A few months ago I was working on a post describing kutnorhorite, a carbonate that was not, at that time, in my vocabulary [Ca(Mn,Mg,Fe)(CO3)2]. The mineral had been collected in the Kalahari Manganese Field, Northern Cape Province, South Africa, a locality I vaguely recognized. However, in reading additional articles about the field I became duly impressed for it is the world’s most prolific producer of manganese. The deposits are in the Hotazel Formation of Proterozoic age (Precambrian) and are the largest land-based (deep sea deposits are larger but nearly impossible to mine) sedimentary manganese deposits in the world, perhaps covering ~425 sq. miles. The ore has been subjected to both hydrothermal alteration (temperatures up to 450°C) and to metamorphism. The origin of the giant manganese deposits has been debated for many years but remain controversial: “Proposed models cover a diverse spectrum of genetic processes, from large-scale epigenetic replacement mechanisms, to submarine volcanogenic-exhalative activity, to purely chemical sedimentation whereby the influence of volcanism is of reduced significance” (Tsikos and Moore, 2006).
Another impressive fact about the Kalahari is that the mining region has produced some of the world’s finest rhodochrosite and hematite, as well as the finest ettringite, sturmanite, olmiite, sugilite, inesite, hausmannite, and thaumasite. Two of these mines (Wessels and N’Chwaning II) are together type localities for 19 species, including diegogattaite, wesselsite, lavinskyite, manganvesuvianite, marshallsussmanite, and sturmanite (www.mineralspecies.com). Impressive, correct?
I did not recognize most of those mineral names except sugilite and sturmanite, the former for the lapidary qualities and the latter for the nice crystals in a specimen purchased a few years ago.
Photomicrograph of sturmanite crystals. Note elongated hexagonal and terminated crystal with a length of ~7 mm.
Sturmanite is a hydrous calcium aluminum iron sulfate with some boron [B(OH)4] ions substituting for some of the sulfate ions: [Ca6(Fe3+,Al,Mn3+)2(SO4)2[B(OH)4](OH)12-25H2O]. It often occurs in sharply terminated crystals (dipyramidal tabular to elongated hexagonal crystals according to MinDat) that are nice looking amber to bright or dark yellow in color. Crystals are transparent to translucent, mostly vitreous, and fairly soft at 2.5 (Mohs). Many are gemmy. Outside of the South Africa mines MinDat noted only one other occurrence—in the Northern Caucasus Region in Russia. It is a metamorphic mineral found in secondary cavities in stratified manganese deposits.
Photomicrograph sturmanite crystals. Note nice termination of gemmy crystal far left. Width FOV ~1.3 cm.
So, I was all set to go with this description when I was browsing one of my favorite web sites, www.dakotamatrix.com, and noted a specimen of sturmanite was listed “for sale” under the July 3 “Mixed Minerals” banner. Out of curiosity I clicked and was rewarded with the following information gleaned by Tom: I learned by Malcolm Southwood's blog that it is virtually impossible to distinguish between members of the Ettringite group…Furthermore, most of the minerals in this five amber group are very hydrated so decomposition occurs and there is also compositional zoning… Crystal habit and color alone will not do it.
I could not locate the Southwood Blog but the Ettringite Group consists of 12 (www.mindat.com) hydrated calcium sulfate hydroxide minerals with a common trigonal structure (www.galleries.com/galleries). But I still would like to call my specimen sturmanite.
Tsikos, H. and J.M. Moore, 2006, The chemostratigraphy of a Paleoproterozoic MnF- BIF succession -the Voelwater Subgroup of the Transvaal Supergroup in Griqualand West, South Africa: South African Journal of Geology, v. 109.