DESCRIPTION AND CRYSTAL-STRUCTURE OF VONBEZINGITE, A NEW CA-CU-SO4-H2O MINERAL FROM THE KALAHARI MANGANESE FIELD, SOUTH-AFRICA

Vonbezingite, empirical formula Ca6.03Cu3.07(SO4)2.87(OH)12.46.2.06 H2O, or ideally Ca6Cu3(SO4)3(OH)12 . 2H2O, space group P2(1)/c (pseudo-C2/c), a = 15.122(2), b = 14.358(1), c = 22.063(4) angstrom, beta = 108.68(1)-degrees, V = 4538.3(9) angstrom3, Z = 8, is a new mineral from the Wessels mine in the Kalahari manganese field of northwest Kuruman Hill, Republic of South Africa. It occurs as euhedral crystals associated with sturmanite, azurite, bultfonteinite, gypsum, calcite, barite, and other minerals. Vonbezingite is a deep azure blue and has a light blue streak, a density of 2.81 g/cm3, and a hardness (Mohs) of 4. Optically it is biaxial negative with alpha = 1.590(2), beta = 1.610(3), and gamma = 1.619(2) and strongly pleochroic, eta(gamma) = dark blue, eta(beta) = gray blue, and eta(alpha) = light blue. The five strongest powder diffraction lines are 3.393x(042), 3.120(9)(323), 3.188(7)(043), 3.098(6)(402), and 3.200,(242BAR) angstrom. The structure was solved by direct methods and refined to R = 0.034 using three-dimensional X-ray diffraction data: 3043 independent reflections (F(o) > 3 sigma(F)) were measured. The structure consists of thick heteropolyhedral slabs parallel to (001), in which two Ca polyhedral sheets sandwich a layer of S1 + Cu1 polyhedra. The structural slabs are bonded together by the planar layers of S2 + Cu2 polyhedra at z approximately 1/4 and 3/4. Vonbezingite has a superstructure of disordered S2O4 tetrahedra based on a C2/c pseudocell (b is halved), which is assumed to represent a high-temperature polymorph. H content and bonding have been modeled through coordination geometry and bond-valence sums of 0 atoms; the results match well with the measured density, H2O content, and structure geometry.