The idealized chemical formula for the serpentine minerals is Mg3Si2O5(OH)4, and the idealized structure consists of superimposed sheets, which are trioctahedral analogues of those that form the kaolinite minerals (fig. 5.1). Each of these sheets has two components; one is a network of linked SiO4 tetrahedra, (Si2O5)n, and the other a brucite type octahedral layer. Two-thirds of the hydroxyl ions at the base of the brucite layer are substituted by oxygens at the apices of Si-O tetrahedra. The repeat distances of an Si2O5 network are considerably smaller than those of a brucite layer, and it is this that is in great part responsible for the fact that serpentine minerals often show departures from the simple form of the structure. The two components may accommodate one another by distortion of their simple networks, or by changes in the network parameters through chemical substitution of larger or smaller ions, or by curvature of the composite sheet with tetrahedra on the inside and octahedra on the outside of the curve. A combination of all three methods of accommodation may, of course, occur.
The main kinds of serpentine mineral are chrysotile, antigorite, and lizardite. The latter seems to approximate most closely to the simple structure, since it has the simple unit cell a ⋍ 5.3, b ⋍ 9.2, c ⋍ 72 Å. Lizardites generally show some chemical substitution of trivalent ions for magnesium or silicon or both, but although they do have platy morphology their crystallinity is in general poor and grain size small. Optical properties are not usually measurable, but the coarser grained lizardite is nearly uniaxial (negative) with α perpendicular to the crystal plates.