Fig. 1: Sample displaying mauve stichtite on a matrix of olive-green to black serpentinized ultramafic rock. In favourable lighting, both stichtite and serpentine display linear fabric that may represent slickensides, striations indicative of movement on a fault plane.
Sample 1994. A gift from Joe Arengi, this sample was bought from a dealer operating south of Montreal, at the Bancroft Gemboree in 1999. The specimen weighs 69.72 grams and is 6 x 4 x 2.5 cm in maximum dimensions.
"Rock of the Month # 250, posted for April 2022" ---
Stichtite is a carbonate of chromium and magnesium, a member of the hydrotalcite supergroup, ideal formula Mg6Cr2CO3(OH)16.4H2O. It occurs in two polytypes, of respective hexagonal (barbertonite) and trigonal symmetries. Stichtite takes its name from Robert Sticht, General Manager at the Mount Lyell Mining and Railway Company in Dundas, where the mineral was found at the Adelaide mine (Blackburn and Dennen, 1997, p.256). Worldwide localities (Bernard and Hyrsl, 2004, p.648: see also Ashwal and Cairncross, 1997) include: the Shetland Islands of Scotland (Read and Dixon, 1933); Bou Azzer in southern Morocco; the Megantic mine in Quebec; the Bon Accord deposit, South Africa (in the Barberton Mountain Land: Cairncross and Dixon, 1995); Mount Keith in Western Australia; and Stichtite Hill, Dundas, Tasmania.
The sample is from the Tunnel Hill locality in Tasmania. The Dundas area in Tasmania is famous for production of the red chromate of Pb and Cr, crocoite. Stichtite and serpentine are soft ornamental stones, both well-suited to lapidary purposes. The association with ultramafic rocks, such as chromite-rich serpentinized peridotites, stems from the common occurrence of Cr (in chromite) and Mg (in olivine, and its alteration product serpentine). The breakdown of olivine, in the process of serpentinization, also produces fine-grained secondary magnetite. Thus it is no surprise that the small sample shown here is distinctly magnetic, uncorrected magnetic susceptibility some 25.7x10-3 SI units.
Ashwal and Cairncross (1997) provided the best review of the occurrence of stichtite, a rare carbonate, which they documented from 14 localities worldwide. Stichtite is confined to Cr-rich serpentinites in ophiolites and greenstone belts. Soft and fine-grained with perfect basal cleavage, it possesses physical properties similar to talc, except for the colour. It forms as irregular rounded masses (<1 to 30 cm) and veinlets (<1 to >20 mm wide) in serpentinite. Petrography indicates that stichtite always forms after serpentinization: it may surround relict grains of Cr-rich spinel, or may totally replace euhedral to subhedral chromite. Stichtite growth apparently postdates the ferritchromit alteration of chromite rims, and seems to form by reaction between serpentine and altered chromite. The perfect cleavage adds another similarity to talc, a greasy feel (Mondal and Baidya, 1996).
A few of the other occurrences include:
Though soft, stichtite has the potential to be a fine ornamental stone, with applications in lapidary and jewellery work (Schumann, 2013). The occurrence in Tasmania is well-known (Lancaster, 1980). Thompson (2010) noted that Tasmania has produced an attractive rock traded as atlantisite, a pale yellow-green to orangey serpentine with speckles of purple stichtite.
Both economically and in terms of distribution, chromite is the prime mineral of chromium, found in many magmatic ore deposits, and in a variety of stony, stony-iron and iron meteorites. Other notable Cr-bearing minerals include pyroxene (Cr diopside) and Cr micas such as fuchsite, and the Ca-Cr garnet uvarovite. In all these, Cr serves as a chromophore favouring shades of green, but it may also generate mauve to red hues, as in stichtite and the red gemmy variant of corundum, ruby. Quite a number of rare Cr minerals are known, as in meteorites - an old review (Wilson, 2011) is now ready for a significant update!
Ashwal,LD and Cairncross,B (1997) Mineralogy and origin of stichtite in chromite-bearing serpentinites. Contrib.Mineral.Petrol. 127, 75-86.
Bernard,JH and Hyrsl,J (2004) Minerals and their Localities. Granite, Prague, Czech Republic / Mineralogical Record Bookstore, Tucson, 807pp.
Blackburn,WH and Dennen,WH (1997) Encyclopedia of Mineral Names. Canadian Mineralogist Spec.Publ. 1, 360pp.
Cairncross,B and Dixon,R (1995) Minerals of South Africa. Geol.Soc.S.Africa, 296pp.
Fischer,W, Amosse,J and Leblanc,M (1988) PGE distribution in some ultramafic rocks and minerals from the Bou-Azzer ophiolite complex (Morocco). In "Geo-Platinum 87" (Prichard,HM, Potts,PJ, Bowles,JFW and Cribb,SJ, editors), Elsevier Applied Science, 422pp., 199-210.
Lancaster,KE (1980) Minerals and Gemstones of Tasmania and their Locations. Gemcraft Publications Pty Ltd, East Malvern, Vic., 56pp.
Mandarino,JA (1966) Pyroaurite and stichtite from Langmuir township, Ontario. Can.Mineral. 8, 668-669.
Mondal,SK and Baidya,TK (1996) Stichtite [Mg6Cr2(OH)16CO3.4H2O] in Nuasahi ultramafites, Orissa, India - its transformation at elevated temperatures. Mineral.Mag. 60, 836-840.
Read,HH and Dixon,BE (1933) On stichtite from Cunningsburgh, Shetland Islands. Mineral.Mag. 23, 309-316.
Satterly,J (1977) A Catalogue of the Ontario Localities Represented by the Mineral Collection of the Royal Ontario Museum. OGS Misc.Pap. 70, 464pp.
Schumann,W (2013) Gemstones of the World. Sterling, New York, 5th edition, 320pp.
Thompson,SE (2010) Serpentine and more. Lapidary Journal Jewelry Artist 64 no.5, 32-33, August.
Wilson,GC (2011) Chromite deposits and the natural history of chromium. TGSL Report 2010-10, slide presentation, 50p., 17 January.
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