About the origin of mica polytypes

Abstract

Abstract The crystal growth aspect of mica polytypism is reviewed with emphasis on the relationships between the growth mechanisms of the basal faces and the ordering of the unit modules of the mica structure. On synthetic micas, it is shown that the most common short-period polytypes (basic structures) and the disordered layer-stacking sequences as well originate during the early 3D-nucleation and 2D-layer growth mechanisms. During those stages, basic structures may be randomly faulted to various extents. Subsequently, spiral growth may take place and originate long-period (or complex) polytypes. Supported by such observations on synthetic as well as natural micas, the principles of the perfect-matrix and faulted-matrix models of the screw dislocation theory or polytypism are remembered and shown to be powerful in explaining most of the complex polytypes of the micas. On natural biotite, original data are given about the very nature of stacking disorder and also about weak but significant differences between the chemical compositions of coexisting 1M, 2M1 and 1Md polytypes. Lastly, the use of polytype inheritance to find out the phase transformation process between biotite and chlorite is illustrated.