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Stardust in the TEM

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DOI: 10.1016/0304-3991(89)90008-9


Abstract We report here high resolution TEM and diffraction studies of two phases, diamond and silicon carbide, recently discovered in dissolution residues from two carbonaceous meteorites (Allende and Murray). Isotopic anomalies of Xe, Ne, C, N, and Si in the residues indicate a presolar origin for these phases in carbon-rich circumstellar environments. Lattice column images and azimuthally-averaged diffraction ring profiles confirm previous reports of the fcc diamond structure, but ring broadening indicates that the number-average crystallite size is ≈ 1 nm. The diamond crystals were therefore likely quenched from the solid phase, or nucleated from the vapor phase under high supersaturation conditions, and given little chance to grow thereafter. The silicon carbide crystals, ranging in size typically from tens of nm to several μm, are fcc β-SiC crystals. High resolution images show frequent 111; twins in the form of 2 nm laths on the twin plane, and occasional “bulk” twins with boundaries near perpendicular to the plane of twinning. Neither dislocations or facets associated with growth, nor structural damage associated with exposure to radiation, are evident from examinations do far. Size distributions for SiC and diamond show little overlap, and thereby cluster only at opposite ends of the 1 μm to 5 nm range observationally associated with dust between stars.

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