CRYSTAL-CHEMISTRY AND MINERALOGY OF MOISSANITE FROM ALKALINE-ULTRABASIC VOLCANIC COMPLEX (KAMCHATKA)

The main rock types of the diamond-containing alkaline-ultrabasic volc anic complex in East Kamchatka are meimechite, lamprotoide, and tephri te. They contain accessory impregnations of several varieties of SiC o f different colors: water-transparent, greenish, blue, and indigo blue . From studies of this mineral by the highly localized method of analy tical transmission electron microscopy, all these varieties were assig ned to polytype 6H with parameters of unit cell alpha(0) = 3.07 +/- 0. 03 Angstrom, c(0) = 15.1 +/- 0.03 Angstrom and identified as moissanit e alpha-SiC. In addition, intergrowths of moissanite with zircon and g raphite inclusions were detected. The obtained experimental data sugge st the absence of correlation between the color and crystalline struct ure of alpha-SiC. The presence of the intergrowths mentioned above ind icates the formation of moissanite in a protomagmatic phase of high-te mperature melt crystallization that is characterized by sharp reductio n conditions. The concentrations of C in silicate melt that are high e nough to develop a mineral form of SiC are locally reached as a result of carbon solution in the solid state, i.e., graphite or possibly dia mond. It is assumed that baric conditions at the beginning of moissani te crystallization correspond to the area of diamond metastability. Ba sed on the above statement, alpha-SiC 6H can be considered as one of t he indirect features that points to the presence of carbon mineral for ms in magma. Some of these forms can be of xenogenic origin, among whi ch diamond can be found.