# Carbon

Element commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with * indicating that it been found in meteorites and impact structures: a. diamond*; b. graphite*; c. lonsdalite*; d. buckminsterfullerene* (C60); e. C540; f. C70; g. amorphous carbon; h. carbon nanotube*.

Carbon has two stable, naturally-occurring isotopes: carbon-12, or 12C, (98.89%) and carbon-13, or 13C, (1.11%), and one unstable, naturally-occurring, radioisotope, 14C (t½ = 5730 y). Variations in carbon isotopic ratio, 13C/12C, are common and often large. They are expressed as d13C (in parts per thousand ‰) relative to the Peedee Belemnite standard (PDB). This material has a higher 13C/12C ratio than nearly all other natural C-based substances and, for convenience, is assigned d13C value of zero. This means that almost all other naturally-occurring samples have negative d13C values.

A basic phase diagram of carbon, which shows the state of matter for varying temperatures and pressures, is shown below. The hashed regions indicate conditions under which one phase is metastable, so that two phases can coexist.

Some or all content above used with permission from J. H. Wittke.

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