Bohemian garnet


Authors: Seifert AV, Vrána S

Published in: Bulletin of Geosciences, volume 80, issue 2; pages: 113 - 124; Received 25 November 2004; Accepted in revised form 29 March 2005;

Keywords: Bohemian garnet, chemical composition, mineral inclusions, origin, Podsedice pyrope deposit, Czech Republic,

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This study presents the chemical and mineralogical characteristics of garnet currently mined in the České středohoří Mts. for the production of Bohemian garnet jewellery. Pyrope samples from the Podsedice deposit range in colour from the prevalent red pyrope with a brownish hue to the rare dark violet Cr-rich pyrope. Electron microprobe analyses of the red pyrope show a consistent content (mol%) of 74.3-75.1 pyrope, 12.5-13.7 almandine, 4.3-7.2 uvarovite, 1.1-4.0 grossular, and 0.7-0.8 spessartine. Minor components include 0.44-0.77 wt% TiO2, 0.05-0.10 wt% Na2O, and 0.05-0.07 wt% V2O3. The dark violet pyrope has an elevated chromium content of 3.23-4.25 wt% Cr2O3. Both types of pyrope are nearly free of compositional zoning. Pargasitic amphibole, Al-Cr spinel, and iron-nickel sulphide are for the first time identified as minor inclusions in the Bohemian garnet. Pargasite forms sub-oval inclusions, 30 to 60 µm long. The Ca contents in pargasite are notably lower than in ordinary calciferous amphiboles (~1.6 Ca pfu), and the increased Na content indicates approximately 20 mol% of the magnesiotaramite end-member in the amphibole solid solution. The pargasite inclusions provide important petrologic and geotectonic information on the metamorphic state of the ultramafic host-rock prior to garnet crystallization. The red and violet garnets considered in this study generally contain a very limited amount of microscopic inclusions and impurities. Chemical data indicate that this pyrope has been primarily derived from pyrope lherzolites and peridotites in a shallow crustal position.