chalcocite

n.
A dark gray mineral, essentially Cu₂S, that is an important ore of copper.

[Alteration of obsolete chalcosine, from Greek khalkos, copper.]

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A mineral having composition Cu₂S and crystallizing in the orthorhombic system (below 217°F or 103°C). Crystals are rare and small, usually with hexagonal outline because of twinning. Most commonly, the mineral is fine-grained and massive with a metallic luster and a lead-gray color which tarnishes to dull black on exposure. The Mohs hardness is 2.5–3, and the density 5.5–5.8. Chalcocite is an important copper ore found at Miami, Morenci, and Bisbee, Arizona; Butte, Montana; Kennecott, Alaska; and Tsumeb, South-West Africa.

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Cu
Orthorhombic -- bipyramidal

Environment

Usually secondary, altered by later, cooler solutions in ore veins and by near-surface enrichment of chalcopyrite in disseminated deposits.

Crystal description

Commonly massive; crystals are usually small and are infrequent. With edge face angles close to 60°, many crystals look six-sided from twinning. Elongated prisms are often twinned in geniculated (kneelike) pairs.

Physical properties

Dark lead gray (some specimens in collections become coated with a soft brown-black sooty film; see discussion under marcasite, p. 126). Luster metallic; hardness 2Ɖ-3; specific gravity 7.2-7.4; fracture conchoidal; cleavage poor prismatic. Moderately sectile.

Composition

Cuprous sulfide (79.8% Cu, 20.2% S).

Tests

Powder, moistened with hydrochloric acid on platinum wire, colors flame bluish green (copper). Careful blowpiping with gas flame will produce a copper bead. Soluble in nitric acid, giving green solution that becomes blue on addition of ammonia.

Distinguishing characteristics

Usually associated with copper minerals, it is less sectile than argentite and gives easy copper tests. The gray color distinguishes it from the related copper sulfides.

Occurrence

Chalcocite is an important ore of copper. It is sometimes primary and sometimes secondary (when associated with chalcopyrite, bornite, and covellite). A higher-temperature, and primary, cubic form of Cu ₂ S is known as digenite and was once abundant at Butte, Montana. Chalcocite's most frequent development results from a process known as secondary enrichment. Solutions descend through oxidizing copper iron sulfides (typically chalcopyrite) near the surface. Because iron sulfate is more soluble than copper sulfate, lower-grade copper iron sulfides are enriched in copper as more of their iron goes into solution. The sequence commences with primary chalcopyrite (CuFeS ₂ ), becoming bornite (Cu ₅ FeS ₄ ), then covellite (CuS), and finally chalcocite (Cu ₂ S).

Chalcocite was found in fine crystals in sulfide veins in Cornwall, England, in an ancient mine in Bristol, Connecticut (the best American occurrence), and at Butte, Montana, where digenite also occurs in 1-2 in. (2.5-5 cm) crystals. French localities included Cap Garonne in Var, Garde-en-Oisans (Isère), Giromagny (Belfort), and Framont (Alsace). Its French name is chalcosine. New specimens were briefly found in a new mine in Ladysmith, Wisconsin, but the prospect of finding more is largely a memory at present.

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