The application of electron backscatter diffraction and orientation contrast imaging in the SEM to textural problems in rocks

In a scanning electron microscope (SEM) an electron beam sets up an omni-di rectional source of scattered electrons within a specimen. Diffraction of t hese electrons will occur simultaneously on all lattice planes in the sampl e and the backscattered electrons (BSE), which escape from the specimen, wi ll form a diffraction pattern that can be imaged on a phosphor screen. This is the basis of electron backscatter diffraction (EBSD). Similar diffracti on effects cause individual grains of different orientations to give differ ent total BSE. SEM images that exploit this effect will show orientation co ntrast (OC). EBSD and OC imaging are SEM-based crystallographic tools. EBSD enables measurement of the crystallographic orientation of individual rock-forming minerals as small as 1 mu m, and the calculation of misorienta tion axes and angles between any two data points. OC images enable mapping of all misorientation boundaries in a specimen and thus provide a location map for EBSD analyses. EBSD coupled to OC imaging in the SEM enables comple te specimen microtextures and mesotextures to be determined. EBSD and OC im aging can be applied to any mineral at a range of scales and enable us to e xpand the microstructural approach, so successful in studies of quartz rock s, for example, to the full range of rock-forming minerals. Automated EBSD analysis of rocks remains problematic, although continuing technical develo pments are enabling progress in this area. EBSD and OC are important new tools for petrologists and petrographers. Pre sent and future applications of EBSD and OC imaging include phase identific ation, studying deformation mechanisms, constraining dislocation slip syste ms, empirical quantification of microstructures, studying metamorphic proce sses, studying magmatic processes, and constraining geochemical microsampli ng. In all these cases, quantitative crystallographic orientation data enab le more rigorous testing of models to explain observed microstructures.