1911 Encyclopædia Britannica/Copper-glance
COPPER-GLANCE, a mineral consisting of cuprous sulphide, Cu2S, and crystallizing in the orthorhombic system. It is known also as chalcocite, redruthite and vitreous copper (German, Kupferglaserz of G. Agricola, 1546). The crystals have the form of six-sided tables or prisms; the angle between the prism faces (lettered o in the figure) being 60° 25′. When twinned on the prism planes o, as is frequently the case, the crystals simulate hexagonal symmetry still more closely, as in the minerals aragonite and chrysoberyl. Twinning also takes place according to two other laws, giving rise to interpenetrating crystals with the basal planes (s) of the two individuals inclined at angles of 69° or 87° 56′ respectively. The mineral also occurs as compact masses of considerable extent. The colour is dark lead-grey with a metallic lustre, but this is never very bright, since the material is readily altered, becoming black and dull on exposure to light. The mineral is soft (H.=2½) and sectile, and can be readily cut with a knife, like argentite; sp. gr. 5.7. Analyses agree closely with the formula Cu2S, which corresponds to 79.8% of copper; small quantities of iron and silver are sometimes present.
Next to chalcopyrite, copper-glance is the most important ore of copper. It usually occurs in the upper part of the copper-bearing lodes, and is a secondary sulphide derived from the chalcopyrite met with at greater depths; sometimes, however, the two minerals are found together in the same part of the lodes. The best crystals are from St Just, St Ives, and Redruth in Cornwall, and from Bristol in Connecticut. Small crystals of recent formation are found on Roman bronze coins in the thermal springs at Bourbonne-les-Bains.
Copper-glance readily alters to other minerals, such as malachite, covellite, melaconite and chalcopyrite. On the other hand, it is found as pseudomorphs after chalcopyrite, galena, and organic structures such as wood; copper-glance pseudomorphous after galena preserves the cleavage of the original mineral and is known as harrisite.
Isomorphous with copper-glance is the orthorhombic mineral stromeyerite, a double copper and silver sulphide, CuAgS, which occurs in abundance in the Altai Mountains. (L. J. S.)