Potassic cordierites: characteristic minerals for high-temperature,very low-pressure environments

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 =0 or exhibit intermediate -values 0<<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 M₂Al₄Si₅O₁₈]. The most highly substituted cordierite from Blaue Kuppe is about (K_0.22Na_0.07)^Ch](Mg_1.33Fe _0.66 ²⁺ )^6]Al_4.16Si_4.79O₁₈]. 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 -value to be attained, the hexagonal nature of the cordierites must be due to very rapid crystallization and subsequent quenching. The higher -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.