Ceramics-Silikáty 28, (4) 289 - 309 (1984)

The MDO polytypes of pyrophyllite and talc family have been classified according to their superposition structures into two subfamilies, and accor­ding to the Y Z projection of their structures into 7 (talc family) and 15 (pyrophyllite family) MDO groups. This geometrical classification which is closely related to the diffraction patterns of these minerals, has been confirmed by systematic calculation of X-ray diffraction patterns for all individual MDO polytypes. These calculations made it also possible to construct identi­fication diagrams for determining the subfamily and the MDO group from the distribution of intensities along the reciprocal rows ±20l or ±13l and 02l, respectively. These two characteristics suffice for the determination of any MDO polytype. The identification is based on visual comparison of single-crystal diffraction patterns with the identification diagrams. A partial identification is possible also using powder diagrams of specimens with random orientation of particles.