An important clay mineral of the approximate composition {Al₄[Si₃AlO₁₀]₂ (OH)₄}²⁻.nH₂O with some K⁺, Na⁺, or Ca⁺ ions also present; it belongs to the 2:1 group of phyllosilicates (sheet silicates) and the montmorillonite or smectite group includes bentonite; most members of this group are designated expansive clays by engineers, because they can accommodate many water molecules into their structure and they all possess an overall negative charge; sp. gr. variable but 2.0–2.7; hardness 2; monoclinic; white to grey with tints of blue, pink, pink-red, and green; dull lustre; usually occurs in massive, microcrystalline aggregates of very fine, scale-like crystals, but in soils as a hydrous aluminosilicate clay mineral with a layer-lattice structure (two sheets of tetrahedral silicon crystals enclosing a sheet of octahedral aluminium crystals) that expands when water enters between layers, thus making a soil material, typically with variable water content, subject to swelling and contraction; found very extensively, it results from the decomposition of volcanic ashes in marine basins and also occurs in the weathering crust of basic igneous rocks, e.g. diabases, basalts, gabbros, and peridotites. It is used extensively as an absorbant, for refining out suspended matter, in the textile and chemical industries.