KOBELLITE
Kobellite – Pb22Cu4(Bi,Sb)30S69 – is a rare sulfosalt typical of high-temperature hydrothermal deposits. It does not constitute the primary ore mineral of the deposit, but occurs within polymetallic ores.
It is classified within the Kobellite homologous series group. It is the bismuth analogue of tintinaite, with which it forms a series. It may contain some iron substituting for copper and can also contain silver. It has two varieties (antimony-bearing and selenium-bearing).
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Lead-gray with blackish to steel-gray tones.
Habit: Tabular, fibrous, massive, granular.
Cleavage: {010} good.
Tenacity: Brittle.
Twinning: Common.
Fracture: No information available.
Mohs Hardness: 2.5 – 3
Parting: No.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 6.334
2. Geology and Deposits
Kobellite is a mineral found in high-temperature hydrothermal deposits—typically polymetallic—forming a Pb-Bi-Ag-Sb-Cu association. In this context, kobellite is one of nearly a dozen sulfides and sulfosalts, many of the latter sharing similar compositions.
3. Mineral Associations
Kobellite occurs with minerals characteristic of hydrothermal deposits such as quartz and carbonates (dolomite, siderite).
In its specific paragenesis, it is associated with common sulfides (pyrite, chalcopyrite, sphalerite) and with complex sulfides and sulfosalts of similar composition (Pb+Sb+Bi+Ag) such as tintinaite, eskimoite, aramayoite, jamesonite, bismuthinite, boulangerite and jordanite. This group generally also contains proustite and electrum.
Other associates are iron oxides (hematite, magnetite), molybdenite and scheelite.
4. Transmitted Light Microscopy
Not applicable, as kobellite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: It takes a high polish but shows numerous polishing scratches (like chalcopyrite). Its polishing hardness is significantly higher than that of native bismuth and lower than that of galena.
PLANE POLARIZED LIGHT – PPL
Reflection color: White. Compared to galena, the color is slightly darker, with a yellowish-green tint in the lightest position.
Pleochroism: Weak; ranging from greenish-white, violet-gray, and yellowish-white to pinkish-white.
Reflectivity: 40.33 – 44.73%
Bireflectance: Weak.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Distinct anisotropy in brownish gray tones: steel gray to gray brown. Extinction is apparently parallel.
Internal reflections: No.
May be confused with: Cosalite (Pb2Bi2S5) possesses very similar properties, but the pleochroism and anisotropy of kobellite are much stronger. It is not possible to distinguish kobellite from other sulfosalts occurring in the same paragenesis based solely on microscopic analysis.
General Characteristics:
Grain shape: granular aggregates or columnar to acicular crystals forming radial aggregates.
Polishing scratches due to low hardness are practically inevitable and, therefore, quite diagnostic. They are more easily observed in XPL.
Cleavage is visible, well developed, parallel to elongation.
Twins are not rare, and are generally simple twins developed parallel to the length. Lamellar twins may occur.
Intergranular boundaries between kobellite grains may contain chalcopyrite and galena.
Substitutions 1: kobellite replaces and involves arsenopyrite, pyrite, pyrrhotite and marcasite.
Substitutions 2: kobellite is replaced by tetrahedrite and chalcopyrite.
Exsolutions of native bismuth may occur.
Inclusions of tellurides may occur.
Myrmekites with tetrahedrite may occur.
