ABSTRACT
The term “ clay minerals” is derived from the definition originally used by sedimentologists and soil scientists for the fraction of particles having very small size, with an equivalent diameter smaller than 2 μιη, which is the “ clay fraction.” Particles in this size range can include quartz, carbonates, metal oxides, and other minerals in addition to clay minerals, as well as amorphous materials. Although most clay minerals occur as particles too small to be resolved by an ordinary microscope, x-ray diffraction analysis shows that most of them, even in their finest size fraction, are composed of crystalline particles and that the number of crystalline minerals likely to be found is limited. Furthermore, a wide distribution of particle sizes is frequently present and certain clay deposits contain welldefined crystalline particles with diameters greater than 2 μπι. In this book a distinction is made between clays and clay minerals. The former term is used for the small particles found in soils and sediments, including crystalline and amorphous oxides and hydroxides of various metals, whereas the latter is used for a certain group of layered crystalline silicate minerals (phyllosilicates and related minerals).
