"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:

• Serpentinized ultramafic rocks in an upper Proterozoic ophiolite in the central Anti-Atlas of Morocco also display chromite locally altered to stichtite (Fischer et al., 1988).
• The Abitibi greenstone belt near Timmins in northeast Ontario (Mandarino, 1966; Satterley, 1977, p.379) also displays stichtite, though rhombohedral pyroaurite is much more abundant, apparently formed by alteration of magnetite.
• Mondal and Baidya (1996) reported stichtite in ultramafic rocks in Orissa, east India. In the Keonjhar district, stichtite occurs mainly in Cr-bearing serpentinized dunite and peridotite, and in chromite-rich units.

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!