Acanthite

What is acanthite? Silver sulphide, Ag2S

Acanthite is silver sulphide with two silver atoms to each sulphur, Ag2S. The Handbook of Mineralogy acanthite sheet gives it monoclinic, pseudo-orthorhombic, point group 2/m, space group P21/n. Primary crystals are rare, prismatic to long prismatic, elongated along [001], to 2.5 cm, and may be tubular; it is also massive. Cleavage is indistinct, the fracture uneven, and it is sectile, with hardness 2.0 to 2.5 and measured density 7.20 to 7.22. It is opaque and iron-black with a black streak and a metallic lustre. The sheet adds the single word photosensitive and says no more; light-sensitive species in general are on which minerals fade in light.

What the IMA list records. The September 2026 list gives acanthite, Ag2S, status G (grandfathered, a mineral described before the IMA existed), 1855, with the Czech Republic as the country of first description. The sheet marks Jáchymov (Joachimsthal) as the type locality and holds type material in the Emperor’s collection, Vienna, number 2592. The name is from the Greek for thorn, for the shape of the crystals.

Its chemistry, worked. On standard atomic weights Ag2S is 87.06% silver and 12.94% sulphur, the figures the sheet prints for the ideal. Its two microprobe analyses, both from Guanajuato, Mexico, give Ag 86.4 with Se 1.6 and S 12.0, and Ag 87.2 with Cu 0.1 and S 12.6 from the Santa Lucia mine, La Luz: atomic ratios of 2.030 silver to each sulphur-plus-selenium and 2.057 silver to each sulphur, close to the ideal 2.

Is argentite the same as acanthite?

A specimen labelled argentite is acanthite: argentite is the name of the cubic high-temperature form, and it does not survive cooling. The acanthite sheet says the high-temperature cubic form (“argentite”) inverts to acanthite at about 173 °C, and that below this temperature acanthite is the stable phase and forms directly. It records acanthite as commonly paramorphic after the cubic phase, of original cubic or octahedral habit, to 8 cm. The IMA-list test for the name, and the weight gap between acanthite and native silver, are on is silver a mineral and are not repeated here.

A paramorph keeps its shape and changes its structure. The acanthite sheet’s own wording, paramorphic after the cubic phase, is the case the what is a pseudomorph page describes: same chemistry, a different polymorph, the outward form preserved. On the sheet’s account, a cube of acanthite grew as the cubic phase, above about 173 °C, and is monoclinic now.

Not the only silver mineral with a cubic past. The Handbook hessite sheet says hessite, Ag2Te, inverts to a cubic form above 155 °C and describes its crystals as pseudocubic; the aguilarite sheet says aguilarite, Ag4SeS, was inverted from a higher-temperature cubic form. A cube-like dark silver mineral may therefore be any of the three; the shape alone does not prove Ag2S, and the tests below are what separate them.

Where acanthite sits among the silver sulphides: the Ag–S–Se–Te ledger

Acanthite carries the most silver of the 10 IMA minerals made of silver with sulphur, selenium or tellurium alone. Method: the September 2026 list was converted with pdftotext -layout. A row parser split each line into name, formula and status; an independent scan took the first token of the second field of every line split on runs of two or more spaces; a third scan tested every whitespace token on every line. Each kept formulas whose element symbols are silver plus one or more of S, Se and Te. All three found the same 10, and acanthite, naumannite, hessite and aguilarite, the positive controls, were all found.

The ledger, worked. Silver by mass from each IMA formula: acanthite 87.06%, aguilarite 79.53%, naumannite 73.20%, cervelleite 72.99%, kurilite 65.14%, hessite 62.84%, stützite 56.66 to 57.29% across the x = 0.24 to 0.36 its formula allows, zavyalovite 49.08%, empressite 45.81% and lingbaoite 21.98%. Every one is a silver mineral; only acanthite is Ag2S. Of the nine Handbook sheets on the ledger (zavyalovite has none), two use the word photosensitive: acanthite’s and cervelleite’s.

The selenium check, worked. Divide each weight percent by its atomic weight. The first Guanajuato analysis, S 12.0 and Se 1.6, gives 0.374 sulphur to 0.020 selenium, so selenium is 5.1% of the two. Aguilarite’s formula, Ag4SeS, puts selenium at half; at 5.1% the analysed material is acanthite. Aguilarite’s type locality is the San Carlos mine, Guanajuato, on its own sheet, and both species are named from the Comstock Lode; where they meet, only an analysis settles a dark sectile grain.

How do you tell acanthite from galena and aguilarite? The 0.07-gram trap

The balance cannot separate acanthite from aguilarite or galena: 20.00 g of each reads within 0.14 g of the others in water. Worked as 20 − 20/D on the sheets’ measured densities: acanthite 7.20 to 7.22 reads 17.22 to 17.23 g; aguilarite 7.40 to 7.53, 17.30 to 17.34 g, only 0.07 g above; galena 7.58, 17.36 g, 0.13 g above acanthite; empressite 7.61, 17.37 g. Naumannite’s measured density is printed as 7.0 to 8.0, which spans 17.14 to 17.50 g and covers all four. The method is on how to measure specific gravity at home.

What does separate them. Tenacity first: the acanthite, aguilarite and hessite sheets say sectile, naumannite’s somewhat sectile, galena’s, empressite’s and stützite’s brittle. Cleavage second: galena is perfect on {001}, acanthite indistinct. Streak third: acanthite black, galena lead-gray. Hardness does not help; acanthite is 2.0 to 2.5, galena 2.5 to 2.75, aguilarite and naumannite 2.5. The scratch method and its limits are on how to test mineral hardness properly; streak technique on how to use a streak plate.

Against native silver and the sulphosalts. Native silver reads 18.02 to 18.20 g and stephanite 16.81 g, about 0.4 g under acanthite. The pin test that separates native silver from acanthite is on the silver page and is not repeated here. Any question about handling silver minerals belongs on toxic minerals in a collection; nothing on this page is health or safety advice.

Where acanthite is found

The Distribution field calls acanthite widespread in silver deposits and names, for fine primary and paramorphic crystals: Jáchymov (Joachimsthal), Czech Republic [TL]; Freiberg, Schneeberg, Annaberg and Marienberg, Saxony, and St. Andreasberg, Harz Mountains, Germany; large paramorphs from Arizpe, Sonora, the Rayas and other mines at Guanajuato, and many mines in Zacatecas and Chihuahua, Mexico; Butte, Silver Bow Co., Montana; Tonopah, Nye Co., and the Comstock Lode, Virginia City, Nevada; various mines at Cobalt, Ontario; and Chañarcillo, south of Copiapó, Chile.

How and with what. The occurrence field reads a common silver species in moderately low-temperature hydrothermal sulfide veins, and in zones of secondary enrichment. The association line is silver, pyrargyrite, proustite, polybasite, stephanite, aguilarite, galena, chalcopyrite, sphalerite, calcite and quartz. The red silver sulphosalts it grows with have their own pages, proustite and pyrargyrite; native silver is on native silver, and the silver chloride from the oxidised zone above such veins is chlorargyrite.

Weighing it. A 20.00 g piece of acanthite reads 17.22 to 17.23 g in water; quartz or calcite matrix pulls the reading down. 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.

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