CRYSTALLINITY, CRYSTALLITE SIZE AND LATTICE STRAIN OF ILLITE-MUSCOVITE AND CHLORITE - COMPARISON OF XRD AND TEM DATA FOR DIAGENETIC TO EPIZONAL PELITES

Crystallinity indices, apparent mean crystallite sizes, lattice strain values and crystallite thickness distributions of illite-muscovite an d chlorite from metapelitic rocks have been determined by powder XRD a nd/or by HRTEM. The studied slates and phyllite derive from the NE-Hun garian Paleozoic and Mesozoic formations affected by Alpine (Cretaceou s) regional metamorphism, and cover a range from late diagenesis to th e epizone (greenschist facies chlorite zone). The average crystallite size for both illite-muscovite and chlorite increases with increasing metamorphic grade, although illite-muscovite crystallites are thicker than those of chlorite. XRD-measured % lattice strain generally decrea ses with grade, with chlorite showing greater % strain than illite-mus covite, consistent with TEM evidence of strain-related textures in chl orite but rarely in illite-muscovite. There is reasonably good correla tion between TEM-and XRD-determined crystallite sizes, especially for mean thickness as determined by the Scherrer method. However, there ar e significant differences in results for four different XRD data-based methods (Scherrer, Voigt, variance and Warren-Averbach), inferred to be caused by approximations in each method, and between results for di fferent standards. Therefore, accurate, absolute values of mean thickn ess cannot be predicted on the basis of illite and chlorite crystallin ity or XRD line-profiles, and it is not possible to correlate precise values with boundaries of the zone of diagenesis, anchizone and epizon e. Nevertheless, illite or chlorite crystallinity can be correlated wi th reasonably narrow ranges of mean crystallite size as a well-behaved function of metamorphic grade.