Illite chlorite facies

Kaolinite as a mineral characteristic of continental deposits is present virtually throughout all facies studied. However, variations in its composition during the early stages of diagenesis, as compared to those of chlorite, illite and mixed-layer clays, is of no special diagenetic significance. Because of this, it wUl not be discussed at this stage. 

Figure 31b. Compositions of chlorites in the mixed-layered mineral facies of pelitic rocks (circles) and from the illite-chlorite facies (barred circles). Shaded area shows chlorite compositions from muscovite-chlorite metamorphic rocks.

Figure 31b indicates the compositional spread of chlorites from six rocks in the illite-montmorillonite mixed layered mineral facies and from the illite-chlorite zone in the French Alps (Velde, unpublished). The grains analyzed with the microprobe are chlorites replacing isolated grains of detrital mica or were newly formed grains. They are usually 15 microns in the smallest dimension. 

The stability conditions of corrensite then cover the low grade clay mineral facies (near 100°C) and extend well into the calcium zeolite-prehnite, muscovite-chlorite facies. In pelitic rocks the upper limit will be somewhat lower near the illite-chlorite zone. It is evident that composition of a rock governs the occurrence of corrensite. It can be... 

The second facies is marked by the instability of the fully expanding dioctahedral phases and the existence of a kaolinite-illite tie-line (Figure 48b). In this facies the siliceous alkali zeolites (other than analcite) become unstable, the compositional range of the trioctahedral expanding phases is reduced and aluminous 14 8 chlorite-"allevardite"... 

Once the illite-chlorite zone is entered, i.e., the facies where dioctahedral mica-montmorillonite mineral solid-solutions are no longer stable, how does the assemblage change into muscovite-chlorite The major... 

The clay mineral spectrum is notably less differentiated than in the other facies, the dominant minerals being trioctahedral chlorites and dioctahedral illites. In the chlorite structure, non-swelling layers predominate whereas the alternation of layers of different types exhibits a trend towards ordering. The proportion of mixed-layer minerals of the iUite-montmorillonite type decreases especially as one approaches the massive layers of rock salt. 

The components making up the fine-grained fractions of the terrigenous-halitic complex are essentially Mg-rich trioctahedral well-crystallized chlorites without swelling layers as well as Fe-illites, the structure of which does not contain swelling layers (see model in Fig. 2.10). At the same time there are no mixed-layer species of the chlorite-montmorillonite type which were so characteristic of the carbonate-terrigenous complex (dolomite-sulfate facies). Mixed-layer clays of the illite-montmoriUonite type also diminish throughout the complex. 

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