This study aims at further understanding of the mechanisms how lattice-preferred orientations (LPO) develop during deformation in the main eclogite minerals. Microstructures and textures of deformed eclogites from the Les Essarts complex (Western France) were investigated using optical microscopy and electron backscatter diffraction (EBSD) in the scanning electron microscope. Microfabric analyses of eclogite-facies minerals are used to identify their deformation mechanisms, which define the rheology at high-pressure metamorphic conditions. Mechanisms of intracrystalline deformation by dislocation movement (dislocation creep) result usually in a non-linear flow law (typically power law), while diffusive processes (diffusion creep) correspond to linear flow laws. General microstructural observations may suggest intracrystalline deformation (dislocation creep) of omphacite. The omphacite LPO vary between S- and L-type and correlate with oblate or prolate grain shape fabrics, respectively. Until now, these LPO types have not been understood by plasticity models based on dislocation glide on the known slip systems in clinopyroxene. An alternative interpretation is given in terms of anisotropic growth and dissolution, with grain boundary diffusion as the rate controlling process. There are further indications suggesting diffusion creep with concomitant anisotropic growth and dissolution as a main deformation mechanism in omphacite. In omphacite around a hollow garnet, crystallographic and shape fabrics align with the c[001] axes parallel to the grain elongations defining the mineral lineation, which rotates locally with the inferred flow direction. In this part, the grain sizes of omphacite and rutile are larger than in the surrounding matrix. The geometry of both the shape and crystallographic fabrics is interpreted to represent the local stress regime (directions and ratios of the principal stresses). The LPO of rutile duplicate the LPO of omphacite and a similar distinction between S- and L-type was used. Rutile deformation mechanisms probably involve dislocation creep as well as diffusion creep. Quartz mainly occurs as an interstitial phase with weak LPO patterns interpreted as random. No representative obliquity of the LPO in omphacite nor rutile with respect to foliation and lineation was observed to be used as potential shear sense criteria. However, the rutile LPO was slightly rotated relative to the omphacite LPO consistently in most samples. The results suggest that diffusion processes are strongly involved in the deformation of eclogites. A linear flow law should be taken into account in tectonic models where eclogites are incorporated. 

Many materials are polycrystalline aggregates of tiny crystallites or grains of various sizes and shapes. Even for aggregates whose crystallites are chemically identical, the crystal lattices of the grains will still differ in their orientation in space. Since each crystallite is anisotropic in its physical properties, the macroscopic properties of a polycrystal will likewise be anisotropic unless the orientations of its constituent crystallites are completely random and the anisotropies of the crystallites even themselves out.

This fully revised and updated edition has been prepared by a very active worker in the field, who has used previous editions extensively both in teaching and in research, together with one of the original authors. Since the first edition, written in the late 1960s, understanding of crystal defects such as dislocations, stacking faults, twin, grain and interphase boundaries and of their effect on the mechanical and electrical properties of materials has grown enormously and has been accompanied by a total change in style of the way in which both research and teaching are carried out through the use of the fast digital computer. This edition takes account of this change. <...>

Some families of inorganic compounds form an array of more or less closely related, very complex structures with a number of independent atomic positions and unit cells of large dimensions. These structures can be broken up into a number of fragments (modules) that have relatively simple substructures but are joined into a more complex whole. Some modules occur in many members of these families, often in variably expanded/contracted forms as well as combined with elements of other kinds.