Quantitative Mineral Analysis of Clays

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Abstract
Quantitative determination of the weight percentages of mineral phases in rocks and soils challenges theorist and analyst alike. Although precision – analytical reproducibility – is often described in great detail, few, if any, commercial laboratories or published methods say much about accuracy – the approach of the determined value to the "real" value. Results are often reported with four significant figures, but for most analyses, only one or two figures have real significance. Technologists and non-technologists, unfamiliar with the above problems, typically assume that quantitative phase analysis (QPA) is similar to elemental chemical analysis, not realizing that the variabililty of minerals is much greater than that of atoms. Accuracy may be of far more that academic interest. In court, the amount of kaolinite in soil adhering to a boot may be critical piece of forensic evidence in a murder case, and in an equity hearing, the effect of clay in a petroleum reservoir on predicted production may result in each 0.1% of smectite being worth millions of dollars. Many geoscientists have a rather casual attitude towards quantitative mineral analysis, perhaps because they realize how semiquantitative such analyses can be. X-ray powder diffraction (XRD) peaks are commonly normalized to give "quick and dirty" results, the accuracy of which is unknown. That these results fail to correlate with other data may be solely due to bad mineral analyses. This volume describes XRD methods including a complete treatment for QPA of oriented clay aggregates. Both theory and practice are detailed, and the section on preferred orientation is especially enlightening.

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