Clay minerals in Upper Permian evaporites from the Northern Calcareous Alps (Alpine Haselgebirge Formation, Austria)

Abstract

The clay mineral assemblages of argillaceous halites, mudrocks, and sandstones from the Upper Permian Alpine Haselgebirge formation are composed of dioctahedral illited and trioctahedral Mg-chlorite (clinochlore) plus varying traces of quartz ± alkali feldspar ± magnesite. Interstratified clay phases are entirely absent. Illite contains <5% smectite and is predominantly of the 1Md/1M polytype with <20% 2M 1 component. Chlorite is likewise non-expandable and belongs to the IIb polytype. SEM and TEM investigations show that most clay particles are well-crystallized, suggesting that they formed diagenetically. Measurements of illite crystallinity' show distinct ranges for each lithology with almost no overlap. Illites from halites yield the lowest IC values (mean 0.42±0.02 Δ°2θ), implying and incipient anchizonal overprint. IC values from mudrock and sandstones samples, however, are significantly higher (mean values of 0.47±0.01 and 0.56±0.03, respectively) and indicate high-grade diagenesis rather than anchimetamorphism. The low IC values of illites from halite samples probably reflect the high availability of potassium in these evaporites