ALABANDITE
Alabandite – MnS – is a relatively rare sulfide, with 63.14% Mn. It is associated with Mn deposits and polymetallic sulfide deposits.
It is classified in the Galena Group and is polimorph with browneite and with rambergite. It frequently contains appreciable levels of FeS. It may also contain Mg and Co.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Black, steel gray, brown, green, fades to brownish black (resembling mold).
Habit: Generally massive to granular. Cubic or octahedral crystals up to 1 cm.
Cleavage: {100} perfect.
Tenacity: Brittle.
Twinning: Lamellar // a {111}.
Fracture: Irregular.
Mohs Hardness: 3.5 – 4
Parting: No.
Streak: Green or brown.
Lustre: Submetallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.0
2. Geology and Deposits
It can occur in large quantities in epithermal polymetallic sulfide veins, where it is associated with rhodochrosite and rhodonite.
It is characteristic of low-temperature Mn deposits.
Alabandite can accompany Ag and Au-Te ores.
It can occur in meteorites and forms during many metallurgical processes.
In one case, it was described in an Alpine-type vein.
3. Mineral Associations
It is associated with typical gangue minerals such as quartz, calcite, Mn-calcite, and fluorite.
With other common sulfides such as sphalerite, galena, pyrrhotite, chalcopyrite, and pyrite.
With other Mn minerals such as pyrolusite, rhodochrosite, rhodonite, and tephroite.
With minerals of the Ag-Au-Te paragenesis such as acanthite, petzite, native tellurium, and other tellurides.
With stannite, jamesonite, enargite, pectolite, tremolite, and Mn-diaspore.
4. Transmitted Light Microscopy
Not applicable, as alabandite is opaque.
The literature reports that in very fine fragments alabandite is translucent in green, brown or red colors.
5. Reflected Light Microscopy
Sample preparation: Alabandite acquires a good polish easily after careful grinding. Its hardness upon polishing is slightly less than that of sphalerite. In many cases, the polished surface becomes covered, after days or weeks, with a brownish-black film resembling mold, as occurs with recent fractures in macro samples. During polishing, the polishing cloth becomes covered with green dust.
PLANE POLARIZED LIGHT – PPL
Reflection color: Grayish-white, may have a greenish tint.
Compared to the color of sphalerite, the color of alabandite is distinctly lighter.
Pleochroism: No.
Reflectivity: 21.91%.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic, completely dark.
Sometimes a very weak anomalous anisotropy can occur, in colors ranging from light gray to gray-black.
Internal reflections: With some attention, and after immersion in oil, internal reflections in deep green can be observed. If oxidized, internal reflections in brown to brownish hues will appear.
May be confused with: Very similar to sphalerite, which has lower reflectivity and internal reflections of different colors.
Some other similar ores are anisotropic and much harder. The deep green internal reflections are very diagnostic.
General Characteristics:
Grain shape: Alabandite generally has coarse, anhedral grains. In crusts, the grains are euhedral, subsequently covered by other minerals. The grains are slightly dentate.
Cleavage is only visible in low-quality polished sections.
Twins, vine lamellar, very similar to those of sphalerite, are often visible due to the relief generated by polishing.
Zoning occurs in some cases, when small grains exhibit idiomorphic pyrrhotite inclusions, arranged parallel to (111). Larger grains show these inclusions in a marginal zone. After chemical etching, zoning is usually observed.
Inclusions, very small (“dust”), of chalcopyrite and pyrite may occur. Pyrrhotite inclusions occur in the form of idiomorphic grains, arranged parallel to (111).
Substitutions are common. Pyrite and marcasite frequently penetrate fractures and cleavages, and may replace appreciable portions of the alabandite. Pyrolusite can, rarely, form in fractures.
