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ENARGITE


Pl. 20(3

Cu3AsS4

Orthorhombic — pyramidal m m 2
Environment: Medium-temperature ore veins. Crystal description: Smaller crystals fairly common, usually prismatic, with several grooved, almost curving, prisms, and a flat truncating base. Rarely in up to 6-in.-long (15 cm) crystals. Sometimes tabular; also massive and granular. Physical properties: Gray-black to iron black. Luster metallic; hardness 3; specific gravity 4.4--4.5; streak grayish black; fracture uneven; cleavage perfect prismatic. Brittle. Composition: Copper arsenic sulfide (48.3% Cu, 19.1% As, 32.6% S), but up to 6% antimony can take the place of arsenic. The far rarer tetragonal antimony equivalent, known as famatinite (Cu3SbS4), has a reddish tint on a polished surface. Enargite is dimorphous with pinkish, tetragonal luzonite. Tests: Fuses on charcoal (with the sublimates and odor of sulfur, antimony, and arsenic), leaving a bead that can produce, with borax fluxes and great care, a copper bead. Touched with hydrochloric acid the melted bead will show a blue copper flame; or in a nitric acid solution plus ammonia will give the copper blue color.
Distinguishing characteristics: The crystals are typical, rather like those of manganite, but the blowpipe response quickly shows the difference. Enargite is difficult to distinguish from many related minerals, though the tests for arsenic and copper separate enargite and luzonite from minerals lacking those elements.
Occurrence: Enargite is an important ore of copper and is usually associated in ore deposits with other copper minerals and sulfides. It is frequently in well-crystallized specimens; individuals once found in Quiruvilca, Peru, were a spectacular 6 in. (15 cm) long.
An abundant ore in the deep sulfide part of the great Chuquimata copper deposit (Chile). Butte, Montana, is the most important locality in the U.S., but it is also found in Colorado, Utah, and California. Microscopic crystals have been found at Picher, Oklahoma, perched on ¼-in. (5 mm) chalcopyrite crystals. Good 1½-in. (4 cm) crystals are found in Japan at the Kaize-mura Mine, Nagano Prefecture.
Remarks: Enargite was originally thought to be isomorphous with the antimonial equivalent famatinite (antimony present to any amount in place of arsenic). It is now known to be tetragonal. At Morococha, Peru, enargite bands seem to be transformed inward from silvery gray enargite at the base of crystal crusts into a pinkish gray sulfide, seemingly a dimorphous tetragonal equivalent without enargite's distinct cleavage planes known as luzonite. Luzonite crystals have been found at the Teine Mine, Sapporo, Japan, and in Taiwan at the Chinkuashih Mine.