Digenite

Digenite
Digenite from the East Colusa Mine, Butte, Montana, USA. Specimen size = 4.3 cm
General
Category	Sulfide mineral
Formula (repeating unit)	Cu9S5
Strunz classification	2.BA.05e
Dana classification	02.04.07.03
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	R3m
Identification
Formula mass	146.45 g/mol
Color	Blue to black
Crystal habit	Pseudocubic crystals are rare, usually as intergrowths with other copper sulfides
Cleavage	{111} (observed in synthetic material)
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	2.5 to 3
Luster	Submetallic to metallic
Streak	Black
Diaphaneity	Opaque
Specific gravity	5.5 to 5.7 observed, 5.628 calculated
Optical properties	Reflectivity: 21.6 at 540 nm
Other characteristics	Distinctly blue in polished sections
References

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.

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.

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 digenus meaning of two origins in reference to its close resemblance with chalcocite and covellite.

Polymorphs of digenite

There are three polymorphs of digenite, high, metastable and low. There is a complete solid solution series between high digenite and berzelianite Cu_2−xSe (x~0.12).

High digenite is stable above 73 °C, with space group Fm3m 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 Fd3m 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 Fd3m 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.

References

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