COLORADOITE
Coloradoite – HgTe – is a very rare telluride, which is part of polymetallic ores containing Au, Ag, and Hg.
It belongs to the Sphalerite Group and contains 61.12% Hg and 38.88% Te. It may contain Pb.
Special care is needed when grinding and polishing coloradoite due to the presence of mercury, which is toxic in both liquid and vapor form!
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Iron black to gray, somewhat reddish. Quickly tarnishes to tobacco brown.
Habit: Massive to granular. No other habits are known.
Cleavage: No.
Tenacity: Brittle, friable.
Twinning: No.
Fracture: Irregular, subconchoidal.
Mohs Hardness: 2.5
Parting: No.
Streak: Black,
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 8.0 – 8.1
2. Geology and Deposits
Coloradoite is a mineral characteristic of subvolcanic to intrusive hydrothermal veins with moderate temperatures, containing precious metals (Te, Au, Ag, Pb) associated with Hg.
3. Mineral Associations
It occurs associated with:
– quartz, calcite, native elements (gold, tellurium),
– other tellurides (altaite, hessite, petzite, calaverite, sylvanite and krennerite) and
– sulfides (pyrite, chalcopyrite, pyrrhotite, sphalerite, bornite, galena, tetrahedrite-tennantite and nagyágite).
4. Transmitted Light Microscopy
This does not apply, as coloradoite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Coloradoite is easy to polish and produces very good quality results, but fine polishing grooves are difficult to remove. It is slightly less hard than calaverite, sylvanite, and petzite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Light gray with a brownish tint, fading to a dull purple or pink or brownish-black color. Compared to gangue, it is white.
Pleochroism: No.
Reflectivity: 37.14%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic, but may exhibit weak anomalous anisotropy ranging from dark brownish-red to bluish-gray. Polishing scratches simulate anisotropy.
Internal reflections: No.
May be confused with: Few other minerals.
Petzite (AgAuTe2), is extremely similar, but weakly anisotropic.
Calaverite and sylvanite are lighter.
General Characteristics:
Grain shape: generally occurs included or framed by hessite.
Cleavage is not observed.
Inclusions IN coloradoite may be chalcopyrite or sylvanite, the latter in lamellae.
Inclusions OF coloradoite occur in bornite.
Substitutions of pyrite by coloradoite may occur.
Separation between coloradoite and krennerite may be intergrowths of sylvanite with hessite.
