.2+][[Si.sub.2][O.sub.6]]

Von Bezing (2006) reports platy yellow hedenbergite crystals in calcsilicate marble from the Tubussis 22 andradite locality west of the Erongo Mountains.

Hematite [alpha]-[Fe.sub.2][O.sub.3]

One discovery produced sceptered crystals of quartz with hematite staining, giving these specimens a pleasing red tinge. Very small crystals occur as crusts on goethite pseudomorphs after siderite (Jahn and Bahmann, 2000); however, recent observations suggest that this bright silver mineral is in fact pyrolusite and not hematite.

Hydroxylapatite [Ca.sub.5](P[O.sub.4])[.sub.3](OH)

Jahn (2003) describes hydroxylapatite associated with orthoclase. The 2 to 3-mm colorless crystals are prismatic with pyramidal terminations and superficially resemble quartz except that the prism faces lack the striations typically observed in quartz. Some of the hydroxylapatite is coated or partially coated by a thin 1-mm film of fibrous fluorite.

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Hydroxyl-herderite CaBe(P[O.sub.4])(OH)

Hydroxyl-herderite is the second beryllium species (the other being beryl) to be found in the Erongo Mountains miarolitic cavities. Hydroxyl-herderite is associated with small (less than 2 cm) schorl crystals, muscovite and orthoclase. A single pocket opened in July 2003 yielded a small amount of this material. Most specimens are thumbnail to small-cabinet-size clusters composed of individual crystals measuring less than 1 cm. The largest crystal found thus far measures 6.2 cm and is studded by small schorl crystals.

Two different colors of hydroxyl-herderite exist at Erongo: pale green and off-white. X-ray diffraction analysis of crystals of both colors at the University of Johannesburg's SPECTRAU analytical facility revealed that they are the same species; the cause of the color difference is not known.

Ilmenite [Fe.sup.2+]Ti[O.sub.3]

Ilmenite occurs as simple and complex crystals to 7.4 cm with slightly radioactive orange-yellow coatings. The ilmenite was quantitatively identified by the Geological Survey in Windhoek, Namibia and by X-ray diffraction at the University of Johannesburg. Ilmenite is associated with schorl, quartz and cassiterite, and some specimens contain microscopic intergrowths of rutile and ilmenite (Jahn et al., 2003).

Ilmenorutile (Ti,Nb, [Fe.sup.3+])[.sub.3][O.sub.6]

Gebhard (2002) describes small, bright, silvery metallic ilmenorutile crystals on goethite pseudomorphs after siderite(?) and magnetite(?). These tiny crystals are difficult to distinguish visually from the pyrolusite.

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Jeremejevite [Al.sub.6][B.sub.5][O.sub.15](F,OH)[.sub.3]

Jeremejevite is a rare mineral, but was abundant from Erongo for a short period. It was first discovered in eastern Siberia in 1883, when a handful of pale yellow crystals were found. Ninety years later, in 1973, a second discovery was made at Mile 72, on the Atlantic seaboard north of Swakopmund, Namibia (Herting and Strunz, 1978; Wilson et al., 2002). Local mineral dealer Sid Pieters worked this deposit, and in 1976 he was rewarded with a small pocket that yielded some of the finest jeremejevite crystals known (see Wilson et al., 2002). In total, a few hundred specimens were removed during the 1970's. In 1999, a collaborative Namibian-American company, Khan River Mining (Pty) Ltd., reopened the deposit in the hope of finding more of the famous blue crystals, but they had only limited success. Three years later, in March 2001, jeremejevite was discovered in the Erongo Mountains (Gebhard, 2002; Wilson et al., 2002; Jahn, 2003). Several miarolitic cavities were excavated in the granite on the farm Ameib 60, almost at the border with the neighboring farms Davib Ost 61 and Brabant 68. At first, the blue prismatic crystals were thought to be aquamarine but analysis proved otherwise.

It is estimated that during the exploitation of these pockets a few thousand specimens were collected, including crystals that were dug from weathered alluvium. At the time, some dealers had small plastic bags filled with loose, pale yellow to colorless needle-like crystals without matrix. The vast majority of these are less than 1 cm long and 2 mm thick; larger crystals to 5 cm were found but are rare. Von Bezing (2006) states that the largest crystal is 6 cm. Matrix specimens of jeremejevite, either on smoky quartz or more rarely on orthoclase, attest to the simple mineralogy of these particular pockets. Large crystals typically taper toward the termination, and the bases of the crystals are bluer than the terminations. Furthermore, some crystals have stripes of blue-white color zoning that run the length of the crystal. Blue crystals exhibit a striking pleochroism from colorless to "cornflower" blue, an optical attribute used to great effect by gem cutters.

A visit to the discovery site on the farm Ameib 60 in August 2005 revealed several small worked-out cavities clustered together on one southeast-facing granite face. Most are less than 1 meter in diameter, and there is evidence of drilling having taken place. In 2005 the area was deserted, in contrast to 2001 when several hundred diggers were (illegally) excavating jeremejevite (Wilson et al., 2002).

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In April-May 2006, another discovery of jeremejevite was made in the Erongo Mountains, not at the previous site on Ameib 60 but at a site whose exact whereabouts have not yet been divulged. The crystals are similar in size to the previous ones from Ameib 60, but apparently fewer have been found. They have very good blue color but are opaque, and they are not clean, equant prisms but are somewhat distorted and misshapen.

Jeremejevite was recently discovered in Madagascar. In 2004, a 7.88-carat faceted stone was purchased in Madagascar as colorless "achroite" tourmaline. Quantitative analysis showed the stone to be jeremejevite (Mocquet and Lulzac, 2004). To date, it appears to be the only jeremejevite sample known from Madagascar.

Magnetite [Fe.sup.2+][Fe.sub.2.sup.3+][O.sub.4]

Unaltered magnetite crystals are rare at Erongo. Goethite forms pseudomorphs after octahedral crystals which are assumed to have been magnetite. Tiny magnetite octahedrons are sometimes included in smoky quartz, and loose, black, metallic octahedral crystals are sometimes found.

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Metanovacekite Mg(U[O.sub.2])[.sub.2](As[O.sub.4])[.sub.2] x 4-8[H.sub.2]O

In September 2003 a small uranium-rich miarolitic cavity was discovered on the farm Tubussis 22. It yielded a few uranium species, including metanovacekite, metazeunerite and uranophane. Uranium anomalies were previously known from airborne exploration just north of Erongo on the farm Omandumba West 137, at the common boundary with the adjacent Anibib 136 (Roesener and Schreuder, 1992). The mineralization is structurally controlled by fractures in the Erongo Granite. Another area of supergene uranium mineralization occurs in pegmatites on Etemba 135, bordering northwest Erongo, at Omandumba West 137 and on southwestern portions of Brabant 68 (Pirajno, 1990).

Metazeunerite Cu(U[O.sub.2])[.sub.2](As[O.sub.4])[.sub.2] x 8[H.sub.2]O

Single crystals and crystal groups of metazeunerite were collected in the uranium pocket described above. Crystals are typically less than 1 cm and are rare. They are associated with purple fluorite, quartz and schorl.

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Molybdenite Mo[S.sub.2]

In October 2005 a small cavity yielded the first specimens of Erongo molybdenite; only about ten specimens were collected. Associated minerals are quartz, muscovite and beryl (Herbert Nagele, personal communication 2006).

Monazite-group (Ce,La,Nd)P[O.sub.4]

Monazite occurs as orange-brown tabular crystals, some associated with muscovite, others with pale green beryl. In May-June 2001, green to yellow beryl crystals to 8 cm associated with brown-yellow monazite crystals to 2 cm were found (Gebhard, 2002). One of the largest monazite crystals found measures 2.5 cm (Jahn and Bahmann, 2006).

Muscovite K[Al.sub.2]([Si.sub.3]Al)[O.sub.10](OH,F)[.sub.2]

Three different types of Erongo mica are described in the literature: muscovite, lithium-muscovite and zinnwaldite (Frommurze et al., 1942; von Bezing, 2006). Jahn and Bahmann (2000) describe muscovite as 1 x 1-cm silver-green crystals but state that the identification of this mica was done visually and not quantitatively. The color of the muscovite is variable from silver-white to off-white, gray-green, pale green, pale yellow and pale orange-yellow. Small, stacked hexagonal "books" of muscovite, some flattened and other tapering, are common. Muscovite is associated with cuboctahedral fluorite, colorless beryl, schorl and topaz, and muscovite crystals commonly stud crystals of schorl, orthoclase and other species. One of the most attractive associations is 1 to 2-cm rosettes of honey-yellow muscovite providing the matrix for crystals of green fluorite and colorless beryl.

Opal Si[O.sub.2] x n[H.sub.2]O

Hyaline opal is relatively common in the miarolitic cavities and in some of the metallic deposits such as the one at Krantzberg. The opal, being the last mineral phase to have formed in the Erongo paragenesis, occurs on all other species. This can be a blessing and a curse because the opal is tenacious and very difficult to remove from schorl, quartz and other minerals. On the other hand, some (not all) opal fluoresces a vivid, intense yellow-green, sometimes even in daylight, adding to the aesthetics of specimens.

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The opal occurs in various habits. Glassy, transparent, botryoidal coatings are relatively common. White ice cream cone-like sprays are sometimes seen perched on fluorite and very commonly on orthoclase feldspar. These sprays have a divergent habit, very similar to Don King's hairstyle!

At the Krantzberg tungsten deposit, hyaline opal coats most minerals. It occurs as botryoidal layers, some stained green by secondary copper minerals or black by hematite.

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Orthoclase Kal[Si.sub.3][O.sub.8]

Alkali feldspars are abundant in the miarolitic cavities at Erongo (Jahn and Bahmann, 2000). Orthoclase and microcline have the same chemical composition, but different crystal structures. Some microcline of the amazonite variety has been said to occur at Erongo, but the specimens most likely came from the nearby Klein Spitzkoppe, where microcline is locally abundant (Cairncross, 2004).

Most of the Erongo Mountains alkali feldspar is considered to be orthoclase. This conclusion is based partly on the abundance of excellent Baveno, Carlsbad and Manebach twins typical of orthoclase. Furthermore, thin sections were made from 15 randomly selected feldspar samples. These were microscopically examined under transmitted polarized light to determine whether any diagnostic "tartan twinning" could be observed that would be indicative of microcline rather than orthoclase, and no such twinning was found. The twinning of the Erongo orthoclase is very diagnostic and spectacular. Combinations of contact (Baveno) and penetration (Carlsbad) twins are pervasive, and multiple and complex twins are common.

Orthoclase usually forms the matrix for later-stage aquamarine, and the combination of perfectly twinned feldspar with blue-green aquamarine produces beautiful and unusual specimens (bright green fluorite perched on white orthoclase is equally appealing.) In some cases the aquamarine is aligned parallel to the orthoclase twin planes, particularly for the Carlsbad twins.

The state of alteration of orthoclase in Erongo specimens is variable. Fresh, unaltered crystals with smooth faces rarely occur. Most specimens show orthoclase crystals with etched, corroded faces, while extreme alteration and replacement of orthoclase is evident in other specimens showing epimorphic molds, some partially or wholly replaced by secondary minerals such as quartz (see pseudomorphs).

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In early 2006, twinned orthoclase was collected from one cavity. The crystals are relatively unaltered, with smooth white faces and minor goethite staining. They occur either as single crystals to 20 cm or in composite groups. Most are highly elongated parallel to the c axis.

Phlogopite K[Mg.sub.3][Si.sub.3]Al[O.sub.10](F,OH)[.sub.2]

Phlogopite occurs as a minor species (von Bezing, 2006).

Prehnite [Ca.sub.2][Al.sub.2][Si.sub.3][O.sub.10](OH)[.sub.2]

Prehnite occurs intergrown with clinozoisite in the calcsilicate-marble andradite locality at Tubussis 22 (Niedermayr, 2000).

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Pyrolusite [Mn.sup.4+][O.sub.2]

Small, lustrous, silvery-metallic, spindle-shaped pyrolusite crystals occur associated with goethite pseudomorphs after siderite(?). Some similar-looking crystals have been identified as ilmenorutile (Gebhard, 2002).

Quartz Si[O.sub.2]

Several types of quartz exist in the miarolitic cavities, including transparent crystals, smoky quartz, milky quartz and amethystine quartz, some with inclusions. The largest quartz crystals reach 50 cm.

In August 2003, a pocket yielded epimorphs of quartz after elongated orthoclase crystals. The quartz forming the epimorphic shell is aligned parallel to the cleavage of the feldspar. Some specimens still have vestiges of the feldspar while in others the feldspar is completely gone, leaving only a hollow shell composed of an interlocking network of quartz crystal domains. The quartz is commonly coated and included by chlorite. The quartz mold can superficially resemble Japan-law twins, but in fact the intercrystal angle is an artifact inherited by the quartz crystallizing in a preferential orientation and alignment along the orthoclase structure. However, some rare Japan-law twins have been collected. These are typically small, flattened, transparent crystals.

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Amethystine quartz is not common. The sceptered terminations of a few dozen specimens collected from a single pocket in November 1999 have partly amethystine tips, and are also stained red by hematite. One specimen is matte black throughout. These are the best Erongo specimens of sceptered quartz known to date; the largest crystal measures 20 cm.

A few rare quartz specimens display the "artichoke" habit (White, 2004). These were collected at Tubussis 22, associated with pale green fluorite. Colorless, transparent, highly lustrous quartz crystals to 2 cm were collected in 2004. These are perched on and intergrown with brilliant black acicular schorl in composite specimens to 15 cm. Quartz containing inclusions of acicular schorl is relatively common; some quartz crystals are blackened by these inclusions.

In April-May 2006 another pocket of "artichoke" quartz was unearthed. The specimens consist of transparent quartz coated by a layer of opaque milky quartz with amethystine terminations. The largest specimen measures approximately 40 x 40 cm. Some sceptered quartz crystals associated with schorl were also collected.

Chalcedonic quartz is rare, and only a few aesthetic specimens are known from the Erongo pockets. Like hyaline opal, the chalcedony is a late-stage mineral in the paragenesis and usually coats previously formed minerals. Chalcedony and jasper-lined vugs in quartz-schorl veins are located approximately 9 km west of Krantzberg Mountain. Doubly terminated quartz crystals were once found in mineralized vugs at Krantzberg.

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Rhodochrosite [Mn.sup.2+]C[O.sub.3]

A specimen of a single, rhombohedral Erongo rhodochrosite crystal is in the Nagele collection in Windhoek. The crystal is pink and was quantitatively identified as rhodochrosite. This find is not too surprising, considering that manganese was present in the miarolitic cavity fluids.

Romanechite (Ba,[H.sub.2]O)[.sub.2]([Mn.sup.4+],[Mn.sup.3+])[.sub.5][O.sub.10] and Cryptomelane K([Mn.sup.4+],[Mn.sup.3+])[.sub.8][O.sub.16]

Botryoidal aggregates and masses of black, amorphous material have been described as "psilomelane." This is no longer a valid species (Mandarino and Back, 2004), but it has not been possible to chemically analyze whether the Erongo specimens are romanechite or cryptomelane. Aesthetic specimens of black and silver metallic botryoidal aggregates and radiating acicular sprays, in some cases associated with schorl, occur at Erongo.

Rutile Ti[O.sub.2]

Rutile occurs as reticulated intergrowths of tiny crystals with ilmenite. Small (2 mm) red rutile crystals are found rarely on the serrated terminations of schorl, producing a pleasant red-black color combination (Jahn et al., 2003).

A famous deposit of rutile is found 10 km by road from Kanona siding on the railway line between Omaruru and Usakos, at the boundary between Erongo Ost 82 and Kanona Ost 81 (Schneider, 1992b). The rutile is hosted in albitized granite partially intruded by pegmatites. Here, rutile occurs as drusy crystals lining fissures, with quartz, albite and Cr-muscovite. Rutile crystals up to 20 cm are still found today at the locality but are embedded in the host rock. The deposit was commercially mined as a source of titanium in 1936 and 1937, when 71.3 tons of hand-sorted 95%-pure rutile concentrate was recovered (Schneider, 1992b).

Scheelite CaW[O.sub.4]

Very rare orange microcrystals of scheelite from Erongo are known from a specimen in the Bahmann collection. The mineral is associated with goethite pseudomorphous after siderite(?), ilmenorutile-pyrolusite and quartz. Scheelite is one of the accessory ore minerals at Krantzberg (Haughton et al., 1939).

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Schorl Na[Fe.sub.3.sup.2+][Al.sub.6](B[O.sub.3])[.sub.3][Si.sub.6][O.sub.18](OH)[.sub.4]

Schorl from the Erongo Mountains has set a high quality standard for this species worldwide. Erongo schorl specimens are certainly the best in the southern African region and most likely Africa as a whole. The attractive associations, the great variety of crystal habits, the large crystal size and the brilliant luster of the larger schorl crystals, combine to make Erongo one of the world's most significant occurrences of the species.

Schorl is the most common species in the miarolitic cavities. Scattered crystals and fragments are strewn around the mountainous area (Jahn and Bode, 2003); most of these have naturally weathered from the outcrop but some are rejected specimens left behind by the diggers. Some schorl eroded from the granite has been transported and deposited as alluvial crystals on the surrounding plains. With time, these crystals become embedded and cemented into calcrete, producing "secondary" matrix specimens. The schorl occurs as individual crystals up to 15 cm, and in large museum-sized plates with associated minerals. Much Erongo schorl is characterized by smooth crystal faces with extremely high luster; other crystals have more of a matte luster, with highly striated prism faces. Many specimens are floaters, either as single crystals or as clusters. Not all schorl appears as pristine crystals. Some specimens are highly corroded, etched and resorbed, and some crystal faces are studded by acicular gray quartz, muscovite and orthoclase. In some cases schorl has pseudomorphically replaced orthoclase.

Schorl belongs to the ditrigonal pyramidal class of the hexagonal system, and many complex habits are found at Erongo. The crystals very commonly display distinct hemimorphic habits, having pyramidal terminations on one end while splaying out into multiple terminations on the opposite end. These terminations (termed "rockets" by Gebhard, 2002) consist of three flattened trigonal surfaces. The "Mercedes Benz" forms {101} and {101} show some extreme variations. Perfect trigonal crystals exist and some specimens have "frayed" terminations or may consist of a mosaic of intergrown subcrystals.

Some terminations are cavernous, arena-like hollows produced by preferential growth of sections of the outer prism faces, accompanied by less rapid crystal growth towards the center of the termination. Later-stage muscovite and fluorite commonly crystallize in these hollow terminations. Another interesting paragenetic process occurs when parts of the thin crystal rims are naturally fractured and collapse into the hollows; these then become cemented together as a network of crystal shards by continued crystallization of the schorl.

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Some specimens are coated by fluorescent and non-fluorescent hyaline opal. Not all schorl is black--some crystals are very dark green or deep red. These tourmalines require further investigation to determine whether they are in fact schorl. One pocket produced crystals from thumbnail size to 6 or 7 cm in diameter that have an equant, pseudo-isometric habit, resembling black garnets. These are either individual floater crystals or aggregates. Associated species include aquamarine, hyaline opal, muscovite, orthoclase and quartz. A handful of specimens of a unique vermiform schorl was collected during January 2006; they are miniatures consisting of contorted finger-like groups of crystals. Most of the farms located in the southwestern, western and northwestern Erongo Mountains have yielded outstanding schorls during the past 5 years.

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One interesting aspect of the Erongo tourmalines is the internal color zonation that is revealed when very thin sections are sliced, either perpendicular or parallel to the c axis of crystals (Rustemeyer and Deyer, 2003); perpendicular sections tend to reveal blue colors while the parallel slices are mostly red-brown. Multiple colored zones that are either trigonal or pseudohexagonal in shape occur in the crystals. These are reminiscent of the the spectacular color-zoning in the famous Madagascar liddicoatite crystals, and colors include yellow, orange, blue and green. Some zones form mosaic patterns that change in shape and color from the base to the termination of the crystal. Hemimorphic schorl, in particular, displays various stages of crystal growth that are reflected by different episodes of color contrast--orange-brown in the prism sections of the crystal, violet-red at one end, and dark brown on the opposite termination (Rustmeyer and Deyer, 2003). However, black tourmaline, and schorl in particular, typically displays varying colors when viewed as thin sections in transmitted light (Zang and Fonseca-Zang, 2002), so this phenomenon is not confined to the Erongo schorls.

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Siderite [Fe.sup.2+]C[O.sub.3]

Pristine, unaltered siderite is virtually unknown at Erongo. There is, however, a large specimen in the Nagele collection composed of unaltered siderite with interlayerings of rhodochrosite. Goethite pseudomorphs after siderite are very common, the rhombohedral crystals of the original siderite being now partially or wholly replaced by goethite. These specimens have periodically been collected from the diggings on the western and southwestern sides of the Erongo Mountains. In September 2001, some excellent examples of these pseudomorphs were collected; the most notable one is a single 27-cm crystal weighing 9.4 kg (in the Bahmann collection).

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Stolzite PbW[O.sub.4]

Rare stolzite microcrystals have been found. They are pale orange and occur on orthoclase. In May 2006, two large, pale gray stolzite crystals were collected. One is tabular, euhedral and looks superficially like wulfenite. The other specimen has no distinct crystal faces but is composed of smooth, curved surfaces suggesting partial resorption.

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Topaz [Al.sub.2]Si[O.sub.4](F,OH)[.sub.2]

Erongo topaz is characteristically partly etched, with relatively well-developed crystal faces. It is typically colorless but can be pale green or pale blue, ranging from partly translucent to transparent. Associated species are quartz, orthoclase, fluorite and schorl. The largest known crystal measures 20 cm. The crystals occur singly, with no matrix, or in small clusters. Small schorl crystals are occasionally found included in topaz.

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Tridymite Si[O.sub.2]

Tridymite has been reported from the Erongo Volcanic Complex (Pirajno, 1990).

Uranophane Ca(U[O.sub.2])[.sub.2](Si[O.sub.3](OH))[.sub.2] x 5[H.sub.2]O

Stellate groups of uranophane microcrystals associated with botryoidal hyaline opal, muscovite, topaz and orthoclase came from the uranium pocket (see under metanovacekite for details). Some of the clusters of acicular yellow uranophane crystals are coated by opal that masks the uranophane's habit and color. Amorphous yellow uranophane aggregates from the Brabant (van der Made) pegmatites are described by Roering (1963).

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Walpurgite (BiO)[.sub.4](U[O.sub.2])(As[O.sub.4])[.sub.2] x 2[H.sub.2]O

Walpurgite, a rare bismuth uranium arsenate, occurs together with violet fluorite, topaz and quartz, as small (maximum 1 mm), flat, tabular amber-brown to yellow crystals. Jahn (2003) speculates on the source of the bismuth, and comments on its absence (to date) in the miarolitic cavities. However, Krantzberg contains sporadic bismuth mineralization and the nearby Etiro pegmatite is famous for its massive and crystalline native bismuth (Miller, 1969; Cairncross, 2004).

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Xenotime-(Y) YP[O.sub.4]

Small, yellow xenotime crystals are associated with aquamarine, fluorite and quartz (von Bezing, 2006). The xenotime-(Y) has a typical oily-resinous luster. Small crystals have been found perched on metazeunerite from the uranium pocket. The Erongo crystals contains up to 4% Dy and 3% Gd (Jahn, 2003).

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Zinnwaldite Kli[Fe.sup.2+]Al(Al[Si.sub.3])[O.sub.10](F,OH)[.sub.2]

There is some confusion regarding the existence of zinnwaldite in the Erongo miarolitic cavities and surrounding pegmatites. Frommurze et al. (1942) provide optical and chemical analytical evidence for gray-green zinnwaldite from the Erongo Schlucht pegmatites, and it has been elsewhere reported from the Erongo Mountains (Charles Key, personal communication, 2003). However, later studies of the same deposit (Roering, 1963) found only lithian muscovite. Small rosettes of gray-green zinnwaldite from miarolitic cavities in the Erongo Mountains are reportedly in the Harvard collection (von Bezing, 2006).

Pseudomorphs

Several interesting pseudomorphs have been found in the Erongo miarolitic cavities. Some of the most unusual, collected in August 2003, are quartz after orthoclase. They are either single crystals measuring several centimeters or groups of crystals to 30 cm. Various stages of pseudomorphic replacement are evident, from quartz coatings (epimorphs), to partial or complete replacements of feldspar by quartz. Some of the quartz is oriented epitactically on the orthoclase. The latter specimens usually have a hollow framework shell of quartz, the inner part of the hollow cavity being lined by drusy schorl and small quartz crystals. The replacement quartz consists of flattened plates of variably aligned, short quartz crystals that superficially resemble Japan-law twins (Jahn et al., 2003).

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Several examples of goethite pseudomorphing other species are known. The best of these are rhombohedral, brown to rust-red goethite after siderite(?), goethite after octahedral magnetite(?) and goethite after trigonal ilmenite. These goethite pseudomorphs have scattered drusy crystals of pyrolusite, ilmenorutile and romanechite-cryptomelane on the altered crystal surfaces.

Muscovite after orthoclase crystals has also been found. Excellent examples of these pseudomorphs are seen where the finegrained muscovite has completely replaced Carlsbad-law twinned crystals. Pseudomorphs of aquamarine beryl after twinned orthoclase are described and illustrated by Gebhard (2002).

Inclusions

Some of the Erongo minerals, notably quartz, aquamarine and fluorite, have interesting included minerals. These are listed in Table 5.

Unknowns

Several species from Erongo have yet to be identified and are awaiting analysis. Most have been found as microcrystals.

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AUGUST 2005 FIELD TRIP TO THE ERONGO MOUNTAINS

During August 2005, we spent several days on a fact-finding mission in the Erongo Mountains. The trip was undertaken to obtain up-to-date information for this article, to get photographs of the site, and to interview some of the people in Namibia who have been instrumental in collecting Erongo specimens. One of us (UB) set up a number of meetings and contacts with the local experts so that we could view their collections and also be guided in the field to some of the famous pockets that had produced specimens during the previous five years.

Windhoek is a two-hour flight from Johannesburg International Airport, and I (BC) flew up on Saturday August 6, 2005; Uli had already made the trip by road a few days before. The quickest way to drive to Namibia from South Africa is via the Trans Kalahari Highway that transects west-central Botswana. This trip takes approximately 12 hours, depending on time taken at the border crossings into Botswana from South Africa and into Namibia from Botswana. We spent Sunday in the company of Windhoek dealers and collectors Herbert Nagele and Ernst Schnaitmann, who have been collecting Erongo specimens since late 1999. They have a systematic collection of most species from Erongo, and some of their specimens are illustrated in this article. This visit was followed by an afternoon trip to see Andreas Palfi, a local dealer who is in partnership with Ralf Wartha. After viewing their stock of specimens (not only from Erongo, but from many other Namibian localities too) we decided to turn in for the evening, as we had an early start the next morning. After a long day of looking at minerals, "Joe's Beer Garden" in Windhoek provided a welcome respite. No visit to Windhoek would be complete without a meal at this famous watering hole.

On Monday morning, we left our bed-and-breakfast at 6:00 sharp, as we had an appointment to meet Gerd Bachran at 9:00 at the Erongo Mountains. The trip from Windhoek is via the main paved road that ultimately ends at the coastal town of Swakopmund. Driving from Windhoek, one passes through Okahandja and then on to the village of Karibib, then, 40 km further on, Usakos. These two towns are well-known for being in the heart of the Usakos-Karibib pegmatite belt that has produced excellent tourmalines and other mineral specimens. The journey westwards leads through progressively more arid regions, with thornveld and white-bleached grass slowly giving way to rocky and sandy terrain. This region of Namibia is traditionally dry, but absence of seasonal rains made it even more so in 2005 (4).

In Usakos, a dirt road leads northwestward, skirting the western extremity of the mountains. This particular morning, there was an eerie fog blanketing the whole region--this is commonplace along the Namibian Atlantic coast, particularly at Swakopmund, where the cold Atlantic Benguela current causes early morning mist that can extend some distance inland. It was unusual to see it that far from the coast, and the morning was chilly. The Erongo Mountains could be seen looming off in the distance in this foggy mist. As we approached the southwestern part of the mountains, the fog began to lift, providing unusual views of the mountain peaks jutting through the mist. The road we were on continued to the settlement of Tubussis, but we turned off to enter the farm Bergsig, where the Hohenstein Lodge is located. We were met on arrival at the lodge by our host for the next few days, Gerd Bachran. He lives in Swakopmund and knows the area and the local Damara who dig for minerals. He had arranged for one of the locals, David, to guide us up into the mountains to view the diggings. The tracks that lead up to the foothills of the mountain are rugged and ideally require a four-wheel-drive vehicle. We parked at the bottom of the mountain and prepared to hike up. The first part of the hike is along a well-worn path that winds through the undergrowth between granite boulder scree that has weathered and rolled down the mountain over the years. Even at this low-lying section, the potential for minerals is evident: "nests" of black schorl tourmaline within coarse quartz protrude out of many of the boulders. These are more resistant than the granite host rock, so they give a distinct knobby appearance to the rock faces.

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Our hike was aimed at a valley, or "Schlucht," as this provides the easiest access to the collecting areas. Otherwise one has to negotiate nearly vertical granite cliff faces. In some sections of the mountains, the local diggers rely on ropes to pull themselves up the steep slopes, sometimes with dire consequences. Shortly before our visit, one of the diggers was killed while making the climb: he had been carrying a jack-hammer on his back, and while he was pulling himself up, one of the ropes snapped and he tumbled to his death. Having this information passed on to us just before our ascent was rather sobering!

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The granite is very coarse-grained, composed of coarse quartz and interlocking feldspar laths up to 5 cm. This rock texture is advantageous for climbing up the steeply dipping rock faces because the rough surface provides a firm grip for rubber-soled hiking boots. Nevertheless, "walking" up 40[degrees]-60[degrees] slopes takes some getting used to. It goes without saying that our nimble guides were hopping up and down the outcrop with casual abandon! Early on in the climb, we heard a voice from on high shouting down, and when we looked up the rock face we could see one of the diggers in a crevice about 100 meters up, waving down to us--that was where we were heading.

Upon reaching some of the old excavated holes, we met up with another group of local diggers. After posing for the obligatory photographs (some with the subjects very seriously posed and staring off pensively into the distance) we proceeded upwards. We soon came to the zone where many specimens had been collected (GPS location S: 21[degrees]45'6.4"; E: 15[degrees]31'50.2"). All of the productive sites are now empty cavities in the granite, some with small tailings of waste material filtering down the slope from the mouths of the openings. The cavities vary in size; some are small, less than 10 cm in diameter and as deep, whereas others are tubular, 50 to 80 cm wide, and winding down to over 2 meters depth.

The diggers randomly select pockets to excavate, identifying them from the telltale "nests" of schorl and quartz. Not all yield prize specimens and some are merely clay-filled and barren. There is not much to be self-collected from the odd tailings that remain. Because the climb up and down the mountain is arduous, even for the nimble-footed, the local diggers have set up semi-permanent camps in areas where fallen rocks provide shelter. Some have built rudimentary tents and lean- to shelters while others use natural caves formed by huge granite boulders. Water can be a problem, as it does not rain often, but at this particular locality in Bergsig, one of the larger pockets had previously filled with rainwater and this provided a supply that David said would last for months.

We photographed several of the pockets while David explained what minerals had been taken from each pocket. While we were there in August, some excellent green fluorite was being excavated.

In the afternoon, we returned to the Tubussis road and then turned off heading west to visit the well-known green andradite operation several kilometers to the west of the Erongo. When we arrived at the workings, which are fenced in, we were faced with a locked gate and greeted by an amicable machine-gun-carrying security guard. We asked permission to enter and take some photographs but this was politely denied. No mining was taking place on the day of our visit, so we decided not to waste any time and drove on to the farm Tubussis 22, in the northwestern part of the Erongo Mountains, where the original, major discovery of schorl had been made in 1999. Gerd Bachran had been instrumental in the that discovery.

After obtaining the key to the farm gate, we proceeded to the pocket where large (greater than 10-cm) orthoclase feldspar crystals had been excavated, together with schorl and yellow hyaline opal. This was another pipe-like cavity about 2 meters deep and 60 to 70 cm in diameter (GPS location S: 21[degrees]34'28.4"; E: 15[degrees]31'4.5"). On a nearby rock face, several Bushman paintings could clearly be seen in the late afternoon sun. These depict various antelope, notably kudu and eland, as well as giraffes and several caricatured humans.

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We spent that Monday evening at the Ameib Ranch, located on the farm Ameib 60, adjoining Davib Ost 61, the other privately owned farm from which Erongo minerals have been (illegally) collected. On Tuesday morning, we returned to the Hohenstein Ranch on Davib Ost 61, but this time drove on farther to the southeast of the mountain range. The purpose of this morning's trip was to visit the 2001 excavations that had produced the jeremejevite. Some of these crystals had been collected from alluvium while others had been removed from in situ pockets in the granite.

Tuesday was somewhat warmer than the previous couple of days had been, and our climb up the granite this time was far more arduous because the slopes are steeper and one had to be careful not to lose footing and tumble down the rock face. The ascent was made more comfortable by zigzagging up the slope, again with David leading the way. We climbed up from the farm Davib Ost 61, but then began to move southeastwards towards the boundary fence with the neighboring Ameib 60 farm--the jeremejevite came from close to the junction point between these two farms. En route, we passed several excavations that had yielded schorl, quartz and aquamarine (GPS location S: 21[degrees]45'20.4"; E: 15[degrees]34'47.6").

[FIGURE 337 OMITTED]

[FIGURE 338 OMITTED]

[FIGURE 339 OMITTED]

We had to traverse around the front lobe of the granite mountain in order to get a route across to the jeremejevite diggings. Some of the cavities that we observed are large, a few meters across and a couple of meters deep. As at the Bergsig diggings, there was evidence of diggers occupying boulder overhangs in the mountains and using these for shelter.

We then stopped at an excavation that had the largest "tailings dump" we'd so far encountered. This material had been removed from a substantial pocket about 5 to 6 meters wide and 4 to 5 meters deep. Apart from aquamarine, smoky quartz and opal, David explained, this particular pocket had yielded the highly lustrous, complex cassiterite crystals in July 2004. Scratching around in the residue, it was easy to find smoky gray to black quartz crystals up to 6 cm and some highly fluorescent lime-green, botryoidal hyaline opal (GPS locality S: 21[degrees]45'26.1"; E: 15[degrees]35'0.4"). Most of the single quartz crystals display an interesting tapering of the prism faces towards the termination.

Standing at this cassiterite cavity, one looked downslope towards the southeast, and a few hundred meters away the empty jeremejevite pockets and rubble were clearly visible. We clambered down to this area, which is pockmarked with small excavations, most less than a meter across and a meter deep, although a few are larger. There has been some systematic work carried out here, as indicated by the cavities left behind by jackhammers. The exact locality of the jeremejevite discovery is GPS location 21[degrees]45'26.4"; E: 15[degrees]35'2.7".

We scratched around in the tailings for anything of interest, but only found one tiny chip of blue jeremejevite in feldspar--the diggers do not leave much behind. The view from the top of this granite koppie is breathtaking, however. One can clearly see the Gross Spitzkoppe to the northwest and the vast, flat plain leading away from the Erongo foothills. The dry Khan River snakes off into the distance, and one of the old cassiterite pegmatites on the plains farm Ameib 60 can also be seen.

That concluded our trip to the western and southwestern fringes of the Erongo Mountains. Our next goal was to visit the northeastern section, where the Krantzberg mine had operated. This mine is now closed but was worked for tungsten and cassiterite intermittently between the 1920's and 1979. It is located on the upper slopes of Krantzberg Mountain, a prominent feature that has a circular "wreath" of sedimentary rocks near the crest of the mountain: hence its German name, Krantzberg or "wreath mountain."

The mine is on private farmland, and at first it was not easy to get permission to access the mine. The problem was that the mine workings fall on the boundary between two neighboring farms, and on one of these the farmer was entertaining a hunting party, so he was reluctant to allow us on the property. We did not like the idea of getting shot, so we tried to contact the adjacent farmer, Herr Decker. He happened to be away in Germany, but his resident manager was happy to allow us in to see the mine. In fact, he even drove us up to the locality in his four-wheel-drive vehicle. The old track leads up to what appeared to be an adit in the side of the mountain where some old mining equipment was lying around. We soon noticed that it was in fact not an adit but a loading area where ore had been dropped from an upper level and then trucked away.

[FIGURE 340 OMITTED]

The view from Krantzberg is magnificent. Looking up the precipitous slope, we could see that the access adit was in fact higher up the hill. There are some old concrete steps that obviously had been used to communicate between these two levels. Uli and the farm manager's assistant, Nelson, climbed up, pulling themselves up over the last 20 or 30 meters on an old telephone cable tethered to an iron pole anchored in the ground. The memory of the fatality at Bergsig a few weeks earlier was still fresh in my mind, so I passed on that option; instead, I used the old access road to reach the adit on the upper level.

This adit had been driven horizontally into the mountain. We explored the underground mine for a distance of a few hundred meters into the old workings. The air is very fresh because several openings where rock had caved in from higher levels had broken through to the surface, admitting light and fresh air. Further into the mine the air becomes foul, exacerbated, no doubt, by the many bats we found dangling off various old cables and rock hangings. We also found leopard droppings and spoor (tracks), but the owner of these was thankfully not home at the time! We found evidence of some drusy cassiterite and blue-green secondary copper staining, but no sulfides, ferberite or fluorite.

That concluded our excursion to the Erongo Mountains. We capped off our Namibian trip by spending two days driving to Swakopmund via Uis and the Goboboseb Mountains. In the old tin mining village of Uis we met with mineral dealer and adventure tour leader Monty van der Smit, who showed us some of his recent acquisitions from the Erongo region and environs. We also visited several dealers in Swakopmund, which is now the main focal point for Namibia's mineral shops. However, there was nothing really exciting to be seen, neither in their stock nor in the collection of the Swakopmund Museum, located on the beachfront. This museum has a suite of Erongo minerals, but nothing extraordinary.

On the road back from Swakopmund to Windhoek, one drives by the turnoff to Henties Bay, and this road passes close by the Klein Spitzkoppe. About two dozen or so local diggers who have set up tables at this intersection offer a variety of mineral specimens from numerous Namibian localities. We stopped for a while and found some interesting Erongo specimens, including small jeremejevite crystals on orthoclase matrix, unusual orange hyaline opal that we had not seen before, thumbnail specimens of fluorite included in aquamarine and an assortment of other specimens such as colorless beryl on yellow muscovite, schorl and quartz.

[FIGURE 341 OMITTED]

CONCLUSION

The Erongo Mountains offer huge potential for minerals in the future, as vast areas are still unexplored. Much of the northern, central and western parts of the Erongo Mountains form part of the Erongo Mountain Nature Conservancy, a protected wilderness area where private landowners take a dim view of any trespassers and especially iterant mineral collectors. There are other private farms in areas outside of the Nature Conservancy as well, so one would be well advised to seek permission from the owners before embarking on any mineral collecting expeditions. The same applies to Ameib 60, Davib Ost 61 and Bersig 167, on all of which specimens have been collected. This is also privately owned land. Furthermore, it is illegal to remove any mineral, gemstone or geological specimens from Namibia without the necessary official export documents. These can be obtained from the Geological Survey offices in Windhoek.

ACKNOWLEDGMENTS

We are grateful to the individuals and institutions who assisted us in various ways to help bring this article to the light of day. Gerd Bachran of Swakopmund gave assistance in the field and mineralogical information on the various pockets, and donated jeremejevites for analyses. David from Tubussis was our guide for a few days in the Erongo Mountains. Andreas Palfi and Ralf Wartha of "Namibia Minerals," Windhoek, shared their knowledge and showed us their recent Erongo specimen acquisitions. Monty van der Smit of "Adventure Tours & Minerals," based in Uis, granted us access to his stock of Erongo minerals. Frau Kogel, the owner of Ameib Ranch where we stayed, allowed access to her farm and property. Herr Decker of the farm Otjohotozu provided transport and granted permission to visit the Krantzberg mine. Desmond Sacco provided historical information on the Krantzberg mine and permitted photography of his specimens. Dr. Ludi von Bezing provided pictures of his Krantzberg specimens. Peter Tandy, Curator of Minerals (Curation & Mineral Systematics) at the Natural History Museum, London, assisted BC in examining historic Erongo specimens in the Natural History Museum's collection. Rob Smith provided aquamarine specimens for analyses. Ellen de Kock searched the Museum Africa's Geological Museum collection database for Erongo specimens. Last but not least we thank Anka Bahmann for sustenance during the August 2005 field trip to the Erongo Mountains. Notwithstanding all this collaboration, we are responsible for the final content of this article.

We are particularly grateful to Herbert Nagele and Ernst Schnaitmann of Windhoek's "House of Gems." Ernst photographed specimens from their Erongo collection and further provided up-to-date information on Erongo and its minerals.

Finally, Dr Wendell Wilson is gratefully acknowledged for his thorough editing of the manuscript. Tom Moore and Dr. Bill Birch also reviewed the article text.

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Bruce Cairncross

Department of Geology

University of Johannesburg

P. O. Box 524 Auckland Park, 2006

Johannesburg, Gauteng, South Africa

Email: bc@rau.ac.za brucec@uj.ac.za (from 2007)

Uli Bahmann

P. O. Box 873

Halfway House, 1685

Gauteng, South Africa

Email: ubahmann@baitsa.co.za

RELATED ARTICLE: KRANTZBERG MINE

One of South Africa's best known mineral collectors, Desmond Sacco, has historical links to mining at Krantzberg. African Mining and Trust, under the Chairmanship of Guido Sacco, Desmond's father, mined the deposit during the mid-20th century. One of us (BC) recalled hearing Des Sacco's story about his father and specimens from the mine, and this prompted an informal interview:

BC: What was your family's involvement in mining at Krantzberg?

DS: My father, Guido Sacco, was Chairman of African Mining & Trust, a company started in 1932, but they only began to mine the tungsten deposits at Krantzberg in the 1950's. The company mined the deposit from the 1950's until mid-1960. A mechanical engineer, Walter Parker, was dispatched to Krantzberg and he constructed a processing plant and other infrastructure to get the mine up and running. He also recruited miners.

All the mine personnel lived in nearby Omaruru. In fact, several years ago when I was in Namibia, I visited Omaruru and went to the hotel to have a drink. There was this old-timer sitting at the bar and we struck up a conversation. I said that I'd come to Omaruru just to see where my father had worked. He asked who my father was and I answered "Guido Sacco." The old guy replied: "What? That famous man!" He clearly recalled Guido's involvement at Krantzberg and even the presence of Walter Parker, the engineer. He then proceeded to take me to the Omaruru River to a spot where at the time of mining, my father's car got stuck in the river bed during a rain storm and they just managed to free it before the area was hit by a flash flood that would undoubtedly have washed them all away.

I remember that my father used to take three to four days to make the trip from Johannesburg to Omaruru by car--he traveled in a blue Buick Roadmaster, with the luggage tied to the roof of the car. He would spend several weeks at the mine before returning to South Africa. Even though he was Chairman of the company and of other companies, he loved going to the field; he spent a large part of his life in the Postmasburg and Kalahari manganese fields, that he was instrumental in discovering. He traveled to some other well-known South African deposits such as Palabora and the Consolidated Murchison mine. He also traveled extensively in Zimbabwe exploring for chromium.

BC: What were early field conditions like at Krantzberg?

DS: Conditions were very tough. Malaria was still prevalent in the region and black water fever was also sporadically contracted. But the grade of the ore at the mine was spectacular. My father said that he could not believe how rich the tungsten (ferberite) ore was. They would stope areas of solid ore and crush it and there was hardly any need to concentrate the ore because it was so pure. It was merely put into containers and shipped out.

BC: What are your personal memories of specimen production from the mine?

DS: Krantzberg was not a prolific specimen-producing mine and nobody really seriously collected minerals, like at Tsumeb, for example. But my father would occasionally bring back specimens. I remember that he once brought home two dinner-plate-sized ferberite crystals. They were impressively large, with well-developed crystal faces. This was in the early 1960's. A few years after this, I personally donated two ferberites to the Johannesburg Geology Museum, to the curator, Townsend. I remember that Townsend had a few other Krantzberg ferberite crystals in a small display case in his office, but I believe these have subsequently disappeared. (1)

BC: What Krantzberg specimens do you have in your collection and how and when did you acquire them?

DS: I have no ferberites in my collection, but I do have three Krantzberg cassiterites. One is a very large, brown crystal (see Fig. 99) that I got from my father. The other two are highly lustrous black crystals.

BC: Did African Mining and Trust company donate specimens to any other local or overseas museums?

DS: No. As I mentioned, the mine was not really known for producing masses of quality specimens. Apart from the few specimens I donated to the local Johannesburg museum, none were formally donated to other museums. (2)

(1) During research for this article, the current collection and inventory of the Johannesburg Geological Museum was searched for Krantzberg specimens, but none were found, corroborating the loss of these minerals from this institutional collection.

(2) Krantzberg did produce mineral specimens, and most appear to be in southern African collections. An acquaintance of Windhoek dealer-collector Herbert Nagele has searched a number of private and museum collections in Germany for Krantzberg specimens, but to no avail (Herbert Nagele personal communication, March 2006). The Nagele collection in Windhoek has a suite of Krantzberg specimens, as does the Ludi von Bezing collection in Kimberley. Von Bezing has an outstanding museum-size specimen of fluorite and quartz, which he acquired from the Nambian collector Gawie Cloete.

(3) The Erongo tourmalines are currently under investigation to determine which species occur at this locality (Frank Hawthorne, personal communication, 2005). For the purpose of this article, specimens that have been identified as foitite are described here; all others are described under schorl.

(4) In January-February 2006, the highest rainfall in decades produced waist-high grasslands throughout the region.
Table 1. Chronology of some significant discoveries at Erongo (Data from
U. Bahmann collection and Gebhard, 2002).

Date                   Description

February-March 1999    First major pocket of schorl and large topaz
                       crystals
May 1999               Schorl pocket with trigonal "Mercedes Benz"
                       terminations
September 1999         First major pocket of green beryl
October 1999           "Cauliflower" schorl with siderite
November 1999          Quartz scepters
April 2000 (Easter)    First major aquamarine pocket of approx. 250-300
                       highgrade specimens
May 2000               Schorl, foitite and monazite-(Ce)
August 2000            First discovery of fluorapatite and first large
                       pocket of colorless beryl
October 2000           Associations of quartz, fluorite and dolomite
November 2000          Network of interlocking aquamarines
January-February 2001  "Garnet-like," pseudo-isometric schorl
March 2001             Jeremejevites
May 2001               Yellow-green beryl, monazite and Japan-law
                       twinned quartz
June 2001              Second discovery of yellow beryl and cassiterite
September 2001         Aquamarine "cotton-reel" habit and large siderite
                       crystals
August 2002            Discovery of zinnwaldite crystals and ilmenite
May-June 2003          Cassiterite
July 2003              Pocket of hydroxyl-herderite
August 2003            Pseudomorphs of quartz after orthoclase and third
                       major pocket of yellow beryl
September 2003         Metazeunerite, metanovaceckite and associated
                       uranium species
October 2003           Gold
July 2004              Second pocket of cassiterite
August 2004            Goethite aggregates
July 2005              Major fluorite discovery associated with white
                       orthoclase
August 2005            Another pocket of colorless beryl
January-February 2006  Stalactitic fluorite; orange-zoned beryl;
                       stellate colorless beryl; vermiform schorl;
                       elongate, twinned orthoclase, galena
April-May 2006         Jeremejevite

Table 2. Erongo Mountains Deposits and Associated Pegmatites.

Pegmatite-Mine       Farm Name          Economic Minerals

Sandamap             Sandamap           Cassiterite; ferrotantalite
                     North 115
Cameroon pegmatite   Goabeb 63          Cassiterite-equivalent to
                                        Sandamap
Sidney pegmatite     Davib West 62 and  Cassiterite; lepidolite
                     northern boundary
                     with Goabeb 63
Borna pegmatite      Davib Ost 61       Cassiterite
Carsie pegmatite     Davib Ost 61       Cassiterite; ferberite;
                                        columbite (Fe?); ferrotantalite;
                                        monazite; molybdenite
Davib mine           Davib Ost 61       Cassiterite; ferrotantalite;
                                        ferberite; amblygonite;
                                        lepidolite
Tubussis pegmatites  Tubussis 22        Gem green andradite garnet
Ameib pegmatites     Ameib 60           Cassiterite
(several)
Drews pegmatite      Kudubis 19         Cassiterite (up to 50 kg
                                        aggregates)
Brabant (also known  Brabant 68         Cassiterite; lepidolite;
as Erongo Schlucht,                     ferrotantalite
Van der Made
pegmatite)
Pietershill          Erongorus 166      Cassiterite; ferrotantalite
Elliot claims        Erongorus 166      Cassiterite
Schimanski's claims  Erongorus 166      Cassiterite
Wendroth's workings  Erongorus 166      Cassiterite
Krantzberg (not the  Kranzberg 59 and   Cassiterite
tungsten mine in     Onguati 52
the northwest)
Riverplaats          Riverplaats 97     Cassiterite
Etiro pegmatite      Etiro 50           Beryl, muscovite, columbite-
                                        tantalite, K-feldspar, bismuth
Kanona-Erongo        Erongo West 83     Cassiterite (2.5 cm crystals)
workings             and Kanona West    in southwest portion of Erongo
                     84                 West 83)
Krantzberg           Pistelwitz 128     Ferberite; cassiterite
                     and Omaruru
                     Townlands 85
Giftkuppe            Boundary between   Rutile--in drusy fissures, lined
                     Erongo Ost 82 and  with quartz, albite and Cr-
                     Kanona Ost 81      muscovite. Rutile crystals up to
                                        20 cm

                     Accessory-Secondary
Pegmatite-Mine       Minerals                   References

Sandamap             Dark green elbaite;        Frommurze et al. (1942);
                     schorl; triplite;          Diehl (1992a); Gevers
                     K-feldspar as large,       and Frommurze (1930).
                     euhedral crystals
Cameroon pegmatite   Schorl; triplite           Diehl (1992a)
Sidney pegmatite     Schorl; grossular garnet;  Frommurze et al. (1942);
                     "huge feldspar crystals";  Diehl (1992a)
                     triplite
Borna pegmatite      Albite; schorl; triplite   Frommurze et al. (1942)
Carsie pegmatite     Muscovite as "large        Frommurze et al. (1942)
                     books"; schorl of
                     "tremendous size";
                     nontronite
Davib mine           Schorl; green elbaite      Frommurze et al. (1942)
Tubussis pegmatites  Aquamarine, diopside,      Grolig (2005);
                     calcite, epidote,          Niedermayr (2000)
                     magnesio-axinite,
                     prehnite, quartz,
                     vesuvianite
Ameib pegmatites     Large K-feldspar           Frommurze et al. (1942);
(several)            crystals; apple-green      Wagner (1916)
                     fluorapatite
Drews pegmatite      Schorl; muscovite;         Frommurze et al. (1942);
                     triplite                   Gevers and Frommurze
                                                (1930)
Brabant (also known  Schorl; zinnwaldite;       Frommurze et al. (1942)
as Erongo Schlucht,  garnet; fluorapatite;
Van der Made         muscovite; pink albite in
pegmatite)           vugs; purple fluorite in
                     vugs
Pietershill          Schorl; blue-black         Frommurze et al. (1942)
                     tourmaline
Elliot claims        Schorl; triplite;          Frommurze et al. (1942)
                     andradite; spessartine
Schimanski's claims  Schorl; andradite          Frommurze et al. (1942)
Wendroth's workings  Muscovite; hematite        Frommurze et al. (1942)
                     nodules; schorl; dark-
                     green elbaite
Krantzberg (not the  Schorl; blue elbaite on    Frommurze et al. (1942)
tungsten mine in     Onguati 52
the northwest)
Riverplaats          Brown, green pink, blue    Frommurze et al. (1942)
                     tourmaline; garnet
Etiro pegmatite      Topaz, brazilianite,       Miller (1969)
                     eosphorite
Kanona-Erongo        Schorl; garnet             Frommurze et al. (1942)
workings
Krantzberg           Fluorite; beryl            Frommurze et al. (1942);
                     (aquamarine); schorl       Schlogl (1984)
Giftkuppe            Schorl; muscovite;         Frommurze et al. (1942)
                     K-feldspar; pyrite;
                     pyrrhotite; chalcopyrite;
                     marcasite

Table 3. Minerals of Krantzberg (pro parte from Haughton et al.,
1939; Schlogl, 1984; von Bezing, 2006; and the authors' own
observations).

Albite
Arsenopyrite
Azurite
Beryl
Biotite
Bismuth
Calcite
Cassiterite
Chalcopyrite
Danburite
Dravite
Ferberite
Ferro-axinite
Fluorapatite
Fluorite
Garnet (almandine?)
Goethite
Goyazite
Hematite
Malachite
Molybdenite
Monazite
Muscovite
Opal variety hyalite
Plumbogummite
Pyrite
Quartz
Rutile
Scheelite
Schorl
Scorodite
Titanite
Topaz

Table 5. Mineral inclusions

Species          Included Minerals

Aquamarine       Fluorite, orthoclase, schorl
Fluorite         Aquamarine, schorl
Colorless Beryl  Orthoclase, schorl
Quartz           Fluorite, magnetite, orthoclase, schorl
Topaz            Orthoclase, schorl
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Author:Cairncross, Bruce; Bahmann, Uli
Publication:The Mineralogical Record
Article Type:Cover story
Geographic Code:6NAMI
Date:Sep 1, 2006
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