Occurrence and formation of avicennite, Tl2O3 , as a secondary mineral at the Carlin gold deposit, Nevada

Avicennite, Tl₂O₃, occurs as grains disseminated in silicified limestones in the upper part of the East ore zone of the Carlin gold deposit, Nevada. The avicennite is formed by the oxidation of carlinite, Tl₂S, found in primary unoxidized carbonaceous ore immediately below the avicennite. The grains of avicennite closely resemble carlinite in size and shape. Some avicennite occurs as thin coatings on carlinite, but the time of its formation is unclear. Avicennite grains are polycrystalline, porous, dark gray to black, with a hackly fracture but no discernible cleavage. The Mohs hardness is 2.0±0.5; Vickers hardness ranges from 46.0 to 80.5 kg mm^-2. Measured density is distinctly low, 8.9 g cm^-3 (or Mg m^-3), relative to the calculated density, 10.34 g cm^-3. The mineral is isometric, space group Ia3. a=10.5468±0.0003 angstroms or 1.05468±0.00003 nm, Z=16, and the volume of the unit cell is 1173.17±0.04 Ǻ³ or 1.17317±0.00004 nm³. The most intense X-ray diffraction peaks are 3.044 (100), 1.864 (38), 2.637 (37), and 1.590 (30). In reflected light, avicennite is pale to medium gray and isotropic and lacks discernible bireflectance. Reflectance in air ranges from 10.6 to 13.0 percent. Avicennite is nearly pure Tl₂O₃, containing 89.6 weight percent Tl and 10.5 weight percent O by microprobe analysis. Other elements present, detected by emission spectrographic analysis, are: Pb, 300 ppm; Ca, 100 ppm; and Si, Al, Fe, Mg, Ag, Cr, Cu, Ni, and Ti, all <50 ppm.