Oldhamite

Abstract: Fifteen of the sixteen known enstatite chondrites were studied microscopically in reflected and transmitted light, and their modal compositions were determined by point-counting techniques. Compositions of clinoenstatite, orthoenstatite, plagioclase, kamacite, taenite, troilite, oldhamite, daubreelite, niningerite, ferroan alabandite, and schreibersite were determined with the electron microprobe X-ray analyzer. Chemical composition, mineral occurrence, and mineral composition were found to depend on degree of recrystallization of the chondrites as judged by, for example, distinctness of chondrules and coarseness of silicates. On the basis of these parameters, three groups of enstatite chondrites can be distinguished and are referred to as type I, intermediate type, and type II. Differences between types I and II are pronounced, whereas intermediate type is transitional. The suggestion of Van Schmus and Wood that type II enstatite chondrites originated from type I by reheating is reviewed in the light of the new data. It is concluded that, although many of the chemical-mineralogical parameters of type II chondrites could be explained as being the result of reheating of type I chondrites, there are some that would require rather stringent environmental conditions during reheating. For example, lower iron and sulfur contents in type II chondrites would presumably require reheating of type I chondrites to ≥975°C, the lowest temperature at which a melt would appear in the Fe-Ni-S system of type I composition and at which physical separation of the liquid from the silicates could occur. Differences in Si/Mg ratios would require reheating to even higher temperatures and fractionation in an open system. Furthermore, observed differences in nitrogen and sinoite contents between type I and type II are difficult to explain unless reheating took place in a closed system, or under oxygen fugacities low enough to allow nitrogen to react with SiO2 and Si to form Si2N2O. An alternative model to the one by Van Schmus and Wood is discussed; it assumes that major differences in chemical and mineralogical composition between type I and type II were essentially established before or during chondrule formation and agglomeration by, for example, igneous differentiation or fractionation during condensation from a solar nebula, and that differences in texture are due either to different cooling rates of type I and type II chondrites during and after agglomeration of chondrules or to mild reheating to temperatures ≤975°C. This model does not, however, readily explain why only enstatite chondrites of type II bulk chemical composition (i.e. low Fe, S) cooled slowly or were reheated, and why chondrites of type I composition (high Fe, S) were always quenched to temperatures low enough to prevent recrystallization and were not reheated.