Abstract

After the proof of ultrahigh-pressure mineral inclusions in the Variscan Greifenstein granite, we looked for further hints of supercritical indications in the granite-related tin mineralization of Ehrenfriedersdorf, German Erzgebirge. Here we found an unusual moissanite-diamond-graphite paragenesis in a small beryl-quartz vein. That is the first found of SiC and diamond grown in a granite-related pegmatite-hydrothermal system under a low-pressure regime (≤ 2kbar). The study of rare melt inclusions showed a crystallization temperature of 700-720 °C for the simultaneous growth of beryl and moissanite. Raman spectroscopy was the primary method for identifying moissanite, nanodiamond, and graphite. According to these measurements, the SiC crystals and needles are mainly of the hexagonal 6H-SiC polytype and twisted along the c-axis. Moissanite in beryl generally forms needle- or whisker-like crystals parallel to the c-axis of beryl. Nanodiamond crystals are directly in the moissanite, or the moissanite-graphite matrix distributed, showing a clear crystallographic relationship to the beryl substrate. Besides the whisker-like moissanite crystals in beryl grown by two different mechanisms, there are also more isomorphic and spheric moissanite crystals in beryl and quartz. Up to now, the growth mechanism of the whisker-like crystals is not precisely known. Two mechanisms compete for a natural supercritical Vapor-Liquid-Solid method (VLS) and a low-pressure heteroepitaxial Chemical Vapor Deposition process (CVD).

Keywords:Moissanite; Nanodiamond; Graphite; Beryl; Quartz; Raman spectroscopy; Supercritical fluids