Modular aspects of layer silicates
Published:January 01, 1997
The ideal composition of lizardite is (Mg,A1)3[(Si,Al)2O5(OH)4]; its structural layer (Ruzklidge & Zussman, 1965; Krstanović, 1968; Mellini, 1982) consists of a tetrahedral silicate sheet joining with apical oxygens to a continuous, Mg-centred octahedral sheet (Fig. 1). The dimensions of a structural layer are a = 5.332, b = 9.235, the layer thickness = 7.233 Å and γ = 90° (Mellini, 1982). The apical oxygens of tetrahedral sheets are assumed to point into the positive sense of c. Although the symmetry of the structural layer is P31m, for easier indexing of SAED patterns and comparing different polytypes, the C-centred orthogonal unit cell () is used and all indices refer to this setting throughout.
The early structure determinations of trioctahedral 1:1 (T:O) phyllosilicates (Steadman & Nuttal 1962, 1963, 1964; Rucklidge & Zussman, 1965; Jahanbagloo & Zoltai, 1968) led to the recognition that polytypism is their common structural feature.
hydroxyl planes of the octahedral sheet and the basal planes of the tetrahedral sheets are the same between every pair of structural layers (Bailey, 1969). Under this condition polytypes differ from each other at least in one of the following three aspects (Zvyagin et al., 1965; Bailey, 1969, 1988):
– the sequence of slanting of octahedra in subsequent layers (hereafter the orientation of octahedra as in Fig. 2a will be denoted ‘+’, whereas the orientation as in Fig. 2b will be denoted ‘−’)
– the sequence of a/3 shifts along <100>, <110> (i.e. : ,
Figures & Tables
Modular Aspects of Minerals
Since the first beginning of the crystal chemical study of the inorganic compounds, a simple modular approach was developed, by looking at the crystal structures as built up through the assembling of simple polyhedral units. This approach was no only useful for a vivid and insightful description of the complex atomic arrangements of natural and synthetic compounds, but, through the use of simple and powerful rules for assembling polyhedral units, it displayed an extraordinary heuristic power, suggesting reliable models for many complex structural assemblages. The polyhedral approach also laid the basis for meaningful classifications which were applied to all the classes of inorganic compounds.