Murchison carbynes

Another recent interesting finding is that previously unknown organic polymers or "amorphous carbon," which are noble gas carriers in meteorites, are actually carbynes. Five different carbynes have been identified in the Murchison and Allende carbonaceous... 

Carbon in the carbonaceous chondrites does not exist as polymer or organic molecules alone. Carbonates are also present in relatively small amounts 20,23) and the same is true for elemental carbon. Elemental carbon seems to exist as carbynes (triple-bonded allotropes of carbon). At least three types of carbynes have been described in Murchison 341 but these results were questioned in 1982 by Smith and Buseck63). According to these authors, sheet silicates mixed with elemental carbon could be misidentified as carbynes in X-ray diffraction patterns. These particular carbonaceous phases (carbynes or otherwise) and other carbonaceous phases (polymer and amorphous carbon phases called C-oe and C- 3) are carriers of noble gases trapped in the chondritic material. Some of these carbynes seem to be condensates from the protosolar nebula while others are probably of presolar origin34 >. 

At least 4 carbynes — carbon VI ( ), VIII, X, and XI — have been identified in Allende on the basis on x-ray diffraction (Whittaker et al., 1980). Ion microprobe data (specifically, the predominance of even-numbered carbon fragments) suggests that at least 80% of the acid-insoluble carbon fraction is carbyne. Some of the carbynes are thermally labile, and at 250-330 °C give off various fragments, in-clpding 2 ( = 1-5) and (CsC) CN (n = 1-3). Smaller amounts of carbynes have been found in the Murchison C2 chondrite (Whittaker et al., 1980), where they are masked by the abundant organic polymer (Hayatsu et al., 1977). 

Yet another HRTEM study of a Murchison acid-resistant residue (MIL) showed, for the first time, carbyne in a petrographic context. An amorphous carbon matrix contained scattered 1. single-crystal cubic nanodiamonds (3-4 nm) identified by the 0.2 nm (111) /-spacing and in polycrystalline ("ring ) electron diffraction patterns ( 5% error of measurement), and... 

Murchison and Allende meteorites contained a substantial number of individual crystalline carbyne grains [21] was almost certainly incorrect. In yet another ultrathin section and acid-resistant residue of the Murchison meteorite C=C functional groups were identified but they were linked to aromatic carbons [61]. The only reliable evidence for carbyne (i.e. chaoite) in meteorites [20] suggests it formed in situ by solid-state carbon annealing that is an acceptable geological process. A tenet of cosmochemistry is that all solids initially formed by condensation from a cooling vapor phase. Whether condensation proceeded at (near) thermodynamic equilibrium, kinetically controlled, metastable equilibrium, or a combination, remains open to debate. Kinetically controlled, metastable condensation is likely for silicates [6,79] and was proposed for carbyne condensation from interstellar polycyanoacetylenes [80]. 

S4 [90,91] (Table 16.2). A typical comet composition is based on the relative amounts of the different gas species measured in the coma of comets from the Oort cloud, an immense spherical cloud surrounding the solar system and extending from approximately 5000 to 100,000 AU. About half of the short-period comets arc from the Kuiper Belt, a disk-shaped region containing many small icy bodies extending beyond Neptune s orbit from 30 to 50 AU, which are depleted in carbon-chain molecules. The other short-period comets have the typical composition of Oort cloud comets. So far, carbyne molecules detected in the coma of active comets are very short chains unlike the longer chains characteristic of natural carbyne crystals such as chaoite in the Murchison meteorite [20,67]. 

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