DIGENITE
Digenite – Cu9S5 – is a fairly common sulfide, an important Cu ore.
The planetary abundance of copper and sulfur, their wide distribution, and the high reactivity of the two elements have generated nearly 300 different mineral species that contain them. There are nine different minerals formed only by Cu + S. Copper sulfide formed at low temperatures (T<~105ºC) is monoclinic chalcocite. At high temperatures (T>~105ºC) (including in blast furnaces!) digenite (trigonal Cu9S5) is formed. Another copper sulfide, djurleite (monoclinic Cu31S16), is very similar to chalcocite. To complicate matters, another copper sulfide, anilite (orthorhombic Cu7S4), transforms into digenite during the preparation of the polished section, by polishing. Other copper sulfides include roxbyite (monoclinic Cu9S5), spionkopite (trigonal Cu39S28), yarrowite (trigonal Cu9S8), geerite (trigonal Cu8S5), and covellite (hexagonal CuS).
The situation becomes even more difficult because in several of these minerals there are substitutions of Cu by varying amounts of Ag, Fe, and Mn, in addition to substitutions of S by Se and Te. Furthermore, there are crystals with digenite x chalcocite and digenite x covellite mixtures.
And there is a complete series of solid solutions between high-temperature digenite and berzelianite. There are serious sample preparation problems, which are discussed further below. This set of complications has historically caused much confusion and identification errors (More: Pósfai & Buseck 1994. American Mineralogist, 79, 308-315).
Digenite is classified in the Chalcocite-Digenite Group. It forms pseudocubic crystals that can reach up to 3 cm, but these are rare. It usually occurs intergrown with other copper sulfides.
1. Characteristics
Crystal system: Trigonal scalenohedral.
Color: Gray to grayish-black, it turns bluish after exposure to air.
Habit: Usually found as intergrowths with other copper sulfides. Rare crystals.
Cleavage: {111} good, in synthetic material.
Tenacity: Brittle.
Twinning: No.
Fracture: Conchoidal.
Mohs Hardness: 2.5 – 3
Parting: No.
Streak: Grayish black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 5.54
2. Geology and Deposits
Digenite forms under a wide range of conditions. It is a primary and secondary mineral in hydrothermal copper deposits. It occurs in mafic intrusives, as an exhalative volcanic product, and in pegmatites.
It is also found in porphyry Cu deposits and in Mississippi Valley Type Pb-Zn deposits.
3. Mineral Associations
It occurs with:
– several other Cu sulfides (chalcocite, djurleite, bornite, chalcopyrite, covellite, carrollite, enargite, idaite, luzonite, tetrahedrite-tennantite, wittichenite),
– other sulfides (pyrite, galena, sphalerite),
– secondary Cu minerals (malachite, brochantite, linarite, native copper), hematite, rutile, and quartz.
It is also associated with some rare minerals such as neptunite, joaquinite-(Ce), hessite, and bohdanowiczite. It occurs with some uranium ores, associated with curite, soddyite, kasolite, cuprosklodowskite, and vandenrandite.
4. Transmitted Light Microscopy
This does not apply, as digenite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Digenite has a low polishing hardness, approximately similar to that of galena and chalcocite. Chalcopyrite has a much higher hardness. It acquires an excellent polish easily. Large amounts of blue polishing scratches are almost inevitable, especially if there are small pyrite grains embedded in the digenite. Long polishing times should be avoided, otherwise there will be a strong relief between the digenite and the surrounding minerals, and the surface film of the digenite will become very blurred.
Sections with low-quality polishing and/or poorly dried after polishing and/or produced with too much pressure during polishing have a much bluer appearance than correctly prepared sections. Coarse abrasives should be completely avoided during the digenite polishing process. The mineral anilite transforms into digenite during the polishing process of the polished sections.
PLANE POLARIZED LIGHT – PPL
Reflection color: Distinctly bluish.
Pleochroism: No.
Reflectivity: 19.89%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
Internal reflections: No.
May be confused with: other bluish copper sulfides such as djurleite, chalcocite, etc.
General Characteristics:
Grain shape: digenite generally occurs intergrown with other Cu sulfides. It can form pseudocubic crystals, but they are rare.
Twins parallel to (111) can occur, they follow the Spinel Law. Often the lamellae can already be recognized macroscopically.
Inclusions of covellite sometimes occur, arranged parallel to (111).
Cleavage according to {111} – octahedral – is barely visible. Some literature reports that it is often visible.
Alteration of digenite leads to the formation of covellite.
Pseudomorphoses on chalcocite can occur.
Pseudo-structures can be generated through deep polishing scratches generated by grinding and filled with mineral paste during polishing.
