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Genetic significance of the 867 cm−1 out-of-plane Raman mode in graphite associated with V-bearing green grossular

journal contribution
posted on 2023-05-19, 21:31 authored by Thomas, R, Rericha, A, Pohl, WL, Paul DavidsonPaul Davidson
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.

History

Publication title

Mineralogy and Petrology

Volume

112

Issue

5

Pagination

633-645

ISSN

0930-0708

Department/School

School of Natural Sciences

Publisher

Springer-Verlag Wien

Place of publication

Sachsenplatz 4-6, Po Box 89, Vienna, Austria, A-1201

Rights statement

Copyright 2018 Springer-Verlag GmbH Austria, part of Springer Nature

Repository Status

  • Restricted

Socio-economic Objectives

Expanding knowledge in the earth sciences

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