Vermiculite is a hydrated magnesium aluminum silicate mineral. The formation of vermiculite has been variously ascribed to natural weathering, hydrothermal action, percolating ground waters or combinations of all these three actions. Research over recent years, however, tends to exclude an exclusively hydrothermal origin for vermiculite, and the main agents for vermiculite formation are believed to be due to natural weathering and percolating ground waters. Commercial vermiculite deposits are normally derived from either iron bearing phlogopite or biotite micas, by the natural alteration of the mineral structure to reduce the alkali metal levels and to incorporate crystalline water into the structure.

The basic structure of the mineral is identical to the micas and to talc: a 2:1 silicate sheet composed of two flat layers of silica and alumina tetrahedra (the tetrahedral layers), which are joined together in a layer composed of apical oxygen atoms, and magnesium, iron, and hydroxyl molecules (the octahedral layer). To see a detailed discussion of the ‘Crystal Structure of Magnesium Vermiculite’ see the Matheson and Walker paper on this website which is reproduced by kind permission of the Mineralogical Society of America from their journal American Mineralogist Vol. 38, pp. 231-256

Various chemical formulae are given for vermiculite. The general formula given below has been calculated from 65 vermiculite analyses.

(Mg5,Fe+20.2,Fe+30.8) [Si5.5,Al2.5,O20] (OH)4

Where the components in the first bracket represent the ion-exchangeable layer, the components in the second bracket compose the cations of the octahedral layer, and the components within the brackets comprise the tetrahedral layer.