Clay minerals are the most important chemical weathering product of the soil. They are formed by the alteration of existing minerals or by synthesis from elements when minerals weather to their elemental form. It is not possible to make clays by grinding up silt or sand particles. Clay has many uses today including, pottery, ceramics, linings for landfills, computer chips, cosmetics, and pharmaceuticals.
Clay minerals are important in soils because of the negative charge they contribute for cation exchange. Understanding about soil clays is also important because of their shrink and swell potential upon wetting and drying. When soils are unsuitable for building because of a high shrink/swell potential this is because of the amount and type of clay present. Our study of clay minerals will involve looking at their structure and why this contributes to their properties.
Clay minerals are part of the larger class of silicate minerals: the phyllosilicates. Included in the phyllosilicate family are the larger true micas which include the familiar minerals muscovite and biotite. Much of what we know about clay minerals is from the macroscopic (i.e., single crystal) study of the true micas.