In situ observation, X-ray diffraction and Raman analyses of carbon minerals in ureilites: Origin and formation mechanisms of diamond in ureilites

Carbon minerals in sixteen monomict ureilites which are variable in shock features in olivine were observed in situ by optical microscopy, scanning electron microscopy, X-ray powder diffraction, and Raman analyses. Euhedral blade-like shaped or amoeboid shaped graphite crystals occur in the very low-shock ureilites, not associated with diamond. In the low, medium, and high-shock ureilites, diamond occurs together with graphite. X-ray diffraction and Raman analyses reveal that crystallinity of diamond is relatively well in the low-shock ureilites, although it varies in a wide range in the high-shock ureilites. Diamond formation in the low-shock ureilites occurred as a result of solid-state catalytic transformation of graphite at a pressure lower than 10 GPa, with metallic iron as the catalyst, while diamond in the high-shock ureilites formed by spontaneous transformation of graphite together with the catalytic transformation at a pressure higher than 12 GPa. The relative crystal-axes orientations between graphite (Gr) and diamond (Di) determined by EBSD analyses in the low- shock Y-8448 ureilite is the same as that observed for graphite and diamond in a carbon grain from the high- shock Goalpara ureilite. The metallic iron catalyst promoted the sliding and puckering of the hexagonal carbon planes of graphite in the solid-state transformation processes of graphite to diamond after intercalating into its interplanar spaces. It is shown that noble gas systematics in ureilites can be matched with the shock formation origin of diamond in ureilites.