Investigation of polytypes in lepidolite using electron back-scattered diffraction

In this paper we show that electron back-scattered diffraction (EBSD) is an effective technique for identifying polytypes of micas in a scanning electron microscope (SEM). Platy crystals mounted on a specimen holder are instantly analyzed and the polytypes are determined by comparing observed and calculated Kikuchi patterns. Subfamilies A and B are easily distinguished by trigonal and hexagonal symmetries, respectively, around the [001]* direction of the Kikuchi bands corresponding to family reflections. In subfamilyA, 1M and 2M, polytypes can be identified by means of several intense bands characteristic to each polytype, but 3T and 1Md are probably difficult to distinguish from each other. The EBSD identification has been applied to the investigation of polytype occurrence in lepidolite from a lithium pegmatite. 1M, 2M, and 2M, polytypes were unambiguously distinguished. Some crystals contained polytypes of the two subfamilies stacked along the [001]* direction. A combination of X-ray chemical analyses and EBSD in an SEM showed that the chemical compositions of 1M and 2M(1) crystals were segmented, whereas those of 2M, were in the range of both 1M and 2M, compositions.