Other
Scientific paper
Jun 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.2629k&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 12, pp.2629-2638
Other
9
Scientific paper
Carbonado diamonds from Central African Republic exhibit very intense laser-induced photoluminescence due to the presence of the radiation damages. From the spectral profiles of photoluminescence obtained for these samples, the carbonado stones were grouped into three groups depending on a variation in the structure of radiation-damage products; Group-A carbonado with 504 nm band (3H band), Group-B carbonado with 575 nm band in addition to weak background peaks centered at 512 and 580 nm, and the Group-AB carbonado with features intermediate between Group-A and Group-B. When heating Group-A carbonado, Group-A was found to transform to Group-B at temperatures higher than 400°C. Thermogravimetry and Differential Thermal Analysis (TG-DTA) clarified the difference in thermal properties between Group-A and Group-B. While Group-A carbonado yields one intense and narrow exothermic peak at 800°C, Group-B carbonado exhibits a broad exothermic peak at 785°C. The distinct difference in the oxidation rate could be attributed to the difference in the density of micropores in carbonado. Chondrite-normalized rare earth element (REE) patterns for the bulk carbonado samples show negative Eu anomalies, and the inclinations of the patterns are similar to those of kimberlites and much steeper than that of continental crust as represented by continental shales. In the chondrite-normalized value, La/Lu ratios were of the order of 10 2 and the light REE abundances were as high as 10 3 relative to chondrite. Further, 1384 cm -1 absorption band was observed in IR (infrared) spectra of both Group-A and Group-B, which suggests the presence of nitrogen platelet in carbonado. These REE and IR results imply that the seed crystal of carbonado was formed at high pressure and high temperature and was subjected to the annealing process to generate nitrogen platelets. The genesis of Central African carbonado is discussed as a reconciliation of these experimental results.
Hidaka Hiroshi
Kagi Hiroyuki
Masuda Akimasa
Takahashi Kazuya
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