Electron petrography of shock effects in a gas-rich enstatite-achondrite

Abstract

The enstatite-achondrite Khor Temiki has been studied by high-voltage electron microscopy. The “normal” Khor Temiki lithology has a fine-grained matrix in which individual grains show the well-known effects of unshielded solar irradiation. Intensity of deformation varies greatly from grain to grain; this material has a varied history of impact deformation, and must have formed in an environment similar to that of the lunar regolith. The meteorite is traversed by veins of “extra-dark” material. This was produced in situ from the normal lithology by intense shock, sufficient to erase its irradiation record almost completely. Instead of the enstatite that dominates the bulk meteorite, optically twinned clinoenstatite is found to be the major mineral in such a shock-vein. It is highly defective, and its electron diffraction patterns contain diffuse “orthopyroxene maxima”. It is interpreted in terms of inversion of protoenstatite produced by the thermal effect of the shock. Recrystallization phenomena, and the occurrence of enstatite in close association with the clinoenstatite, are described. This occurrence of twinned clinoenstatite is contrasted with those due to quenching of primary protopyroxene in small bodies such as chondrules. The effects of shock have eliminated porosity from the vein material, and indurated it. Less severe such effects, outside the veins themselves, must have contributed to the lithification of the meteorite.