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Licensed Unlicensed Requires Authentication Published by De Gruyter March 31, 2015

Relationship among metamorphic grade, vesuvianite ”rod polytypism”, and vesuvianite composition

Edwin Gnos and Thomas Armbruster
From the journal American Mineralogist

Abstract

Single-crystal X-ray study of different vesuvianite samples of known origin shows that different metamorphic grade results in different arrangements of structural rods oriented parallel to the vesuvianite c axis, interpreted as .rod polytypism.. There is a systematic dependence of space-group symmetry and rod arrangement on crystallization temperature: P4nc-dominant < 300 °C, P4/n-dominant ~300.500 °C, and P4/nnc > 500 °C. Partial occupancy of the T sites (B, Al, Fe3+) and increased F-content seem to stabilize rod disorder causing P4/nnc space-group symmetry. All studied vesuvianites in calcsilicate rocks and marbles from regional- and contact-metamorphic upper amphibolite facies have disordered rods (P4/nnc symmetry). Electron-microprobe analyses of metamorphic vesuvianites from alpine and non-alpine occurrences, supported by structural investigation, showed that in addition to homo- and heterovalent substitution, partial occupancy of the commonly vacant T sites by B, Al, or Fe3+, and the (O4H4)4. → SiO44- (hydrogarnet-type) substitutions are significant in nature. With few exceptions, T-site occupancy seems to be restricted to high-grade metamorphic rocks whereas the .hydrovesuvianite. substitution is only found in vesuvianites formed at very low metamorphic grade. The cell parameters of vesuvianite with empty T sites increase with increasing Ti + Mg → 2 Al substitution, and this increase is even more pronounced with increasing .hydrovesuvianite. component. An increase in boron on T sites leads to a decrease of c but an increase in a. Fluorine incorporation and T-site substitution (B, Al, Fe3+) in vesuvianite are coupled with a decrease in hydroxyl groups. This causes vesuvianites to be stable under higher XCO2 conditions, and in an assemblage with quartz at conditions above the experimentally determined upper stability of quartz + T site vacant, F-free vesuvianite. Optically anomalous vesuvianites have ordered rods and are generally characterized by an intergrowth of P4/n and P4nc domains. In addition to B-rich vesuvianite and wiluite, P4nc-dominant vesuvianites are also commonly optically positive.

Received: 2005-4-29
Accepted: 2005-11-29
Published Online: 2015-3-31
Published in Print: 2006-5-1

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