Tri octahedral micas

Changes with orientation are also clearly evident in the XANES spectra of a number of di- and tri-octahedral micas and one brittle mica, respectively at the Mg (phlogopite Fig. 9), Si (muscovite Fig. 10, and tetra-ferriphlogopite Fig. 11), K (muscovite Fig. 12), and Fe (clintonite Fig. 13, and tetra-ferriphlogopite Fig. 14) K edges. Such changes... 

XAS studies on micas a catalogue. Table 1 lists all XAS studies carried out on micas that could be retrieved in the relevant literature. They are presented in the alphabetical order of the di- and tri-octahedral mica species nomenclature approved recently (Rieder et al. 1998) and are further subdivided on the basis of the investigated atom. 

Figure 21. Experimental Mg TT-edge spectra for the powders of four natural tri-octahedral micas.

Figure 25. Experimental Fe A -edge spectra for the powders of two natural tri-octahedral micas and one natural brittle mica. The pre-edge regions are shown as inset.

We have thus far discussed only the most simple and best known types of interlayering, those between expandable and mica structures. It is possible however, that several types of 2 1 lattice can coexist in the same structure. For example di- and tri-octahedral forms of various types of each species. Because of their similarity under X-ray investigation, it is almost impossible to detect their presence in a mixed layered structure. 

Figure 22. Experimental A1 AT-edge spectra for the powders of three natural tri-octahedral and one natural di-octahedral mica.

The 2-3 subscript for the B site in the formula expresses the fact that there are two families of mica structures, the dioctahedral and trioctahedral micas, based on the composition and occupancy of the intralayer octahedral sites. The trioctahedral micas have three divalent ions—for example, Mg or a brucitelike [Mg(OH)2] intralayer, and the dioctahedral group—two tri-valent ions—for example, Al or a gibbsitelike [AlfOHfa] intralayer, between the tetrahedral sheets. In the dioctahedral micas, therefore, one-third of the octahedral sites are vacant or unoccupied (Fig. 2.12C). 

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