COSALITE
Cosalite – Pb2Bi2S5 – is a fairly rare sulfosalt, typical of hydrothermal veins of the Pb-Bi-Cu-Ag association. It is part of polymetallic ores, but is never the main ore mineral of the deposit.
It always has some Cu and almost always contains some Ag; both seem to be essential, resulting in a formula CuxAgi-sPb8-2s-0,5(x+i)Bi8+sS20 (Topa & Makovicky, 2010). It may contain Fe.
There are four varieties (with Sb, with Se, with Ag and with Ag+Cu).
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Lead gray, steel gray, silver white.
Habit: Prismatic, acicular, capillary. Fibrous masses, granular mass. Prismatic aggregates, radial to fibrous. Crystals up to 8 cm.
Cleavage: Very rare.
Tenacity: Fibers are flexible.
Twinning: No.
Fracture: Irregular.
Mohs Hardness: 2.5 – 3
Parting: No.
Streak: Black,
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 6.86 – 6.99
2. Geology and Deposits
Cosalite occurs in hydrothermal veins of the Pb-Ag-Cu-Bi paragenesis, formed in temperatures that can be hypothermal, mesothermal and near-epithermal.
It is also found in metasomatic contact deposits (skarns) and in epithermal replacements.
It can occur in pegmatites and in Alpine-type tectonic clefts.
3. Mineral Associations
It is associated with common minerals such as quartz (including smoky and rose varieties), albite (including clevelandite variety), fluorite, and carbonates (calcite, siderite, rhodochrosite).
It occurs with:
– other Bi minerals (native bismuth, bismuthitite, bismuthinite, galenobismuthite, aikinite, lillianite, joseite, emplectite, and tetradymite).
– sulfides (galena, sphalerite, pyrite, pyrrhotite, marcasite, arsenopyrite, chalcopyrite, bornite, enargite, glaucodote, hessite, cobaltite, molybdenite, gersdorffite, tetrahedrite, stromeyerite),
– sulfosalts (boulangerite) and tungstates (scheelite),
– phosphates (lithiophosphate, dorfmanite)
– arsenides (löllingite, eskutterudite, niqueline),
– oxides (“wolframite”, uraninite, Mn minerals),
– native gold and silicates (diopside, epidote, tremolite, chlorite, titanite, tourmaline (dravite), stilbite).
4. Transmitted Light Microscopy
This does not apply, as cosalite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Cosalite acquires a good polish without difficulty. Its hardness when polished is slightly higher than that of galena.
PLANE POLARIZED LIGHT – PPL
Reflection color: White with a faint cream or pink tint.
Compared to the color of galena, the color of cosmolite is very similar, only with a subtle cream to green tint.
Pleochroism: Very weak.
Reflectivity: 40.37 – 43.56
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very weak anisotropy between pinkish-yellow, bluish, or gray-violet. If with Se, the anisotropy is more intense.
Internal reflections: No.
May be confused with: Other Pb-Bi-Ag sulfosalts.
Identification of cosalite is difficult. Decomposition is a typical feature, generating small grains of native Bi. Very weak anisotropy is an important diagnostic feature in relation to other minerals of the paragenesis.
General Characteristics:
Grain shape: generally in masses of acicular to fibrous crystals, forming aggregates where these crystals are arranged in a subparallel or radial manner. Isolated crystals in quartz or calcite may occur. Granular aggregates may occur.
Cleavage is very rare, similar to that of bismuthinite.
Lamellar intergrowths of cosalite with boulangerite may occur.
Myrmekitic intergrowths with galena, bismuthinite, and galenobismuthite have been observed on several occasions.
Twinning does not occur.
Decomposition of cosalite generates a mixture of native bismuth, galena, and wittichenite(?).
Substitutions 1: Cosalite substitutions are rare, but substitution by Bi tellurides occurs.
Substitutions 2: Cosalite substitutes pyrite and glaucodote.
Inclusions 1: Cosalite may contain inclusions of native gold, native bismuth, pyrrhotite, chalcopyrite, galenobismuthite, hessite, and pyrite.
Inclusions 2: Cosalite occurs as inclusions in galena, sphalerite, pyrite, pyrrhotite, galenobismuthite, and bismuthinite.
Bismuthinite rims may form around cosalite grains.
