Stichtitic serpentinite (8.6 cm across) from Tasmania. Greenish = serpentine; purplish = stichtite; black crust near center = magnetite; tiny black spots within stichtite masses = chromite.

This is the prettiest ophiolite rock I’ve yet encountered. It is a serpentinite with nice, purplish- to lilac-colored masses of stichtite, a rare hydrous magnesium chromium hydroxy-carbonate mineral (Mg6Cr2CO3(OH)16·4H2O). This rock comes from Stichtite Hill in western Tasmania, the type locality for stichtite. The host rock is chromitic-magnetitic serpentinite from the Dundas Ultramafic Complex, a Cambrian-aged ophiolite that's been cut up by thrust faults. Before metamorphism, the serpentinite was a chromitic dunite (as were several of the rocks in this ophiolites photo album).

The origin of stichtite is not yet completely resolved. The stichtite masses have tiny black specks of chromite, which was a component of the original dunite. What's seems to be clear is that the stichtite is derived from the chromite by hydrothermal metamorphism. The details of the alteration process are still discussed in the literature. The timing of stichtite formation is also unclear. Several orogenic and thermal events affected the rocks of western Tasmania during the Devonian, Jurassic, Cretaceous, and Cenozoic. Stichtite formation could have accompanied any one of those events.

Ophiolites are fragments of oceanic lithosphere (basaltic crust + uppermost mantle) that have been metamorphosed and plastered onto the edges of continental lithospheric plates by obduction (the opposite of subduction).

Stichtite info. synthesized from:

Ashwal & Cairncross (1997) - Mineralogy and origin of stichtite in chromite-bearing serpentinites. Contributions to Mineralogy and Petrology 127: 75-86.