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Representation of the Axial Settings of Mica Polytypes

Abstract : 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 introduced and shown to be useful for classifying non-orthogonal polytypes of micas 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 setting of a polytype with a number N of layers equal to an integral multiple of 3". All the polytypes having the same basic axial setting belong to the same Series. Fixed-angle setting is the axial setting of a general polytype showing the same angle as the corresponding basic axial setting. The total layer stagger of stacking classifies polytypes into two Classes: their c axis is inclined towards respectively the shortest (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 (mod3) (Subclass1) and N=2(mod3) (Subclass 2). By expressing N as 3"(3K + L), the two integers n and L (1 or 2) establish the Series and the Subclass, respectively. This definition allows an effective classification of the polytypes and a systematic approach to the indexing of diffraction patterns, independently of their complexity, which increases with N. The transformation rules between settings are given and examples are discussed.
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Massimo Nespolo, Hiroshi Takeda, Giovanni Ferraris. Representation of the Axial Settings of Mica Polytypes. Acta Cryst., 1998, 54, pp.348 - 356. ⟨10.1107/S0108767397019910⟩. ⟨hal-01713439⟩



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