Comprehensive chemical analyses of natural cordierites: implications for exchange mechanisms

• Published in: Lithos 2004-12, Vol.78 (4), p.389-409
• Main Authors: Bertoldi, Christian, Proyer, Alexander, Garbe-Schönberg, Dieter, Behrens, Harald, Dachs, Edgar
• Format: Article
• Language: English
• Subjects: Beryllium; Cordierite; Exchange mechanisms; Lithium
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2004.07.003

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 H 2O and CO 2 (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 Fe 3+Al −1 on the T 11 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 T 11 site (NaBe□ −1Al −1 and Na(Mg,Fe 2+)□ −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,Fe 2+,Mn,Li) 2 IVSi 5 IVAl 3 IV(Al, Be, Mg, Fe 2+, Fe 3+)O 18 * x Ch(H 2O, CO 2…). Our results show that the population of (Mg, Fe 2+) on the T 11-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–M 2+–Na.