cuprite

n.
A natural red secondary ore of copper, essentially Cu₂O, that forms as a result of weathering.

A mineral having composition Cu₂O and crystallizing in the isometric system. Cuprite is commonly in crystals showing the cube, octahedron, and dodecahedron. It is various shades of red and a fine ruby red in transparent crystals which have a metallic to adamantine luster. The hardness is 3.5–4 (Mohs scale) and the specific gravity is 6.1.

Cuprite is a widespread supergene copper ore. Fine crystals have been found at Cornwall, England, and Chessy, France. It has served as an ore in the Congo, Chile, Bolivia, and Australia. In the United States it has been found at Clifton, Morenci, Globe, and Bisbee, all in Arizona. See also Copper.

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Cu
Cubic -- gyroidal

Environment

Very common in uppermost oxidation zone of copper sulfide ore bodies; usually best developed in desert regions.

Crystal description

Crystals are commonly cubes and octahedrons and combinations of these forms. Frequently in a lattice of square red needles drawn out into distorted cubes; sheets in parallel growths, lining cavities in limonite (called chalcotrichite). Often scattered throughout limonitic rocks, or forming red films on native copper crystals.

Physical properties

Red to dark red. Luster adamantine; hardness 3Ɖ-4; specific gravity 5.8-6.1; streak red; fracture conchoidal; cleavage poor octahedral. Brittle; translucent, transparent.

Composition

Cuprous oxide (88.8% Cu, 11.2% O).

Tests

Fuses on charcoal and easily reduced to copper bead. Dissolves in acid, giving a blue copper color, and colors blowpipe flame green.

Distinguishing characteristics

Distinguished by crystal shape and copper tests from the ruby-silvers. The blowpipe tests for copper and sulfur also distinguish it from cinnabar, realgar, and zincite. Rutile is usually too dark and brown and has a hardness of 6-6.5. There are almost invariably blue and/or green copper minerals associated with cuprite.

Occurrence

An important secondary ore of copper. It forms near the surface during the oxidation and enrichment of copper sulfide ore bodies as a result of weathering. Depending on the nature of the original ore body, minerals of similar origin are its usual associates: native copper, malachite, azurite, and limonite. This group reaches its best development in dry regions where the water table is low and oxidation has gone deep. Often it seems to be characteristic of the deepest oxidation zone, with cuprite just above the enriched sulfides. Native copper is often adjacent to the cuprite in such deposits and frequently the cuprite has grown on the copper surface. It is a common mineral of wide distribution. Only a few localities can be mentioned. The most complex crystals came from Cornwall, England. The Chessy, France, malachite-coated octahedrons are common in large collections (see azurite, p. 199). Sw. U.S. has been the chief American source of fine specimens, especially the Copper Queen Mine at Bisbee, and at Clifton, Arizona. Chalcotrichite has been found at Bisbee, Morenci, and Jerome, Arizona. Namibia is a late entrant in the important locality list with some of the largest crystals known, malachite-coated but gemmy and as much as 2 in. (5 cm) across (Ongonja Mine). Australia has had several good occurrences, including Cobar and Broken Hill (New South Wales), Burra-Burra and Wallaroo (South Australia).

Remarks

Cuprite belongs to an uncommon class of the cubic system, and its crystals sometimes show the required rare faces. Since it is invariably a secondary mineral, the best specimens will be found in the early stages of a mine's development.

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