Crystallinity, crystallite size and lattice strain of illite-muscovite and chlorite: comparison of XRD and TEM data for diagenetic to epizonal pelites
Arkai, Peter; Merriman, Richard J.; Roberts, Brinley; Toth, Maria; Peacor, Donald R.
European Journal of Mineralogy Volume 8 Number 5 (1996), p. 1119 - 1138
published: Oct 30, 1996
manuscript accepted: May 29, 1996
manuscript received: Dec 23, 1995
ArtNo. ESP147050805017, Price: 29.00 €
Abstract Crystallinity indices, apparent mean crystallite sizes, lattice strain values and crystallite thickness distributions of illite-muscovite and chlorite from metapelitic rocks have been determined by powder XRD and/or by HRTEM. The studied slates and phyllite derive from the NE-Hungarian Paleozoic and Mesozoic formations affected by Alpine (Cretaceous) regional metamorphism, and cover a range from late diagenesis to the 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. XRDmeasured % lattice strain generally decreases with grade, with chlorite showing greater % strain than illite-muscovite, consistent with TEM evidence of strain-related textures in chlorite but rarely in illite-muscovite. There is reasonably good correlation between TEM-and XRD-determined crystallite sizes, especially for mean thickness as determined by the Scherrer method. However, there are 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 different standards. Therefore, accurate, absolute values of mean thickness cannot be predicted on the basis of illite and chlorite crystallinity or XRD line-profiles, and it is not possible to correlate precise values with boundaries of the zone of diagenesis, anchizone and epizone. Nevertheless, illite or chlorite crystallinity can be correlated with reasonably narrow ranges of mean crystallite size as a well-behaved function of metamorphic grade.