CALVERTITE, Cu5Ge0.5S4, A NEW MINERAL SPECIES FROM TSUMEB, NAMIBIA

Abstract

The sulfide-rich matrix of a partly oxidized specimen from the Tsumeb mine, Tsumeb, Namibia, consists mainly of renierite, tennantite, gallite, chalcocite, and the new mineral calvertite, simplified as Cu 5 Ge 0.5 S 4 , which is named after Lauriston D. Calvert (1924–1993) of the National Research Council, Ottawa, Canada. Calvertite occurs as anhedral grains, up to 100 μm across, that are outlined by a network of chalcocite veinlets of micrometric width. In reflected light, calvertite is opaque, pale bluish grey, and isotropic. Reflectance percentages in air are 26.3 (470 nm), 23.1 (546), 22.2 (589), and 21.5% (650). The mineral is brittle, macroscopically black with a black streak, and has a hardness of 4 to 5, a conchoidal to uneven fracture, and no cleavage. Single-crystal X-ray study indicated F -centered cubic symmetry, a 5.337(3) A; the structure has not been solved, and the powder-diffraction pattern has only four lines [ d in A( I )( hkl )]: 3.053(100)(111), 2.639(10)(200), 1.869(90)(220), and 1.595(30) (311). An electron-microprobe analysis gave Cu 63.10, Fe 1.66, Zn 0.55, Ge 5.76, As 1.50, Ga 0.36, V 0.05, S 26.63, total 99.60 wt.%, corresponding to (Cu 4.782 Fe 0.143 Zn 0.041 Ga 0.025 V 0.005 ) S4.996 (Ge 0.382 As 0.096 ) S0.478 S 4 on the basis of S = 4 atoms and Z = 1. It is likely that the structure is disordered, and the measured unit-cell is the subcell of a compound whose ordered equivalent has the simplified formula Cu 10 GeS 8 .