Polybasite
Polybasite, [Ag9CuS4][(Ag,Cu)6(Sb,As)2S7]: monoclinic black silver sulfosalt, hardness 2 to 3, density 6.1. Antimony over arsenic names it; arsenic, pearceite.
Polybasite is a silver copper antimony sulfosalt, [Ag9CuS4][(Ag,Cu)6(Sb,As)2S7] on the IMA list (Rd, 2006): monoclinic iron-black pseudohexagonal plates to 6 cm, hardness 2 to 3, density 6.1, streak black. Antimony over arsenic makes it polybasite; arsenic over antimony makes pearceite. The end member is 71.04% silver by mass.
In short
- The As/Sb switch. Bindi et al., American Mineralogist 92, 918–925 (2007) redefined the group on a chemical basis: the distinction between pearceite and polybasite is the As/Sb ratio from a microprobe analysis. The Handbook of Mineralogy polybasite sheet’s Arizpe, Sonora analysis prints Sb 1.64 against As 0.32: 83.7% antimony, so polybasite. The pearceite sheet’s two analyses are 97.4% and 96.6% arsenic.
- Documented failure: the Beaverdell column. The sheet’s Beaverdell analysis prints the formula Sb 1.88, As 0.19, but its own weight percentages, Sb 5.15 and As 3.88, work out at Sb 0.91 and As 1.12: 45.0% antimony. Read from the weights, the analysis falls on pearceite’s side of the switch. Recompute the atoms before trusting a printed formula line.
- Two old names, gone. Bindi et al. abandoned antimonpearceite (now polybasite-Tac) and arsenpolybasite (now pearceite-T2ac or pearceite-M2a2b2c). A whole-line scan of the September 2026 IMA-CNMNC Master List finds 0 lines for either. It finds 9 rows ending in -polybasite or -pearceite, among them cupro-, argento- and auro- members approved from 2007 to 2024.
- Documented failure: the balance. Worked as 20 − 20/D, 20.00 g reads 16.72 g in water as polybasite, 16.75 g as pearceite and 16.81 g as stephanite: 0.09 g covers all three, and all streak black. acanthite reads 17.22 to 17.23 g; the red-streaked pyrargyrite 16.56 g and proustite 16.41 g.
- Silver. A USGS silver profile reproduces Heyl and others’ (1973) list of 33 principal silver minerals, polybasite among them (USGS Open-File Report 2004-1251, Mineral Commodity Profiles: Silver, Table 3). Worked, the end member Ag15CuSb2S11 is 71.04% silver; stephanite 68.33%, proustite 65.41%, pyrargyrite 59.76%.
| Species | Formula (IMA) | IMA status | System (sheet) | Hardness | Density (measured) | 20.00 g piece in water | Streak | Sb share of Sb + As, sheet analyses | Silver by mass, worked |
|---|---|---|---|---|---|---|---|---|---|
| Polybasite | [Ag9CuS4][(Ag,Cu)6(Sb,As)2S7] | Rd, 2006 s.p. | Monoclinic, pseudohexagonal | 2 to 3 | 6.1 | 16.72 g | Black | 90.8% (Beaverdell, as printed); 83.7% (Arizpe) | 71.04% (Ag15CuSb2S11) |
| Pearceite | [Ag9CuS4][(Ag,Cu)6(As,Sb)2S7] | Rd, 2006 s.p. | Monoclinic, pseudohexagonal | 3 | 6.15 | 16.75 g | Black | 2.6% (Molly Gibson); 3.4% (Arqueros) | 74.08% (Ag15CuAs2S11) |
| Stephanite | Ag5SbS4 | G, 1845 | Orthorhombic | 2 to 2.5 | 6.26 | 16.81 g | Iron-black | No arsenic in formula | 68.33% |
| Acanthite | Ag2S | G, 1855 | Monoclinic | 2.0 to 2.5 | 7.20 to 7.22 | 17.22 to 17.23 g | Black | No Sb or As | 87.06% |
| Pyrargyrite | Ag3SbS3 | G, 1831 | Hexagonal | 2.5 | 5.82 | 16.56 g | Purplish red | No arsenic in formula | 59.76% |
| Proustite | Ag3AsS3 | G, 1832 | Hexagonal | 2 to 2.5 | 5.57 | 16.41 g | Vermilion | No antimony in formula | 65.41% |
| Tetrahedrite (sheet) | Tetrahedrite-(Fe), -(Zn) and others | Rd, 2019 s.p. | Cubic | 3 to 4.5 | 4.97 | 15.98 g | Black to brown | Antimony series | Not worked: silver is a variable substituent in the sheet formula |
What is polybasite? A two-layer silver sulfosalt
Polybasite is [Ag9CuS4][(Ag,Cu)6(Sb,As)2S7] on the September 2026 IMA list, status Rd, redefined in 2006. The list’s first reference is Annalen der Physik und Chemie 15 (1829), 573, and its country column reads Mexico / Germany. The Handbook of Mineralogy polybasite sheet still writes the older (Ag,Cu)16Sb2S11 and marks the Neuer Morgenstern mine, Freiberg, Saxony, as the type locality.
Why two brackets. Bindi et al., American Mineralogist 92, 918–925 (2007) report that every polytype is built of two layers stacked along [001]: layer A, [(Ag,Cu)6(As,Sb)2S7]2−, and layer B, [Ag9CuS4]2+. They wrote the formulas of pearceite and polybasite in that two-layer form instead of (Ag,Cu)16(As,Sb)2S11 and (Ag,Cu)16(Sb,As)2S11, and the commission approved the rules. The sum of the brackets is the same 16 metals, 2 semimetals and 11 sulfur as the old formula.
On the sheet. Monoclinic, point group 2/m, space group C2/m. Crystals are pseudohexagonal, tabular on {001}, to 6 cm, with triangular patterns formed by striae on {001}; also massive; twinned on {110}, repeated. Cleavage imperfect on {001}, fracture uneven, hardness 2 to 3, measured density 6.1, calculated 6.36. Nearly opaque, dark red in thin fragments by transmitted light, iron-black, black streak, metallic lustre; biaxial (−). The {001} and {110} forms were read from the rendered sheet, because the text layer can drop overbars. The sheet prints no light sensitivity, and this page claims none. Antimony and arsenic are in the formula; that is covered on toxic minerals in a collection, and nothing here is health or safety advice.
How do you tell polybasite from pearceite? The As/Sb switch
By analysis. Bindi et al. say the distinction between pearceite and polybasite is easily done with an electron microprobe analysis, as the As/Sb ratio. The IMA formulas carry the same rule: (Sb,As) in polybasite, (As,Sb) in pearceite. The sheets agree on the hand: the pearceite sheet also describes short tabular pseudohexagonal prisms with triangular striations on {001}, black, black streak, hardness 3, density 6.15.
The switch, worked on the sheets. Antimony’s share of antimony plus arsenic, from the atoms printed for each analysis: polybasite, Beaverdell, British Columbia, 90.8%, and Arizpe, Sonora, 83.7%; pearceite, Molly Gibson mine, Colorado, 2.6%, and Arqueros, Chile, 3.4%. Re-worked from the printed weight percentages on standard atomic weights, Arizpe and both pearceite analyses reproduce their printed formulas.
The Beaverdell trap. Beaverdell does not. Its weights, Ag 67.95, Cu 6.07, Fe 0.76, Sb 5.15, As 3.88 and S 16.37, give Ag 13.61, Cu 2.06, Sb 0.91 and As 1.12 per 11.03 sulfur, against the printed Ag 14.58, Cu 1.31, Sb 1.88 and As 0.19. Antimony is then 45.0% of the semimetal, on the pearceite side. Either a weight or the formula line is misprinted; this page cannot say which, and does not count Beaverdell as evidence either way.
Old labels: antimonpearceite, arsenpolybasite and the new prefixes
Two names were abandoned in 2007. Frondel introduced antimonpearceite and arsenpolybasite in 1963, Bindi et al. write. Their redefinition maps the old names as follows: pearceite to pearceite-Tac; antimonpearceite to polybasite-Tac; arsenpolybasite-221 to pearceite-T2ac; arsenpolybasite-222 to pearceite-M2a2b2c; polybasite-221 to polybasite-T2ac; and polybasite-222 to polybasite-M2a2b2c. A hyphenated italic suffix for the crystal system and cell type should be added, they write, if crystallographic data are available.
The 2026 list. A whole-line scan finds 0 lines containing antimonpearceite or arsenpolybasite. A row parser, cross-checked by the scan, finds 9 rows ending -polybasite or -pearceite: polybasite and pearceite (both Rd, 2006 s.p.); cupropearceite (2007-046, Kazakhstan) and cupropolybasite (2008-004, Canada), written with [Cu6(As or Sb)2S7]; argentopearceite (2020-049, Czech Republic) and argentopolybasite (2021-119, Slovakia), [Ag9AgS4]; auropearceite (2024-025) and auropolybasite (2024-006), both Slovakia, [Ag9AuS4]; and selenopolybasite (2006-053, USA). A label older than 2007 reading antimonpearceite now reads polybasite; one reading arsenpolybasite now reads pearceite.
Worked. On standard atomic weights the end members read: polybasite Ag15CuSb2S11 71.04% silver, pearceite Ag15CuAs2S11 74.08%, argentopolybasite Ag16Sb2S11 74.33%, cupropolybasite Ag9Cu7Sb2S11 48.26%. Silver content alone does not name the species. Other sulfosalts are covered on tetrahedrite, tennantite and is enargite a sulfosalt.
Can a balance or a streak plate tell polybasite from stephanite?
No. The stephanite sheet gives Ag5SbS4, orthorhombic, hardness 2 to 2.5, density 6.26, iron-black with an iron-black streak, and twinning commonly repeated to form pseudohexagonal groups. Worked as 20 − 20/D, 20.00 g reads 16.81 g in water, 0.08 g above polybasite. Pseudohexagonal shape, black streak and weight are shared; the method is on how to measure specific gravity at home and the streak plate on how to use a streak plate.
What the balance does do. It sets the black silver sulfosalts apart from acanthite, 17.22 to 17.23 g, which is sectile on its sheet, and from tetrahedrite, 15.98 g. Against the ruby silvers the streak is enough: purplish red for pyrargyrite, vermilion for proustite. Native silver and the Ag–S minerals are compared on is silver a mineral.
Where is polybasite found?
The Distribution field calls it common in small amounts and only rarely in fine specimens. It names the Neuer Morgenstern mine [TL] and other mines, Freiberg, and St. Andreasberg, Harz, Germany; Banská Hodruša, near Banská Štiavnica, Slovakia; Příbram, Czech Republic; the Ouray, Gilman and Red Mountain districts, Colorado; Tonopah, Goldfield and the Comstock Lode, Nevada; fine crystals from the Husky mine, Elsa, Yukon; Tres Puntas, near Copiapó, Chile; especially fine examples from the Las Chiapas mine, Arizpe, Sonora, and Guanajuato, Mexico; and Sabana Grande, Honduras. Its occurrence line: silver veins.
Companions. The association field gives pyrargyrite, tetrahedrite, stephanite, other silver sulfosalts, acanthite, gold, quartz, calcite, dolomite and baryte. The pearceite sheet names the Husky mine and Banská Štiavnica too, so on those labels the As/Sb analysis decides. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.