Kaolin is a soft, white, hydrous mineral-clay that is based on aluminum silicate [Al2(Si205)(0H)4], but which also contains minor amounts of quartz, feldspar, and sheet silicate minerals (mica, illite, smectite, and chlorite). Kaolin acts as a binder and provides plasticity. Its plate-like structure allows particles in a wet clay mass to slide across each other. Kaolin-based refractories and ceramics use natural kaolin or a mixture of clay and other ceramics such as alumina, calcium aluminate or silicon carbide. Kaolin is also referred to as clay, anhydrous aluminum silicate, aluminum silicate dihydrate, nacrite, dickite, kaolinite, calcined, kaolinite, china clay, bolus alba, porcelain clay, aluminum, silicate hydroxide, or aluminum silicate (hydrated). Geologically, there are two types of kaolin deposits: primary and secondary. Primary kaolin is formed through the alteration, or kaolinization, of in-situ minerals of feldspar and other aluminum silicates to kaolinite. Secondary kaolin is laid down as sediments, usually in fresh water, far from the place of origin. Various types of secondary kaolin are referred to as ball clay, fireclay, or flint clay depending on kaolinite content and their properties.