Oldhamite
A calcium sulfide mineral, CaS, found in enstatite chondrites and aubrites, which reacts readily with moisture.
Oldhamite is calcium sulfide, sometimes with magnesium and manganese, and is almost unknown in terrestrial rocks. It occurs in highly reduced meteorites, chiefly enstatite chondrites and aubrites, where calcium, which normally enters silicates, was instead bound to sulfur because oxygen was so scarce. It was described from the aubrite Bustee, which fell in India in 1852, and named after Thomas Oldham, first director of the Geological Survey of India.
Oldhamite is a key tracer of formation conditions. It concentrates rare earth elements, and patterns of these elements in oldhamite have been used to argue about whether it condensed directly from nebular gas or crystallised from melts. Norton County and other aubrites contain coarse oldhamite grains, and it is present in enstatite chondrites such as Abee.
Oldhamite is pale, often buff or pale brown, and translucent when fresh. It reacts with water vapour and breaks down, so specimens weather rapidly, releasing hydrogen sulfide with a smell of rotten eggs; samples should not be sniffed or tasted. Preparing sections requires water-free methods, and identification uses reflected-light microscopy and microprobe analysis.
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