The dynamic behaviour of the Earth's lithospheric plates is controlled by the rheological properties of polyphase rocks. Lithosphere deformation is often concentrated into shear zones, which contain very fine grained rocks. Previous work on deformation mechanisms has predominantly concentrated on only one of the minerals within these rocks. This has served well as a first approach, but several shortcomings have become evident concerning the highest strained and most fine grained parts of shear zones, which are composed of polyphase mixtures. Therefore, models describing the rheology of shear zones, based on single phase flow laws are inadequate and can lead to erroneous interpretations of lithospheric rheologies. The aim of the proposed research is to investigate the relation between microstructure and physical properties of some fine grained polyphase materials which can serve as a first understanding of the deformational behaviour of polyphase rocks. As a result of the proposed research, microstructural development and physical properties of polyphase rocks can be modelled. These models will be used to predict the rheology of polyphase rocks in lithospheric shear zones and their influence on lithosphere dynamics.