Wesselsite, SrCu[Si4O10], is a new mineral species from the Wessels mine, Kalahari Manganese Field, South Africa, and it belongs to the gillespite group. Wesselsite is tetragonal, space group P4/ncc; the unit cell parameters, refined from Gandolfi film data, are a = 7.366(1), c = 15.574(3) Å V = 845.01 Å3. The strongest lines are (dobs/lobs/hkl) (7.79/35/002), (4.33/20/112), (3.89/20/004), (3.44/40/104), (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 hennomartinite, embedded in a matrix of sugilite, xonotlite, quartz and pectolite. Microprobe analyses of 111 samples show that it is the end-member of a solid solution series with effenbergerite, BaCu[Si4O10], with substitutions of Sr by Ba up to 50 mol.%. Wesselsite forms tiny subhedral plates in sizes not exceeding 50 × 50 × 5 µm, arranged in clusters of up to 200 µm. It shows a perfect cleavage parallel to {001}, has blue colour, white to light blue streak, and is uniaxial negative with ω = 1.630(2), ε = 1.590(5), strongly pleochroic from blue (ω) to pale blue (ε). The calculated density is 3.32 g cm−3, the measured density is 3.2(1) g cm−3.

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 ∼4. It occurs as subhedral crystals <100 μm 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 pigmentreferred 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 ω = 1.740 (20), ε = 1.735 (20) (420 nm) and ω = 1.745 (20), ε = 1.730(20) (650 nm). The lustre is vitreous and no fluorescence is observed under either shortwave or longwave ultraviolet radiation. Avery 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 I41/acd with Z = 16, anda = 9.967(1), c = 22.290 (2) Å. 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 Å, 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.