Genetic significance of the 867 cm−1 out-of-plane Raman mode in graphite associated with V-bearing green grossular
Thomas, R and Rericha, A and Pohl, WL and Davidson, P, Genetic significance of the 867 cm−1 out-of-plane Raman mode in graphite associated with V-bearing green grossular, Mineralogy and Petrology, 112, (5) pp. 633-645. ISSN 0930-0708 (2018) [Refereed Article]
Copyright 2018 Springer-Verlag GmbH Austria, part of Springer Nature
SE Kenya is the world’s largest producer of green vanadium grossular gemstones (tsavorite). Samples from one of the mines near Mwatate, and of occurrences in Tanzania yielded remarkable new insights into the genesis of tsavorite. Graphite is intimately associated with V-grossular and is one of the keys to understanding its origin. In the course of this study we found five different types of graphite. Surprisingly, in one graphite type the "Raman-forbidden" and IR-active 867 cm-1 band was observed. In this communication, we attempt to find an explanation for this unusual phenomenon. Additionally, our observations also address some of the issues pertaining to the origin of the green grossular-dominated rocks (grossularites), as well as the gem quality tsavorite crystals, since we propose that the anomalous spectroscopic behavior of the graphite is related to the unusual conditions during crystallization of both the grossular and graphite from a near-supercritical volatile- and sulfur-rich silicate melt. The massive green vanadium grossular contains abundant unequivocal crystallized melt inclusions, while the transparent gem quality grossular (tsavorite) displays only fluid inclusions. On the basis of inclusion studies we suggest that anatectic melts originated in the peculiar evaporitic host lithology of the tsavorite deposits. Near peak metamorphic temperatures (∼700 °C) these liquids occurred as a supercritical volatile-rich "fluid/melt phase" characterized by complete miscibility between H2O and silicate liquid. Relatively dry liquid batches precipitated non-transparent green grossular, whereas wet batches segregated fluids that formed transparent tsavorite.
tsavorite, greenV-grossular, graphite: raman scattering, fluid and melt inclusions, sulfur