Review of XRD-based quantitative analyses of clay minerals in soils: the suitability of mineral intensity factors

The determination of the types and relative amounts of the minerals present in soil forms an essential component of most soil characterization efforts. This paper reviews protocols for XRD-based quantitative clay mineral analysis in soils, with emphasis on methods using mineral intensity factors in combination with the so-called 100% approach. We summarize methodological differences and characteristic features and give information about problems, procedural flaws and the comparability of results of some widely distributed procedures. We found that XRD-based protocols are abundant and inconsistent. Procedural flaws exist independent of the quantification approach due to the need of inevitable suppositions and unknown variability in phase chemistry, degree of structural disorder and of interstratification of the soil clay minerals. Quantification protocol instructions using mineral intensity factors in combination with the 100% approach differ in every methodological step. Variance is increased through absent standardization of sample preparation, clay mineral identification or determination of the integrated peak area. Even if the whole quantification process is held constant and precautions are adhered to, results may only be considered as a rough estimate of reality. Quantitative aspects of mineralogical variability within one soil profile and among different soil profiles related by identical parent material may be assessed, if standardized procedures are employed. Different quantification protocols will yield different phase quantities. We are not able to recommend the uncritical use of mineral intensity factors as derived from the literature.