The change of Fe-Ni alloy inclusions in synthetic diamond crystals due to annealing

The influence of annealing under high P-T parameters on the inclusions of Fe-Ni alloys in synthetic diamond crystals was studied. The experimental annealing of the grown diamond crystals was held under a pressure of 6.5-7.0 GPa, at temperatures of 1700-2200 degrees C and the duration of the experiments was 1-30 h. Depending on the annealing temperature, the color of diamonds changed, along with the corresponding variations of their IR spectra, i.e. diamond of Ib type transformed into Is type. The form of metal inclusions, entrapped by diamonds, varied, they experienced change of their faces with their orientation in one direction, in accordance with the temperature gradient in the reaction cell. Besides, the migration of the inclusions of the Fe-Ni alloy within diamond volume in the direction of higher temperatures took place. The mechanism of metal inclusions' migration in diamond as a result of recrystallization in the gradient field of temperatures is considered. The assumption about possible participation of carbon-containing melt of transition metals at the early stage of natural diamonds crystallization with further recrystallization and self-purification of diamonds from the transition metals' inclusions during the long period of their existence in the Earth's mantle at high temperatures and pressures was made. (C) 2000 Elsevier Science S.A. All rights reserved.