Weissite from Gambatesa mine, Val Graveglia, Liguria, Italy: occurrence, composition and determination of the crystal structure

Author： Bindi L. Carbone C. Belmonte D. Cabella R. Bracco R.

Publisher： Mineralogical Society

ISSN： 0026-461X

Source： Mineralogical Magazine, Vol.77, Iss.4, 2013-06, pp. : 475-483

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Abstract

Weissite, Cu_2–xTe (x ≈ 0.21), a very rare copper telluride, occurs in a sample from the Gambatesa mine, Val Graveglia, Liguria, Italy, where it occurs as purplish black anhedral grains up to 0.1 mm in length and shows a black streak. No cleavage is observed and the Vickers hardness (VHN₁₀₀) is 142 kg/mm². Weissite is dark bluish black, weakly pleochroic, and moderately anisotropic in bluish tints. Reflectance percentages in air for R_min and R_max are 37.0, 38.4 (471.1 nm), 33.2, 34.2 (548.3 nm), 31.2, 32.1 (586.6 nm), and 28.6, 31.0 (652.3 nm), respectively.Weissite is trigonal and belongs to the space group P3m1 with the following unit-cell parameters: a = 8.3124(7) Å, c = 21.546(1) Å, V = 1289.3(2) Å³, and Z = 24. Electron microprobe analyses gave the chemical formula (Cu_1.62Ag_0.04Au_0.04Fe_0.04Sb_0.04)_Σ=1.78(Te_0.96S_0.02Se_0.02). The crystal structure has been solved and refined to R = 1.95%. It consists of Cu and Te polyhedra forming complex crystal-chemical environments as is typical of many intermetallic compounds. The exceedingly short bond distances observed among the metals are discussed in relation to other copper tellurides and pure metals.