The Warren-Averbach method, an X-ray diffraction (xrd) method used to measure mean particle thickness and particle-thickness distribution, is used to restudy sericite from the Silverton caldera. Relative mean particle thicknesses determined by this method for the Silverton illites correlate well with cation-exchange capacity, with fixed interlayer chemistry, with an xrd intensity ratio, with the wave number of a 824-834 cm−1 infrared absorption band, with the Kubler index, and with the apparent K-Ar age for the samples. Apparent particle-thickness distributions indicate that the clays may have undergone Ostwald ripening and that this process has modified the K-Ar ages of the samples. The mechanism of Ostwald ripening can account for many of the features found for the hydrothermal alteration of illite.

Expandabilities measured by the xrd peak-position method for illite/smectites (I/S) from various locations are smaller than expandabilities measured by transmission electron microscopy (tem) and by the Warren-Averbach (W-A) method. This disparity is interpreted as being related to the presence of nonswelling basal surfaces that form the ends of stacks of illite particles (short-stack effect), stacks that, according to the theory of interparticle diffraction, diffract as coherent X-ray scattering domains. Previous determinations of the charge of the illite interlayer have been based on expandability measurements that have not been corrected for the presence of these nonswelling surfaces. Thus, the value for the illite-layer charge, previously thought to be about −0.75 equivalents per O10(OH)2, needs revision. By using tem methods for measuring maximum expandabilities, the fixed cation content of illite layers in I/S is determined to be approximately −0.9 equivalents per O10(OH)2. This charge is independent of illite origin and expandability. Based on a long extrapolation, the mean charge on exposed basal surfaces of illite particles from the Silverton caldera is approximately −0.48 equivalents per O10(OH)2.

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