An experimental investigation of oldhamite, CaS; and the petrologic significance of oldhamite in meteorites

Abstract

Oldhamite, a mineral found only in enstatite-rich stony meteorites, was studied under controlled atmospheric and temperature conditions over the pO2 range 10−15 to 10−10 atm and at temperatures between 800° and 1000°C. A maximum upper stability limit was determined by studying the reaction 12CaS + O2 = 12CaSO4. The results are expressed by the equation log pO2 = 8.58 − 25,350T. These results along with previous studies may be used to estimate (1) the temperature at which a meteorite last equilibrated and (2) the composition of the coexisting gas phase during the formation of enstatite chondrites. For the Jajh deh Kot Lalu enstatite chondrite in which the composition of the phases has been determined accurately, the temperature at which this chondrite last equilibrated is estimated to be 720 ± 140°C. The presence of CaS, graphite, metallic Si, and Fe poor pyroxene indicates that the coexisting gas phase during the formation of enstatite chondrites strongly resembled a gas of cosmic composition. However, relative to a gas of cosmic composition, the required gas phase must have had a somewhat lower H2OH2 ratio and a higher H2SH2O ratio. Several processes by which such a gas could evolve from a gas of cosmic composition are considered, and it is suggested that a slight CO fractionation may have occurred in certain regions of the solar nebula.