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.
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Photomicrograph of sturmanite crystals. Note elongated hexagonal and terminated
crystal with a length of ~7 mm.
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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.
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Photomicrograph sturmanite crystals. Note nice
termination of gemmy crystal far left.
Width FOV ~1.3 cm.
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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.
REFERENCES
CITED
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.