FANGITE, TL3ASS4, A NEW THALLIUM ARSENIC SULFOSALT FROM THE MERCUR AUDEPOSIT, UTAH, AND REVISED OPTICAL-DATA FOR GILLULYITE

Fangite, Tl3AsS4, is one of several TI minerals that occur at the Merc ur disseminated Au deposit, Tooele County, Utah. The mineral is descri bed on the basis of one specimen found in a sulfide ore stockpile at t he mine. It occurs in a vug with pyrite and other sulfide material of complex composition. Realgar and orpiment occur in calcite veins in th e host rock, but not in the vug. Fangite has a deep red to maroon colo r and an orange streak. It is translucent but tarnishes to a nearly me tallic luster. The calculated density is 6.185 g/cm3, and the measured density (determined from synthetic Tl3AsS4) is 6.20(4) g/cm3. The Moh s hardness is 2.0-2.5, and the mean VHN100 is 60.7. Fangite is blue-gr ay in polished section and has very low bireflectance, with a differen ce in Y% of 0.4. Red internal reflections are abundant. Reflectivity v alues in air range from 2 1.1 to 30.15%; immersion values range from 8 .08 to 14.7%. A single-crystal X-ray diffraction study shows fangite t o be orthorhombic, space group Pnma, with unit-cell parameters a = 8.8 94(8), h = 10.855(9), c = 9.079(9) angstrom, V = 877(1) angstrom3, and Z = 4. The structure was solved by direct methods and refined to a fi nal residual of 0.046 using 637 observed reflections. The As atom is i n tetrahedral coordination with S, whereas the T12 atom is in fivefold coordination with S, producing trigonal dipyramids. The dipyramids an d tetrahedra are interconnected to form polyhedral layers parallel to (010). Interlayer T11 atoms link the layers. Because of problems with internal reflections in the original published data, new optical data for gillulyite (another Tl mineral from this deposit) are presented. G illulyite, Tl2(As,Sb)8S13, is distinctively bireflectant, with a diffe rence in Y% of 3.8%. Reflectivity values in air range from 21.2 to 32. 0%; immersion values range from 8.14 to 18.6%.