Investigation of carbon phases in ureilite meteorites using electron microscopy techniques

Ureilites contain up to 7 % carbon, which is much higher than other meteorites, and most contain higher concentrations of diamond than rocks on Earth. Graphite, amorphous carbon and lonsdaleite have also been reported in ureilites, but the petrogenesis of carbon phases, their crystallinity and structural inter-relationships are strongly debated. Here, we employ advanced electron microscopy techniques to investigate the morphology and microstructure of carbon phases in ureilite meteorites. Diamonds are present as single crystals (up to 20 μm in size) containing nano-FeS crystals, and smaller defective diamonds surrounded by FeS/FeNiS crystals, suggesting that a catalytic effect may be assisting their formation. Two main types of graphite were observed: well-ordered euhedral graphite and nanocrystalline graphite. The nanocrystalline graphite coexists with diamond and lonsdaleite in all ureilites containing these phases, whereas coarse euhedral graphite is only present in ureilites lacking diamond and lonsdaleite, suggesting the former is associated with diamond/lonsdaleite formation. The lonsdaleite was frequently found to contain cubic stacking faults and exhibited an orientation dependence in which the [$210]$ direction of lonsdaleite aligned with the [001] direction of adjacent nanocrystalline graphite. Therefore, given that the lonsdaleite-bearing regions contain folds and kink bands resembling the euhedral graphite, we suggest that lonsdaleite + nanocrystalline graphite formed by replacement of the well-ordered graphite following a catastrophic impact that disrupted the ureilite parent body.