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) include: the Shetland Islands of Scotland; Bou Azzer in southern Morocco; the Megantic mine in Quebec;the Bon Accord deposit, South Africa; 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.
To be added: with notes concerning locality, mineral assemblage, genesis.
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.
Graham Wilson, draft version posted 22-23,25 November 2021
See the Rock of the Month Thematic Index
or, visit the Turnstone "Rock of the Month" Chronological Archives!