Distortions of octahedra and octahedral sheets in 1 M micas and the relation to their stability

Abstract

Distortions of octahedra and octahedral sheets in 1 M micas have been systematically investigated on the basis of recently published structure data. A survey on the variations of individual interatomic distances has revealed that there is a marked difference in the modes of the distortions between two crystallographically independent octahedra M(1)O₄(OH, F)₂ and M(2)O₄(OH, F)₂. There is a general trend that vacancies or cations with small charge are preferentially located at M(1); the octahedra about M(1) tend to be larger compared with those about the other site. Such a mode of cation ordering may be accounted for in terms of the variation of shared edge lengths which change to relieve the mutual repulsion between octahedral cations. Variations in dimensions of the octahedral sheet along directions perpendicular and parallel to the sheet are related to each other, and the rate of change in thickness of the octahedral sheet is largely affected by the variation of the lateral dimensions of the tetrahedral sheet. The relative stability of mica structures has been discussed in connection with the variation of the size and shape of octahedra. A structural mechanism was proposed for thermal decomposition of mica structures.