X‐ray diffraction (XRD) studies of about 2000 soil clays show that 1500 kPa water retention, in addition to Atterberg limits, cation exchange capacity, 33 kPa water retention, volume of water retained between 33 and 1500 kPa, and linear extensibility varies with kind of soil clay and the intensity of the XRD peak of the dominant clay mineral. Hence, knowledge of soil clay mineralogy is particularly useful for predicting the behavior of soils. In standard characterization work, however, determining the property of interest is usually more efficient than determining mineralogy and then estimating the property. Determining both is not necessary if their relationship is understood. In predicting the behavior of soils, identification of the dominant clay mineral is sufficient if that mineral is smectite. If the mineral is kaolinite or clay mica, somewhat greater accuracy of prediction of properties is possible if measurements of the relative intensities of the other clay minerals in the suite are also known.