Thickness of the mica module

As mentioned at the beginning of this chapter, a mica module is intended to consist of an M layer plus the interlayer cation. The use of the module thickness tm = csinp/n n is the number of M layers in a unit cell) allows the comparison of data from different polytypes. 

Incease of h diigonal oUton. An increase of a (e.g., under compression) causes 

Decease of h diigonal otiion. A decrease of a (e.g., under expansion) causes the difference between I-Ob(outer) and I-Ob(mner) distances to become smaller the interlayer coordination approaches the hexagonal prismatic coordination for both polytype subfamilies A and B. As far as the antiprismatic coordination is a stabilizing factor, the decrease of the ditrigonal rotation at high T should weaken a structure. 

Note that, under the combined causes which influence the I-Ob distances and discussed above, overall decreases of the I-Ob(mner) has been reported in high-temperature refinements of micas (Catti et al. 1989, Guggenheim et al. 1987 Ivaldi et al. 1998 Russel and Guggenheim 1999 Takeda and Morosin 1975). 

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