Origin of opaque minerals in an unequilibrated enstatite chondrite, Yamato-691

This paper describes the petrographic and chemical features of opaque minerals in an unequilibrated enstatite chondrite, Yamato-691,and discusses the genetic relations among opaque minerals. Most of them occur in and/or around Fe-Ni metal. Opaque minerals formed from the primary metal alloy through various reactions such as exsolution and reaction with surrounding sulfur gas. The sequence of the formation is graphite, schreibersite, troilite with perryite and sphalerite in metal alloy. Niningerite and oldhamite formed independently and attached to the metal alloy at the high temperatures. The primary metal alloy had condensed in the nebula under a very low oxygen fugacity condition, and included P, Si, Cr, Mn and Ti, in addition to Fe, Ni and Co. The condensation sequences of minerals calculated previously under the oxygen depletion condition agree with the results obtained here in general. Fe-Ni metal frequently contains silicate inclusions which are mostly silica minerals. They formed later through the oxidization process of Fe-Ni metal under slightly more oxidizing conditions than the primary stage