Abstract
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 P21/c(pseudo-C2/c), a = 15.122(2), b = 14.358(1), c = 22.063(4) Å, β = 108.68(1)°, V = 4538.3(9) Å3, 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 biaxiil negative with α = 1.590(2), β = 1.610(3), and γ : 1.619(2) and stronglv pleochroic, ηγ = dark blue, ηβ = gray blue, and ηα = light blue. The five strongest powder diffraction lines are 3.393x(042), 3.1209(323), 3.1887(043), 3.0986(402), and 3.2005(242) Å.
The structure was solved by direct methods and refined to R = 0.034 using three-dimensional X-ray diffraction data: 3043 independent reflections (F0 > 3σF) were measured. The structure consists of thick heteropolyhedral slabs parallel to (001), in which two Ca polyhedral sheets sandwich a layer of Sl + Cul polyhedra. The structural slabs are bonded together by the planar layers of S2 + Cu2 polyhedfa at z ~ ¼ and ¾. Vonbezingite has a superstructure ofdisordered 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 O atoms; the results match well with the measured density, H2O content, and structure geometry.
