Polytypism of Micas. II. Classification and Abundance of MDO Polytypes

The maximum-degree-of-order (MDO) polytypes in all three mica families have been classified into two subfamilies and five homomorphous MDO groups according to their superposition structure and the YZ projection of their structure, respectively. This classification, which is closely related to the diffraction pattern of micas, can be derived directly from the fully descriptive polytype symbols and facilitates the calculation of the identification diagrams as well as the recognition of the relations of homomorphy between the three mica families. All mica structures refined so far by the least-squares method according to the present standards, are those of MDO polytypes.