WESSELSITE, SRCU[SI4O10], A FURTHER NEW GILLESPITE-GROUP MINERAL FROMTHE KALAHARI MANGANESE FIELD, SOUTH-AFRICA

Wesselsite, SrCu[Si4O10], is a new mineral species from the Wessels mi ne, Kalahari Manganese Field, South Africa, and it belongs to the gill espite group. Wesselsite is tetragonal, space group P4/ncc; the unit c ell parameters, refined from Gandolfi film data, are a = 7.366(1), c = 15.574(3) Angstrom, V = 845.01 Angstrom(3). The strongest lines are ( d(obs)/I-obs/hkl) (7.79/35/002), (4.33/20/112), (3.89/20/004), (3.44/4 0/104), (3.33/100/202), (3.12/55/114), (3.03/50/212), (2.68/25/204), ( 2.61/30/220) and (2.32/30/116). Wesselsite is associated with hennomar tinite, embedded in a matrix of sugilite, xonotlite, quartz and pectol ite. Microprobe analyses of 111 samples show that it is the end-member of a solid series with effenbergerite, BaCu[Si4O10], with substitutio ns of Sr by Ba up to 50 mol.%. Wesselsite forms tiny subhedral plates in sizes not exceeding 50 x 50 x 5 mu m, arranged in clusters of up to 200 mu m. It shows a perfect cleavage parallel to {001}, has blue col our, white to light blue streak, and is uniaxial negative with omega = 1.630(2), epsilon = 1.590(5), strongly pleochroic from blue (omega) t o pale blue (epsilon). The calculated density is 3.32 g cm(-3), the me asured density is 3.2(1) g cm(-3).