Clay minerals in Upper Permian evaporites from the Northern Calcareous Alps (Alpine Haselgebirge Formation, Austria)
European Journal of Mineralogy Volume 4 Number 6 (1992), p. 1407 - 1420
published: Dec 15, 1992
manuscript accepted: Mar 18, 1992
manuscript received: Nov 1, 1991
ArtNo. ESP147050406017, Price: 29.00 €
Abstract The clay mineral assemblages of argillaceous halites, mudrocks, and sandstones from the Upper Permian Alpine Haselgebirge Formation are composed of dioctahedral illite 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 % 2M1 component. Chlorite is likewise non-expandable and belongs to the lib 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. Hikes 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.