One-, two- and three-layer polytypes are common in muscovite (sheet silicate). Based on high-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) examinations, we report in the discovery of complex 5-layer and 7- layer polytypes in natural muscovite for the first time (Fig. 1) and the implications of polytype transitions.

Three main structures are found in muscovite crystals in a sample from Oreana, Nevada, which are 1M, 2M1, and a disordered structure often containing short-range ordered stacking sequences. On the scale of a few micrometers, muscovite occurs as ordered crystals, disordered crystals, and crystals with regions of ordered and disordered layer stacking.

Short-range ordered 5- or 7-layer repeats are found in some disordered structures where single 4-, 6-, 9- or 10-layer intergrowths occur occasionally with 2- and 3-layer repeats. Such complex polytypes have not been observed before in either natural or synthetic muscovite, although 5- and 7-layer structures are commonly present in trioctahedral micas (i.e., biotite) (Baronnet, 1992).