Composite twins of 1M mica: derivation and identification

摘要

1M 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.