OCCURRENCE AND IMPLICATIONS OF SILICON-NITRIDE IN ENSTATITE CHONDRITES

Silicon nitride, Si3N4, has previously been observed to be a common co nstituent of acid residues of Qingzhen (EH3) and Indarch (EH4). Ion pr obe analysis of the Si, N and C isotopic compositions of individual Si 3N4 grains from Qingzhen and Indarch acid residues suggest most, if no t ail, grains are Solar System in origin. A few grains have isotopical ly anomalous C but this is probably due to small presolar SiC grains a dhering to them. In situ observations of the Si3N4 in Qingzhen show th at it is only present within, and probably exsolved from, host phases which contain elemental Si in solid solution. Thermodynamic calculatio ns suggest that the Si3N4 probably formed during metamorphism and not in the nebula. Thermodynamic calculations also show that sinoite (Si2N 2O) and not Si3N4 should be the stable phase during metamorphism. It a ppears that kinetic factors must have inhibited the formation of sinoi te in Qingzhen and Indarch.