Cobalt minerals of the Katanga Crescent, Congo.
HISTORY OF COBALT
Unlike copper, which has been known for millennia, metallic cobalt was not definitely isolated until the 18th century. The coloring properties of its oxides have nonetheless been known since the most ancient times, particularly in the Middle East, Egypt and China. Materials as varied as pottery, earthenware, glass, enamelware and pearls were tinted every shade of blue by heating with cobalt oxides at high temperatures in clay furnaces. Objects could also be decorated with paints derived from these same cobalt oxides.
The cobalt ores used initially came from Persia and Burma. After roasting at red heat, followed by fine grinding, a pigment with a very high coloring capacity was obtained; the "Egyptian blue" and the "China blue" of those times were simply "cobalt blue."
It is likely that the alchemists of the Middle Ages, who had mastered the technique of roasting sulfide ores, followed by reduction to obtain the native elements, produced cobalt under another name well before the 13th century. Subsequent to the Middle Ages and in Central Europe, cobalt ores closely associated with silver, lead, nickel and bismuth ores were extracted from mines in Saxony and Bohemia. Since cobalt was not of any economic value at that time, it posed a considerable problem in the metallurgical processing of the so-called "useful" metals. In addition, since the cobalt was associated with sulfur and arsenic, the roasting process released noxious fumes. As a result, this mineral was regarded by the miners as a divine curse.
The term "cobalt" comes from the German Kobold, which means "demon," or, according to an old Germanic legend, the evil-minded gnome-like spirits of the mines. The etymology that evokes goblins or gnomes also raises another possibility. The origin of the word is perhaps to be found in the height of the miners. Perhaps the exploitation of the mines in Saxony required workers of small size, as it did earlier in the ancient mines of Laurium in Greece.
In any case, the name cobalt appeared in the literature on mining for the first time (in the Latin forms cobaltus and cobaltum) in the celebrated treatise on the mining operations in Saxony and Bohemia entitled De Re Metallica (1556) by Georgius Agricola. It was not until 1735 that the Swedish chemist Georg Brandt isolated cobalt as a pure element. The research subsequently undertaken on this new element led to industrial applications that were far more important than its simple use as a coloring agent.
COBALT MINING WORLDWIDE
By the early 19th century, cobalt deposits had been found more or less everywhere in the world, particularly in Germany (the Black Forest, Harz, Hesse), Norway, Sweden, Transylvania (now part of Romania), France, Spain, England, Chile, Argentina, Tasmania, etc. The chronology of the most recent discoveries of large deposits is as follows:
1874: New Caledonian deposit
1904: Ontario deposits (Canada)
1907: Katanga deposits (Democratic Republic of the Congo)
1930: Bou Azzer deposit (Morocco)
1933: Deposits of the Zambian Copperbelt which overlaps
1939: Outokumpu deposits (Finland)
Other deposits of varying importance have also been discovered in the United States (which is also the leading consumer of cobalt), in Kazakhstan, in Azerbaijan, and in the lateritic soils of Cuba.
Cobalt has been mined intermittently in the United States for more than a century, at localities in Idaho, Maryland, Missouri, Pennsylvania, Virginia and Wisconsin. Known reserves are abundant but generally too low-grade to be economical. Today only a few mines in Pennsylvania are producing cobalt, as a byproduct of iron mining.
The old Blackbird mine in Idaho is among those that once produced cobalt, despite a very low ore grade (0.015% Co). Reserves there are extensive, consisting primarily of copper ore with trace amounts of cobalt. The mine was shut down for purely environmental reasons.
At best, domestic U.S. production of cobalt has rarely exceeded 1,000 tons per year. Consequently, the United States remains heavily dependent upon foreign producers for the roughly 20,000 tons of cobalt it consumes each year.
Cobalt production worldwide is almost invariably a byproduct of mining for other metals, and ore grades are typically very low (a few hundredths to a few tenths of a percent Co). The rare exceptions where ore containing over 1% Co is found include Bou Azzer, Morocco; the hydrothermal veins in the Cobalt-Gowganda area, Ontario; and of course the Katanga Crescent copper-cobalt deposits. High-grade cobalt ores in Katanga occur mainly in the upper enrichment zones of oxidized orebodies. Despite the enrichment, cobalt content alone is generally too low to make mining profitable; as elsewhere, the cobalt has in the past been produced primarily as a byproduct, in this case of copper mining.
The most important producers of cobalt today are Zambia, Canada, Kazakhstam and Azerbaijan. Congo was the leader until the 1970's, producing up to 16,000 metric tons per year. At present, cobalt production in Congo is negligible; but with reserves estimated at 600,000 tons of metallic cobalt, and with good market conditions forecast, Congo may soon be a major cobalt producer once again.
KATANGA COBALT DEPOSITS
All cobalt produced in Katanga is taken from stratiform deposits. Thus far it has been a byproduct of copper production, but plans for the future call for cobalt to be extracted as the main ore metal, with copper as the byproduct. In the Katanga Crescent, the Union Miniere du Haut-Katanga (U.M.H.K.), which operated mines there from 1906 to 1966, has identified over 100 deposits containing both cobalt and copper. The size and grade of the orebodies vary over a substantial range, but they are always in the form of continuous layers in the lower part of the Roan System (R.2 group). The R.2 group has been dislocated and fragmented to some extent by tectonic movements and separated by breccias, some of which are also mineralized (by Cu or Cu+Co). Where cobalt is found, it is always associated with copper, both in the primary ores and in the oxidation zone.
There are two principal strata which tend to be mineralized (known generally as the "upper" and "lower" orebodies). The mineralization is always parallel to the stratification and somewhat patchy in distribution, with cobalt-rich patches overlapping copper-rich patches. Patches are typically separated by barren or very low-grade zones. Individual patches range from a few hundred meters to several kilometers in maximum dimension.
In general the upper orebody is richer in cobalt, but the lower orebody is thicker and more extensive (except at sites such as Kambove West). Cobalt-rich patches are generally smaller in extent than copper-rich patches. Cobalt distribution is somewhat irregular; in the huge Kolwezi Klippe (in which there are many mines including the Musonoi, Kamoto, Mutoshi, Mupine, Mashamba and Dikuluwe) cobalt content decreases steadily from south to noah, disappearing entirely in the northern half of the Klippe. Cobalt content is generally lower than copper, though still substantial relative to other world deposits (see Table 1).
Cobalt recovery in Katanga began in 1925 at the Panda-Likasi electrical refinery. An iron-cobalt mixture called "white alloy" was produced from this pilot plant. Regular production began in the 1940's, at the Shituru-Likasi electrolytic plant, and in the 1960's at the Luilu-Kolwezi plant.
Actual production of cobalt ore first began in 1907 with the opening of the famous Star of the Congo mine near Lubumbashi, the capital of Katanga province. Table 1 shows the ore grades for Cu and Co at mines in the Katanga Crescent which contain at least 0.3% cobalt.
It should be noted that in the upper portion of the oxidized zones of the deposits there are local enrichments, mainly of cobalt, in which a heterogenite heap is more resistant than the copper oxides to being washed out by meteoric waters. This is the case in particular, at an outcropping of the Musonoi deposit, where exploitation of the particularly well developed "cobalt cap" was begun in 1940-41, while the mine itself did not begin operation until 1945. It is also the case with the upper zone of the Oliveira fragment, in the K.O.V. (Kamoto-Oliveira-Virgule) deposit, also in the western sector, where there is a superficial enrichment of cobalt, while the copper has been entirely leached away.
During World War II, cobalt became an important strategic metal. From 1939 to 1943, the U.M.H.K. increased substantially its cobalt ore production with the opening of the rich cobalt mines of Kasombo (southern sector), Mindigi and Kabolela (central sector), Kingamyambo, and the gossan zone of the Kamoto mine (western sector).
Although more than 400 mineral species containing essential copper are known (worldwide), the total number of cobalt species barely exceeds 50. Only 15 of these have thus far been reported from Katanga, 14 from the Cu-Co stratiform deposits and from vein deposits at Shikolobwe, and one from the pipe-like Kipushi deposit. The stratiform deposits have yielded carrollite, linnaeite, trogtalite, cobaltomenite, heterogenite (both 3R and 2H polytypes), julienite, kolwezite and sphaerocobaltite. The Shinkolobwe mine has yielded cattierite, carrollite, linnaeite, penroseite, siegenite, comblainite, oursinite and heterogenite (3R). And the Kipushi mine has produced colbaltite. Julienite (a thiocyanate of cobalt) has been found only at the Shamitumba deposit outcrop and may be quite recent in origin. Bieberite (cobalt sulfate) has formed as a post-mining mineral on timbering in old stopes above the water table.
It should also be noted that the alteration of carrollite (virtually the only primary cobalt ore mineral in the stratiform deposits) proceeds far more readily than the alteration of primary copper sulfides. Consequently, even where alteration of copper and cobalt sulfides begins simultaneously, and copper sulfides significantly exceed carrollite, the concentration of Co ions in solution will dominate during the deposition of Co-Cu secondary minerals until the carrollite has been exhausted. Thereafter only copper secondary minerals will form. This dephasing of the Co and Cu secondary minerals is particularly obvious at one of the Fungurume outcrops where (secondary Co) heterogenite is found associated with (primary Cu) chalcocite; unaltered carrollite is only found at a depth of about 300 feet below the outcrop.
The relatively small number of secondary cobalt species is a result of the limited compatibility and affinity of [Co.sup.2+] and [Co.sup.3+] ions for other possible ions in solution. The affinities are restricted to S, Se, and to a lesser extent Cu and Ni (for primary minerals), and [O.sup.2-], [OH.sup.-], [Mathematical Expression Omitted] and to some extent Cu and Ni (for secondary minerals). Physiochemical characteristics of the alteration environment may further restrict the possibilities. Cobalt, however, can replace copper, calcium and perhaps also magnesium to a limited degree in such minerals as cobaltiferous malachite and cobaltoan calcite and dolomite.
Katanga is the type locality for five of the known cobalt species: cattierite (from Shinkolobwe), comblainite and oursinite (from Shinkolobwe), heterogenite (the 2H polytype, from Mindingi) and kolwezite (from Musonoi).
Primary Minerals - Stratiform Deposits
Cu-Co sulfide mineralization in Katanga is localized into two orebodies. The so-called "lower" orebody of stratified, siliceous dolomite 10 to 15 meters thick, and the "upper" orebody of stratified, sandy, dolomitic shale 5 to 10 meters thick. These two major layers are separated by a massive, barren dolomite (the RSC Formation) of variable thickness ranging from a few meters to several tens of meters. The two orebodies are located at the base of the R, Group, an Upper Proterozoic member of The Roan Supergroup. The only cobalt sulfides present are carrollite and linnaeite, but the selenide trogtalite is also associated.
Katanga carrollite contains 36% Co and virtually no Ni, so the formula can be simplified to that of the pure end-member: Cu[Co.sub.2][S.sub.4] (except at Luishia, where a little nickel is found). Carrollite is the principal primary cobalt mineral in the stratiform deposits. It generally occurs in massive form, associated with massive copper sulfides such as chalcopyrite, chalcocite and bornite. Well-formed crystals have developed in the dolomitic beds of both main orebodies, in calcite veins crossing these dolomites, and in fissures and vugs in the walls of the lodes. The most common crystal forms are the cube and the octahedron in some combination; the cube or octahedron alone are exceedingly rare. Unaltered crystals are steel-gray to silvery white with a bright metallic luster. Altered crystals are dull and frosty, gray to black in color, and may bear a thin coating of heterogenite.
Remarkable crystals of carrollite have been found at four mines. The Musonoi open pit has yielded crystals to 5 cm on an edge. The Kambove West (underground) mine produced a 10-cm crystal which was unfortunately broken by blasting; race twins 3 to 4 cm in size have also been found there. And the Kamoto (underground) workings have been the source of perfect octahedrons and exceptional cubes to 3 cm on calcite. At the Luishia open pit, tiny, perfect, lustrous crystals have been found on a bed of magnesite. These are superb octahedrons nicely modified by truncated edges and low-angle tetrahexahedron faces. Unlike carrollite from other stratiform deposits, the crystals from Luishia contain several percent nickel. In the Shinkolobwe vein deposits a nickeliferous carrollite has also been found, but crystals there are exceedingly rare. Alteration of carrollite above the water table and on the dumps has often led to the deposition of sphaerocobaltite on skeletal, etched carrollite.
Linnaeite, with a cobalt content of 58%, is very rare in Katanga, and occurrences in The Katanga Crescent are few. Not a single crystal visible to the naked eye has been found during more than 90 years of exploitation of the stratiform deposits. Linnaeite has been identified primarily in polished sections of massive sulfide ore from the Luishia and Kabolela open pits, in association with carrollite and copper sulfides.
Cu-Pd-rich trogtalite has been found at the Musonoi-Extension mine in the Kolwezi Klippe (Johann et al., 1970). It occurs in close association with oosterboschite, [(Pd,Cu).sub.7][Se.sub.5], and secondary selenium minerals such as marthozite, demesmaekerite and derriksite in U-Se-rich portions of the oxide zone. The two selenides form porous, blackish nuggets resembling heterogenite, but distinguished from it by their yellowish metallic luster. The mixed nodules are found sporadically throughout the deposit.
When examined in polished section, the patches of trogtalite may measure up to a millimeter in diameter. They reveal an orthogonal network of polysynthetic twins. The trogtalite appears pinkish brown, in contrast to the creamy white of the oosterboschite. The cobalt content is 17%, with 9% copper and 4.7% palladium.
Primary Minerals - Vein Deposits
The second type of deposit in southern Katanga is vein-like in character, but is genetically related to the stratiform deposits of the Katanga Crescent. The Shinkolobwe-Kasolo vein-like deposit is located at the far eastern end of a long anticlinical zone extending to Kalongwe on the west, and on through Swambo, Mindigi and Kasompi along the Monwezi fault. This zone is characterized by cuprocobaltiferous deposits that are richer in nickel, and are often also uraniferous.
The Shinkolobwe-Kasolo deposit is genetically related to the stratiform cuprocobaltiferous deposits in the Katanga Crescent, but the intense local tectonics, accompanied by a later remobilization of sulfide minerals and uranium oxides, have given the deposit a typical veined appearance. This characteristic is clearly seen in the geometry of the mineralized body, and in the mineralization itself. The mine was basically exploited for uranium, associated with high grades of cobalt and nickel; copper is not very abundant. It should be remembered that uranium is not unusual in the Cu-Co deposits in the Katanga Crescent (Gauthier et al., 1989). However, although it is generally only a minor byproduct of copper production from the stratiform deposits (with no economic interest, the grade and the quantity being too low), it was the main mineralization in the vein-like deposit of Shinkolobwe, with high levels of cobalt and nickel. The mineralization at Shinkolobwe also contains selenium, as does that of the Musonoi-Extension uranium deposit in the Kolwezi Klippe.
Before taking up the copper sulfide minerals at Shinkolobwe, it is worthwhile to devote a few lines to the history of this great deposit. Shinkolobwe is the name of a small river that has its source at the base of a high hill. The summit of the hill was chosen as a geodesic signal point during the triangulation of South Katanga by the "Comite Special du Katanga" (the C.S.K.), the state landlord of the Katanga Crescent. This hill was identified as Shinkolobwe-Signal for reference purposes. It was a protruding outcrop in the eroded area of a Cu-Co deposit, about 3 km east of the native village of Kasolo. In 1915, Major Sharp, one of the U.M.H.K. prospectors, noticed at Kasolo an almost vertical cliff several meters high. It was a steep outcropping of hard, cellular siliceous rock (the RSC Formation, well-known throughout the Katanga Crescent) considerably more resistant to erosion than the surrounding sedimentary beds. The RSC Formation is the great reference mark for finding the stratiform orebodies: at depth, it is the barren massive dolomite separating the two orebodies in the alteration zone, this formation is completely silicified and full of cavities, showing frequently the remnants of the Roan Collenias fossils (stromatoporidae-like algal formations). The big Collenias colonies are the only visible fossils in the formation.
At the foot of this cliff, Sharp identified solid blocks of uraninite associated with curite. But it was not until 1920 that the U.M.H.K. decided to register the deposit at the C.S.K. office in Lubumbashi, under the name "Kasolo deposit" with the mention of "radium" (not uranium!) as the ore metal.
Strangely, this mine was opened by the U.M.H.K. in 1924 under the name of "Shinkolobwe." Logically, it should have been named "Kasolo," or at least "Shinkolobwe-Kasolo," to avoid any confusion with the Shinkolobwe-Signal deposit.
Linnaeite is present only in the form of massive ore, in association with other sulfides. It can only be identified with certainty by observation in polished section. There are no idiomorphic crystals in Shinkolobwe.
Cattierite in Katanga has been identified only from the Shinkolobwe mine, which is the type locality (Kerr, 1945). In the ore mass, the cattierite forms mixed associations with linnaeite. The two sulfides form gray to bronze patches with a metallic luster, associated with chalcopyrite, in crystallized pink dolomite. The cattierite may contain up to 0.5% copper and up to 4% nickel.
Beautiful cubes of cattierite have been found in vugs. The crystals measuring 1-5 mm, have sharp edges and a definite cleavage parallel to the faces of the cube. Their color is a metallic gray with purplish glints. With oxidation in the air, the crystals take on a characteristic tobacco-brown color.
Unlike the carrollite in the stratiform deposits, Shinkolobwe carrollite contains nickel. It is very rare in Shinkolobwe.
The cobalt content of Shinkolobwe siegenite is 26%. Centimeter-size octahedral crystals with brilliant faces are found at a number of points in the Shinkolobwe deposit. The mineral is generally associated with masses of gray siegenite and black vaesite. Massive veins of siegenite have resulted from the replacement of vaesite or cattierite veins.
In the lower part of the Shinkolobwe oxidation zone, siegenite rich in selenium is also found, in close association with penroseite (described below).
Peproseite was discovered in the oxidation zone of the Shinkolobwe deposit (Deliens, 1975), in association with seleniferous siegenite and molybdenite. Thus it is not a primary mineral, properly speaking, but rather a species belonging to the upper enrichment zone of the deposit. It is nevertheless being mentioned in this category because it is chemically analogous to the primary sulfides. Penroseite and the associated sulfides form mixed clusters measuring up to several [cm.sup.3] within a gray vuggy dolomite rich in microcrystals of barite and anhydrite. In polished section, the penrosette appears pale gray, in contrast to the creamy white siegenite and the dark gray molybdenite. The penroseite contains 2.6% cobalt, 26% nickel and 1% copper. The unusual characteristic of Shinkolobwe penroseite is its high sulfur content, which, in part, replaces the selenium: 5.8% sulfur, and 66.6% selenium.
The alteration of primary sulfide minerals in Katanga has led to secondary associations that differ greatly in quality and in copper/cobalt ratio. One of the principal characteristics of the oxidation zones in the southern Katanga deposits, whether in the stratiform layers or the vein beds, is the lack of diversity in the cobalt mineral assemblages as compared to the very broad range of copper minerals. From this point of view, the cobalt more resembles nickel, a rarer metal in Katanga, but one that, where it is present (Shinkolobwe, Mindigi), is only involved in the formation of secondary minerals in exceptional cases.
Another notable fact is the extreme development of the oxidation zone of the Katanga deposits. It is not uncommon to observe thicknesses of several score, or even 100 meters (Kambove, for example). The oxidized ores are found at a great depth below the current water table, which suggests an oxidation prior to a new rise in the hydrostatic level. This phenomenon is explained by major climatic variations during the Pleistocene epoch. Moreover, erratic oxidation phenomena independent of depth can be observed. They are directly linked with the permeability (porosity and fissuration) of the host rocks and the lode wall rocks. The presence of numerous faults and impermeable obstacles amplifies the anomalies.
From the genetic point of view, it is believed that the leaching of the primary sulfides created sulfate solutions that have reacted with the dolomitic rocks to produce secondary carbonates (malachite, sphaerocobaltite), and with the siliceous rocks to produce silicates, such as chrysocolla, plancheite, shattuckite and dioptase. More generally, however, the cobalt tends to accumulate in the form of hydrous oxides associated with siliceous rocks, but frequently retaining the indices of the primitive carbonate rock. Katanga malachites may contain an appreciable percentage of cobalt (up to 6.5%). In the western mines (Musonoi, Kamoto), a double carbonate of copper and cobalt (kolwezite) has been found which belongs to the rosasite structural group and contain nearly 20% cobalt (Deliens and Piret, 1980).
Calcite CaC[O.sub.3], and Dolomite CaMg[(C[O.sub.3]).sub.2]
Tiny pink crystals of cobaltoan dolomite on gray dolomite have come from deposits in the Kakanda area, and are well-known on the mineral market. The Mashamba West, Mupine and Mindingi deposits produced pink cobaltoan calcite, and generally in much bigger crystals. The large, pink scalenohedrons of cobaltoan calcite from the Mashamba West mine are the most beautiful in the world; some crystals also have a partial kolwezite coating. Like Shinkolobwe, the Mashamba West mine has also produced spectacular cobaltoan calcite in pink hexagonal prisms to several centimeters, with three-faced rhombohedron terminations.
Energy-dispersive X-ray and X-ray diffraction analyses by Dave Douglass (personal communication, 1998) have confirmed that the larger, paler pink crystals are cobaltoan calcite, whereas the more vividly colored druzes and botryoidal crusts are cobaltoan dolomite. The dolomite crystals tend to be magnesium-rich (Mg:Ca up to 3:1), and in fact are so supersaturated with Mg that they should have spontaneously broken down into a mixture of normal dolomite and magnesite. This appears not to have occurred, since X-ray diffraction analyses show only dolomite being present.
Cobaltoan calcite and dolomite, though brightly colored, actually contain only a very small percentage of cobalt. A series does not exist between sphaerocobaltite (50% Co) and calcite or dolomite (0.5 to 1.5% Co) because the [Co.sup.2+]/[Ca.sup.2+] substitution is so limited.
It should also be mentioned that the Tantara mine has yielded spectacular, massive but gemmy cobaltoan calcite, sometimes in association with dioptase. One such example is displayed in a showcase of the U.M.H.K. Company in their Brussels office, and others are currently on exhibit in the Sengier-Cousin Museum in Likasi, Katanga.
Cobaltite has never been reported from the stratiform and vein-like deposits of Katanga; there is practically no arsenic present in those orebodies. However, cobaltite has been found in the massive ore of the Kipushi mine, a pipe-like deposit similar to the one at Tsumeb, Namibia.
Cobaltomenite CoSe[O.sub.3] [center dot] 2[H.sub.2]O
Cobaltomenite has been found on the 2,400 level of the Musonoi-Extension mine. Seleniferous digenite is the main primary sulfide of copper here, in association with primary uraninite (U[O.sub.2]). The alteration of these minerals has yielded rare selenites of copper (chalcomenite, CuSe[O.sub.3].2[H.sub.2]O) and of cobalt (cobaltomenite). The cobaltomenite occurs as very small, purplish crystals on altered digenite.
Alteration in an environment rich in selenium and uranium at Musonoi has led to the formation of other rare minerals including demesmaekerite, derriksite, guilleminite and marthozite.
Comblainite ([Mathematical Expression Omitted], [Mathematical Expression Omitted])[(OH).sub.2][(C[O.sub.3]).sub.(1-x/2)] [center dot] y[H.sub.2]O
Comblainite, an oxycarbonate of nickel and cobalt, has been found in the uraniferous Shinkolobwe deposit in association with heterogenite-3R and secondary cobalt minerals such as rutherfordine, becquerelite and masuyite. Comblainite is found in millimeter-thick beds of turquoise-blue masses, and also yellow to yellowish green masses in association with gummite. Under the microscope, irregular crystals up to 10 microns in size can be seen (Piret and Deliens, 1980).
From the chemical point of view, all of the cobalt in comblainite is in trivalent form, whereas the nickel is bivalent. The highest cobalt content observed is 14.6%. Structurally, comblainite belongs to the hydrotalcite group. The X-ray diffraction pattern is very close to that of takovite. [Ni.sub.6][Al.sub.2][(OH).sub.16](C[O.sub.3],OH) [center dot] 4[H.sub.2]O, which belongs to the same structural group. Comblainite is very rare at Shinkolobwe, and has only been found in a few specimens.
The compact or powdery black oxyhydroxides of cobalt are by far the preponderant secondary minerals in the supergene alteration zones of the sulfide deposits of cobalt throughout the world. They were given the name "heterogenite" (literally "of a different nature") by Frenzel in 1872 in order to distinguish them from the black oxides of manganese (which have the same appearance) obtained from the cobalt, nickel and manganese ore at the Maasen mine in Schneeberg (Saxony, Germany).
Since their discovery, the black oxides of cobalt have posed a mineralogical problem that is particularly difficult to resolve. For more than a century, material obtained from various deposits has been studied. More than 20 different chemical formulae have been proposed, and a number of names were given to them (including boodtite, heubachite, lubumbashite, mindigite, schulzenite, stainierite, transvaalite, trieuite and winklerite). In 1962, Max Hey came to the conclusion that the heterogenites all have a single formula, CoOOH, analogous to that of manganite (MnOOH) and goethite (FeOOH). Deliens (1974), who studied the southern Katanga heterogenites, confirmed Hey's views and retained the single name "heterogenite" to describe the hydrated black oxides of cobalt. This latter author also revealed the existence of a hexagonal polytype, which he called "heterogenite 2H" (Deliens and Goethals, 1973). Since then, common heterogenite has been termed heterogenite 3R in mineralogical treatises, in order to distinguish it from the 2H polytype.
Heterogenite 3R, which occurs with the widest variety of appearances, is the heterogenite most commonly found in southern Katanga. The following habits are well known:
(1) Microgranular kidney-shaped nodules and irregular masses streaked with little veins that have a metallic luster. These crystallized masses were known in Katanga by the name "stainierite."
(2) Kidney-shaped or botryoidal masses with very shiny jet-black surfaces. The fracture resembles that of anthracite. This heterogenite is found in concentric superposed layers in the outcrops. In Katanga it was known as "trieuite."
(3) Earthy encrustations with subconchoid fractures (formerly "mindigite").
(4) Stalactites, draperies, scoria and oolitic formations.
The principal differences between the crystalline varieties that have a metallic luster and the cryptocrystalline varieties that are rich in copper are discussed below. The data are taken from the work of Deliens (1974).
When examined by X-ray diffraction, the material with a metallic luster yields a spectrum made up of sharp peaks, whereas the kidney-shaped masses produce patterns characterized by relatively feeble peaks, or indeed mere undulations reflecting a cryptocrystalline state.
Microscopic examination of polished sections in reflected light shows a very clear relationship between the degree of crystallinity and the reflectivity value. The value of the upper reflective index is nearly 25% for the crystalline material, while it ranges between 15% and 10% for the other specimens.
From the chemical point of view, three factors characterize the difference between the habits with a metallic luster and the other heterogenites. They are the cobalt valence, the presence of copper, and the water content.
The crystalline varieties are almost exclusively made up of trivalent cobalt ([Co.sup.3+]). They contain very little copper (maximum 3.8%), and the [H.sub.2]O content is approximately 1%. In addition, they contain a low percentage of nickel (not over 1.5%). The formula representing them is CoO [center dot] OH, or, in another form, [Co.sub.2][O.sub.3] [center dot] [H.sub.2]O.
The kidney-shaped or earthy cryptocrystalline habits contain between 14% and 25% bivalent cobalt ([Co.sup.2+]) associated with 25% to 37% trivalent cobalt (C[O.sup.3+]). The copper content is always appreciable, and may reach 15%; nickel is absent. The [H.sub.2]O content ranges between 2.5% and 9%. Thermogravimetric analysis curves (percentages of weight lost during the heating of the minerals) are particularly significant. The general formula arrived at for the poorly crystallized habits containing cobalt in its two valence states and copper is: ([Co.sub.2][O.sub.3] [center dot] CoO [center dot] CuO) [center dot] [H.sub.2]O or again, CoO [center dot] OH + nCo[(OH).sub.2] + mCu[(OH).sub.2]. The Co[(OH).sub.2] and Cu[(OH).sub.2] phases cannot be detected by X-ray analysis. They are, therefore, amorphous, and are not regarded as a part of the heterogenite structure, although they do constitute a factor hindering crystallization. That is why the general formula CoO [center dot] OH is valid for all of the heterogenites. The term "cupriferous" can be added to the cryptocrystalline varieties.
Experiments in synthesizing hydrated oxides of cobalt under normal temperature and pressure conditions have clearly confirmed the role of copper in limiting the degree of crystallinity in the heterogenites, whereas the presence of a little nickel favors crystallization.
The most beautiful heterogenites in the world have been collected at the Kabolela mine. They are found there in botryoidal masses, in stalactites, and in cavern draperies. Some museum specimens measure several tens of centimeters.
Mammillary specimens of good quality are obtained from the Star of the Congo, Luishia, Kalabi, Likasi and Mindigi mines. Stalactitic habits are rarer. In Mindigi, for example, pseudostalactites (stalactites without a feeder canal) with a core of heterogenite (3R) and with concentric areas of malachite on the exterior, have been found.
Heterogenite 2H is a hexagonal polytype of rhombohedral heterogenite 3R. It is very closely associated with the crystals of heterogenite 3R in the little veins with a metallic luster found in certain specimens obtained from the oxidation zone of the Mindigi mine. The polytype 2H crystals appear as flattened prisms with regular hexagonal contours. They are black in color with a bright metallic luster. The largest do not measure more that 1 mm.
Chemical analysis of the 2H polytype reveals the presence of 88% [Co.sub.2][O.sub.3], 2% nickel and 10% water. The formula is thus identical to that for heterogenite 3R: CoO [center dot] OH. The nickel content is too low to justify its inclusion in the formula. However, it nonetheless plays an equally important role here in determining the degree of crystallinity.
The maximum reflective capacity measured on a polished section was 23.5%, which is slightly less than that for the polytype 3R (25%).
The clearest distinction between the two polytypes is obtained by means of X-ray diffraction. If the a parameters of the links in the two polytypes have the same value (2.85 A), the c parameter of the heterogenite 2H (8.81 [angstrom]) is only two-thirds of that for the heterogenite 3R (13.16 [angstrom]). This is reflected in perceptible variations in the spectral lines revealed by diffraction.
The 2H polytype is rare in comparison to the 3R, which is almost always present in the oxidation zones of the Cu-Co deposits in southern Katanga. To date, it has been found only in Mindigi, and only in association with the crystallized habits that have the metallic luster of heterogenite 3R. These habits are themselves uncommon in comparison to the large quantities of earthy or reniform material in the deposit.
Julienite [Na.sub.2]Co[(SCN).sub.4] [center dot] 8[H.sub.2]O
Julienite has only been found on the surface at the Cu-Co deposit in Shamitumba, located at the center of the Kambove-Likasi-Shinkolobwe triangle. This thiocyanate of sodium and copper is organically formed and can very easily be synthesized. Since its formation depends upon biological action, this species does not entirely meet the definition of a mineral. Julienite is a mineralogical curiosity in Katanga. The bright blue orthorhombic prismatic crystals may reach several millimeters in length. They form intergrowths on the surface of specimens of dolomite. Julienite is also found in masses filling small fissures.
Kolwezite (Deliens and Piret, 1980) is a member of the rosasite group, which also includes the mineral glaukosphaerite, [(Cu,Ni).sub.2](C[O.sub.3])[(OH).sub.2]; the latter has been found in stratiform deposits at the Kasompi mine. Kolwezite is a typical secondary cobalt mine in the western sector, occurring at several deposits in the Kolwezi Klippe. It forms hemispheres to 1 cm, and blackish brown to pale tan reniform crusts. The knobs can display a range of differently colored zones. The mammillary surfaces are generally lusterless to powdery and occasionally velvety.
Kolwezite appears commonly as a late-stage coating and, more rarely, as layers inside banded concretions of other minerals. It is often associated with very dark green cobaltiferous malachite, and can be distinguished by its invariably tan streak in comparison to the green streak of malachite. Other associations include black heterogenite masses and knobs, pink cobaltoan calcite and cobaltoan dolomite.
The Co content of kolwezite ranges from 14.75 to 18.50%, with a Cu:Co ratio of 2:1; this is the maximum cobalt content for the species. Cobaltiferous malachite never contains more than 6.5% Co. Thus there is a gap in cobalt content (6.5 to 14.75%) between the two species.
The most important localities for kolwezite are the Musonoi Principal, Kamoto, Mupine and, especially, the Mashamba West mine. At Mashamba West, kolwezite is found as beautiful groups of stalactites. In 1997 it was discovered in a large vug on the upper level, as part of a botryoidal succession beginning with heterogenite at the base, then a layer of kolwezite up to 2 cm thick, followed finally by a top layer of cobaltiferous malachite with surface coatings of chrysocolla and kolwezite knobs. Such concretions are exceedingly rare. A partial coating of kolwezite on groups of large, rose-red cobaltoan calcite crystals from Mashamba West is also quite aesthetic.
Table 1. Cu-Co deposits in the Katanga Crescent. Sector Deposit % Co % Cu Remarks South Kasombo 4.9 8.1 High Co % Luiswishi 1.9 3.6 Lupoto 0.6 0.9 Ruashi 0.6 3.4 Star of the Congo 0.4 7.0 First UMHK mine Central Fungurume 0.5 1.5 Future exploitation Kabanbankola 1.5 1.5 Cu = Co Kabolela 0.7 3.8 Kakanda East 0.5 4.3 Kakanda South 0.3 3.7 Kambove West 0.3 5.4 Kamfundwa 0.3 2.6 Kasompi 1.8 1.2 Kazibizi 0.6 2.8 Luishia ? ? Mindingi ? ? Local contractor Saafi 0.3 0.3 Cu = Co Shamitumba 0.7 3.3 Shinkolobwe 1.0 3.0 Tenke ? ? Future exploitation West Kamoto East 0.3 5.2 Kamoto Etang 0.5 2.1 Kamoto Principal 0.3 4.3 Kamoto Fond (undergr.) ? ? Mashamba East 1.1 ? Mupine 0.6 2.6 Musonoi F.N.S.R. 0.4 5.4 Musonoi Deep 0.6 3.9 Oliveira 0.5 3.5 Virgule 0.3 4.9 KOV mines Copper-Only Mines South Karavia 0 10.0% Highest copper Central Kamatanda 0 4.9 West Kolwezi 0 6.0% Mutoshi Breccia 0 1.5% (= Ruwe Breccia) Cobalt-Only Mines West Kabulungu 1.0% 0 near Mashamba East Nyoka 0.8% 0 South of Musonoi Known Reserves: Star of the Congo, Ruashi, Luiswishi Lupoto, (Lubumbashi Kasombo 0.54 3.65 Dist.) Kambove West, Kamoya, M'Sesa Kazibizi, Kamfundwa, Shangulowe 0.30 2.76 (Kambove Dist.) Kakanda East, North, South, Saafi Bangwe, Kiwana, Luita, Disele 0.17 3.31 (Kakanda Dist.) Fungurume, Kwatebala, Fwalu, Shimbidi, Goma, Kabwelunono, (Fungurume- Kakavilondo 0.32 4.42 Tenke Dist.) Fungurume IIh and VII 4.00 (rich, hand-sorted ore) Kolwezi, Mutoshi, Musonoi, Nyoka East & West, Dima (Dikuluwe-Mashamba), K.O.V., Mupine, Kamoto Prin., North & Etang, Mashamba East, West 0.36 4.30
Oursinite (Co,Mg)(U[O.sub.2])2[Si.sub.2][O.sub.7] [center dot] 6[H.sub.2]O
Oursinite is the only secondary uranium mineral in southern Katanga containing cobalt in any appreciable quantity (5.2%). The mineral belongs to the uranophane structural group, and is analogous to sklodowskite (with magnesium) and cuprosklodowskite (with copper). It has been found only in secondary ores at the Shinkolobwe mine (Deliens and Piret, 1983).
Oursinite occurs in flexible to rigid fibers of a pale to whitish yellow color measuring up to 1 mm in length, with a diameter of 0.02 mm. The fibers are grouped in tufts or rosettes. More rarely, oursinite fibers are found closely intermingled with needles of lepersonnite-(Gd).
Oursinite is associated with a whole series of other secondary uranium minerals such as curite, becquerelite and kasolite. One characteristic of the association that might serve as a guide in the detection of oursinite is the frequent black pigmentation caused by hydroxides of cobalt in the accompanying minerals mentioned above. While cobalt is an integral component of oursinite, it is nothing more than pigmentary impurity in the other species. Wherever the black coloring is present in minerals that are usually yellow or orange, there is an excellent likelihood that the radial aggregates resembling sklodowskite and uranophane are in fact oursinite.
Table 2. Cobalt minerals from Katanga. Katanga Crescent Outside of Stratiform Vein-like Katanga Cresent Cobalt Cobalt Cobalt Deposits Deposits(*) Deposits(**) Primary Linnaeite Linnaeite Linnaeite Minerals Carrollite Carrollite Carrollite Cattierite Cobaltite Siegenite Trogtalite Penroseite Secondary Heterogenite-3R Heterogenite-3R Heterogenite-3R Minerals Heterogenite-2H Oursinite Kolwezite Comblainite Sphaerocobaltite Cobaltomenite Julienite * Shinkolobwe ** Kipushi
Sphaerocobaltite occurs in most of the Katanga Crescent Cu-Co deposits, but only sparsely. Unlike the carbonate of copper, malachite, it never occurs in botryoidal concretions and masses. In rare case, massive, fine-grained, rose-red to purple sphaerocobaltite has been found at the Kambove, Kabolela, and Kamoto Fond ("Fond" = "underground") mines typically as an in situ alteration of massive, lenticular beds of carrollite. The reason for the scarcity of sphaerocobaltite is its easy solubility, making it prone to replacement by the more stable heterogenite.
Sphaerocobaltite occurs mainly in the transition zone between primary and secondary ores, and in carrollite-bearing dolomites high in the oxide zone. Outside of these favorable zones it dissolves readily. Sphaerocobaltite specimens commonly contain remnant carrollite. Although found most often as microscopic inclusions coloring dolomite, or as shapeless, vaguely drusy aggregates, sphaerocobaltite also has been found locally as idiomorphic crystals, always with curved, lustrous faces.
Sphaerocobaltite crystals have been found in the Kambove, Musonoi, Mupine, Kamoto Principal (underground), and Kakanda mines. At Kakanda, crystals reach 3 mm is size and show a discoidal, pseudo-hexagonal habit; groups of singles and twins cluster together, sometimes forming delicate rosettes. Their color is deep rose to deep red, the darkness and intensity of which serve to distinguish to from associated pink cobaltiferous calcite and dolomite. Perfectly shaped crystals to 5 mm have been found at the Kakanda South mine. At the Kambove mine it forms aggregates and tiny crystals on superb carrollite crystals.
The authors wish to gratefully acknowledge the assistance with editing, correcting, modifying and augmenting provided by Armand Francois (Belgium), Director of Geological Service of Union-Miniere-du-Haut Katanga and the Gecamines Society, Joseph Lhoest (Belgium), an exceptional collector of Katanga minerals, and Professor Dave Douglass (Tucson), who shared his interesting work on Ca-Mg-Co substitution in dolomite. Photographs were graciously provided by Nelly Bariand, Eddy Van Der Meersche, Wendell E. Wilson, Joseph Lhoest, Jeffrey Scovil, and Gordon Comblain.
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|Author:||Gauthier, Gilbert; Deliens, Michel|
|Publication:||The Mineralogical Record|
|Date:||Jul 1, 1999|
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