Interstellar graphite in meteorites

IN a continuation of our search for interstellar 'needles' in the meteoritic 'haystack'^1,2, we have now identified graphite grains, 1-4 μm in diameter, in the Murchison C2 chondrite. The interstellar origin of these grains is demonstrated by their ¹²C/¹³C ratio, which ranges from 0.09 to 16 times the Solar System value, and by the presence of the noble-gas component 'Ne-E(L)', nearly monoisotopic ²²Ne from the decay of ²²Na (with a half-life of 2.6 yr). The grains apparently formed in the outflows of novae and red giants, and demonstrate that graphite can form as a circumstellar condensate. Curiously, interstellar graphite is much rarer than interstellar microdiamonds (<2 p.p.m. compared to 400 p.p.m.) or even SiC (6-9 p.p.m.), although diamond is thermodynamically unstable relative to graphite. The main reason may be preferential destruction. Graphite is the third type of circumstellar grain that has become available for laboratory study.