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

Author(s)

Branko Rieck, Helmut Pristacz, Gerald Giester

Abstract

A new silicate, colinowensite, BaCuSi

₂O

₆, 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 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 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 SiO

₂ 34.04 yielding the empirical formula (based on 6 O a. p.f.u.) BaCuSi

₂O

₆. The new mineral is tetragonal, space group I4

₁/acd with Z = 16, and a = 9.967(1), c = 22.290 (2) Å. Colinowensite is a cyclosilicate with [Si

₄O

₁₂]

^8- 4-membered single rings, arranged in sheets parallel to (001). The structure is further characterized by CuO

₄ squares sharing corners with four neighbouring silicate rings within a sheet. Ba

²⁺ 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 Sr

_0.9Ba

_0.1CuSi

₄O

₁₀ are provided.

Organisation(s)

Department of Mineralogy and Crystallography

Journal

Mineralogical Magazine

Volume

79

Pages

1769-1778

No. of pages

10

ISSN

0026-461X

DOI

https://doi.org/10.1180/minmag.2015.079.7.04

Publication date

12-2015

Peer reviewed

Yes

Austrian Fields of Science 2012

105116 Mineralogy, 105113 Crystallography

Keywords

ASJC Scopus subject areas

Geochemistry and Petrology

Portal url

https://ucrisportal.univie.ac.at/en/publications/b4487693-031f-4803-9ae2-ae33946b65e1