MINERALOGY OF TSUMOITE BASED ON SAMPLES FROM THE DASHUIGOU TELLURIUM DEPOSIT, TIBET PLATEAU, CHINA

Tsumoite is not as common as chalcopyrite or most other minerals on the Earth. Discovery of the Dashuigou tellurium deposit at the Tibet Plateau makes it possible to further understand the mineralogical characteristics of this rare and usually associated mineral.

Tsumoite, as the second richest tellurium mineral after tetradymite from the deposit, occurs in the form of irregular vermiform and/or fine veinlet at the contact between pyrrhotite, wall rock breccia including dolomite, and tetradymite, and in fractures of the pyrrhotite. In several cases, tsumoite appears in the form of interlocking crystals with tetradymite, and coexists with native gold, chalcopyrite and so on.

Tsumoite coexists with chalcopyrite indicating unmixing characteristics of solid solution. Tsumoite is anisotropic with glossy white color and low hardness. Its reflectance and micro-pressure hardness are respectively 36.2 (470 nm), 43.5 (546 nm), 44.8 (589 nm) and 47.3 (650 nm), and 192 kg/mm² (Hv25). Its chemical compositions are Te=35.58% ~ 37.54% (average 36.55%), Bi=61.56% ~ 62.20% (average 61.84%). Its chemical formula is calculated as BiTe_0.95-0.99 and the average is BiTe_0.97, indicating slightly poorer tellurium than that of the associated tsumoite (BiTe_1.01) from the Huangshandong Cu-Ni deposit in China. The telluride of the mine formed in an environment rich in Te but poor in S. Results of the quantitative phase analysis (wt.%) XRD-Rietveld and lattice parameters of the tsumoite are respectively a = 4.432 Å, c = 23.995 Å, and v = 408.294 ų.