Chalcocite Group. Orthrhombic, Chancocite, Copper Glance Composition
Composition. Cuprous sulphide, Cu2s = Sulfur 20.2, copper 79.8.
Crystallization. Orthorhombic. Usually in small tabular crystals with hexagonal outline. Striated parallel to the brachyaxis (Fig. 223). Often twinned in pseudohexagonal forms (Fig. 224).
Structure. Massive. Crystals very rare.
Physical Properties. Conchoidal fracture. H. = 2.5-3. G. = 5.5-5.8. Metallic luster. Color shining lead-gray, tarnishing on exposure to dull black. Streak grayish black.
Test. Easily at 2-2.5. In O.T. or B.B. on charcoal gives odor of sulfur dioxide. Roasted mineral, moistened with hydrochloric acid, gives azure-blue flame. Soluble in nitric acid; and the solution with an excess of ammonia turns dark blue. Recognized by its massive structure, its high specific gravity, its color, softness, and black streak.
Occurrence. Chalcocite is a very valuable ore of copper and is widespread in its occurrence. In the majority of cases it is to be found in the enriched sulphide zone of copper veins and is clearly secondary in origin. It has been formed in various ways by the action of descending copper-bearing solutions upon the original sulphide minerals of the veins. In other cases, however, Chalcocite is primary and has been deposited as a direct precipitation from ascending waters. Associated with bornite, chalcopyrite, covellite, tretahedrite, enargite, malachite, pyrite, etc. Found as an ore at Monte Catini, Tuscany; From Tsumeb, South West Africa; French Congo, Mexico, Peru, Chile, etc. Occurs in immense deposits at Butte, Montaña. Found in Alaska at Kennecott, Copper River District.
Use. an important copper ore.
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