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Empirical ferric iron corrections: necessity, assumptions, and effects on selected geothermobarometers

Published online by Cambridge University Press:  05 July 2018

John C. Schumacher
John C. Schumacher*
Affiliation:
Mineralogisch-Petrographisches Institut, Olshausenstraβe 40, D-2300 Kiel, F.R. Germany
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Abstract

The ferromagnesian silicate minerals, such as garnets, pyroxenes, micas, and amphiboles, appear in a variety of geothermometers and geobarometers. Where complete chemical analyses are available and regardless of bulk composition (metamorphosed pelitic or mafic), the aforementioned minerals commonly contain ferric iron. In mineral analyses using the electron microprobe, ferric and ferrous iron are not distinguished, and all the iron is treated as FeO. In ferric Fe-bearing minerals, this treatment results in (1) low analytical sums and (2) excess cations in the mineral formulae. Assuming ideal stoichiometry (ideal formula cations and oxygens) allows direct ferric estimates in garnets and pyroxenes; amphiboles require additional assumptions concerning site occupancies, and, for micas, no acceptable constraint exists for a ferric estimate. Based on ferric iron determinations for some metamorphic ferromagnesian silicates, the proportion of ferric to total iron increases at higher XMg values. The influence of ferric estimates on T and P calculations depends on the model used and on the extent the ferric estimate alters the relative proportions of end-members. Several examples suggest that, in general, if ferric estimates (or determinations) are made, they should be made for all the relevant minerals.

Keywords

ferric ironferromagnesian mineralsgeothermometergeobarometer
Type
Research Article
Information
Mineralogical Magazine , Volume 55 , Issue 378 , March 1991 , pp. 3 - 18
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1991

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