Polytypes, proportions

Subfamily A polytypes. As shown in the section dealing with the family structure, the family structure of subfamily A polytypes has symmetry //r(3)1/w, admitting a primitive rhombohedral cell. Within the Trigonal model the family reciprocal sublattice is rhombohedral both in its geometry and intensity distribution. In the real diffraction pattern the intensity distribution deviates from rhombohedral symmetry proportionally to the deviations of the layer from their archetypes described by the Trigonal model, but the geometry remains rhombohedral. 

Figure 5. Proportions of polytypes in technical-grade SiC as a function of Al content.

Anionic clays are lamellar hydroxylated compounds that exist as minerals, as well as in synthetic phases. A wide diversity of materials can be prepared by varying the nature and proportions of metallic cations and the interlamellar anion. Each compound presents a range of reversible hydration states in relation to temperature and hygrometry. The diversity of the obtained compounds is even extended by the existence of several polytypes. Several synthesis routes are available for the preparation of most of these phases. The synthesis process conditions, washing conditions, and drying conditions are of major importance for the... 

Finally, in the case of chlorite, a small proportion (even up to 30%) of interstratified layers of vermiculite would be very difficult to detect, so the suspicion arises that many chlorites may, in fact, be interstratified chlorite-vermiculites. Johnson [1964] has found 1 1 regular chlorite-vermiculite present in a silt-loam soil in Adams Co., Pa. He suggests that the chlorite present in the parent material may be a two-layer polytype with different degrees of bonding of alternate brucite layers. This problem is related to the corrensite problem (see Section El on special minerals). 

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