Abstract: The basic outlines of most of the hydrous layer silicate structures were determined during the 1930's. Present escalating interest in obtaining additional detail is indicated by (a) publication of over twenty structural refinements (2-D or 3-D) during 1954–64, (b) publication of at least nine structures in 1965 or early 1966, and (c) personal communication that at least fifteen additional refinements are in progress. Points of especial interest in these recent studies follow.
Octahedral cation order is common, but tetrahedral cation order is confirmed in only three cases. Because ordering of Si, Al does not significantly affect the statistical tests for centrosymmetry, centrosymmetric space groups that do not permit order should be avoided during refinement. Oversize tetrahedral sheets articulate with smaller octahedral sheets by tetrahedral rotation and, for dioctahedral species, by tetrahedral tilting around vacant octahedra. The latter mechanism influences the type and regularity of layer sequences. Undersize tetrahedral sheets articulate with larger octahedral sheets by tilting plus octahedral contraction or by inversion of some tetrahedra. The amount and direction of tetrahedral rotation and tilt, length of T—O, M—O, and O—O bonds, sheet thicknesses, and relation of cell dimensions to composition can now be predicted with some confidence. Variation in layer stacking (polytypism) is common. In some cases the stabilities of different polytypes can be explained by the relative amounts of repulsion and attraction between the ions in the structures. The stabilities can be correlated with the energy available in the environment of crystallization.