Polytypes illite

K-Ar dating of sedimentary illite polytypes. Bull. Geol. Soc. Arne., 73, 1167-70. 

In nature the Fe-rich illites (glauconite and celadonite) appear to progress from the lMd to the 1M polytype. The Al-rich illites are predominantly the lMd and 2M varieties. If the 1M polytype is an intermediate phase, it is surprising that it is not more abundant in sediments. Recent studies of unmetamorphosed Precambrian sediments (Reynolds,1963 Maxwell and Hower,1967) have shown that the lMd polytype is relatively abundant in ancient sediments, particularly in the extremely fine fraction. The senior author has noted the relative abundance of the lMd polytype in the fine fraction of most Paleozoic rocks but has considered most of it to be mixedlayered illite-montmorillonite rather than illite. Weaver (1963a), Reynolds (1965), and Maxwell and Hower (1967) have shown that during low-grade metamorphism water is squeezed from the expanded layers and the lMd polytype is transformed into the stable 2M polytype. 

Granthoff, G. H., andD. M. Moore. 1996. Illite polytype quantification using WILDFIRE(c) calculated X-ray diffraction patterns. Clays Clay Miner. 44 835-842. 

Dong H, Peacor DR (1996) TEM observations of coherent stacking relations in smectite, I/S and illite of shales Evidence for MacEwan crystallites and dominance of 2Mi polytypism. Clays Clay Minerals 44 257-275... 

Velde B, Hower J (1963) Petrological significance of illite polytypism in Paleocene sedimentary rocks. Am Mineral 48 1239-1254... 

The kaolinite mineral species studied are kaolinite, kaolinite d (disordered kaolinite), dickite and nacrite. These polytypes have been described by Bailey (1963) on the basis of sense and degree of displacement of 1 1 layers and the position of vacant octahedral positions in the layer sequence. For the hydrated kaolinitic minerals, we have used the terminology of Keller and Johns (1976) which is based on endellite as the completely hydrated species and halloysite as the partly or completely dehydrated species. The polytypes of chlorite have been described by Bailey and Brown (1962) and Hayes (1970). In Fig. 8.3b it is shown that montmorillonite, the mixed-layer clays and illite are located between pyrophyllite without interfoliar charge and the dioctahedral... 

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