The Status of Clay Mineral Structures

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 a t least nine structures in 1965 or early 1966, and (c) personal communicat ion tha t a t least fifteen additional refinements are in progress. Points of especial interest in these recent studies follow. 1. Octahedral cation order is common, bu t te trahedral cation order is confirmed in only three cases. Because ordering of Si, A1 does not significantly affect the statistical tests for centrosymmctry, centrosymmetric space groups tha t do not permit order should be avoided during refinement. 2. Oversize te trahedral sheets articulate with smaller octahedral sheets by tetrahedral rotat ion and, for dioctahedral species, by tetrahedral t i l t ing around vacant octahedra. The lat ter mechanism influences the type and regularity of layer sequences. Undersize tetrahedral sheets articulate with larger octahedral sheets by ti l t ing plus octahedral contraction or by inversion of some tetrahedra. 3. The amount and direction of te trahedral 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. 4. 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 attract ion between the ions in the structures. The stabilities can be correlated with the energy available in the environment of crystallization.