CRYSTALLOCHEMISTRY AND GENESIS OF CARBONADO FROM THE MELANOCRATIC BASALTOIDS OF THE AVACHA VOLCANO, KAMCHATKA PENINSULA

The paper reports first, in the Russian literature, data on the minera logy and crystallochemistry of carbonado from melanocratic basaltoids - avachites (SiO2 approximately 50 wt%, MgO approximately 14-16 wt%) - from the Avacha volcano, Kamchatka. Carbonado occurs in the rocks as irregular, angular grains with uneven, rough surfaces. Studies with an electron microscope, equipped with Kevex-5100 EDS and ASID scanning s ets, revealed that the carbonado is composed of polymineral aggregates , in which the following minerals were definitely detected: diamond mi crocrystals up to 7 - 8 m in size (cubic face-centered cell, a = 3.55 +/- 0.03 angstrom, space group Fd3m); fine-dispersed polycrystalline a ggregates of cubic silicon carbide, beta-SiC (cell parameters: a = 4.3 6 +/- 0.03 angstrom, space group F4BAR3m); discrete particles of nativ e silicon (cubic face-centered cell with a = 5.42 +/- 0.03 angstrom, s pace group Fd3m); and graphite with specific high periodicity (approxi mately 40 angstrom) along the c axis. The major accompanying phases ar e quartz, potassium, and sodium feldspars, occasional chlorite, and si ngle rutile grains. Our experimental studies and data compiled from th e literature, including those on diamond synthesis, suggest that the c arbonado crystallized at reduced conditions, a relatively low temperat ure (1800 - 1118-degrees-C), and a lithostatic pressure of no more tha n 1 kbar. The diamond microcrystals grow on a matrix of cubic silicon carbide and inherit the beta-SiC polycrystalline texture, which accoun ts for the similar carbonado texture. The beta-SiC is hypothesized to form in a melt, due to the reaction of native silicon or quartz with c arbon-bearing magmatic gases. Analysis of the data obtained demonstrat es that the principal specific feature of the carbonado-generating mag ma is the high pressure of the carbon-bearing fluid and balanced chemi stry, which is required to suppress the growth of graphite.