DETERMINATION OF WHITE K-MICA POLYTYPE RATIOS - COMPARISON OF DIFFERENT XRD METHODS

Six different X-ray diffraction (XRD) methods for determining the rela tive amounts of 2M1 and 1M white K-mica polytypes were compared. Unori ented samples of different concentrations of the two polytypes were pr epared using the technique described by Handschin & Stern (1989) and S tern (1991). An evaluation of different XRD techniques reported in the literature revealed that the method proposed by Caillere et al. (1982 ) is the most accurate. The main reasons are the use of the 1M 112 and 2M1 025 reflections with relatively strong intensities, showing no su perposition with feldspar reflections, and being only slightly affecte d by sample preparation effects.