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Acanthite Pseudo-octahedron

The Mineral acanthite




Acanthite and Argentite are usually grouped together in mineral guides as one mineral. However, they are scientifically categorized as two unique minerals, even though they are composed of the same substance. They only differ in crystal structure. As explained below, Argentite cannot exist at normal temperatures. Specimens labeled as Argentite are sometimes sold by dealers, however, these specimens are really Acanthite pseudomorphs after Argentite.

Because Acanthite is a significant ore of silver, and occurs in odd and unique forms, its specimens command high prices and are sought after by collectors.
Chemical Formula Ag2S
Composition Silver sulfide
Color Dark gray to nearly black.
Streak Black. Streak shiny.
Hardness 2
Crystal System Monoclinic
3D Crystal Atlas
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Crystal Forms
and Aggregates
Acanthite crystallizes in the monoclinic system, and Argentite crystallizes in the isometric system. However, the crystal structure of Argentite is unstable at temperatures below 356° F (180° C), and below that temperature its crystal structure will change to Acanthite, though retaining the isometric crystals of Argentite. Therefore, Argentite specimens are really Acanthite pseudomorphs after Argentite.

Two types of Acanthite exist: monoclinic Acanthite  (Acanthite that formed naturally as Acanthite, and is not a pseudomorph after Argentite) and Acanthite pseudomorph after Argentite. Monoclinic Acanthite occurs as distorted elongated prisms, and Acanthite pseudomorph after Argentite occurs as distorted groups of cubes, octahedrons, and dodecahedrons. However, both forms have the same crystal structure, and both are scientifically Acanthite.

Crystals are usually in parallel groups. The most prevalent aggregates are groupings of distorted octahedrons and dendritic growths, but it also occurs as a coating and massive.
Transparency Opaque
Specific Gravity 7.2 - 7.4
Luster Metallic. Heavy tarnish can cause it to become dull.
Cleavage 3, all sides. Indiscernible.
Fracture Subconchoidal
Tenacity Malleable and sectile
Other ID Marks May tarnish dark gray.
In Group Sulfides; Simple Sulfides
Striking Features Unique crystal aggregates, tarnish, and occurrence in Silver deposits.
Environment In hydrothermal deposits and epithermal veins.
Rock Type Metamorphic
Popularity (1-4) 2
Prevalence (1-3) 3
Demand (1-3) 1

Acanthite ON EBAY
OTHER NAMES
Silver Glance

POLYMORPHS
Argentite

USES
Acanthite is perhaps the most important ore of silver. It is also a popular mineral in high-end collections.

NOTEWORTHY LOCALITIES
Fine Acanthite specimens have come from Kongsberg, Norway, where it occurs with Silver. It also occurred in numerous places in Germany, notably the Clara and Wenzel mines in the Black Forest, and in Freiberg and Scheenberg, in Saxony, Germany. An excellent producer of Acanthite is the Imiter Mine in Morocco, as well as the Hongda Mine, Shanxi Province, China.

In South America, Acanthite is found in the Uchucchacua Mine, Oyon Province, Peru. There are numerous localities in Mexico. The most significant are Batopilas, in Chihuahua; the Reyes Mine in Guanajuato; and Fresnillo, in Zacatecas.

In the U.S., Acanthite occurs in the Comstock Lode, Virginia City, Nevada; Butte, Silver Bow Co., Montana; the Bulldog Mountain Mine, Creede District, Mineral Co., Colorado; and the White Pine Mine, Ontonagan Co., Michigan. In Canada, Acanthite was found in the silver mines of Cobalt, Timiskaming District, Ontario.

COMMON MINERAL ASSOCIATIONS
Silver, Barite, Bornite, Galena, Gold, Pyrargyrite, Pyrite

DISTINGUISHING SIMILAR MINERALS
Silver - Lighter color, more malleable, not sectile.
Chalcocite - Different fracture (conchoidal).
Galena - Good cleavage, different tarnish, no sectility.


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