Computer Science
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000m%26ps...35..487n&link_type=abstract
Meteoritics & Planetary Science, vol. 35, no. 3, pp. 487-494 (2000).
Computer Science
9
Scientific paper
The X-ray powder diffraction patterns of 50-100 μm carbon-rich grains from five ureilitic meteorites, Kenna, ALH-78019, Y-82100, Y-791538 and ALH-77257, were obtained by using a Gandolfi camera. The results reveal that the basal spacing of part of the graphite coexisting with diamond is slightly smaller compared to the normal spacing. Compressed graphite is experimentally known to occur at the initial stage of the direct transformation from graphite to diamond structures at high pressures and temperatures. The presence of the compressed graphite in ureilites, therefore, gives clear evidence that the diamond formed by high-pressure conversion of graphite. The modes of occurrence of carbon minerals observed with reflected light through an optical microscope reveal that graphite coexisted with olivine and pyroxene during igneous or metamorphic processes and, furthermore, that part of the graphite was converted to diamond by impact. The relative X-ray intensity of diamond to graphite increases in the following order: ALH-78019 and Y-82100 < Y-791538 < Kenna < ALH-77257, and correlates with the shock level which is estimated mainly based on the shock features of silicates. Therefore, the relative amounts of diamond to graphite suggested by X-ray intensities may be useful as a measure of the degree of shock.
Aoki Yasumichi
Nakamuta Yoshihiro
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