WITTICHENITE
Wititchenite – Cu3BiS3 – is a rarer sulfide, occurring in hydrothermal veins rich in Cu + Bi. It contains 38.45% Cu, 42.15% Bi, and 19.40% S.
It commonly contains Ag, and there is a variety with a significant Ag content. It is the BiS analog of bytizite.
1. Characteristics
Crystal system: Orthorhombic bisphenoidal
Color: Steel gray, bronze, tin white. Tarnishes to lead gray and yellow.
Habit: Tabular, short prismatic, acicular, granular. Crystals up to a few centimeters.
Cleavage: No.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular, conchoidal.
Mohs Hardness: 2 – 3
Parting: No.
Streak: Black.
Lustre: Metallic, tarnishes fast.
Diaphaneity: Opaque.
Density (g/cm³): 6,3 – 6,7
2. Geology and Deposits
Wittichenite is a mineral characteristic of hydrothermal veins rich in Bi and Cu, belonging to the Ag-Co-Ni type vein group, basically with gangue formed by baryte.
3. Mineral Associations
It is associated with typical gangue minerals such as barite, calcite, aragonite, fluorite, and quartz.
It is also associated with common sulfides such as pyrite.
It occurs with Cu minerals such as chalcopyrite, bornite, chalcocite, digenite, djurleite, rochesite, enargite, famatinite (“stibioluzonite”), stromeyerite, and covellite. Its occurrence in chalcocite + enargite deposits is especially common.
It is also associated with oxides (cassiterite and hematite), native gold, native silver, other silver minerals, rammelsbergite, and sphalerite.
4. Transmitted Light Microscopy
This does not apply, as wittichenite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Witchichenite exhibits medium hardness upon polishing and achieves a high-quality polish.
Its hardness is lower than that of tetrahedrite, tennantite, chalcopyrite, bornite, and annivite.
It is slightly higher than that of chalcocite, higher than that of emplectite, and significantly higher than the hardness of native bismuth.
PLANE POLARIZED LIGHT – PPL
Reflection color: Cream to grayish-white, may show an olive hue.
Compared to the color of tetrahedrite, the color of wittichenite is lighter.
Compared to the color of tennantite, the color of wittichenite is lighter, pinkish.
Compared to the color of chalcocite, the color of wittichenite is somewhat greenish or brownish.
Compared to the color of emplectite, the color of wittichenite is slightly darker.
Compared to the color of famatinite (“klaprothite”), the color is very similar, slightly darker.
Compared to the color of aikinite, the color of wittichenite is brownish-gray.
Pleochroism: Very faint, only observable with oil immersion.
Reflectivity: 33.37 – 35.26%
Bireflectance: No information available.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Weak anisotropy in shades of dark brownish-gray.
Internal reflections: No.
May be confused with: many other minerals. Identifying wittichenite is very difficult.
General Characteristics:
Grain shape: Wittichenite generally occurs coarse-grained, in irregular grains. It can be idiomorphic in acicular crystals or with elongated rectangular boundaries or form elongated tabular crystals parallel to (001).
Cleavage was not observed.
Twinning is not present.
Myrmekites of wittichenite with tetrahedrite-tennantite may occur.
Intergrowths of wittichenite may be with bornite or digenite.
Reaction rims of wittichenite occur between chalcopyrite and bismuth or bismuthinite.
Inclusions of wittichenite occur in bornite and chalcopyrite, as well as between grains of bornite and chalcopyrite. Inclusions in chalcopyrite may be corroded, as they are inherited from substituted bornite. It rarely occurs as inclusions in magnetite and hematite.
Inclusions in wittichenite can be tennantite, native bismuth, and annivite.
Substitutions 1: wittichenite can be replaced by covellite, chalcocite, chalcopyrite, emplectite, native bismuth, and bismuthinite. Chalcocite can form pseudomorphs on wittichenite.
Substitutions 2: wittichenite can replace native bismuth, annivite, and aikinite; the substitution of the latter can generate a checkerboard network with fine veins of wittichenite in the aikinite.
