On the indexing of 3T mica polytype

Abstract 3T mica polytype is a trigonal polytype that can be indexed both in a hexagonal primitive and in an orthohexagonal double cell. The reflection conditions clearly show that when k = 0(mod3) (orthohexagonal indexing) the geometry of the corresponding rows is the same as that of the 1M or 2M 1 polytypes, differing just for the violation of the additional reflection conditions. However, while the last two polytypes require an oblique setting, the pattern of the 3T one should be always indexed in a right-angle setting. Spiral twinning of 1M polytype according to the commonest mica twin law [310] leads to a situation ideally indistinguishable from that of the 3T polytype. However, the distortions from the ideal geometry of the layers building up the polytype result in the appearance of weak reflections that allow the distinction. The ambiguity of the axial choice and its solution is shown comparing the theoretical calculations with the experimental pattern of a phengite sample.