Ferri-annite

For large cations such as Cs and Rb (e.g., Cs-tetra-ferri-annite, Rb-,Cs-rich phlogopite, and nanpingite) the small a value corresponds to a large interlayer separation, whereas for small cations such as Na and Ca (e.g., preiswerkite, paragonite, and... 

Figure 6. Bulk modulus as a function of interlayer cation size, calculated using the interlayer chemical composition. Na(Pg) paragonite (Comodi and Zanazzi 1997) Na(mu) Na-rich muscovite (Comodi and Zanazzi 1995) K(phl) phlogopite (Hazen and Finger 1978) K(mu) K-muscovite (Comodi and Zanazzi 1995) Cs(Cs-tfa) Cs-tetra-ferri-annite (Comodi et al. 1999) Rb(Rb-tfa) Rb-tetra-feni-annite (Comodi et al. 2001).

Ferri-annite composition Wittenrom (Austria) Miyano 1982... 

Donnay et al. [1964] have refined the structure of a synthetic iron mica KFei+fSisFe ) Oio(OH)2, which may be called ferri-annite by analogy with annite, whose composition is KFej+(Si3Al)Oio(OH)2. It is unfortunate that the inherent errors in the original measured intensities did not allow the full power of modem least-squares refinement to be used, but the final positional parameters still give much more useful information about mica structures than was available a few years previously. This structure conforms to the general features of the mica structures discussed above. 

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