THE DESERT
Hot Kalahari
Mining dark black
manganese
Finding pink
olmiite
This
last fall I was able to purchase, at a great price, several back issues of
Rocks and Minerals, issues starting before my subscription begin. I enjoy, immensely, reading the journal from
cover to cover. The May/April 2012 issue
had, as its cover photo, a specimen of olmiite, a newly discovered mineral
(2007) from the N’Chwaning Mine II, Kalahari manganese fields, Republic of
South Africa. I thought the mineral and photo were pretty spectacular and decided to look for such while touring
the Tucson Shows in February. I was
always peering at the different booths and tables and was about ready to admit
defeat when there it was, olmiite in a perky box on a table with many other
thumbnails. The proprietor told me he wasn’t
certain what olmiite was except a manganese mineral. He originally had two specimens but had sold
one and since the show was drawing to a close I could have it for half price,
$7. I grabbed it out of his hand and
handed him the cash. At least it seemed
like a good deal to me.
The photo that started it all!
I
was interested in the mineral, not only for its good looks, but also due to its
rarity. According to MinDat most olmiite
specimens on the market have from the N’Chwaning Mine II, the Type Locality,
but a few specimens are known from the nearby Wessels Mine (four photos in
MinDat) and N’Chwaning III Mine (one photo in MinDat). Olmiite is also listed
by MinDat as being found in the Långban Mine (no photos) in Sweden. For a little interesting trivia, note that
the iron ore and manganese mining at Långban has produced ~300 different minerals and is the Type
Locality for ~60!
The
Kalahari, home to the N’Chwaning and associated mines is
the world’s most prolific producer of manganese (and also home to a famous
desert). 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).
Olmiite, CaMn++[SiO3(OH)](OH), is best known for its reddish pink color (pale to intense) but also occurs in various shades of raspberry, honey, pinkish tan, white to gray, while the crystals at N’Chwaning III are gemmy clear and some at Wessels are translucent clear. However, the reddish pink is the color that sticks in the minds of collectors.
Olmiite with white crystals of bultfonteinite. Width FOV ~1.3 cm.
Orthorhombic olmiite, a product of hydrothermal alteration, is transparent, has a vitreous luster and a white streak; hardness is ~5.0-5.5 (Mohs). Crystal habits include prismatic, spherical radial bundles, crystal sprays and what has been termed wheat-sheaf-like aggregates. The olmiite in my collection, as do most other olmiite specimens, has a deep red fluorescence under short wave UV.
My specimen of olmiite, as do many others, is associated with a little-known
mineral (at least to me) named bultfonteinite, a
fluorine bearing calc-silicate---Ca2(HSiO4)F-H2O.
Calc-silicates are rocks, or their composing minerals (common minerals include
diopside, wollastonite, the “garnets” grossular-andradite, and epidote) that
usually form in high-temperature, contact metamorphic zones where a mafic magma
(high magnesium and iron content, low silica content) intrudes into limestone
or other carbonate rocks, as in a skarn. The Type Locality of bultfonteinite is at the famous diamond-bearing kimberlites
in present-day Kimberley, Northern Cape, South Africa. Bultfonteinite was
first found in a large, isolated block of dolerite (an igneous rock like
basalt)) and shale fragments that were enclosed in a kimberlite pipe. These
pipes consist of an igneous rock known as peridotite (lots of iron, silica, olivine,
amphibole, magnesium) that form deep within the earth’s mantle and then
rapidly and violently eject to the surface. Evidently the ejecting pipe
picked up this large hunk of dolerite and shale on its way to the surface. In the U.S. bultfonteinite is uncommon but
is found in the famous calc-silicate rocks from the Crestmore Quarries in
California.
Prismatic acicular crystals of bultfonteinite associated with olmiite shown above. Width FOV: top (spray) ~1.5 mm, middle ~1.1 cm, bottom ~1.0 mm.
Bultfonteinite is usually found as small, transparent, colorless to pale pink or white, radiating prismatic acicular crystals. Crystals are vitreous, have a white streak and a hardness of ~4.5 (Mohs). They seem to look like many other white acicular globs of crystals I have seen since beginning my quest for nifty micromounts and thumbnails.
REFERENCES
CITED
Cook, R. B., 2012, Connoisseur’s Choice, Olmiite: Rocks and
Minerals, Vol. 87, No. 2.
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.
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