Identification of two allotwins of mica polytypes in reciprocal space through the minimal rhombus unit

The X-ray investigation (precession method) of the Ruiz Peak oxybiotite, wh ich is well known for the occurrence of a large number of polytypes and twi ns, revealed two complex diffraction patterns, which cannot be identified a s long-period polytypes. These patterns are analysed in terms of the minima l rhombus, a geometrical asymmetric unit in reciprocal space which permits the decomposition of the composite reciprocal lattice of a twin or allotwin into the reciprocal lattices of the individuals. Both the recorded pattern s correspond to a 1M-2M(1) allotwin: the relative rotation between the indi viduals is 120 degrees in one case and 60 degrees in the other. The geometr ical criteria for evaluating the presence of twinning or allotwinning are a nalysed through these two natural examples.