The white mica crystallinity index has been analyzed as a function of the following parameters: 1) the domain size, 2) lattice distortion, and 3) the presence of other phyllosilicates overlapping the 10 Å XRD peak, particularly I/S mixed-layer. (2) is determined by the variance method suggested by Wilson (1963) and subtracted from the experimental XRD profile. The remanent peak width is compared with the statistical values obtained from direct measurements of (1) on HRTEM images. The relation between 10 Å and 5 Å peak widths was used for the evaluation of (3). When R3 I/S mixed-layers are absent, the progressive decrease of the illite crystallinity index is a consequence of a parallel improvement in both quantity of defects and crystallite size. The contribution of the latter to the XRD peak width is expressed by the Scherrer formula using K=0,5. A direct relationship Between size and peak width, without subtraction of the lattice distortion contribution, responds to the same formula with K=0,83. Since mica crystallite sizes are highly heterogeneous in all the samples and their distributions are asymmetrical, the mode differs from the mean and shows better correlation with the XRD peak width. According to the experimental relationship, the Kübler anchizone limits equivalent to 900 and 300 Å in terms of crystallite size.

12 páginas, 4 figuras, 2 tablas

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