Single crystals of polybasite are unknown, it universally occurs as pseudo hexagonal tablets or rosettes to 6 cm¹. It occurs in low to mid temperature hydrothermal silver veins. It is associated with pyrargyrite, tetrahedrite, stephanite, and other silver and lead sulfosalts, silver, argentite, gold, various simple sufides,and with quartz. calcite. dolomite, barite. Oriented overgrowths of Stephanite and Chalcopyrite (see Arizpe below) have been noted². Often the crystals are small making for excellent micromounts. It is common in small quantities and rarely in large amounts. Of the Ruby Silvers only miargyrite is rarer. Fine specimens with crystals over a cm are quite rare, especially undamaged and not on black matrix. Such aesthetic specimens on matrix (minatures and larger) are worth over a thousand (US$) and as the specimen and crystals get bigger, to tens of thousands of dollars. Good silver minerals have always commanded higher prices.

Now at the dawn of the new millenium, due to the current IMA lattice site rules, the Polybasite group contains Polybasite, Pearceite, Selenopolybasite, Cupropolybasite and Cupropearceite. The ideal formula for the group is 6T₂S₇>9CuS₂X₂> with M = Ag, Cu; T = As, Sb; and X = S, Se, with all of the sulfur positions able to accept some selenium³ and the copper site able to accept some silver⁴.


Only Polybasite and Pearceite have been found in cm sized xls, the others are found only as study grade material or as a single museum specimen. Often sulfosalts in an arsenic antimony series, like the antimony rich Bournonite and Tetrahedrite are more grey and get blacker as the arsenic content increases to the point where you can't visually separate them from the arsenic dominant members (such as Tennantite and Seligmannite, repectively) which are quite black. Sadly you can't visually distinguish any of the minerals in the Polybasite group. If a locality has both arsenic and antimony, at least a probe analysis is required to distinguish Pearceite from Polybasite and both a quantitative probe and crystal structure analysis is required to identify the others. Since Parceite is rarer there is an incentive to label Polybasites as Pearceites. In 1987 I saw a small Polybasite from the Husky Mine in the Humboldt Museum in Berlin labeled as Pearceite and I have my doubts about some recently marketed Mexican Pearcites.

One of the four "Ruby Silvers" (Pyrargyrite, Proustite and Miargyrite are the other three), Polybasite was recognized as distinct in the early 19^th century and was named by Rose in the same journal that Einstein published his relativity theories nearly a hundred years later. He named it from the Greek πολύς, poly, "many" and βάσις, basis, "a base" in allusion to the many base elements (Cu, Sb, As) present, one more (Cu) than the other Ruby Silvers².

The saga of Polybasite in the 19^th and 20 ^th century has had many twists, turns and surprises. Like many sulfosalts the formula contains (Sb, As) indicating there is also probably an arsenic rich mineral with almost the same formula except for (As. Sb). This arsenic dominant mineral was recognized by Rose in 1833, and reasonably (I think) he called it Polybasite². It wasn't until 1896 that Penfield named the arsenic analogue Pearceite⁵. In the 20^th century Frondel, who had the habit of showing up at conferences and announcing results with no supporting data, saw that the x-ray powder patterns were nearly the same for both minerals⁶, but there were some extra diffraction lines with some crystals. Real "pearceite" had these extra lines, real "polybasite" did not. To make matters worse there were "polybasites" (antimony rich) that had such lines⁷ (Antimonpearceite) and arsenic rich "polybasites" that had no extra lines⁷ (Arsenpolybasite). So by the end of the 20th century there were four members of the polybasite group (Polybasite, Pearceite, Antimonpearceite, Arsenpolybasite). The tremendous improvement in computerised x-ray technology that occured in the late 20^th century finally allowed the deciphering of these extra lines in polybasites. By the mid 20^th century single crystal work recognized that such extra lines were usually due to crystal growth features called polytypes, but the ubiquitous twinning (actualy trillings to give the pseudohexagonal rosettes) in this group precluded sorting this out until the people got around to it in the new millenium. Fortunately the IMA decided in 1998 that polytypes were no longer a species definer, but they would grandfather those names already published. Thus the polybasite group narrowly missed expanding by over half a dozen as now the polytypes designated by numbers and letters after the name had to be all varieties! Thus there are two structural varieties of the antimomy rich polybasite (trigonal, with space group P321 and monoclinic with C2/c)⁸. Further the species antimonpearceite was replaced by the variety polybasite-Tac, arsenpolybasite-221 by pearceite-T2ac, arsenpolybasite-222 by pearceite-M2a2b2c, polybasite-221 by polybasite-T2ac, and polybasite-222 by polybasite-M2a2b2c^4.9 So now it would seem that we are back to 1896 with only two members of the polybasite group: Polybasite and Pearcite, repectively antimony and arsenic rich members. Perhaps the next century will take us back to 1833 with only Polybasite and lots of varieties (I hope).

However in many sulfosalts selenium can replace some or most of the sulfur. When the selenium dominates two of the sulfur sites we get the new species Selenopolybasite <(Ag,Cu)₆(Sb,As)₂(S,Se)₇>9Cu(S,Se)₂Se₂> from the De Lamar mine, Owyhee County, Idaho, USA.³. Although overall the sulfur dominates the selenium, it is the last two Se₂ lattice sites that creates the species according to current IMA rules. And of course there will be polytype varieties, so far only Selenopolybasite-Tac has been observed from the San Carlos mine, La Luz,Guanajuato, Mexico⁹. One should also expect a similar selenium pearceite analogues, but so far none have been observed.

Further there are silver deficient Polybasites with idealy only copper in one of the silver positions. So we get Cupropolybasite 6Sb₂S₇>9CuS₄> from the Silbak Premier Mine, Stewart, Skeena Mining Division, British Columbia Canada¹⁰. If the antimony dominant member can do this so should the arsenic dominant one and we get Cupropearceite 6As₂S₇>9CuS₄>¹⁰ from the Sarbaiskoe deposit (Sarbai; Sarbay Mine), Qostaney Oblysy (Kostanai Oblast'), Kazakhstan. Both of these occured in the Tac polytype, so if more is found one can expect lots more varieties. Again one should expect the selenium analogues of these, but so far none have been observed.

Recently¹¹ the structure of Fettelite 6As₂S₇>10HgAs₂S₈> was found. As can be seen from the fomula the first layer in the formula is the same as in the polybasite group formula. So either Fettelite is a related mineral or the polybasite group should be redefined to include it. Fettelite occurs as tiny pseudohexagonal twins resembling polybasite and much of the marketed Fettelite is in fact polybasite



1) Anthony et al., Hanbook of Mineralogy, vol.1, pg 417, (1990)
2) Dana's System of Mineralogy, 7th ed., vol 1, pg 351-353, (1944)
3) Bindi, L., Evain, M., Menchetti, S.; Selenopolybasite, <(Ag,Cu)6(Sb,As)2(S,Se)7>, a new member of the pearceite–polybasite group from the De Lamar mine, Owyhee County, Idaho, USA. Canadian Mineralogist, 45, 1525-1528. (2007)
4) Luca Bindi and Silvio Menchetti; Adding further complexity to the polybasite structure: The role of Ag in the B layer of the -M2a2b2c polytype. American Mineralogist, 94, 151-155 (2009).
5) Dana's System of Mineralogy, 7th ed., vol 1, pg 353, (1944)
6) Ibid. pg 355
7) Frondel, Am. Min. 48, pg 565 (1963)
8) Evain, M., L. Bindi and S. Menchetti; Structural complexity in minerals: twinning, polytypism and disorder in the crystal structure of polybasite, (Ag,Cu)₁₆(Sb,As)₂S₁₁. Acta Cryst. B62, 447-456 (2006).
9) Bindi, Luca, Michel Evain, Paul G. Spry, and Silvio Menchett; The pearceite-polybasite group of minerals: Crystal chemistry and new nomenclature rules. American Mineralogist, 92, 918-925 (2007).
10) Bindi, L., M.Evain, P.G.Spry, K.T. Tait, S. Menchetti; Structural role of copper in the minerals of the pearceite-polybasite group: the case of the new minerals cupropearceite and cupropolybasite: Mineralogical Magazine 71,641-650 (2007)
11) Bindi, L., Keutsch, F.N., Francis, C.A., Menchetti, S. (2009): Fettelite, from Chanarcillo, Chile: Crystal structure, pseudosymmetry, twinning, and revised chemical formula. American Mineralogist, 94, 609-615.

Canada

Husky Mine, Elsa, Galena Hill, Mayo mining district, Yukon, Canada

There is little to no arsenic at Husky and all those x-rayed and probed have been end member Polybasite and with no polytypism observed¹. Though Polybasites often show internal red reflections, a truly remarkable property of Husky Polybasites is that when held at a certain angle under strong light the iridescence disappears and the basal face turns blood red. The iridescence of these pieces is also quite remarkable, but sadly darkens with age². Recently Alan Sharpe donated a Husky Stephanite and Chi Ma and George R. Rossman at Caltech tried to analyse the thin coating causing the iridescence. The folk lore was that either Bornite or Covellite, both copper bearing, was the cause of the iridescence. So a Stephanite, containing no copper, and shows the same iridescence as Polybasite was chosen to analyze the coating. The results are given in the photos at: http://www.mindat.org/photo-539963.html . Both aluminum and silicon are in the probe of the coating, indicating that a silicate causes the iridescence! The large amounts of silver and sulfur in the probe of the coating seem to confirm Mark Mauthner's observation of these specimens darkening with time. If so the silver cleaner should restore the original brightnes, but try it on a scrap piece first!

In spring 1977,"Little Joe" Weinholtzner and Rod Shiletto collected some of the world's finest Polybasite and Stephanite from Stope 302 of the Husky Mine. Sadly Rod Shilleto died the following year and some of his pieces were given to friends as in memoriam gifts³.Some of the other miners there at the time gave wives and girlfriends these irridescent crystals for buttons and broaches. These delightful crystals even made it into folk art. In one such piece they were glued onto a set of mounted antlers³! There were occurrences of these irridescent crystals in other stopes, and indeed other mines in the camp, but little was saved.

Rod Tyson got there in1978 and Little Joe let him select two little pieces from a beer mug whose loose contents were poured out onto a bed. Rod got them out of the beer mugs and into specimen boxes.Rod was more successful with the Shilleto family and got a few small miniatures and quite a few thumbnails which he brought to market³. Word soon spread and people began making a pilgrimage to Little Joe's to photograph and marvel at the specimens. Someone told Little Joe that one of the specimens was worth $8,000.00 and Little Joe decided that this small collection would be his retirement nest egg. Many tried to pry them out of Little Joe and many failed. Finally when Mark Mauthner was in the Yukon looking for crystallized gold, he made the pilgrimage too. He and Little Joe got on well and it turned out that Little Joe had worked with Mark's father years early at the Husky. Little Joe trusted Mark and when he found out that Mark was well connected in the specimen community, he asked Mark to market his collection². One small cabinet (pictured above), a few miniatures and three dozen thumbnails and well over a hundred micros and kerbles were in the collection. The best of these polybasites were sold for many thousands of US dollars to well over ten thousand dollars. I was lucky enough to get a few of the best pieces from Mark. Shortly after Mark's success, Rod Tyson phoned the Shilletos and they agreed to sell the rest of what they had for roughly Mark's appraised value. Again there were a few small miniatures and more thumbnails³.

The mine following hydrothemal veins in greenstone is now closed and water filled and is slowly freezing. Like many northern mines it is in permafrost and had to be timbered well. Once a shaft was driven the permafrost began melting and rock falls were common. There was even permafrost in the 302 stope³.


1) Gary Ansell, private communication.
2) Mark Mauthner, private communication.
3) Rod Tyson, private communication
Ref.: (in part): Rocks & Min.:64:489.

Germany

Clara Mine, Oberwolfach, Ortenaukreis, Freiburg Region, Baden-Württemberg, Germany

Sebastian Möller has contributed the following about the Clara Mine and its polybasites.

"The polybasite minerals from Clara Mine are the former species Polybasite, Antimonpearceite (polybasite-Tac) (the most abundant one, xls up to about 1 cm, I have some ore specimen with xl fragments of 3-4 cm) and pearceite.

The Clara Mine is the only mine still active in the Black Forest. Now over 400 mineral species have been discovered at the Clara, some of them being unique. The mine is type locality for about 20 species. It has been worked without a gap from the 1880'ies, but sporadic mining occured much earlier. There is the legend of a small mining town called Benau (mineral benauite) on the mountain above today's mine. It was destroyed by a flashflood in 11^th or 12^th century and the legend tells that it was due to the habitants worshipping a golden calf ! Up to the 19^th century silver ore (probably fahlore, which now is really abundant) and maybe copper ores were mined. In the 19^th century production of baryte began. It was then used mainly for paints, later on for paper additives and medical products as well as a barium ore for fireworks. In 20^th century fluorite production began.

There are three main vein systems of about 1 km in length and up to 8-10 cm in width: 1) a baryte vein with minor fluorite and chalcopyrite, fahlore (tetraedrite-tennantite) and even Sb sulphides (polybasite-Tac, pyrargyrite, proustite); 2) a fluorite vein with minor sellaite; 3) a diagonal trum with mainly fluorite and minor baryte, often silicified areas with quartz and minor galena/chalcopyrite ore. These veins are situated in a melange of rocks containing mostly gneisses, with minor silicified serpentinites, basic rocks and felsic granulites."

Mexico

Fresnillo, Fresnillo Municipality, Zacatecas, Mexico

Guanajuato Municipality, Guanajuato, Mexico

Chispas Mine (Pedrazzini mine), Arizpe, Arizpe Municipality, Sonora, Mexico

Morocco

Imiter mine, Imiter mining district, Tinghir Cercle, Tinghir Province, Drâa-Tafilalet Region, Morocco

Initially there were few Polybasite specimens from Imiter, but now there is lots of material available. Unfortunately the xls are all micros, but the associations with xanthoconite and proustite make spectacular micromounts. Have any of these been analysed? The arsenic present in proustite lets one suspect the presence of Pearceite here. Many "Fettalites" from here are in fact polybasites

Peru

Uchucchacua Mine, Oyón District, Oyón Province, Lima, Peru

Both arsenic and antimony occur at Uchucchacua and so do both Polybasite and Pearceite. To know which is present, they must be analysed.