Digenite

 ( Sulfide Minerals ) 

Digenite is a copper sulfide mineral with formula: Cu₉S₅. Digenite is a black to dark blue opaque mineral that crystallizes with a trigonal–hexagonal scalenohedral structure. In habit it is usually massive, but does often show pseudo-cubic forms. It has poor to indistinct cleavage and a brittle fracture. It has a Mohs hardness of 2.5 to 3 and a specific gravity of 5.6. It is found in copper sulfide deposits of both primary and supergene occurrences. It is typically associated with and often intergrown with chalcocite, covellite, djurleite, bornite, chalcopyrite and pyrite. The type locality is Sangerhausen, Thuringia, Germany, in copper slate deposits.

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Occurrence Digenite occurs in the transitional zone of supergene oxidation of primary sulfide ore deposits, at the interface between the upper and lower saprolite ore zones. It is rarely an important mineral for copper ores, as it is more usually replaced by chalcocite further up in the weathering profile, and is a minor weathering product of primary chalcopyrite. Natural digenite always contains a small amount of iron and is considered to be stable only in the Cu-Fe-S system.American Mineralogist (1970) 55:106

In the Deflector and Deflector West Cu-Au lode deposits of the Gullewa Greenstone Belt, Western Australia, digenite is an important constituent of the transitional Cu-Au ore. However, it is difficult to treat metallurgically and remains a refractory ore type. In this locality digenite is found with covellite, chalcocite and bornite.

It was first described in 1844 from the type locality of Sangerhausen, Saxony-Anhalt, Germany. The name is from the Greek language digenus meaning of two origins in reference to its close resemblance with chalcocite and covellite.

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Polymorphs of digenite There are three polymorphs of digenite, high, metastable and low.American Mineralogist (1963) 48:110 There is a complete solid solution series between high digenite and berzelianite Cu_2−xSe (x~0.12).Canadian Mineralogist (2008) 46: 219-231

High digenite is stable above 73 °C, with space group Fmm and unit cell parameters a = 5.57 Å and Z = 1 for formula Cu_7.2−xS₄. High digenite is isostructural with bornite Cu₅FeS₄.

Metastable digenite forms on cooling from 73 °C. It appears to be isometric with space group Fdm and a = 27.85 Å. This symmetry, however, is due to twinning of fine domains with rhombohedral symmetry, trigonal -3m, point group R-3m, a = 3.92 Å, c = 48 Å, Z = 15 for formula Cu_1.8S. Metastable digenite changes with time to stable low temperature digenite or a mixture of anilite Cu₇S₄ and djurleite Cu₃₁S₁₆.

Low digenite is isometric, space group Fdm and a = 27.85 Å, i.e. the same as the apparent isometric symmetry of the metastable polymorph, Z=4 for formula Cu_1.8S.

• Palache, C., H. Berman, and C. Frondel (1944) Dana’s system of mineralogy, (7th edition), v. I, pp. 180–182.

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