- © The Mineralogical Society Of America
An intrinsic property of polycrystalline materials is the orientation distribution of crystallites. In some cases this distribution is random, yet often there is preferred orientation of crystallites relative to macroscopic axes that may have been attained during a deformation process. Many rocks—metamorphic, igneous as well as sedimentary—display non-random orientation distributions that are the cause for anisotropy of macroscopic physical properties. Interpretation of textures in materials has to rely on a quantitative description of orientation characteristics. Two types of preferred orientations need to be distinguished: The shape preferred orientation (or often abbreviated SPO) describes the orientation of grains with anisotropic shape. The lattice preferred orientation (LPO) or “texture” refers to the orientation of the crystal lattice. (LPO is an unfortunate term since the lattice does not always uniquely describe the crystal orientation, as in the trigonal mineral quartz with a hexagonal unit cell. “Crystallographic preferred orientation,” CPO, would be more appropriate). Shape and crystal orientation can be correlated, such as in sheet silicates with a flaky morphology in schists, or fibers in fiber reinforced ceramics. In other cases they are not. In a rolled cubic metal or a plastically deformed quartzite, the grain shape depends on the deformation and is not directly related to the crystallography.
Many methods have been used to determine preferred orientation. Geologists have applied extensively the petrographic microscope equipped with a Universal stage to measure the orientation of morphological and optical directions in individual grains (e.g., Wahlstrom 1979; Wenk 1985). More recently electron diffraction, both with transmission (TEM) and scanning electron microscopes (SEM) have been used to measure orientation of crystals (e.g., Schwarzer and Weiland 1988; Randle and Engel 2000). In this case, the location of a grain can be determined, which permits to correlate microstructures, neighbor relations and texture. Also from individual orientation measurements …