P. Arkai et G. Lelkesfelvari, THE EFFECTS OF LITHOLOGY, BULK CHEMISTRY AND MODAL COMPOSITION ON ILLITE CRYSTALLINITY - A CASE-STUDY FROM THE BAKONY MTS, HUNGARY, Clay Minerals, 28(3), 1993, pp. 417-433
Modal and bulk chemical compositions, illite and chlorite ''crystallin
ity'' indices (IC and ChC), white mica b(o) and vitrinite reflectance
values of a Palaeozoic variegated elastic sequence have been determine
d. The strongly varying IC values are inadequate for the determination
of metamorphic zones. Based on statistical analyses of the factors wh
ich might affect IC, the presence of paragonite, even in very small qu
antities as a discrete phase and/or as mixed-layers with illite-muscov
ite, proved to be the decisive factor. In contrast, no significant cor
relations were found between the modal composition of the main phases,
bulk chemical composition (including Fe3+/Fe2+ ratio) and IC, ChC, or
white mica b(o) values. Thus, neither lithofacies (granulometric), no
r redox conditions had any effect on ''crystallinity'' and b(o) parame
ters. Chlorite crystallinity indices and vitrinite reflectance values,
together with IC values of selected samples, indicate ''anchizonal''
conditions. The distribution of white mica b(o) values indicates that
this Hercynian metamorphism was of low-pressure type.