Enargite

What is enargite? A copper arsenic sulfide with a perfect cleavage

Enargite is Cu3AsS4, a grayish black to iron-black orthorhombic mineral named for its cleavage. The Handbook of Mineralogy enargite sheet gives point group mm2, space group Pmn21, crystals tabular on {001} or prismatic along [001], up to 15 cm, the prism zone typically deeply striated along [001]; twin plane {320} common, rarely as interpenetrating pseudohexagonal trillings. It cleaves perfectly on {110}, distinctly on {100} and {010}, indistinctly on {001}; fracture uneven, brittle, hardness 3, measured density 4.45 against a calculated 4.40.

Colour and optics. Grayish black to iron-black; in polished section gray to pale pink-brown, where deep red internal reflections may be observed; streak grayish black; lustre metallic to dull; anisotropism very strong, in dark violet-red or olive-green tones. The sheet names the mineral from the Greek for distinct, in allusion to its cleavage, and records the type material at the Mining Academy, Freiberg, Germany, number 6929.

What the IMA list records. The September 2026 list gives enargite Cu3AsS4, status G (grandfathered, described before the IMA existed), 1850, Peru, which agrees with the sheet’s Morococha [TL]. The sheet’s occurrence line: in hydrothermal vein deposits formed at medium temperatures, also as a late-stage mineral in low-temperature deposits. Its associates are pyrite, sphalerite, galena, bornite, tetrahedrite–tennantite, chalcocite, covellite, barite and quartz. Whether it counts as a sulfosalt is answered, with the sources’ limits, on is enargite a sulfosalt.

Enargite or luzonite? One formula, two crystal systems

Enargite and luzonite are both Cu3AsS4; enargite is orthorhombic and grayish black, luzonite tetragonal and deep pinkish brown. The luzonite sheet says dimorphous with enargite, forms a series with famatinite, and gives the stannite group, tetragonal, space group I−42m; crystals rare, to 2 mm, commonly massive and intimately intergrown with enargite; good {101} cleavage, distinct {100}; hardness 3.5, density 4.38, black streak, colour deep pinkish brown, similar to bornite although darker. Its type locality is the Lepanto mine, Mankayan, Luzon, Philippines.

The Cu3MS4 six. A field scan of the September 2026 IMA list, splitting each line on runs of two or more spaces and checked by a token scan of the second token, finds 6 rows whose formula has the shape Cu3MS4: enargite and luzonite, Cu3AsS4; famatinite, Cu3SbS4 (G, 1873, Argentina); sulvanite, Cu3VS4; idaite, Cu3FeS4; kuramite, Cu3SnS4. Exactly two share a formula, and they are the dimorphs. The antimony analogue on the sheets is famatinite, which the famatinite sheet places with luzonite in the stannite group; the enargite sheet prints no group at all.

Documented failure: the balance. At the sheets’ measured densities a 20.00 g piece reads 15.51 g in water as enargite and 15.43 g as luzonite, 0.07 g apart, and luzonite is commonly intergrown with enargite. Colour and crystal system separate them, and in a polished section an ore microscopist would; a balance will not. The method is on how to measure specific gravity at home.

Enargite or tennantite? The cleavage key, and the copper worked

The cleavage key: of the six sheets in the table above, enargite alone prints a perfect cleavage, and tennantite and tetrahedrite print none. Tennantite’s sheet gives cubic, tetrahedral crystals as large as 20 cm, subconchoidal to uneven fracture, hardness 3 to 4.5, density 4.62; tetrahedrite’s gives tetrahedral crystals to 15 cm, hardness 3 to 4.5, density 4.97. A black copper sulfide that breaks along flat, striated prism faces is behaving as the enargite sheet describes. The weight cannot do it: tennantite reads 15.67 g for a 20.00 g piece, 0.17 g from enargite. Both old names now cover several IMA species, 8 tennantite rows and 7 tetrahedrite rows, as set out on tetrahedrite. The method is on cleavage or fracture: how to tell.

Copper and arsenic, worked. On standard atomic weights (Cu 63.546, As 74.922, S 32.06), Cu3AsS4 is 48.41% copper, 19.03% arsenic and 32.56% sulfur; the sheet’s ideal column prints 48.42, 19.02 and 32.56, a 0.01 difference from the atomic weights used. The Cerro Blanco, Chile, analysis (47.96 Cu, 1.22 Fe, 0.57 Zn, 18.16 As, 32.21 S) normalises to Cu 3.00, Fe 0.09, Zn 0.03 and As 0.97 per four sulfur; the Bor, Serbia, microprobe analysis to Cu 3.11, Sb 0.03 and As 1.00, antimony 3.2% of the semimetal. Tennantite-(Fe), by comparison, is 43.42% copper and 20.48% arsenic.

Arsenic. Enargite carries 19.03% arsenic by mass in the ideal formula; what that means for handling is not covered here. See toxic minerals in a collection; nothing on this page is health or safety advice.

Where enargite is found

The Distribution field calls enargite widespread but not commonly well crystallized, and names only a few localities. In Peru, Morococha [TL] and Cerro de Pasco, with exceptional crystals from the Mina Luz Angelica, Quiruvilca. In Argentina, the Sierra de Famatina, La Rioja. In the Philippines, Mancayan, Luzon. The Chinkuashi mine, Keelung, Taiwan. Kaize-mura, Nagano, and the Teine mine, Hokkaido, Japan. In the USA, splendent crystals from Butte, Montana; the Tintic district, Utah; and fine specimens from mines in the Red Mountain district, San Juan Co., Colorado. Matzenköpfl, Brixlegg, Austria; Bor, Serbia; Alghero and Calabona, Sardinia; and Tsumeb, Namibia.

What forms from it. The Handbook conichalcite sheet records conichalcite as an uncommon secondary mineral in the oxidized zone of copper deposits, typically an alteration product of enargite. The black copper sulfides it is confused with have their own pages: tetrahedrite, bournonite and covellite; the arsenic sulfosalts proustite and the antimony analogue pyrargyrite are silver minerals. The reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.

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