CALAVERITE
Calaverite – AuTe2 – is a rarer telluride, one of the few Au ores besides native gold. Moreover, it is the most common Au mineral after native gold. It was the mineral of the Western Australian gold rushes in the 1890s, where it was initially mistaken for pyrite and used as a building and landfill material. Once the misunderstanding was cleared up, the “landfills” generated a new gold rush in 1896.
Calaverite is dimorphic with krennerite (Au3AgTe8). Approximately 3% of the Au is replaced by Ag. It may contain some Cu and Sb.
The structure of calaverite is very complex, it has been an object of fascination and frustration for the scientific community during the last century and is still not fully understood. Even the crystalline forms of the mineral are complex and require detailed analysis.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Brass yellow to silver white.
Habit: Thin, tabular, striated prisms. Massive, granular.
Cleavage: No. Striations // to [010]
Tenacity: Brittle.
Twinning: Common on {110}, less common on {031} and {111}.
Fracture: Irregular, conchoidal.
Mohs Hardness: 2.5 – 3
Parting: No.
Streak: Green to yellowish gray.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 9.1 – 9.3
2. Geology and Deposits
Calaverite occurs in low-temperature hydrothermal veins and in medium- and high-temperature hydrothermal deposits, generally in very subordinate quantities; it is not the main ore mineral of the deposit.
It can occur in epithermal subvolcanic gold veins.
3. Mineral Associations
It occurs associated with the characteristic paragenesis of these Au-Ag-Te veins, which contain:
– native elements such as native gold, native silver, and electrum;
– tellurides such as altaite, coloradoite, krennerite, hessite, sylvanite, tellurobismuthite, tetradymite, and rickardite;
– sulfides typical of polymetallic deposits such as pyrite, arsenopyrite, sphalerite, stibnite, chalcopyrite, galena, molybdenite, jamesonite, tennantite-tetrahedrite (“fahlore”), bismuthinite, and minerals of the Freibergite Group.
4. Transmitted Light Microscopy
This does not apply, as calaverite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Calaverite polishes easily, but due to its low hardness, it exhibits negative relief when in the presence of much harder minerals. Its hardness upon polishing is less than that of galena and slightly greater than that of pyrargyrite. It is much softer than chalcopyrite and tetrahedrite-tennantite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Very light cream (or white with a yellow-brown tint). Tarnishes to slightly darker colors over time.
Compared to the color of pyrite, the color of calaverite is lighter and more pink.
Compared to the colors of galena and sylvanite, the color of calaverite is lighter.
Compared to the color of krennerite, in its lightest position it has a golden hue.
Compared to the color of altaite, the color of calaverite is slightly pink.
Pleochroism: Weak, perceptible only at intergranular boundaries: white-yellow to yellowish-brown. Both the colors and the reflectivity vary considerably.
Reflectivity: 63, 46 – 67,92%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Weak anisotropy, in light and dark brownish grays, can be reddish-gray, green, brownish-green. Color variations become more distinct in the diagonal position with the nicols slightly uncrossed. Extinction is oblique.
Internal reflections: No.
May be confused with: Several other very similar minerals.
Pyrite has a similar reflective color, but is much harder; it has high relief and old polishing scratches.
Sylvanite is almost identical, its color is slightly darker and it is more anisotropic.
Krennerite is very similar, but has well developed cleavage and stronger anisotropy.
Nagyàgite shows perfect cleavage and a different color.
Montbrayite is identical to calaverite under the microscope.
The Au and Te contents are diagnostic against many other minerals.
General Characteristics:
Grain shape is typically idiomorphic, forming short prisms with fringed edges, resembling idioblasts. Crystals can be acicular in radial or lamellar aggregates. Also anhedral as impregnation. Subparallel aggregates may occur.
Twinning is not normally visible in polished section. Sometimes, intensely lamellar twinning occurs at the edges of the crystals.
Cleavage is not visible.
Inclusions IN calaverite can be pyrite, coloradoite, altaite, melonite, and others.
Inclusions OF calaverite occur in chalcopyrite and pyrrhotite.
Calaverite substitutes for petzite, altaite, gold, chalcopyrite, and pyrite. The substitution of gold by calaverite generates spectacular gold paramorphoses.
Calaverite is substituted for gold, resulting in gold pseudomorphoses on calaverite.
