Figure 1. Claystone. Credit: Mineral Information Institute.
Figure 2. Fire clay.
Clay is any soil material with a particle size of less than 2–4 micrometers in diameter, i.e., finer-grained than silt or sand; an earthy particulate which becomes plastic when wet, including mud (which is used in oil drilling). Clays are used as catalysts in petroleum refining, for making molds for casting and, when molded and fired, for ceramics, porcelain, and pottery, bricks and tiles. They are also used in making cement and rubber, and as ion-exchange agents for softening hard water.
Clay rocks, including mudstones and shales, are microcrystalline rocks composed mainly of clay-size particles. Their mineralogical composition is highly variable, but they usually contain a high proportion of clay minerals, hydrated aluminum and magnesium silicates, including bentonite, chlorite, diaspore (hydrated aluminum oxide), illite (hydrated mica), kaolinite, and meerschaum.
Fire clay (see Figure 2) is a type of clay which, being refractory (melting point above 1,500°C), is used to make firebrick. It has a high content of aluminum oxide and silica, approaching kaolin in composition; better grades contain at least 35% (Al2O3) when fired.
Clay in prebiotic evolution
J. B. S. Haldane and, more recently, Graham Cairns-Smith, have been among those to promote the idea that clay minerals may have played a vital role in the manufacture of large prebiotic molecules – their lattice structures acting as templates for the organization of organic matter into polymers.