Pregraphitic and poorly graphitised carbons in porous chondritic micrometeorites

Abstract

Two forms of crystalline carbon in porous chondritic micrometeorites W7029E5, U2011C2, and U2022C7/C8 are mixed layered, pregraphitic carbons with C (C+H+O+N) (at%) ≈ 0.45. This bulk composition is inferred from the presence of three groupings of d 002 basal spacings at 0.45 nm, 0.37 nm, and 0.348 nm. Mixed layered carbons represent incomplete carbonisation and graphitisation of precursor material. In U2022C7/C8, carbonisation mostly involved volatile loss. The formation of pregraphitic carbons indicates a sustained thermal regime in parent bodies of these micrometeorites (i.e., short-period comets, outer-belt asteroids, or protocomet nuclei). Temperatures of the sustained thermal regime remain unspecified, but carbon reactions were probably facilitated by catalytic support from layer silicates in these samples. Poorly graphitised carbon in U2022C7/C8 formed during a transient thermal event which is most likely flash-heating during micrometeoroid deceleration in the Earth's atmosphere.