X-RAY POWDER TRANSMISS ION D IFFRACTOMETRY DETERMINAT ION OF MICA POLYTYPES: METHOD AND APPL ICAT ION TO

natural micas have been recorded and compared with those calculated from structural data with the aid of the DIFK computer program odified to correct for preferred orientation. I tensities calculated from structural data for random and for highly oriented samples ( ymmetrical and oblique settings) are presented for 1M, 2M~, 2M2, 3T polytypes of muscovite, 1M, 2M~, 3T polytypes of paragonite and 1M (C2/m and C2 space groups), 2M~, 2M 2, 3T polytypes of lepidolite. Among theoretically-derived simple mica polytypes the 1M, 2M 1, 2M 2 and 3 T structures are frequently identified in both natural (Bailey, 1980) and synthetic specimens (Velde, 1965a,b; Munoz, 1968; Kotov et al., 1980). Micas are abundant in igneous and in pneumatolytic rocks, where they form megascopic rystals, and are widespread in sedimentary rocks down to clay-size flakes. It is important to have a quick and reliable method for..their identification. The application of single-crystal methods for identification of phyllosilicate polytypes is limited to coarsely crystalline materials. Highly dispersed minerals must be studied by powder methods. The most effective is oblique-texture electron diffraction (Zvyagin, 1964