REPRESENTATION OF THE AXIAL SETTINGS OF MICA POLYTYPES

The basic unit of mica polytypes has monoclinic symmetry and the layer stagger is a submultiple of the periodicity along the a axis. Because of these features, more than one suitable axial setting can be chosen for non-orthogonal micas. Three types of axial settings are introduce d and shown to be useful for classifying non-orthogonal polytypes of m icas and indexing their diffraction patterns. Standard setting is the axial setting of a polytype leading to the shortest projection of the c axis onto the (001) plane. Basic axial setting is the standard setti ng of a polytype with a number N of layers equal to an integral multip le of 3(n). All the polytypes having the same basic axial setting belo ng to the same Series. Fixed-angle setting is the axial setting of a g eneral polytype showing the same angle as the corresponding basic axia l setting. The total layer stagger of stacking classifies polytypes in to two Classes: their c axis is inclined towards respectively the shor test (Class a) or the longest (Class b) of the two orthohexagonal axes in the plane of the layer. Each Class is further divided according to N = 1 (mod 3) (Subclass1) and N = 2 (mod 3) (Subclass 2). By expressi ng N as 3(n)(3K + L), the two integers n and L (1 or 2) establish the Series and the Subclass, respectively. This definition allows an effec tive classification of the polytypes and a systematic approach to the indexing of diffraction patterns, independently of their complexity, w hich increases with N. The transformation rules between settings are g iven and examples are discussed.