Synthetic micas with unusual properties

In some naturally occurring true micas. Si nearly fills all the tetrahedral sites (e.g., polylithionite, tainiolite, norrishite, and celadonite), whereas in the most common mica species (i.e., muscovite and phlogopite) Al substitutes for Si in a ratio near 1 3. In some true micas and brittle micas, the Al for Si substitution corresponds to a ratio of Al Si = 1 1 (e.g., ephesite, preiswerkite, siderophyllite, margarite, and kinoshitalite), whereas the 

Hazen and Burnham (1973) related (T-0) distances of trisilicic micas to tetrahedral composition by the linear relationship (xai and xsi represent Al and Si apfu, respectively) 

A more general relationship derived here including both trioctahedral and dioctahedral true and brittle micas (Tables 1-4, Appendix II) between tetrahedral mean bond distances (T-O) and tetrahedral chemistry (in apfu) is  

In the regression analysis, structures containing B, Be, and Ge in tetrahedral sites were not considered, as well as structures with symmetry lower than ideal owing to tetrahedral cation ordering (differences in T-0 values greater than 5o). Only structures containing tetrahedral Si, Al, and Fe were examined. 

Geometrical considerations of tetrahedral distortion parameters have been considered earlier (e.g., Drits 1969, 1975 Takeuchi 1975 Appelo 1978 Lee and Guggenheim 1981 Weiss et al. 1992). We further discuss these relationships here and relate them to layer composition on the basis of data from a large number of structure determinations. 

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