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Famous mineral localities: Cerro Rico de Potosi, Bolivia.

Cerro Rico ("Rich Mountain") de Potosi is among the oldest and most famous orebodies in South America, has yielded billions of dollars in silver, is the type locality for two tin sulfides - berndtite and ottemannite - and has produced the world's finest specimens of phosphophyllite.

INTRODUCTION

Charles V of Spain was the fortunate ruler in charge when Cortez conquered Mexico (1519-1521) and Pizarro conquered Peru and what is now Bolivia (1531-1535). Tons of silver began to pour from mines in the New World, and for a time Potosi was the world's leading producer. Oddly enough, despite its enormous output of silver, Potosi is famous among mineral collectors for a small occurrence of a rare zinc-iron-manganese phosphate, phosphophyllite, discovered there in the 1950's.

Mineral specimens considered to be from Cerro Rico de Potosi (except perhaps phosphophyllite) must bear documentation specifically for "Cerro Rico" and not just "Potosi." Although the town of Potosi is indeed at the foot of Cerro Rico, the department in which it is situated is also named "Potosi." Consequently confusion is possible with the other important localities in the department, which include Colquechaca and Llallagua, not to mention other less notable occurrences such as Chocaya, Chulchucani, Carguaicollo, Cerro Tazna, Cerro Malmisa, Huayna, Machacamarca, Maragua, Ocuri, Porco, Pulacayo, San Vincente and Uncia (all of which, incidentally, are represented by specimens in the Smithsonian collection).

HISTORY

Discovery by the Incas

According to legend, the silver deposits at Potosi were first discovered by the Inca King Huaina Ccapac in 1462, while traveling from Cantumarca to Colque-Porco. He is said to have admired the grandeur and beauty of the mountain, remarking to his vassals that within its bowels there must, no doubt, be much silver. Subsequently a well-equipped mining expedition was dispatched to the site, but after ascending the mountain and being on the point of commencing work, the miners heard thunderous noises which seemed to be warning them away. They returned immediately to their king, referring to the mountain as Potocsi, which in the Quechua language means "thunder" or "place of great noises" (Wendt, 1891). This is not an unlikely story, considering that strong thunderstorms are commonly seen playing around the summit. Bancroft (1984), however, maintains that the name derives from the Quechua word potojchi, meaning "fountain of silver" . . . certainly a more romantic interpretation.

Discovery by the Spanish

Following the Spanish conquest, a Quechua Indian by the name of Diego Guallca is said to have rediscovered the deposits in 1545. Guallca was a native of Chunvivillca, a town near Cuzco, and had become a servant of the Spanish captain Juan de Villarroel, who was involved in mining at Porco about 75 km away. He had been chasing some lost llamas up the side of the mountain but was forced to make camp there at nightfall. Because of the cold he built a campfire; in the morning he was surprised to see that the rock beneath it had melted, and a small rivulet of molten silver had formed. He quickly returned to Porco and told captain Villarroel of his discovery, who registered claims in April of 1545.

In December Juan de Villarroel traveled to Potosi with Diego Centeno, Pedro Cotamito and a man named Santandia, and they began mining the rich surface ores. Production continued through 1572, yielding native silver and chlorargyrite ore containing 25% silver. The soft ore was melted in small furnaces called guairachinas, using litharge as a flux. No air needed to be forced through the furnaces; they were simply constructed with their mouths facing the prevailing wind. A local friar, Joseph de Acosta, estimated that a quarter of a billion dollars in silver was removed between 1545 and 1572. Spanish government records show that duty was paid on roughly a billion dollars in silver from 1556 to 1791, although far less silver was reported as being dutiable than was actually mined.

Zarate's Description

Agustin de Zarate was sent from Spain to Peru in 1543 as Comptroller-General ("Auditor") by Charles V (Kuhner, 1980). He wrote a book about the country, which was first published in Europe in 1555; an English translation of 1581 is entitled: The strange and delectable History of the discoverie and Conquest of the Provinces of Peru. in the South Sea. And of the notable things which there are found; and also of the bloudie civil warres which there happened for government . . . And also of the ritche Mines of Potosi. A copy exists in the Herbert Hoover collection, now a part of the Sprague Library at Claremont College in Claremont, California. Because this is such an extremely rare work, and the entry regarding Potosi is short, it is quoted here in full. Some spellings have been updated to make it more easily understood but no words have been omitted or otherwise much altered.

By 1572 the surface ores had been exhausted and production temporarily declined. The Spanish government sent a new viceroy, named Toledo, to Potosi in order to restore production. He and Don Pedro Fernandez de Velasco introduced the "patio" process in 1574, which had been invented only 17 years earlier by Bartolome Medina at Pachuca, Mexico. The process involves the cold amalgamation of ore with mercury in large open troughs stirred by slave labor. (In 1793 mules and horses were first used in the process, saving 75% in labor cost, as well as the lives of many enslaved Indians who would have continued to die of mercury poisoning.) The mercury was available in ample quantities from the Peruvian mines at Huancavelica.

Barba's Work

In 1588 another Spaniard was sent to Peru, a priest by the name of Alvaro Alonso Barba. He further developed the amalgamation process which proved so important to ore processing at Potosi. He presented to the Spanish government a detailed report on mining technology as it had evolved at Potosi in 1637. This landmark work was first published in Spain in 1640, under the title El Arte de los Metales ("The Art of Metallurgy"). It is the first significant treatise on metals to be written in Spanish. Unfortunately, the book was banned and burned by the Spanish Inquisition, and only four copies seem to have survived: three owned by the British Museum and one in the Herbert Hoover collection at Claremont College, Claremont, California. It was reprinted in 1675 and 1729. The Spanish government wished to keep the work proprietary (an early "industrial secret"). But Edward Montagu, the Earl of Sandwich and the British Ambassador Extraordinary to Spain, obtained a copy and published a poor translation of the first two of the five major chapters in 1669. German translations and reprints subsequently appeared in 1676, 1696, 1726, 1739, 1749 and 1767. French editions were published in 1730, 1751 and 1752. Three Spanish-language editions published in South America are said to contain many errors. Finally, in 1923, an excellent English translation was published by Ross E. Douglass and E. P. Mathewson.

Decline of the District

In 1599 the mine operators began to complain that rich ore of easy access was becoming scarce, and profits were poor. In 1618 they complained again to the King of Spain, begging for a reduction in duties because the ore was running only 52 ounces of silver per ton.

In order to make the stamping and amalgamating works more efficient, large water reservoirs were constructed in 1621 to supply waterpower and water for processing. These reservoirs continued in use for nearly three centuries, although in a progressively worse state of decay and disrepair. On March 3, 1626, one of the reservoir dams gave way, and the resulting flash flood destroyed nearly all of the 136 individual ore-reduction facilities, a loss estimated at over $10,000,000. With the help of a Spanish government loan, most of these had been rebuilt by 1633. Ore was still coming primarily from surface workings, yielding about 50 ounces of silver per ton.

Again in 1636 the mine operators complained of meager profits. Only about 1500 Indian slaves were actively engaged in mining, although 70,000 Indians were in residence at Potosi. Disputes between mine operators over the possession of various complex ore veins at depth had been resulting in lawsuit expenses exceeding $200,000 per year. Losers in these disputes had been known to sabotage the mines before vacating them, some veins having been yielding ore containing 150 ounces of silver per ton at the time.

In 1690 a miner named Ouiroga is on record as having paid $21,000,000 in duty to the crown, then set at 20%. Therefore he had removed over $100,000,000 in silver from his one mine, the Cotamitos mine. With some of the profits he built the Cathedral of San Francisco in Potosi, and was eventually buried there.

By 1693 production was beginning to dwindle, and in 1712 an epidemic killed many of the Indian miners. To assist the mine operators, the Spanish government reduced the required duty from 20% to 10%. Difficulties nevertheless gradually worsened. Finally, in 1759, the Miners' Corporation prepared for the King a detailed report on the district. According to this report, the silver veins vary in width from paper-thin to several meters. Workings had reached a depth of several hundred meters, and the deepest mines were suffering from a high water flow combined with bad ventilation. The adits, shafts, veins and orebodies formed a complex labyrinth which was not understood by anyone. In the upper 300 meters of the 900-meter mountain there was said to be no veins which had not been worked out. Mine accidents were also reported: in the Mendieta vein, for example, a cave-in had killed 300 Indians and two Spaniards in a rich zone which had never been reopened. The miners found themselves unable to accurately follow veins at depth, even though these often led to some of the richest pockets. It is reported that in the lowest workings they had recently stumbled upon metal "like fringes of solid white silver, without any tarnish, and some of the finest kind of sulfides."

Following extensive surveys, it was recommended that a 3000-meter-long adit be driven at depth to intersect the lower reaches of the vein system and to ventilate and drain the entire mountain. However, since total output had already fallen to less than $2,500,000, the Spanish government delayed the project for many years. From 1778 to 1790 roughly a third of the planned adit was driven. Nevertheless, the Governor of Potosi described the mines in 1794 as being in bad condition, all shafts having filled with water up to a depth of 80 meters. By 1799 only 35 ore-reduction facilities were still in operation.

In 1809 the War of Independence broke out and Potosi suffered repeatedly in the cross-fire, putting a temporary end to all mining activities.

Attempts at Revival

In 1825 an English company shipped a large quantity of machinery intended for use in reopening and dewatering the Potosi mines. but the financial panic of the following year caused the machinery to be abandoned on the coast, and it never reached Potosi.

In 1886, Arthur Wendt was sent to Potosi by an American mining company intending to reopen the mines. After three years of work and half a million dollars in expenses he succeeded in pumping out and reopening the Cotamitos vein, where he was rewarded by finding ore averaging 70 ounces of silver per ton.

Tin mining began in Cerro Rico in the late 19th century as the exploitation of silver was declining. The colonial Spanish knew about the tin, of course, but had little use for it. The Spanish had, however, produced small quantities of tin for such purposes as local manufacture of liquor stills. Tin mining received a welcome boost from the high demand in WWI. But production methods remained haphazard until several mines were unified with the Pailaviri to form the Unificada, permitting the introduction of rational techniques. By the 1930's silver production had ceased completely, and it was often stated that the silver minerals had been totally worked out. During WWII, tin production enjoyed another surge, due in part to the demand for canned food for U.S. servicemen.

During the early 20th century the "tin baron" in possession of the Unificada (Pailaviri) mine was one Moritz (Mauricio) Hochschild, black sheep of a German-Jewish family. Hochschild had been sent off to South America to make his fortune, and proved quite successful. When he was finally expelled from Bolivia in 1952, he offered the German mineralogists in Bolivia (Ahlfeld, Herzenberg and Berndt) new jobs at his mines in Peru. However, the three felt an attachment to Bolivia's mines and minerals, and elected to take their chances with the new Revolutionary Government. The Hochschild Group today still owns important mines in Peru.

In the mid-20th century Bolivia was still a feudal society, with the "tin baron" billionaires controlling the political system while the majority of the population, the Indians, lived as illiterate serfs. This unstable social system died with the revolution and land reform of 1952, which also nationalized the largest mines (including the Pailaviri/Unificada on Cerro Rico) under the grossly inefficient bureaucracy of COMIBOL (Corporacion Mineria de Bolivia). the State mining corporation. The many small and medium-sized mines on Cerro Rico remained in private hands.

With the government's financial inability to continue subsidizing COMIBOL's losses from corruption and inefficiency, and the worldwide pressure in the 1980's for privatization of state-owned industries. the Bolivian government gave up most of its mines, including the Unificada. The present situation at Cerro Rico is that the Unificada, the largest mine, is operated as a joint venture, while several small to medium-size mines are operated by either private companies or miner-owned cooperatives. In total, a few thousand miners still work in Cerro Rico. with the number fluctuating according to world metal prices.

A surprising development in the early 1990's was the discovery of a rich new vein of massive pyrargyrite-miargyrite which was named the Veta Potosi (Potosi Vein). It was discovered by a cooperative about 50 meters above the Zero Level, on the southeast side of the mountain: on working it downwards it was found to cut the busy main haulageway (zero level) of the Pailaviri mine, a couple of kilometers from the entrance on the north side of the mountain. where it had apparently not been noticed. One retired COMIBOL geologist says the vein was in fact known to Pailaviri geologists in the 1960's, but was ignored because silver prices were very low and COMIBOL's policy at the time was to promote increased tin production. Decades later, after successive politically motivated personnel changes, no one was left who remembered the silver vein. The Veta Potosi is now known to extend 70 meters below the Zero Level. In 1998 it caused armed conflict, including underground hostage-taking, between the Pailaviri and the cooperative. So the Cerro Rico is once again a producer of silver. Silver is also being produced from tetrahedrite at other mines on the mountain.

After centuries of producing only silver, it was only in the 20th century that the full polymetallic potential of the mountain was realized. Tungsten and bismuth were produced while the deep levels (5 to 10) of the Pailaviri were operational, reaching over 400 meters below the zero level, or over 1,000 meters below the peak of the Cerro. The deep levels were abandoned starting in the 1960's because of declining ore values (1% Sn, plus small amounts of W and Bi), combined with rising temperatures and even the eruption of an underground hot spring. These levels are now unventilated, and are hot and poisonous due to intensive pyrite decomposition. Production is limited to no further down than Level One below the Zero Level. Nevertheless, the upper and middle levels of Cerro Rico still produce tin, silver, antimony, zinc, lead and copper.

GEOLOGY

The central Bolivian tin belt is one of the world's richest metallogenic provinces, stretching from eastern Peru through Bolivia and into northern Argentina. The belt follows the eastern Andes, mineralization being most intense in a restricted area between Oruro and Potosi where the Andean Cordillera curves toward the south. The northern portion of the belt hosts primarily hypothermal and mesothermal deposits, whereas the southern and central sections are characterized by xenothermal mineralization, of which Cerro Rico is a prime example.

The entire belt of mineralization, which includes silver, tungsten, lead, zinc, antimony and gold as well as tin, is related to Middle Tertiary intrusions representing the uppermost cupolas and apophyses of an underlying batholithic basement complex. Nearly all of the metalliferous deposits in the belt appear to have been emplaced under low-pressure, near-surface conditions (Lindgren and Creveling, 1928, estimate a depth of 3,000 feet or less). Veins, sheeted zones and stockworks were the sites of deposition for the ore minerals.

The oldest rocks in the Potosi area are Ordovician shales and slates, unconformably overlain by Tertiary volcanic rocks; these are called the Caracoles tuff, the Venus breccia and the Pailaviri conglomerate, together known as the Cerro Rico Series. Probably during late Miocene or early Pliocene time this series of rocks was intruded by the Cerro Rico Stock, a funnel-shaped body of altered dacite now forming the large pyramidal hill called Cerro Rico. (A small hill protruding from the north slope of Cerro Rico and genetically related to it is known as Cerro Chico or Huayna Potosi.) The dacite in the lower part of the stock is marked by strong propylitization, sericitization and pyritization, whereas silicification dominates in the upper levels. Silicification is commonly so intense (over 95%) that no trace remains of original feldspar crystals in the dacite; feldspar-shaped vugs are commonly hollow but may be lined or filled with quartz, hematite or jarosite. The stock is riddled by shear fractures and normal faults of only slight displacement. In general, the mineralized veins trend northeasterly with a steep dip. The shallower veins are primarily in the Cerro Rico Stock, but as the stock narrows with depth, veins are found in the surrounding Paleozoic shale as well.

The Cerro Rico Stock has an oval-shaped outcrop measuring about 1200 x 1600 meters; by a depth of 700 meters the stock has decreased to 100 x 400 meters. At least 35 veins and vein branches have been located thus far, all of which unite at depth into five principal vein systems: (1) the Tajo Polo, (2) the Encinas, (3) The Mendieta, (4) the San Miguel, and (5) the Alco Barreno systems. The Mendieta is the most important; the Tajo Polo and Encinas probably merge with it at greater depth. The veins have a distinct en echelon structure, especially below the Caracoles Level; each vein seems to divide into a succession of strongly mineralized sections separated by weakly mineralized stringers.

The Cerro Rico deposits have been divided into at least four zones: (1) the upper oxide zone, (2) the lower oxide zone, (3) the upper sulfide zone, and (4) the lower sulfide zone. Mineralization in the sulfide zones probably began with quartz, pyrite and cassiterite, followed by stannite and chalcopyrite, then andorite and tetrahedrite, and lastly galena, pyrargyrite, lead sulfosalts and late-stage gangue minerals. This sequence represents a gradual drop in temperature, from 400-500 [degrees] C down to 100-150 [degrees] C. Spatially this conforms to the deep central zone characterized by bismuthinite and wolframite, surrounded by a zone of silver sulfosalts and, beyond this, a lead-zinc zone. Differences in wall-rock chemistry appear to have had no effect on mineralization; deposition was controlled solely by changes in temperature and pressure.

The early stages of deposition have been characterized as xenothermal, although part of the later stages appear to be the near-surface equivalent of mesothermal (i.e., kryptothermal), while the sulfosalts of the upper stage were probably deposited in the epithermal range. Metallogenically the silver is primarily epithermai whereas the tin mineralization was xenothermal. The kryptothermal mineralization has proven to have little if any economic value.

The above discussion of Cerro Rico geology and paragenesis has been synthesized from the various works of Ahlfeld, and of Turneaure (1960), and Ridge (1972).

MINERALS

It is important to note that, in general, there are no well-crystallized secondary minerals in the Cerro Rico oxidized zone. Although species such as chalcocite, covellite, anglesite and cerussite have been identified, they are actually present only as earthy films and masses in piles of decomposing sulfosalt ores: hardly of interest to the collector. This may come as a surprise to people more familiar with lead-copper-silver oxide-zone mineralization in other arid regions such as Arizona and Mexico, but the cold, dry, high Andes provide a much less productive environment for weathering and oxidation. All minerals of collector interest at Cerro Rico (as at Oruro, Colquechaca, Huanuni, Llallagua, Morococala, etc.) are of primary hydrothermal origin.

The following data have been compiled from published sources, from our own observations of specimens, and from private communications regarding the observations and analyses conducted by others. We are especially indebted to Richard Kosnar for information on several Cerro Rico species which he has had identified by X-ray diffraction, electron microprobe analysis and polished section studies over the years.
Table 1. Important veins in the Cerro Rico de Potosi.

Alcko Barreno                  Mendieta
Animas                         Mesapata
Bolivar 1                      Potosi
Bolivar 2                      Ramo San Miguel
Bolivar 3                      Ramo Bolivar Nueva
Bolivar 4                      Ramo Tajo Polo
Bolivar 5                      Ramo San Antonio
Bolivar 13                     Rica 1
Bolivar Nueva                  Ricuch
Claudio                        San Antonio
Cotamitos                      San Migue 1
Don Mauricio                   Tajo Polo
Eduardo                        Utne 2
Encinas                        Utne 3
Este                           Utne 9
Exaltacion                     Utne 12
Krause                         Vasquez
Machume                        Zalle


Acanthite [Ag.sub.2]S

Acanthite ("argentite") was once an important ore mineral in the oxide-sulfide transition zone at Potosi, finely disseminated in limonite, but is now rarely encountered. Ahlfeld and Reyes (1955) described it from the San Antonio vein of the Bolivar mine as microcrystals covered by blebs of chlorargyrite on a dacite matrix. There is, however, a superb specimen with octahedral crystals (after argentite) to 1.8 cm in the Terry Wallace collection.

Alunite Group

Unidentified white, massive minerals of the alunite group occur as late-stage hydrothermal gangue in several veins. The material is especially abundant in the Bolivar-Santa Rita section on the west flank of Cerro Rico (Ahlfeld and Reyes, 1955).

Andorite PbAg[Sb.sub.3][S.sub.6]

Lindgren and Creveling (1928) report andorite as occurring in compact masses with pyrite, quartz and cassiterite in the Santa Rita adit, and give an analysis which shows more silver (13.12%) and less lead than Oruro andorite. Ahlfeld and Reyes (1955) reported massive andorite with pyrargyrite. Recently found andorite specimens characterized by 3-mm, longitudinally striated prisms, some of them tubular (!), have been identified by Jaroslav Hyrsl (personal communication).

Arsenic As

Native arsenic has been found in fist-size metallic masses at Cerro Rico. A 12-cm specimen is illustrated in Perez and Tenago (1992).

Arsenopyrite FeAsS

Arsenopyrite occurs sporadically at Cerro Rico, especially in the deeper levels of the Rica and Tajo Polo veins (Ahlfeld and Reyes, 1955). Many specimens showing thin prisms to 1 cm long were found in 1993, with quartz and stannite crystals. It is the most abundant sulfide accompanying phosphophyllite.

Arsenosulvanite [Cu.sub.3](As,V)[S.sub.4]

Microcrystals of arsenosulvanite intergrown with stannite and arsenopyrite crystals (on the matrix of phosphophyllite crystals) were identified by the late Gene Foord through XRD (R. Kosnar, pers. comm.)

Barite BaS[O.sub.4]

Ahlfeld and Reyes (1955) reported barite crystals with cassiterite from the Rosario mine near the peak. Excellent, gemmy, colorless crystals to 1 cm, showing an unusual double wedge-shaped habit, can still be collected as crusts on dacite near the peak.

Berndtite Sn[S.sub.2]

Cerro Rico is the type locality for this rare tin sulfide, and also for ottemannite ([Sn.sub.2][S.sub.3]). Moh and Berndt (1964) first described it from Cerro Rico ore samples, and Moh named it for Berndt in 1966. It occurs as minute, tabular, hexagonal crystals included in pyrite crystals that are replacing stannite ([Cu.sub.2]FeSn[S.sub.4]). In polished section it appears gray with intense orange-yellow internal reflections.

Berthierite Fe[Sb.sub.2][S.sub.4]

Kosnar (pers. comm.) reports having had berthierite identified in polished sections, associated with andorite and prismatic crystals of zinkenite.

Bismuthinite [Bi.sub.2][S.sub.3]

Small amounts of bismuthinite occur sporadically in the deepest levels of the Mendieta-Tajo Polo vein system as small lance-like prisms associated with quartz, cassiterite, stannite and pyrite (Ahlfeld and Reyes, 1955). A white, 7-cm group of crude, corroded crystals from Cerro Rico is in the Museo D. Felipe de Borbon in Madrid.

Boulangerite [Pb.sub.5][Sb.sub.4][S.sub.11]

Jaskolski (1933) described boulangerite from the Cerro Chico, in compact masses with galena and sphalerite.

Bournonite PbCuSb[S.sub.3]

Ahlfeld and Reyes (1955) described large, dull, corroded crystals of bournonite from the Encinas vein. Sharp "cogwheel" twins to 1 cm occur rarely, on tetrahedrite. A single crystal and a penetration twin are illustrated in Dufrenoy's (1856) Mineralogie, with the locality given simply as "Potosi."

Calcite CaC[O.sub.3]

Carbonate minerals, calcite included, are rare at Cerro Rico. The only reference to crystals is Bournon's 1808 work on carbonates in which he attributes three crystal drawings to "Potosi."

Cassiterite Sn[O.sub.2]

Fine-grained, almost invisible disseminations of cassiterite are virtually ubiquitous at Cerro Rico, and currently constitute the main ore mineral (Ahlfeld and Reyes, 1955; Jaskolski, 1933). It occurs both as an early and late-stage hydrothermal mineral in veins and disseminations through silicified volcanic rocks. It has also formed as a supergene alteration product of stannite and "varlamoffite." Black to honey-yellow druses of microcrystals can still be found lining vugs in pyrite on the dumps.

Chalcanthite Cu[SO.sub.4] [center dot] 5[H.sub.2]O

Blue efflorescences of post-mining chalcanthite can be found on tetrahedrite ore piles at Cerro Chico. Long, deep blue stalactites in the mine workings are referred to by the miners as "chalcanthite" but in fact usually prove to be cuprian melanterite.

Chalcopyrite CuFe[S.sub.2]

As an early-stage hydrothermal mineral, chalcopyrite is rare in the upper levels but common in the deeper levels, where it occurs associated with stannite, cassiterite and arsenopyrite. It also occurs as a late-stage hydrothermal mineral associated with sphalerite and tetrahedrite. None of the Cerro Rico occurrences have produced significant crystals.

Childrenite [Fe.sup.2+]Al(P[O.sub.4])[(OH).sub.2] [center dot] [H.sub.2]O

Mrose, Strunz and Ahlfeld, in an unpublished paper on Potosi phosphophyllite, list childrenite as one of the minerals Ahlfeld found at Cerro Rico.

Chlorargyrite AgCl

Ahlfeld and Reyes (1955) state that: "The enormous wealth of silver in the Cerro [Rico] de Potosi came not from any abundance of proustite-pyrargyrite and other noble minerals of silver, nor from native silver, but rather principally from low-grade chlorargyrite ores." After 1640, when amalgamation was introduced at Potosi, oxide-zone rock containing as little as 0.06% silver in the form of chlorargyrite became the main ore and was exploited in huge quantities. Specimens today are very rare, consisting mainly of small blebs on altered silicified dacite from near the peak.

Copiapite Group

Abundant yellow masses of unidentified copiapite-group minerals tend to form wherever acidic water is draining out of the mines. Surfaces of these masses are featureless, but tiny pseudo-hexagonal scales can sometimes be found in protected or enclosed areas.

Diaphorite [Pb.sub.2][Ag.sub.3][Sb.sub.3][S.sub.8]

Diaphorite has recently been identified (by X-ray diffraction) as complex, 3-mm crystals associated with miargyrite in vugs in massive pyrargyrite. The specimens are from the recently discovered Veta Potosi (Potosi vein).

Fluorapatite [Ca.sub.5][([PO.sub.4]).sub.3]F

Well-formed, high-quality crystals of apatite (presumably fluorapatite) have been recovered at Potosi, mostly in the 1950's. Recently only colorless, blocky microcrystals have been found. But Kosnar (pers. comm.) recalls seeing a lilac-colored short hexagonal prismatic crystal in Ahlfeld's collection in 1976.

Franckeite [Mathematical Expression Omitted]

During the last couple of years, several specimens of franckeite have been recovered at Potosi. The mineral occurs as 5 to 6-mm spherical aggregates associated with prismatic to tabular andorite.

Freibergite [(Ag, Cu, Fe).sub.12][(Sb,As).sub.4][S.sub.13]

Published analyses of ore minerals have cited freibergite: Kosnar (pers. comm.) reports having one specimen confirmed by X-ray diffraction and EDX as freibergite, and also having several other crystals of exactly the same appearance.

Galena PbS

Argentiferous galena occurs sporadically as unremarkable specimens, especially at Cerro Chico where it is found with sphalerite.

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

Earthy goethite is the primary component of the limonitic cassiterite ore found in the upper parts of the Cerro Rico. Vugs sometimes contain shiny, brownish black, mammillary to stalactitic growths.

Goslarite ZnS[O.sub.4] [center dot] 7[H.sub.:2]O

Goslarite is occasionally found at Cerro Rico as white acicular to filiform crystals on sphalerite.

Halotrichite [Fe.sub.2+][Al.sub.2][(S[O.sub.4]).sub.4] [center dot] 22 [H.sub.2]O

Fast-growing fibrous masses of halotrichite and pickeringite are common in many of the Cerro Rico adits and drifts. (This is the infamous, so-called "asbestos" with which Potosi guides have frightened gullible tourists!)

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

Hematite, in the form of iridescent botryoidal "turgite," is common as thin crusts on silicified dacite on the upper slopes. It also forms earthy red pseudomorphs after feldspar phenocrysts.

Hopeite [Zn.sub.3][([PO.sub.4]).sub.2] [center dot] 4[H.sub.2]O

An unpublished paper by Mrose, Strunz and Ahlfeld lists hopeite as being among the species Ahlfeld found at Cerro Rico.

Jamesonite [Pb.sub.4]Fe[Sb.sub.6][S.sub.14]

Massive jamesonite occurs sporadically in peripheral veins, as for example at the Lourdes mine on the south flank of Cerro Rico, where it is found with wurtzite.

Jarosite [Mathematical Expression Omitted]

Minute, golden yellow rhombs of jarosite abundantly impregnate a Pliocene tuff on the west flank of the mountain, forming about 16% of the rock. Analysis (Milton, 1935) indicates that this is a potassium-rich jarosite. The mineral also constitutes a component of the limonitic cassiterite ore, and forms druses of small crystals lining feldspar-shaped vugs in dacite. Hydronium jarosite is probably also present, and perhaps even argentojarosite (thought by mining engineers to be the main ore mineral at a new operation on the east slope).

Lazulite (?) Mg[Al.sub.2][([PO.sub.4]).sub.2][(OH).sub.2]

On the dumps of the Potosi tunnel, on the southeastern flank of Cerro Rico, and on other small dumps nearby, Ahlfeld and Reyes (1955) observed pale blue pseudomorphs after feldspar in the dacite: they speculated that this material is "probably" lazulite but give no reasons for this assumption. Specimens (labeled "lazulite") are preserved in the Smithsonian collection.

Lepidocrocite [Fe.sup.3+]O(OH)

Iridescent, botryoidal to vermiform coatings of lepidocrocite on matrix were returned to Spain by Heuland's 1803 expedition, and are now in the National Museum of Natural Science in Madrid. Two specimens are pictured in Perez and Tenago (1992).

Lollingite Fe[As.sub.2]

Kosnar (pers. comm.) reports having five supposed specimens of arsenopyrite prepared for polished section analysis, three of which proved instead to be lollingite. Gene Foord confirmed the analysis.

Matildite AgBi[S.sub.2]

Matildite has been reported from Cerro Rico as brilliant, gray to black, imperfectly formed crystals in granular aggregates and compact, laminary masses associated with tetrahedrite, sphalerite, andorite and pyrargyrite (Lindgren and Creveling, 1928; Perez and Tenago, 1992).

Melanterite [Fe.sup.2+][SO.sub.4] [center dot] 7[H.sub.2]O

Melanterite is abundant at Cerro Rico as beautiful, transparent, sea-green stalactites to several decimeters long in old tunnels and stopes. Ahlfeld and Reyes (1955) report finding it on old wooden beams in the Lourdes mine (southern flank of Cerro Rico). The miners commonly break off these stalactites and present them as gifts to tourists visiting underground. Unfortunately, these invariably dehydrate to a white powder within a few hours of their removal from the moist mine environment. Blue stalactites (called "chalcanthite" by mine engineers) often turn out to be cuprian melanterite.

Miargyrite AgSb[S.sub.2]

Ahlfeld and Reyes (1955) report that miargyrite was once abundant in the upper portions of the Tajo Polo vein system, as compact masses in pyrite with associated pyrargyrite and cassiterite. The recently discovered Potosi vein is producing complex, lustrous, 4-mm to 5-mm crystals associated with crystals of diaphorite, pyrostilpnite and pyrargyrite, in vugs in massive pyrargyrite and miargyrite. Somewhat flattened, rectangular crystals from "Potosi" were illustrated in 1865 by Weisbach in Poggendorf's Annalen.

Nacrite [Al.sub.2][Si.sub.2][O.sub.5][(OH).sub.4]

Nacrite, a clay mineral, occurs in the Veta Potosi as 1-mm spherules composed of minute, radiating, pearly white crystal plates. Associations include pyrite, franckeite, and silver sulfosalts.

Ottemannite [Sn.sub.2][S.sub.3]

Cerro Rico is the type locality for this rare tin sulfide, and also for berndtite (Sn[S.sub.2]). Moh and Berndt (1964) first described it from Cerro Rico ore samples, and Moh named it for Professor Joaquin Ottemann of Heidelberg. It occurs as minute, twinned, orthorhombic laths replacing stannite. In polished section it is gray with orange-brown internal reflections.

Pharmacosiderite [Mathematical Expression Omitted]

Kosnar (pers. comm.) reports finding cubic microcrystals of greenish brown pharmacosiderite with jarosite in 1978.

Phosphophyllite [Zn.sub.2][Fe.sup.2+][([PO.sub.4]).sub.2] [center dot] 4[H.sub.2]O

Phosphophyllite was first described from Hagendorf, Bavaria, in 1920, and may have been known to the miners at Potosi as early as the 1930's. Not recognizing its specimen value, the miners usually hammered the bigger crystals into pretty cleavages as toys for their children. It was not until 1957 that Dr. Fritz Berndt, chief chemist for COMIBOL, correctly identified the gemmy blue-green crystals as phosphophyllite. Berndt found that, unlike its Bavarian counterpart, the Potosi phosphophyllite contains very little, if any, manganese: Avila-Salinas (1982) confirmed Berndt's observation (Table 2). At Potosi it is thought to have formed by hydrothermal alteration of sphalerite and pyrite in the presence of phosphate ions supplied by associated hypogenic apatite (Avila-Salinas, 1982).
Table 2. Composition of Cerro Rico phosphophyllite

(Avila-Salinas, 1982).

                        (1)       (2)       (3)

ZnO                    36.30     34.26     36.42
FeO                    11.35     12.24     14.97
MnO                     4.62      4.96      0.12
[P.sub.2][O.sub.5]     31.66     32.51     32.72
[H.sub.2]O             16.07     16.52     16.40

Total:                100.00    100.49    100.63

(1) Ideal formula, with Fe:Mn = 17:7 (Palache et al., 1951)

(2) Hagendorf

(3) Cerro Rico


Cerro Rico phosphophyllite has been found mainly on the 7th-10th levels of the Kraus vein, about 1,000 feet below the main haulage tunnel of the Unificada mine. Those deep workings have long been abandoned and unventilated, and suffer from the effects of intense pyrite decomposition (which liberates sulfuric acid and heat). According to a recent rumor in Potosi, three miners intent on recovering more phosphophyllite descended into the acidic heat of the abandoned levels in March of 1996 and at least one of them (some say all three) died of asphyxiation. Peter Bancroft had descended alone to the abandoned 535-meter level in 1957 and spent several hours trying unsuccessfully to work thin seams of phosphophyllite in solid, massive chalcopyrite, only to find that he was nearly unable to crawl out due to exhaustion and bad air (Bancroft, 1984).

Many phosphophyllite specimens that have reached the mineral market came out by way of Aurelio Bustos, head of the geology office of COMIBOL in Oruro. For over 20 years he was the major source of Bolivian mineral specimens, and was well known to every traveling international mineral dealer. Bustos claimed (though some dispute him) to have personally collected the "Swoboda Phosphophyllite," a 7-cm twin now in the Houston Museum of Natural Science, in 1960 and has even pinpointed the exact location of the vug.

Edward Swoboda, in his memoirs, talked about the initial encounters the miners had with phosphophyllite, as recounted to him by Aurelio Bustos:

Many years ago, when mining was focussed on stoping between the seventh and ninth levels [of the Unificada mine], they had mined along a section of the vein which produced many vugs of a bright green, transparent mineral initially thought to be fluorite. During this period an unknown quantity of phosphophyllite was discarded on the waste dumps. Vugs up to 30 cm in diameter were found filled with the green crystals, which were systematically destroyed. Not long after these splendid crystals were finally correctly identified and sporadically collected and saved, mining was discontinued on these levels.

Some rough unpublished notes on the phosphophyllite occurrence, written during the early 1960's, were found among Friedrich Ahlfeld's papers following his death. Following is an (edited) excerpt:

The mineral [phosphophyllite] was first observed by miners and mining engineers who mistook it for vivianite, a mineral common in Bolivian ore deposits. It is not known how many crystals have been found. Many were carried away by the miners to their homes and were destroyed by their children. I have seen eight good crystals, all twinned. Two of these are in the collection of Mr. P. Zubrzyski, a mining engineer. The largest crystals, however, are said to be in a private collection which I have not seen [according to Kosnar, this is a reference to Urquidi's collection]; 11 crystals to 6 cm in size are preserved there. I saw the place [Potosi] in 1958, when it was completely exhausted, but I was able to purchase some good crystals from the same miners who had earlier supplied study specimens to Hugo Strunz and Mary Mrose.

The "Wilbur Phosphophyllite," by far the largest twin at 13 cm and on matrix besides, was recovered by a man named Urquidi in 1962; according to Richard Kosnar (pers. comm.). Urquidi was a mining engineer at the Unificada mine from 1956 to 1964 and saw a great many fine crystals, most of which came out in 1957, But in 1964 the "phosphophyllite stope" in the Unificada mine was sealed off by a concrete bulkhead to prevent further collecting by miners. The 13-cm crystal on matrix was sold by Urquidi to Kosnar in 1977; Kosnar sold it to David Wilbur in 1978; Wilbur later sold it to the Research Charitable Trust in New York, in whose possession it remains today. Though repaired, it is widely regarded as one of the finest mineral specimens in the world.

According to Kosnar, Urquidi told him that on that fateful day in 1962, when the miners returned to the stope following a blast, the entire floor of the drift was completely covered by broken and loose crystals of phosphophyllite; it looked as if someone had smashed hundreds of green glass bottles in the stope! It took several days for the miners to muck out all of the debris. Kosnar also purchased a plastic bag containing 48 small, relatively undamaged, highly gemmy loose twins from this find; one fine thumbnail-size specimen was sold to Ralph Clark, and the rest went to a New York gem cutter who turned them all into faceted stones.

Another fine specimen, the 6-cm "Eidahl Phosphophyllite" once pictured on the cover of the Mineralogical Record (vol. 12, no. 1) was also found around 1962 and was first purchased by Phil Gregory in 1970. Gregory then sold it to Dave Eidahl (now deceased); the specimen remained in Eidahl's family after his death but was recently accidentally destroyed while on loan for an exhibition.

Kosnar also reports that a superb, doubly terminated, gemmy crystal measuring 1.9 x 1.9 x 5.7 cm, with a little matrix, was sold to the late mineral dealer Ed McDole in 1969, and after his death was retained by his brother in northern Idaho; Kosnar rates it as being better than the Phil Gregory/Dave Eidahl specimen. Other smaller but nonetheless excellent twins are currently in the collections of Richard Kosnar (obtained from Keith Proctor in 1980), Ernie Schlichter, Wendy Melanson, William Larson, the Carnegie Museum of Natural History in Pittsburgh, John Barlow, Martin Zinn, the Canadian Museum of Nature in Ottawa and the Smithsonian Institution.

A phosphophyllite crystal apparently originating from a 1965 find was pictured on the cover of Der Aufschluss by Weninger (1974). He wrote:

Through a meeting with Mr. Xavier M. Prevost of La Paz in June of 1974, I was able to briefly view an outstandingly beautiful crystal [of phosphophyllite] from Cerro Rico de Potosi and photograph it for this report. The crystal is a swallow-tail twin . . . pale blue-green, transparent . . . measuring 1.5 x 3.1 x 7.9 cm. According to Mr. Prevost, it was found in 1965 in a 2.5-meter cavity in the stannite zone of the Cerro Rico, together with a few smaller crystals which (because of their excellent cleavage) had all broken off their matrix during blasting.

This is clearly not the "Bustos Cavity" found in 1960. The current location of the specimen is not known. However, Kosnar reports that a superb, doubly terminated, 6-cm crystal from this pocket (which he dates at 1962-1964) was in the collection of Kurt Kontrus in Vienna, Austria, in 1969.

Mining may someday resume in the phosphophyllite zone of the vein; ore values there are still good. Only time will tell whether new specimens will be found.

Potosi phosphophyllite ranges from nearly colorless to pale green to a rich, slightly bluish green which is quite distinctive. Crystals occur in sizes from a few millimeters to perhaps 13 cm (the Wilbur crystal), but most specimens that have survived measure from 1 to 5 cm.

Avila-Salinas (1982) studied the crystal morphology of Cerro Rico phosphophyllite, and reported that {100}, {110}, {210}, {011}, {111}, {211} and [Mathematical Expression Omitted] are commonly seen forms whereas {001}, {120}, {013}, {101}, [Mathematical Expression Omitted] and [Mathematical Expression Omitted] are present but relatively rarely. In the specimens we have studied, {100}, {111} and {001} are usually dominant, and a variety of small modifications are_typically present as well (provisionally identified as {010}, [Mathematical Expression Omitted], [Mathematical Expression Omitted], [Mathematical Expression Omitted], and possibly also[Mathematical Expression Omitted][Mathematical Expression Omitted], {124}, [Mathematical Expression Omitted], [Mathematical Expression Omitted] and [Mathematical Expression Omitted]). Some of these forms have not previously been reported for the species; but the large crystal sizes, damaged edges, oscillatory development and other problems render positive identification of some forms difficult. Additional forms beyond those listed above have been observed but remain unidentified.

The twinning appears to be simple contact twinning on (100), and penetration twinning according to the same law. In some cases the twins are flattened markedly parallel to the composition plane, and in other cases there is no noticeable flattening, the twin members each being relatively equant. The re-entrant angle, looking rather like a fish-tail or swallow-tail twin, is bounded primarily by two corresponding triangular c faces meeting at an apex. Untwinned crystals are very rare, especially in the larger crystal sizes. Single crystals of an elongated prismatic habit are also known. Complete penetration twins, having a bow-tie aspect, are also very rare. Most twins are simple V-twins broken off at the base. Most of the crystal drawings shown here (prepared by R. Peter Richards) are in non-standard orientations so as to be more recognizable in comparison to actual specimens and the way they are typically mounted in collections. No crystal drawings of Cerro Rico phosphophyllite have previously been published.

Phosphophyllite crystals are generally very transparent, often showing incipient cleavage planes along {100} (perfect), {010} (distinct), and {102} (distinct). Some crystals show rounded edges and surface features which may indicate solution etching. Matrix, where present, usually contains associated arsenopyrite, stannite, and occasionally other sulfides.

The phosphophyllite crystals from Potosi, for size, color and quality, are by far the finest in the world.

Pickeringite Mg[Al.sub.2][([SO.sub.4]).sub.4][center dot]22[H.sub.2]O

Fast-growing fibrous masses of pickeringite and halotrichite are common in many of the old workings at Cerro Rico.

Proustite [Ag.sub.3]As[S.sub.3]

Bolivians tend to call any fresh pyrargyrite which has not yet darkened, "proustite." However, all analyzed specimens have thus far turned out to be pyrargyrite. In all of the subvolcanic Sn-Ag mines in Bolivia, Sb sulfosalts greatly predominate over their As counterparts.

Pyrargyrite [Ag.sub.3]Sb[S.sub.3]

Ahlfeld and Reyes (1955) reported pyrargyrite/miargyrite intergrowths, often intimately mixed with cassiterite, to be locally abundant in the upper levels of the Tajo Polo, Rica, and other veins. Thin, poorly developed prisms to 1.8 cm were also found.

In the early 1990's a rich new vein of mostly massive pyrargyrite (now named the Potosi vein) was discovered in a cooperative mining operation just above the Pailaviri mine, and was subsequently found to actually cut across the main haulageway of the Pailaviri mine, where it had gone unnoticed! Small vugs in this vein contain short, prismatic pyrargyrite crystals to 2 mm, associated with crystals of miargyrite, pyrostilpnite, valentinite, pyrite and quartz.

Pyrite [FeS.sub.2]

Much of the lower portions of the Cerro Rico volcanic stock have been pyritized. Pyrite is the most abundant vein mineral at Potosi, in both the early and late assemblages. Most pyrite is massive, however, and good crystal specimens are scarce. Small cubes, pyritohedrons and complex combinations can still be found, associated with cassiterite and quartz crystals, in vugs in boulders of massive pyrite on the dumps.

Pyrostilpnite [Ag.sub.3]Sb[S.sub.3]

Small (1-2 mm), well-formed orange-red crystals of pyrostilpnite having typical gypsum-like habit and flexibility have been found since 1995 in the newly opened Potosi vein. They occur in small vugs in massive pyrargyrite and miargyrite. Filiform-dendritic crystals have also been found. Associated species include crystals of diaphorite, miargyrite, quartz, pyrargyrite, pyrite, siderite and valentinite.

Quartz [SiO.sub.2]

Quartz, massive and as small, simple crystals, is an abundant gangue mineral in both early and late-stage hydrothermal veins. On the southern flank of Cerro Rico, the main Tajo Polo vein grades into more or less pure quartz. Supergene or late-stage hydrothermal quartz also forms yellowish to pale blue micro-botryoidal chalcedonic crusts on sphalerite from Cerro Chico.

Ramdohrite [Ag.sub.3][Pb.sub.6][Sb.sub.11][S.sub.24]

Ramdohrite from "Potosi, Bolivia" was reported by Harcourt (1942), who commented on the close similarity of its X-ray diffraction pattern to that of andorite. No other description was given regarding the specimen or its precise locality.

Siderite [Fe.sup.2+][CO.sub.3]

Siderite occurs in small quantities as a late-stage hydrothermal mineral. Sharp, translucent, yellow scalenohedrons to a few millimeters in size accompany silver sulfosalts in the recently opened Potosi vein. Hollow, thin-shelled siderite scalenohedrons found on sphalerite at Cerro Chico may be pseudomorphs after smithsonite.

Sideronatrite [Na.sub.2][Fe.sup.3+][([SO.sub.4]).sub.2](OH)[center dot]3[H.sub.2]O

Ahlfeld and Reyes (1955) report silky orange sideronatrite associated with voltaite in the ancient workings.

Siderotil [Fe.sup.2+][SO.sub.4][center dot]5H.sub.2]O

White, pulverulent masses of siderotil form abundantly as a post-mining mineral on the walls of tunnels, especially near entrance adits, as a dehydration product of melanterite.

Silver Ag

Native silver was abundant at Cerro Rico in the early colonial days, but the oxide zone in which it occurred has been completely mined out. Very fine-grained disseminations in dacite, associated with chlorargyrite, also occur. Highly fragile specimens of loose, micro-filiform silver "moss" are still occasionally dug out of the "cementation zone" separating the oxide and primary zones.

Very few colonial-era silver specimens from Potosi have survived. Massive, compact silver specimens from several veins are preserved in the Gumucio Collection. The Vienna Natural History Museum has an 11-cm specimen of reticulated silver and also a sub-dendritic mass weighing 8 kg, acquired during colonial times. Kosnar (pers. comm.) reports having once owned a 1.9-kg chunk of massive native silver from Cerro Rico, associated with sharp miargyrite and pyrargyrite microcrystals.

Smithsonite ZnC[O.sub.3]

Jaskolski (1933) and Ahlfeld and Reyes (1955) describe smithsonite from Cerro Chico de Polosi. It occurs as small, transparent, honey-yellow crystals and as reniform masses containing remnant inclusions of sphalerite, in cavities in galena. A recent find of hollow, thin-shelled siderite scalenohedrons on sphalerite may be pseudomorphs after smithsonite crystals.

Sphalerite (Zn,Fe)S

Sphalerite occurs sporadically in the Cerro Rico and Cerro Chico as both early and late-stage hydrothermal deposits, the earlier crystals tending to be more iron-rich.

Stannite [Cu.sub.2]FeSn[S.sub.4]

Good specimens of stannite from Cerro Rico were recognized and preserved as far back as the 19th century. Stelzner (1897) described crystals 1 to 3 mm associated with acicular arsenopyrite in vugs in pyrite from the Cotamitos vein. Similar specimens are still occasionally recovered, in association with crystals of arsenopyrite and quartz. The dominant habit of Potosi stannite is p{111}. Generally the crystals range from lustrous to dull gray, but in recent years several cabinet-size druses have been found which are brightly iridescent. Kosnar (pers. comm.) reports having owned "relatively sharp, somewhat lustrous, sphenoidal crystals to 1 cm on arsenopyrite microcrystals." The "Proctor/Kosnar phosphophyllite" [ILLUSTRATION FOR FIGURE 36 OMITTED] matrix contains dull stannite crystals to 1 cm as well.

In the Mendieta-Tajo Polo vein system the abundance of stannite increases with depth, becoming quite common as botryoidal or concentric growths with pyrite and cassiterite. Chemically the stannite from Potosi is of usual composition in the upper levels but becomes increasingly zinc-rich with depth and takes on a dark brown to olive-green color (Ahlfeld et al., 1935).

Stephanite [Ag.sub.5]Sb[S.sub.4]

Kosnar (pers. comm.) reports having polished sections made in which minute amounts of stephanite have been identified.

Stibnite [Sb.sub.2][S.sub.3]

Stibnite is a rare, late-stage hydrothermal mineral at Potosi. In 1995 some vugs in sphalerite in the Cerro Chico area produced fine acicular sprays of stibnite to 1 cm.

Teallite PbSn[S.sub.2]

Kosnar (pers. comm.) reports the identification of lath-like crystals of teallite to 5 mm in polished sections.

Tetrahedrite [(Cu,Fe,Ag,Zn).sub.12][Sb.sub.4][S.sub.13]

Tetrahedrite is a common mineral at Potosi, as simple tetrahedrons to 1 cm. It is, at present, considered to be the most important silver ore mineral. Published references may cite tetrahedrite or freibergite, but analyses are absent.

Valentinite [Sb.sub.2][O.sub.3]

Excellent sprays of transparent, adamantine valentinite blades to 1.5 cm were first found at Potosi in 1995, associated with pyrostilpnite in vugs in massive pyrargyrite-miargyrite in the Potosi vein. Dark gray valentinite colored by abundant microfiliform metallic inclusions is also known. The mineral is a primary late-stage hydrothermal product.

Varlamoffite (Sn,Fe)[(O,OH).sub.2]

Herzenberg (1946) described an amorphous hydrated tin oxide from Cerro Rico and named it souxite (after Luis Soux, a mine owner). It was found to occur abundantly as pale yellow, pulverulent masses with ferric iron and admixed silica, apparently having formed by the oxidation of stannite in the Utne vein, Cotamitos mine. It was described as being easily soluble in acids (unlike cassiterite), but unstable in the air, altering further to a very fine-grained, acid-insoluble cassiterite. His work was not convincing, however, and proved only that tin in some form soluble in acids was present. Reviewer Michael Fleischer wrote: "This may be a new mineral, but the evidence is certainly insufficient to justify a name or the assignment of a formula" (American Mineralogist, 32, 372).

The following year DeDycker (Buttgenbach, 1947) described a similar yellow, earthy alteration product of stannite from tin-bearing veins in the Belgian Congo, naming it varlamoffite (after Nicolas Varlamoff, a mining engineer in the Belgian Congo who discovered it). Only analyses of mixtures were provided. Fleischer's comments appended to his abstract of the description read simply: "Compare the similar souxite" (American Mineralogist, 34, 618); Fleischer remains unconvinced of its validity as a species, listing it in his Glossary of Mineral Species 1995 as "perhaps a variety of cassiterite."

In 1982 the material was again analyzed, this time from a granite in China, by Deng et al. They retained the name varlamoffite, and [TABULAR DATA FOR TABLE 4 OMITTED] provided a fuller description of apparently better-formed material: orange-red, semi-transparent, vitreous to greasy, sp. gr. 3.21-3.26, nearly amorphous with X-ray lines at 3.30, 2.5810 and 1.4105, unit cell parameters a = 4.6671, c = 3.0959 [Angstrom], and space group p [4.sub.2]/mnm. This was apparently sufficient evidence for Nickel and Nichols (1991), who gave the mineral full species status in their Mineral Reference Manual.

Consequently it appears that Cerro Rico was very nearly the type locality for the questionable species currently referred to as varlamoffite.

Voltaite [Mathematical Expression Omitted]

Lindgren and Creveling (1928) reported waxy, olive-green voltaite crystals associated with orange sideronatrite in the ancient workings.

Wavellite [Al.sub.3][(P[O.sub.4]).sub.2][(OH,F).sub.3][center dot]5[H.sub.2]O

Crusts of tiny, colorless wavellite crystals lining vugs in stannite were described by Ahlfeld and Reyes (1955) from the Tajo Polo vein. Recently some yellowish, micro-botryoidal crusts have been found coating pyrite and silver sulfosalts in the newly opened Potosi vein. The Roberto Herzenberg collection contained wavellite with pyrite, arsenopyrite and cassiterite from the San Miguel vein.

Wolframite series [(Mn,Fe).sup.2]+[WO.sub.4]

Thin, tabular crystals of wolframite (exact composition undetermined) occur in the deeper levels of the Utne and Tajo Polo vein systems, associated with early hydrothermal quartz and pyrite. The Roberto Herzenberg collection contained specimens of Cerro Rico wolframite with cassiterite and stannite. Although wolframite is not especially abundant at Cerro Rico, the deeper levels of the Unificada mine at one time yielded several tons annually.

Kosnar (pers. comm.) contends that Cerro Rico wolframite is probably ferberite, the iron end-member of the series. Six specimens he has had analyzed from other mines in the Bolivian tin belt have all proven to be ferberite, and there seems to be (judging by the thousands of specimens he has seen) a general paucity of Mn throughout the Bolivian deposits. Kosnar once held a specimen concession at Ahlfeld's tungsten mine at Chicote Grande, which produced hundreds of "wolframite" specimens containing less than 1% Mn.

Wurtzite (Zn,Fe)S

Ahlfeld and Reyes (1855) reported compact wurtzite with jamesonite from the Lourdes mine on the southern flank of the Cerro Rico. Sharp, translucent, brownish red hexagonal platelets to 2 mm can still be found in vugs in sphalerite and tetrahedrite from ore piles on the northern flank of Cerro Rico.

Xanthoconite [Ag.sub.3]As[S.sub.3]

In 1996 a single 1-mm, orange, thin-tabular hexagonal platelet of twinned xanthoconite was found with pyrostilpnite and pyrargyrite in the newly opened Potosi vein.

Zinc-melanterite (Zn,[Cu.sup.2_],[Fe.sup.2_])S[O.sub.4][center dot]7[H.sub.2]O

Small, 2-mm masses of compact, fine-grained, yellow-green zinc-melanterite were found in 1994 as an alteration product of zincian stannite.

Zinkenite [Pb.sub.9][Sb.sub.22][s.sub.42]

Zinkenite was listed by Jaskolski (1933) as a late-stage hydrothermal mineral at Cerro Rico. Although we have not been able to confirm that report, zinkenite is a fairly common mineral in the Bolivian tin belt, so there is no reason to doubt its occurrence at Potosi. Kosnar (pers. comm.) has confirmed the presence of slender zinkenite prisms associated with stannite, franckeite and andorite (by X-ray diffraction analysis).

CURRENT COLLECTING OPPORTUNITIES

Nearly all of the surface of Cerro Rico is today covered by dumps, and there are no fences to keep out mineral collectors except in the vicinity of the Pailaviri mine. Most of the dump material consists of boulders of massive pyrite and tough, silicified volcanic rocks. With patience and a heavy hammer one can find rugs containing crystals (mostly microcrystals) of quartz, pyrite, arsenopyrite, cassiterite, jarosite and barite. Some of the smaller mining cooperatives have ore piles accumulating outside their adits, and the miners are quite happy to sell a vuggy lump out of the pile for a dollar to two. In such fresh sulfide ore one can find crystals of sphalerite, wurtzite, siderite, wavellite, tetrahedrite and other less common sulfosalts. Remember that, in the dry tropics at an altitude of 14,000 to 15,000 feet the sunlight (especially ultraviolet) is intense, and sunstroke and dehydration are serious dangers. Take a hat, sunscreen, and plenty of fluids.

Because of recent political conflicts, tours are no longer being offered at the Pailaviri mine, but some of the impoverished Indian-owned cooperatives are still willing to accept a few dollars for an underground tour of their workings. Climbing and sliding around inside working stopes which are two or three hours hike deep into the mountain seems to be the favorite entertainment lately for backpacker tourists visiting Potosi. Several tourist agencies are now organizing such excursions commercially. The independent mineral collector may be able to arrange a visit to a working stope by offering some traditional Bolivian miners' gifts such as liquor, cigarettes, a pound of coca leaves, or a few sticks of dynamite (all of which can be purchased legally on the streets of Potosi). It is also good to remember that every adit contains a small shrine or statue of Tio. the subterranean god who "owns" the minerals and who must be compensated for their removal. It is customary to leave some alcohol, coca leaves, cigarettes or money there as an offering.

A few of the old Spanish silver mines are still accessible in the upper oxide zone near the peak of Cerro Rico. Human bones are more common there than good mineral specimens, but for silver it is the only place to look because all of the old workings farther down the mountainside are buried under younger dump material.

POSTSCRIPT: BOLIVIAN COLLECTIONS

Most so-called Bolivian "collections" of minerals are merely shelves piled with massive, unlabeled ore samples. As a historical note it is worth mentioning that only four important collections of classic Bolivian specimens were built during the mid-20th century. These are the collections of Mark Bandy, Friedrich Ahlfeld, Roberto Herzenberg, and Julio Gumucio.

Mark Chance Bandy (1900-1963) was an American mining engineer and mineral collector. With a 1938 Harvard PhD degree under his belt, he set off on a career primarily in foreign mining geology that took him to Mexico, Europe, Africa, Venezuela, Chile and Bolivia (Chief Geologist for 11 years at Llallagua). He was also a Latin scholar, translating and publishing Agricola's De natura fossilium. A Chilean mineral he found was named bandylite in his honor. After retiring to Wickenburg, Arizona, in 1958 he set up his large and fine mineral collection, rich in Bolivian specimens, in a special out-building near his home. Ten years after his death in 1963 his widow, Jean Bandy, arranged for his collection to go to the Natural History Museum of Los Angeles County, where it is preserved today. (See "The Mark Chance Bandy collection" by Bob Jones in vol. 4, no. 5, and series of Bandy's Chilean collecting memoirs published in vol. 14, no. 6 and vol. 15, no. 2 and 3.)

Roberto Herzenberg (1885-1956), after whom the mineral herzenbergite was named, was a Latvian-born German mineralogist who was especially adept at quantitative analysis. He discovered several new species in Bolivia (including rooseveltite), but some of them described purely in chemical terms were later discredited. He worked in Oruro, Bolivia, as Chief Chemist for the German-Jewish "tin baron" Mauricio Hochschild. His extensive mineral collection of 2,731 specimens, most of them cabinet-size to museum-size Bolivian minerals, was sold to Pablo Biggemann, owner of several Bolivian mines. Biggemann gradually dispersed the better part of the collection through visiting foreign dealers (notably Martin Ehrmann), and left the remainder to the Technical University of Oruro where it is currently on display. Herzenberg's catalog lists 46 specimens from Cerro Rico de Potosi, including one alunite, 27 cassiterites, two sphalerites, ten stannites, two jamesonites and two wolframites (but no phosphophyllite, which was not recognized at Potosi until a year after his death).

Little seems to be known about Julio Gumucio, but he is the only collector of the four who was a native-born Bolivian. He was a mining engineer during the pre-revolution years at the Siglo Veinte mine in Llallagua, and in his old age managed the railroad yard of tin baron Simon Patino in Machacamarca. His collection was obtained by the Banco Minero in La Paz, and was displayed there for many years. When the Banco Minero failed, the collection went into storage for a time, then reappeared recently in the old Spanish mint building (the Casa Moneda) in downtown Potosi, where it is open to the public. It still contains some good specimens, but many were probably pilfered during its years out of public view.

Friedrich Ahlfeld (1892-1982) was a German mining engineer and geologist who devoted himself to Bolivian mineralogy, and wrote important reference works including Las Especies Minerales de Bolivia (1937, 1943 and 1955). Ahlfeld was a geologist at the Unificada mine, Cerro Rico de Potosi from 1948 to 1950, but had relatively few specimens from there in his collection. He did sell specimens to museums and visiting dealers, including in later years a specimen of phosphophyllite said to two young American dealers, Charles Key and Jack Young, for $50. Even after his death, specimens from his collection were being said piecemeal by his widow. Presumably it is now totally dispersed.

ACKNOWLEDGMENTS

We wish to thank Terry Szenics for providing a number of illustrations; Dr. Jeff Post and the Mineral Sciences staff of the U.S. National Museum of Natural History (Smithsonian Institution) for a print-out of the museum's Bolivian specimens; Wendy Melanson and Marc Wilson of the Carnegie Museum of Natural History for the loan of study specimens; Dr. Jaroslav Hyrsl of Charles University in Prague for his very helpful research on Cerro Rico over the last three years and especially for the many X-ray diffraction analyses he has provided for Cerro Rico sulfosalts; R. Peter Richards for preparation of the crystal drawings; Bryan Lees and Abraham Rosenzweig for reviewing the manuscript; and Richard Kosnar for providing much useful information on Cerro Rico species and specimens.

BIBLIOGRAPHY - Potosi, Bolivia

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RELATED ARTICLE: The Discovery of the Rich Mines of Potosi, & how Captain Caravajall took it into his Power

Fortune having showed herself so amiable to Captain Caravajall, it appears now that she has brought him to the top of the Hill of Prosperity. It happened that after certain days, that the Indians and Anaconas of Juan de villa Roell, citizen of the town of Plata [= "silver"], went traveling in the country; about 18 leagues distant from the said town, they chanced to come to a high hill, situated in a plain, in the which, they found manifest tokens of silver, whereupon they began to melt out a vein which seemed rich.

The riches was so great which here they found, that almost in every vein where they made their assay, they found the greatest part of ore to be fine silver, and the basest mines were by valuation 480 ducats in every hundredweight of ore, which is the greatest riches that ever has been seen or written of. When the Justice of the town of Plata had understanding of this fortunate success, the ruler of the said town came and divided the said mines among the townsmen, and each of them made his choice according to his lot.

The Indians and Anaconas which came thither to work were many in number, in such sort that in a short time they built at the foot of the said hill a town to dwell in, which multiplied in such wise, that there inhabited of Indian workmen above 7,000 persons, which did so well understand that business, that they came to agreement with their masters to allow unto them a weekly pension: but their gain was twice so much as they paid unto their masters.

The veins of these mine are of such quality that the ore will not melt with the winds of bellows, as in other mines are accustomed: but their melting is in certain little furnaces called Guayras, wherein they use to melt with coals and sheep's dung, with only the force of the air, without any other instrument. These mines are called Potosi, by reason that all the borders thereabouts are so called.

These Indian workmen are rich, for he that hath but four or five thousand poyzes [of silver] is counted but poor: notwithstanding the great pension or tribute which they pay unto their masters, and all such workmen which come thither to work will not willingly depart from thence, by reason that their pains and peril is not comparable to other mines, by blowing of the bellows, with the great smoke of the coals: and likewise the sulfur of the veins of ore.

When the repartitioning[?] of these mines were made, they began to provide all sorts of necessary victuals for the workmen, which daily came to serve in those works, whereby victuals grew to such a dearth that the bushell of maize came to be worth twenty castlins of gold, and a bushell of wheat as much; and a sack of the herbs called Coca, was worth 30 poyzes, yea, and afterward it came to be dearer: yet though the great treasure which there was found, all the other mines were left uninhabited, and especially the mines of Porco, where Hernando Pisarro had a great portion of ground, out of the which he gathered great riches.

The miners also which gathered gold in Cambaya, and other rivers, left there works and came to Potosi, because they found there greater profit without comparison. They which understand in these works, hold opinion that by manifest tokens these mines are of perpetuity.

With this good success, Captain Caravajall began to gather great sums of treasure, in such sort that he took possession of all the [property of the] Indians and Anaconas, of such Spaniards as were slain or fled, or had been against him in his former war: so that in a short time he obtained into his power the sum of seven hundred thousand poyzes, and would not thereof aid his soldiers with anything, who had followed him in all his wars: whereupon they began to murmur among themselves, and were minded to kill him.

The chiefs of this mutiny, were Luis Pardomo, Alonso de Camargo, Diego de Balmazeda, and Diego de Luxan: there were near 30 persons, which determined to execute the said pretense within one month after Caravajell was come to the town of Plata: But through a mischance which happened, they deferred the matter til another day. This practice was not so secretly wrought, but that Caravajall came to knowledge thereof, whereupon he commanded Luis Pardomo, Camargo, Orbaneja, Balmazeda and another 10 or 12 persons of the principallest to be quartered, and others banished: so that with the execution of such cruel justices, in causes of mutinies, the people were so afraid that they never dared at any time after to deal in the like practices.
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Title Annotation:includes related article on the discovery of the Potosi mines and rule of Captain Caravajall
Author:Wilson, Wendell E.; Petrov, Alfredo
Publication:The Mineralogical Record
Date:Jan 1, 1999
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