PECULIARITIES OF DIAMONDS FORMED IN ALKALINE CARBONATE-CARBON MELTS AT PRESSURES OF 8-10 GPA - SCANNING ELECTRON-MICROSCOPY AND CATHODOLUMINESCENCE DATA

Diamond crystallization with both spontaneous and seeded nucleation wa s realized in the system Na2Mg(CO3)(2)-K2Mg(CO3)(2)-C (graphite) at 8- 10 GPa and 1700-1800 degrees C. The crystallization products were tran sparent colorless diamond single crystals of octahedral habit up to 10 0-150 mu m in size. Scanning electron microscopy (SEM) showed that the growing diamond material was precipitated on both octahedral {111} an d cubic {100} faces of synthetic and natural diamond seed crystals by layers of octahedral orientation, much like the growth of the natural diamond. The physicochemical conditions for diamond crystallization ar e interpreted as a crystal growth from carbon solutions in alkaline-ca rbonate melts. Color cathodoluminescence scanning electron microscopy (CCL-SEM) and cathodoluminescence (CL) spectroscopy studies suggested specific peculiarities of the synthetic ''carbonate-carbon''(CC) diamo nds resembling natural crystals in comparison with diamonds produced b y metal-carbon (MC) synthesis. The main feature of the CC product is t he lack, for both spontaneous and seed stimulated diamonds, of surface color cathodoluminescence as in the case for natural diamonds with lo wer concentrations of nitrogen impurity (type II). The CL spectra of t he CC diamonds showed the simultaneous luminescent three-band system - H3, 575 nm, and a weak blue A-band - the H3 band structure of which r esembles that of natural diamonds of type IIa.