Occurrence and implications of silicon nitride in enstatite chondrites

Silicon nitride, Si 3 N 4 , has previously been observed to be a common constituent of acid residues of Qingzhen (EH3) and Indarch (EH4). Ion probe analysis of the Si, N and C isotopic compositions of individual Si 3 N 4 grains from Qingzhen and Indarch acid residues suggest most, if not all, grains are Solar System in origin. A few grains have isotopically anomalous C but this is probably due to small presolar SiC grains adhering to them. In situ observations of the Si 3 N 4 in Qingzhen show that it is only present within, and probably exsolved from, host phases which contain elemental Si in solid solution. Thermodynamic calculations suggest that the Si 3 N 4 probably formed during metamorphism and not in the nebula. Thermodynamic calculations also show that sinoite (Si 2 N 2 O) and not Si 3 N 4 should be the stable phase during metamorphism. It appears that kinetic factors must have inhibited the formation of sinoite in Qingzhen and Indarch.