Comprehensive chemical analyses of natural cordierites: implications for exchange mechanisms

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Authors / Editors

Proyer, Alexander

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Publication Details

Author list: Bertoldi C, Proyer A, Garbe-Schonberg D, Behrens H, Dachs E

Publisher: Elsevier

Place: AMSTERDAM

Publication year: 2004

Journal: Lithos: Journal of Petrology, Mineralogy and Geochemistry (0024-4937)

Journal acronym: LITHOS

Volume number: 78

Issue number: 4

Start page: 389

End page: 409

Number of pages: 21

ISSN: 0024-4937

Languages: English-Great Britain (EN-GB)

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Abstract

Cordierite samples from pegmatites and metamorphic rocks have been analysed for major [electron microprobe analysis (EMPA)] and trace elements [inductively coupled plasma mass spectrometry (ICP-MS), secondary ion mass spectrometry analyses (SIMS)] as well as for H2O and CO2 (coulometric titration), and the results evaluated in conjunction with published data in order to determine which exchange mechanisms are significant. Apart from the homovalent substitutions FeMg-1 and MnMg-1 on the octahedral site, some minor KNa-1 on the Ch0 channel site, and Fe3+Al-1 on the T(1)l tetrahedral site, the three most important substitution mechanisms are those for the incorporation of Li on the octahedral sites (NaLisquare(-1)Mg(-1)), and of Be and other divalent cations on the tetrahedral T(1)l site (NaBesquare(-1)Al(-1) and Na(Mg,Fe2+)square(-1)Al(-1)). The dominant role of the last vector is clearly demonstrated. We propose a new generalized formula for cordierite: (Ch)(Na,K)(0-1) (Vl)(Mg,Fe2+,Mn,Li)(2) Si-IV(5) Al-IV(3) (IV)(Al, Be, Mg, Fe2+, Fe3+)O-18 *x(Ch)(H2O, CO2...). Our results show that the population of (Mg, Fe2+) on the T(1)l-site is limited to about 0.08 a.p.f.u. Other exchange mechanisms that were encountered in experiments operate only under P-T conditions or in bulk compositions that are rarely realized in nature. Routine analyses by electron microprobe in which Li and Be are not determined can be plotted as (Mg+Fe+Mn) versus (Si+Al) to assess whether significant amounts of Li and Be could be present. These amounts can be calculated as Li (a.p.f.u.)=Al+Na-4 and Be (a.p.f.u.)=10-2Al-M2+-Na. (C) 2004 Elsevier B.V. All rights reserved.

Keywords

beryllium, cordierite, exchange mechanisms, lithium

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