TY - JOUR

T1 - Comprehensive chemical analyses of natural cordierites

T2 - Implications for exchange mechanisms

AU - Bertoldi, Christian

AU - Proyer, Alexander

AU - Garbe-Schönberg, Dieter

AU - Behrens, Harald

AU - Dachs, Edgar

N1 - Funding Information: We gratefully acknowledge the donors of the cordierite samples listed in Appendix A . The first author would like to thank Charles Geiger for the invitation to the University of Kiel to work on this thrilling topic and for his helpful comments, and L. Ottolini for performing the SIMS analyses. Reviews by Ed Grew, Angelika Kalt and Edward Manning helped to improve the manuscript. This work was supported by DFG, project no. GE 659/6-2.

PY - 2004/10/5

Y1 - 2004/10/5

N2 - 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 T11 tetrahedral site, the three most important substitution mechanisms are those for the incorporation of Li on the octahedral sites (NaLi□-1Mg-1), and of Be and other divalent cations on the tetrahedral T11 site (NaBe□-1Al-1 and Na(Mg,Fe2+) □-1Al-1). The dominant role of the last vector is clearly demonstrated. We propose a new generalized formula for cordierite: Ch(Na,K)0-1 VI(Mg,Fe2+,Mn,Li)2 IVSi5 IVAl3 IV(Al, Be, Mg, Fe2+, Fe3+)O18 *xCh(H2O, CO2...). Our results show that the population of (Mg, Fe2+) on the T11-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 whethers ignificant 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.

AB - 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 T11 tetrahedral site, the three most important substitution mechanisms are those for the incorporation of Li on the octahedral sites (NaLi□-1Mg-1), and of Be and other divalent cations on the tetrahedral T11 site (NaBe□-1Al-1 and Na(Mg,Fe2+) □-1Al-1). The dominant role of the last vector is clearly demonstrated. We propose a new generalized formula for cordierite: Ch(Na,K)0-1 VI(Mg,Fe2+,Mn,Li)2 IVSi5 IVAl3 IV(Al, Be, Mg, Fe2+, Fe3+)O18 *xCh(H2O, CO2...). Our results show that the population of (Mg, Fe2+) on the T11-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 whethers ignificant 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.

KW - Beryllium

KW - Cordierite

KW - Exchange mechanisms

KW - Lithium

UR - http://www.scopus.com/inward/record.url?scp=7244253205&partnerID=8YFLogxK

U2 - 10.1016/j.lithos.2004.07.003

DO - 10.1016/j.lithos.2004.07.003

M3 - Article

AN - SCOPUS:7244253205

VL - 78

SP - 389

EP - 409

JO - LITHOS

JF - LITHOS

SN - 0024-4937

IS - 4

ER -