TEM and XRD determination of crystallite size and lattice strain as a function of illite crystallinity in pelitic rocks

Abstract

A B S TR A CT Average crystallite size and mean-square strain of illite in rock specimens and clay separates were measured independently in TEM images and by single-line Fourier (Voigt method) profile analysis of the c. 1 nm peak of XRD patterns for a prograde sequence of pelitic rocks (illite crystallinity indices=0.17-0.58°D 2h) from the Gaspé Peninsula, Quebec. The TEM-determined crystallite sizes in clay separates approximate those determined by Fourier profile analyses and those calculated from illite crystallinity indices by the Scherrer equation, with the exception of the diagenetic sample. The crystallite sizes and mean-square strains of illite in rock samples exhibit a trend similar to that determined by profile analyses, but the average crystallite sizes are up to five times larger than those measured for clay separates.TEM images show that all rock samples have a wide range of crystallite sizes, and the proportions of larger crystallites increase with metamorphic grade. The diagenetic illite is defect-rich, fine-grained (mean thickness by volume=c. 70 nm), 1M d material. Anchizonal illite tends to occur as separate aggregates of small 1M d and larger 2M 1 crystals (c. 200 nm), comprising arrays of subparallel coalescing packets. The epizone sample has thick (c. 400 nm), defect-free crystals of muscovite occurring in stacks of parallel layers, or subhedral crystals intergrown with large-angle boundaries. Cleaved crystals that are free of intracrystalline layer terminations are dominant in clay separates of all samples, having ranges of smaller sizes with volume-average thicknesses of c. 43, 43, and 81 nm (c. 14, 28, 67 nm by the Voigt method), respectively, for the three zones.The results suggest that illite crystallinity indices do not provide a direct measure of a single microstructural state of illite in rocks, although they yield consistent limits for average crystallite sizes for the anchizone (23 & 48 nm here). Therefore, they serve as a general parameter of the degree of recrystallization on a relative basis, in part because the contributions of all peak-broadening variables (mixed layering, size and strain) decrease regularly with prograde regional metamorphism of pelites. The microstructural changes caused by rock disaggregation are probably a function of those variables as well. The data collectively demonstrate a trend from metastable, defect-rich, small crystals towards a stable assemblage of larger, defect-free crystals, through dissolution of strained crystals and neocrystallization, consistent with the Ostwald step rule.

Illite “Crystallinity” Revisited

“…Applying the principle of equivalence between Nfp and IW2w (IWAD), Arkai et al (1996) may have measured the number of consecutive mica-like layers separated by swelling layers. Thus, it is not surprising that our resuits are apparently in accordance with the values of Weber et aL (1976), Krumm (1992), Merriman et al (1990, Rice (1994), ,~rkai et al (1996), Dalla Torre et al (1996), andJiang et al (1997), all of whom used the Scherrer equation (Table 2). Our resuits are in accordance with the mean N of previous studies, 24 _+ 5 (ltr) and 55 ---17, for the anchizone lower and upper limits, respectively.…”

“…Determination of the average of T (2P) is possible by estimating K for different CSD thickness distributions using XRD pattern simulations or experimental XRD data correlated with transmission electron microscope measurements (TEM) (Nieto and Sanchez-Navas, 1994). T determinations obtained using XRD techniques were corroborated by direct measurements of T using TEM observations (Eberl and Srodo~, 1988;Merriman et al, 1990;Eberl and Blum, 1993;Nieto and Sanchez-Navas, 1994;,~rkai et al, 1996;Dalla Torte et al, 1996;Jiang et al, 1997;Wan" and Nieto, 1998). Mean thicknesses obtained from TEM and XRD analysis are comparable but both methods can be inaccurate if swelling layers are not considered.…”

“…Another source of FWHM broadening is disorder in the crystal structure , such as lattice strain which involves small variations in d value within CSDs of a single layer type (Drits and Tchoubar, 1990). Although lattice strain clearly affects KI values (,~rkai et aL, 1996;Wan" and Nieto, 1998;Drits et aL, 1998), its role is difficult to assess accurately in peak broadening analysis, because many authors interchange small variations of a single d value within one layer type with interstratification and CSD thickness (e.g., ,~rkal et al, 1996;Jiang et aL, 1997). According to Eberl et al (1998a), the 10-,~ peak is not significantly affected by strain.…”

Effect of Urea Excess on the Properties of the MgAl2O4 Obtained by Microwave-Assisted Combustion

“…However, the values were not equal, since an exact correlation between the crystallite sizes obtained by the two techniques would be fortuitous, as it was pointed out by Jiang et al 51 . In recent studies 24,26 , non-stoichiometric magnesium aluminate spinel powders were prepared by solution combustion method and presented small and comparable crystallite sizes, estimated at 13-15 nm.…”

Deshidroxilaciones y efectos Ostwald ripening en pizarras de techar

“…En filosilicatos dispersos (illita, clorita), se puede asumir que los principales modificadores de la geometría del pico de difracción de rayos X (DRX) a 10 Å de la clorita-illita son el tamaño de cristal y el hinchamiento de las capas 00l (4)(5). Los mecanismos de deshidroxilación de la moscovita-illita son conocidos, cabiendo destacar que hacia los 475ºC, la red cristalográfica expande doble según el eje c (001) que según los otros ejes (6-7).…”