Carbon — How Many Allotropes Associated with Meteorites and Impact Phenomena?

Abstract

Virtually all the allotropic forms of carbon known naturally have been at some time or another recognised within meteorites or as a part of phenomena associated with meteorite impacts. Thus, three types of graphite the well known cubic and rhombohedral, plus a unique cubic form (cliftonite) have been encountered. There are several reported occurrences of buckminster-fullerene and its siblings. Diamonds are amongst the most interesting form of carbon in meteorites, their incidence having been noted for as long as the mineral was known to be found on Earth in kimberlites. Both the cubic and hexagonal (Lonsdaleite) forms are seen, the latter almost always denoting the advent of shock conditions. A particularly unusual type of crystal, which has been found to be ubiquitous in primitive unmetamorphosed chondrites, consists of nanometre sized grains. Originally the acid resistant residues which have transpired to be diamond were first thought to be the sp carbon allotrope generically known as carbyne. This identification proved to be highly controversial, but chaoite, one of the initial carbyne candidates, was definitely located in the impact breccia suevite found at the Ries Crater, S. Germany. Tiny crystals of chaoite seem to be elusive grains coexisting with meteorite nanodiamonds where they give EELS data which demonstrate they are different from other types of carbon. Molecules with multiple triple bonds as the main structural unit are common in the interstellar medium.