Any geotechnical engineering textbook includes an introductory chapter on soil properties such as grain size distribution and Atterberg limits, and on soil classification. Typically, there is a dedicated sub-section on soil mineralogy with special emphasis on the influence of clay minerals on soil behavior. However, geotechnical engineering practice largely relies on the USCS classification and parameters such as % finer than the #200 sieve (0.075 mm) to assess soil behavior for problems such as swell, chemical stabilization, compressibility etc. This approach is quite limited, since USCS classification does not capture differences in soil behavior between a single class (e.g. CH) that are attributed to soil mineralogy. X-ray Diffraction has been used widely to qualitative identify the type of soil minerals present in a soil, but the lack of quantitative information precludes the translation of this information in quantitative guidelines for soil behavior. This paper presents a new approach to clay characterization, which combines quantitative mineralogical analysis using the Rietveld method, with physicochemical characteristics including specific surface area, cation exchange capacity and potassium content. The combination of several techniques enables the quantification of soil minerals and of the amorphous content of a soil with high accuracy and precision, so that the engineering behavior of different soils can be interpreted quantitatively as a result of chemical/mineralogical and mechanical properties. Such an approach can lead to an improved paradigm for various design problems of clay soils.