Polytypism of trioctahedral 1:1 layer silicates

Abstract-Polytypism in trioctahedral 1:1 phyllosilicates results from two variable features in the structure. (1) The octabedral cations may occupy the same set of three positions throughout or may alternate regularly between two different sets of positions in successive layers. (2) Hydrogen bonding between adjacent oxygen and hydroxyl surfaces of successive layers can be obtained by three different relative positions of layers: (a) direct superposition of layers, (b) shift of the second layer by a[3 along any of the three hexagonal X-axes of the initial layer, with a positive or negative sense of shift deter-mined uniquely by the octahedral cation set occupied in the lower layer, and (c) shift of the second layer by + _ b/3 along Yt (normal to X1) of the initial layer egardless of octahedral cation sets occupied. Assuming ideal hexagonal geometry, no cation ordering, and no intermixing in the same crystal of the three possible types of layer superpositions, then twelve standard polytypes (plus four enantiomorphs) with periodicities between one and six layers may be derived. Relative shifts along the three X-axes lead to the same layer sequences derived for the micas, namely 1M, 2M1, 3T, 2M2, 2Or, and 6H. Polytypes l T and 2H1 result from direct superposition of layers. Layer shifts of b[3 lead to polytypes designated 2T, 3R, 2H2, and 6R. The twelve standard 1: 1 structures can be divided into four group