argentite

(är'jən-tīt')
n.
A valuable silver ore, Ag₂S, with a lead-gray color and metallic luster that is often tarnished a dull black.

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A sulphide ore of silver, Ag₂S. It crystallizes in the cubic system but most commonly occurs in massive form. It is dull grey-black in colour but bright when first cut and occurs in veins associated with other silver minerals. Important deposits occur in Mexico, Peru, Chile, Bolivia, and Norway.

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Ag
Cubic -- hexoctahedral

Environment

In fairly low-temperature ore veins formed at some distance from the primary source.

Crystal description

Frequently occurs in poorly formed, dull black crystals, usually cubic but often so distorted and branching that it is difficult to recognize their faces. Commonly massive.

Physical properties

Dark lead gray. Luster metallic, usually tarnished to dull black; hardness 2-2Ɖ; specific gravity 7.3; fracture subconchoidal; cleavage poor cubic and dodecahedral. Like lead, can be cut by a knife (sectile).

Composition

Silver sulfide (87.1% Ag, 12.9% S).

Tests

Blowpipe fuses it into a bead on charcoal, which in an oxidizing flame gives silver button. Tests for silver then apply.

Distinguishing characteristics

Sectility distinguishes it from other sulfides, particularly galena (which shows far better brittle cleavage). Grayer color distinguishes it from very white native silver; colorless nitric acid solution distinguishes it from chalcocite; copper nitrate solution is green.

Occurrence

Argentite is the most important primary ore of silver. Of common occurrence in veins with native silver. It seems to have catalytic power, with wire silver appearing to rise from argentite crystal surfaces. Fine crystals are found in Mexico, in Saxony, at Kongsberg, Norway, and at Cobalt, Ontario. Encountered near the surface of ore veins; there are no good commercial occurrences remaining in the U.S.

Remarks

Argentite has a cubic structure only at temperatures above 180°C, so its presence indicates that the vein formed at temperatures higher than 180°C. Actually all Ag ₂ S specimens are structurally rearranged into acanthite (orthorhombic Ag ₂ S) at room temperature and are now pseudomorphs after the original cubic argentite.

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For a list of words related to argentite, see:

• Mineral compounds

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