Colinowensite, BaCuSi2O6, a new mineral from the Kalahari Manganese Field, South Africa and new data on wesselsite, SrCuSi4O10

摘要

A new silicate, colinowensite, BaCuSi2O6, has been found in the Wessels mine, Kalahari Manganese Field, South Africa. It is associated with effenbergerite, wesselsite, lavinskyite, scottyite, diegogattaite, as well as with pectolite, quartz, aegirine, richterite, minerals of the garnet group and a number of different manganese and iron oxides, especially hausmannite and hematite. The mineral was named for the mineral collector and finder of the new species, Colin R. Owens, of Somerset West, South Africa. Colinowensite is brittle, with uneven fracture, and the estimated Mohs hardness is similar to 4. It occurs as subhedral crystals < 100 mu m in size. The forms {100} and {110} are observed while {001} is always present in cleavage plates. The calculated density is 4.236 g cm(-3). It is the natural analogue of the synthetic pigment referred to as Chinese or Han purple, which is found on artifacts from ancient and imperial China. The mineral is of dark blue to purple colour, with a purple streak, and is uniaxial (-), with omega = 1.740 (20), epsilon = 1.735 (20) (420 nm) and omega = 1.745 (20), epsilon = 1.730 (20) (650 nm). The lustre is vitreous and no fluorescence is observed under either shortwave or longwave ultraviolet radiation. A very strong pleochroism occurs from purple along the c axis to blue in a perpendicular direction. Colinowensite is not soluble in acids except HF. Electron microprobe analyses gave an average composition (wt.%) of CuO 22.53, BaO 43.43 and SiO2 34.04 yielding the empirical formula (based on 6 O a. p.f.u.) BaCuSi2O6. The new mineral is tetragonal, space group I4(1)/acd with Z = 16, and a = 9.967(1), c = 22.290 (2) angstrom. Colinowensite is a cyclosilicate with [Si4O12](8-) 4-membered single rings, arranged in sheets parallel to ( 001). The structure is further characterized by CuO4 squares sharing corners with four neighbouring silicate rings within a sheet. Ba2+ cations are bonded to ten O atoms in irregular coordination. Average Si-O, Cu-O and Ba-O bond lengths are 1.619, 1.934 and 2.943 angstrom, respectively. Colinowensite belongs to subdivision 9. CE of the Strunz Mineralogical Tables. In addition, based on single-crystal X-ray work, new structural data for wesselsite of chemical composition Sr0.9Ba0.1CuSi4O10 are provided.