Modification of illite-muscovite crystallite-size distributions by sample preparation for powder XRD analysis

Mean crystallite sizes of illite-muscovite have been obtained by direct measurements using TEM on ion-milled (original rock) samples and separates prepared for XRD analysis, and by XRD profile analysis of separates in a prograde sequence of metapelites from southern New Zealand. The mean crystallite sizes determined by the analyses of XRD peak profiles increase with metamorphic grade, consistent with the trend determined by TEM from ion-milled rock samples. However, the sizes obtained by TEM on the ion-milled rock samples are approximately two to four times larger than those by XRD profile analyses. This difference is greater in higher-grade than lower-grade samples. The mean sizes of crystallites measured using TEM on the separates that were used to obtain XRD profiles correspond well to those from XRD profile analyses. Further TEM and AFM observations of separates show that crystals have been broken across and cleaved along (001) planes. These relations indicate that the XRD data were obtained from samples that differ significantly from the original rock samples; the data indicate that sample preparation for XRD analysis significantly altered the original size-distributions. Crystallite-size distributions are poor approximations to lognormal in original rocks, but are modified to lognormal or nearly lognormal distributions in separates. Interpretation of growth conditions from size distributions of separates is therefore subject to error. Only size distributions of unseparated samples, or separates where the effects of the separation process are known, should be used as indicators of growth conditions.