caa a22 4500 465791824 CHVBK 20180323112027.0 cr unu---uuuuu 170327e19900301xx s 000 0 eng 10.1007/BF00678983 doi (NATIONALLICENCE)springer-10.1007/BF00678983 Potassic cordierites: characteristic minerals for high-temperature, very low-pressure environments [Elektronische Daten] [W. Schreyer, W. Maresch, P. Daniels, P. Wolfsdorff] The mineral chemistry of cordierites from three different sanidinite facies localities-1) volcanic xenoliths from the Eifel, Germany; 1) buchites of the Blaue Kuppe, Germany; 3) paralavas from the Bokaro coalfield, India-is characterized by unusually high potassium contents up to 1.71 wt%, equivalent to 0.22 K atoms per formula unit (p.f.u.) based on 18 oxygens. Significantly, these cordierites are either hexagonal highcordierites (indialites) with Δ=0 or exhibit intermediate Δ-values 0<Δ<0.20 relative to well Al,Si-ordered orthorhombic low-cordierite. Based on microprobe analyses, the predominant substitutional mechanism for alkali incorporation is Alk[Channel]+Al[4] for □+Si[4], thus leading to Al/Si-ratios deviating considerably from the value 4:5 in ideal cordierite M2[Al4Si5O18]. The most highly substituted cordierite from Blaue Kuppe is about (K0.22Na0.07)[Ch](Mg1.33Fe 0.66 2+ )[6][Al4.16Si4.79O18]. Bokaro cordierites are further characterized by obvious (Al+Si)-deficiencies against the ideal value of 9.0 p.f.u., a tendency of which is apparent in most Blaue Kuppe analyses as well. As the tetrahedral deficiencies are often equivalent to excess cations in the octahedra, we assume that ferric iron fills up the remaining tetrahedral sites, again linked with the introduction of potassium according to K+Fe3+ for □+Si. In comparison with the available experimental data, these natural potassic cordierites are considered stable high-temperature phases regarding their compositions, but not their structural states. Although the substitution KAl for Si in Mg-cordierite is known to lower the maximum Δ-value to be attained, the hexagonal nature of the cordierites must be due to very rapid crystallization and subsequent quenching. The higher Δ-values of the Blaue Kuppe cordierites might be caused by their topotactic origin from preexisting biotite. The complicated twin and domain patterns of the hexagonal Eifel and Bokaro cordierites as observed in thin section could perhaps be attributed to structural modulations as postulated recently for hexagonal cordierite shortly after its growth. Springer-Verlag, 1990 Schreyer W. Institut für Mineralogie, Ruhr-Universität Bochum, D-4630, Bochum 1, Federal Republic of Germany aut Maresch W. Institut für Mineralogie, Ruhr-Universität Bochum, D-4630, Bochum 1, Federal Republic of Germany aut Daniels P. Institut für Mineralogie, Ruhr-Universität Bochum, D-4630, Bochum 1, Federal Republic of Germany aut Wolfsdorff P. Institut für Mineralogie, Ruhr-Universität Bochum, D-4630, Bochum 1, Federal Republic of Germany aut Contributions to Mineralogy and Petrology Springer-Verlag 105/2(1990-03-01), 162-172 0010-7999 105:2<162 1990 105 410 https://doi.org/10.1007/BF00678983 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/BF00678983 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Schreyer W. Institut für Mineralogie, Ruhr-Universität Bochum, D-4630, Bochum 1, Federal Republic of Germany aut NATIONALLICENCE 700 1- Maresch W. Institut für Mineralogie, Ruhr-Universität Bochum, D-4630, Bochum 1, Federal Republic of Germany aut NATIONALLICENCE 700 1- Daniels P. Institut für Mineralogie, Ruhr-Universität Bochum, D-4630, Bochum 1, Federal Republic of Germany aut NATIONALLICENCE 700 1- Wolfsdorff P. Institut für Mineralogie, Ruhr-Universität Bochum, D-4630, Bochum 1, Federal Republic of Germany aut NATIONALLICENCE 773 0- Contributions to Mineralogy and Petrology Springer-Verlag 105/2(1990-03-01), 162-172 0010-7999 105:2<162 1990 105 410 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer