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High-resolution transmission electron microscopy (HRTEM) reveals a wide variety of deviations from perfect crystalline periodicity within many mineral groups. Among the defects, the linear and planar ones are particularly abundant in synthetic and natural phyllosilicates which show a quasi-one-dimensional structural organization. Directly related to the layerby-layer or spiral growth stage, various dislocations and stacking faults are observed. They may explain, for example, the complex polytypic behaviour of micas and chlorite. In the same manner, specific deformation defects are easily visible. They characterize different degrees of deformation in phyllosilicates related to the tectonic history of their host rocks. Furthermore, HRTEM reveals that intercalations of different structural units may also occur in sheet silicates, accompanying their growth and deformation mechanisms. In this case, more or less important chemical changes are involved in addition to the structural changes, with all the consequences this may induce concerning the meaning and development of such 'mixed minerals'. |
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