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M is the simplest and one of the most common mica polytypes. Because of the special cell dimensions, twinning by reticular pseudo-merohedry is frequently observed. The possible orientations of the individuals in the twin are approximately the same as the possible orientations of layers in polytypes. Because of the presence of a pseudo-hexagonal twin lattice, five
pairs of twin laws are possible and they give rise to five
pairs of corresponding twins. Thirty-one composite twins can be enumerated, which however give rise to ten different diffraction patterns. In the twin lattice there are nine geometrically independent rows parallel to
c*, defining a
minimal rhombus in reciprocal space, which allows us to identify the ten independent patterns. A larger unit of translation, called
tessellation rhombus, can be built on it; by tessellation of this unit all the reciprocal space can be covered.
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