1911 Encyclopædia Britannica/Cuprite

CUPRITE, a mineral consisting of cuprous oxide, Cu2O, crystallizing in the cubic system, and forming an important ore of copper, of which element cuprite contains 88.8%. The name cuprite (from Lat. cuprum, copper) was given by W. Haidinger in 1845; earlier names are red copper ore and ruby copper, which at once distinguish this mineral from the other native copper oxide—cupric oxide—known as black copper ore or melaconite. Well-developed crystals are of common occurrence; they usually have the form of the regular octahedron, sometimes in combination with the cube and the rhombic dodecahedron. A few Cornish crystals have been observed with faces of a form {hkl} known as the pentagonal icositetrahedron, since it is bounded by twenty-four irregular pentagons. In this class of cubic crystals there are no planes or centre of symmetry, but the full number (thirteen) of axes of symmetry; it is known as the trapezohedral hemihedral class, and cuprite affords the best example of this type of symmetry. The etching figures do not, however, conform to this lower degree of symmetry, nor do crystals of cuprite rotate the plane of polarization of plane-polarized light. The colour of the mineral is cochineal-red, and the lustre brilliant and adamantine to sub-metallic in character; crystals are often translucent, and show a crimson-red colour by transmitted light. On prolonged exposure to light the crystals become dull and opaque. The streak is brownish-red. Hardness 3½; specific gravity 6.0; refractive index 2.85.

EB1911 Cuprite.jpg

Compact to granular masses also occur, and there are two curious varieties—chalcotrichite and tile-ore—which require special mention. Chalcotrichite (from Gr. χαλκός, copper, and θρίξ, τριχός, hair) or “plush copper ore” is a capillary form with a rich carmine colour and silky lustre; the delicate hairs are loosely matted together, and each one is an individual crystal enormously elongated in the direction of the diagonal or the edge of the cube. Tile-ore (Ger. Ziegelerz) is a soft earthy variety of a brick-red to brownish-red colour; it contains admixed limonite, and has been formed by the alteration of chalcopyrite (copper and iron sulphide).

Cuprite occurs in the upper part of copper-bearing lodes, and is of secondary origin, having been produced by the alteration of copper sulphides. Beautifully crystallized specimens were formerly found in Wheal Gorland and Wheal Unity at Gwennap, and in Wheal Phoenix near Liskeard in Cornwall; they also occur in the copper mines of the Urals, and in Arizona. Isolated crystals bounded by faces on all sides, and an inch or more in diameter, are found embedded in a soft white clay at Chessy near Lyons; they are usually altered on the surface, or throughout, to malachite. Chalcotrichite comes from Wheal Phoenix and Fowey Consols mine in Cornwall, and from Morenci in Arizona; tile-ore from Bogoslovsk in the Urals, Atacama in South America, and other localities. Small crystals of cuprite, together with malachite, azurite and cerussite, are sometimes found encrusting ancient objects of copper and bronze, such as celts and Roman coins, which have for long periods remained buried in the soil. Artificially formed crystals have been observed in furnace products. (L. J. S.)