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Infrared spectroscopy study of nitrogen centers in microdiamonds from Ukrainian Neogene placers

¹ Institute of Geochemistry, Mineralogy and Ore Formation, National Academy of Sciences of Ukraine, Palladin Ave. 34, 03680 Kyiv-142, Ukraine
² Institute of Applied Geosciences, Technical University of Berlin, D-10623 Berlin, Germany

^* Corresponding author, e-mail: taran{at}mineral.freenet.kiev.ua

Microdiamonds from two different Ukrainian Neogenic placer deposits, Samotkan and Zeleny Yar, and from kimberlite pipe Udachnaya, Yakutia, Russia, were studied by FTIR absorption spectroscopy. The spectra were normalized to sample thickness of 1 mm, background corrected and analyzed by curve fitting with Peakfit 4.0 software. For this purpose, four user-defined functions which simulate the absorption of the nitrogen-bearing-centers A, B, C and D were synthesized using each seven to nine Gaussians or Lorentzians out of the list of spectroscopic functions of the Peakfit 4.0 program.

One result was that with respect to the IR active forms of nitrogen, most diamonds from Ukrainian Neogenic deposits are nitrogen-free. The rare nitrogen-bearing crystals from Samotkan (25% of total amount studied) contain nitrogen mainly in the A-form (pairs of adjacent nitrogen ions in regular sites of the structure) and, partly, in the C-form (single atoms randomly dispersed over regular sites of the diamond structure). This indicates either a relatively low temperature or a relatively short residence time of such crystals under Mantle conditions. The rare ( 10% of total) nitrogen-bearing crystals from Zeleny Yar contain nitrogen mainly in the A- and B-forms (four nitrogen atoms arranged around a carbon vacancy) with predominance of B. According to existing theory this indicates higher temperatures or longer Mantle residence times of such diamonds compared with those from Samotkan.

Most of the Yakutian diamonds studied (95%) contain nitrogen-bearing defects. The curve fitting analysis shows a wide range of N_B/N_total ratios which suggests a wide range of Mantle residence temperatures. With respect to the temperatures, the Yakutian diamonds may be regarded as intermediate between those from Samotkan and Zeleny Yar.

Key-words: diamond, nitrogen impurities, infrared spectroscopy, curve fitting analysis.