Structural study of brittle micas (II)

Statistical mode of stacking sequence in a valuevite crystal as deduced by computer simulation

Abstract

Achmatovsk valuevite, Ca_0.99(Mg_2.20Fe²⁺_0.14Al_0.70) (Al_2.77Si_1.23)·O₁₀(OH)₂, a=5.21(1)Å, b=9.00(4)Å, c=9.83(4)Å, β=100°10(6)′ (for 1 M subcell), exhibits X-ray diffraction patterns which assume, unlike common xanthophyllites, a 2 M₁ type. A salient feature was observed in the diffraction patterns that reflections with k≠3n are drawn out along c^*, and that the diffuse maxima are slightly but significantly displaced from the positions expected for those of a regular 2 M₁ structure, showing that the structure is based on a statistical mode of layer sequence. By comparing observed periodic intensity distribution function with ones calculated based on the computer-generated random models of long-period polytypes, which consist of around 100∼150 layers, we have been successful in deriving the probable model for the layer sequence. It is based on the 2 M₁ type but faults based on the stacking mode of 1 M occur with a probability of ∼0.1. A notable feature of the model is that the stacking-fault vectors align in only one direction.