Abstract The basic serpentine structure is extremely simple. In spite of the simple crystal-chemical features involving the nearest neighbours (namely, the coordination polyhe-dra), complexity arises because of the different ways in which the basic polyhedra assemble together, forming flat layers in lizardite, curled layers in chrysotile, alternating layers in antigorite, flat kinked layers in polygonal serpentine, and flat geodesically kinked layers in polyhedral serpentine. Further complexity is derived from not-nearest-neighbour relations, such as polytyp-ism and polysomatism, that may occur as ordered and disordered distributions. A peculiar feature of chrysotile and polygonal serpentine is the presence of local fivefold symmetry. Chrysotile shares many nanoscale properties with synthetic nanotubes and nanowires. Serpentine minerals may be mutually associated, or interleaved with other layer silicates. Serpentinization and deserpentinization reactions have important implications for many extremely important large-scale processes occurring in the Earth’s crust and outer mantle. Due to their occurrence as tiny disordered crystals, meaningful structural study of serpentine minerals deals mostly with the nanoscale and may require alternative, unconventional methods. For this reason, electron microscopy techniques have long been used widely in the study of serpentine minerals, revealing a fascinating microstructural world.

Abstract The idealized chemical formula for the serpentine minerals is Mg 3 Si 2 O 5 (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 SiO 4 tetrahedra, (Si 2 O 5 ) 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 Si 2 O 5 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.