1. Clay refers to a soft sediment or deposit that is plastic when wet and comprised of very fine- grained materials, mainly hydrous aluminum silicates. 2. A natural, earthy, fine-grained material which develops plasticity when mixed with a limited amount of water; composed primarily of silica, alumina, and water, often with iron, alkalies, and alkaline earths. 3. The fraction of an earthy material containing the smallest particles, that is, finer than 3 micrometers. 4. A clastic mineral particle of any composition that has a grain size smaller than 1/256 mm. The term is also used in reference to a broad category of hydrous silicate minerals in which the silica tetrahedrons are arranged into sheets.
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