RECONDITE INTERSTELLAR CARBON COMPONENTS IN THE ALLENDE METEORITE REVEALED BY PREPARATIVE PRECOMBUSTION

Prolonged combustion (preparative precombustion) at low temperatures (390-450°C and 450-510°C) removes large quantities of isotopically normal carbon from an acid (HF/HCl) residue of the Allende meteorite to reveal two isotopically heavy carbon components. One of them, called Cא (carbon aleph) because of its possible relationship to Cαthe host of Ne-E(L), has δ13C of+345א. and a combustion temperature of 700-750°C. It is believed that the presence of Cא is masked in the bulk acid residue because of a "tailing effect" in the combustion of less stable forms of carbon. The second heavy carbon component burns over the temperature regime 900-1000°C and has a δ13C of >527א. This component called Cκ (carbon kappa) is distinguishable from the well known types of heavy carbon called Cβ and Cε{lunate} the hosts of s-Xe and NeE(H), respectively, and now recognised as forms of SiC. Cκ, however, could also be a form of very poorly crystalline SiC; it appears to be released from the Allende acid residue because the precombustion slowly degrades a secondary host mineral, which is possibly a variety of spinel. Likewise, a component of light carbon, possibly Cθ, poorly crystalline graphite, is also liberated by the precombustion reaction after between 52 and 108 h of treatment. Presumably Cθ too is located within a specific type of spinel. The discovery of C%, Cκ, and Cθ, together with Cλ, a component very rich in 12C (ASH et al. 1988a), confirms that primitive meteorites are a prolific source of interstellar grains. Preparative precombustion demonstrates that the legacy to be found in Allende is far more complicated than at first envisaged from the noble gas record of this meteorite. © 1990.