instrumental techniques applied to mineralogy and geochemistry

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10 Elisabetta Mariani et al successfully. Nevertheless particular care must be taken when polishing hydrophilic minerals such as for example NaCl or gypsum. Ion-beam milling techniques may be explored for polishing phyllosilicates, EBSD analysis of which is currently limited to the basal planes due to difficulties in achieving satisfactory polishing of any other orientation. EBSD geometry The specimen and the phosphor screen are positioned in the chamber so that a large (~90°) angular range can be obtained on EBSPs. The projection of the source of BSE on the phosphor, along a trajectory at 90° to it, generates the patter centre (PC). The closer the phosphor and the specimen (i.e. the shorter the distance, DD, between the source point and the pattern centre) the larger the angular range of the EBSP imaged. This yields better quality pattern images that are then easier to index. A digital camera, with axis orthogonal to the phosphor screen, is positioned behind the latter. The positioning of the phosphor is generally restricted by chamber geometry, number of existing detectors and in-situ experimental requirements. To date only 2 SEMs (at Liverpool and Montpellier) have been purposely built to optimize EBSD data acquisition, both standard and during in-situ experiments. Data acquisition set-up EBSD data can be acquired using dedicated software packages supplied by Oxford Instruments HKL and TSL Crystallography. In order to index EBSPs it is necessary to calculate the solid angle between the cones that project as bands on the phosphor screen. Thus a calibration of the EBSD geometry is needed where the position of the pattern centre and the detector distance may be obtained from the EBSP image of a known material (e.g. Si) in a known orientation. Refinement of the initial calibration should be performed before any manual or automated EBSD work is carried out. The polychromatic component of electron scattering generates a background signal that affects the quality of EBSPs. Correction for the background signal can be applied within the dedicated software packages. This involves collecting the signal at very low
Electron backscatter diffraction (EBSD) in the SEM: applications to microstructures in minerals and rocks and recent technological advancements 11 magnification in scanning mode and then averaging and subtracting it from the EBSP signal. Indexing an EBSP involves calculating the position and orientation of bands with respect to the PC thus obtaining the specimen crystallographic orientation at the source point. Indexing algorithms require knowledge of the crystal symmetry, the lattice parameters and the number of lattice planes that give bands on EBSPs (refractors). Correct indexing by the software can be assessed by rigorous comparison of the simulation bands with the live EBSP bands (Winkelmann et al. 2007). Whilst interactive (manual) indexing is available in the software, fully automated EBSD (e.g. Juul Jensen and Schmidt 1990) has become common practice on a large number of rock forming minerals. Very recent advancement in the EBSD detector and software technology allow faster data collection than was ever possible before, at top speeds of 600 data points per second. Additionally combined acquisition of EBSD and EDS data is now possible, although this hampers the speed of the acquisition. EBSD provides us with orientation and misorientation distribution datasets for a given crystalline material (Wheeler et al 2001). Knowledge of the relationship between the specimen surface and the microscope, kinematic and geographic reference frames allows interpretation of data from rock samples at the regional scale. Also, information on the 2D geometry of grain boundaries can be obtained from EBSD data. Recently developed non-standard data processing analysis enables calculation of the distribution of grain boundary planes from EBSD data in cubic materials (Randle 2006). Applications of EBSD in mineralogy and petrology A comprehensive review of a number of different applications of the EBSD technique in petrology is given by Prior et al. 1999. Since then significant progress in sample preparation and in both EBSD hardware and software technology has occurred and the effective application of EBSD to mineralogy and petrology has increased exponentially (e.g. Storey and Prior 2005, Halfpenny et al. 2006, Pennock et al. 2006, Barrie et al. 2007). Excellent to satisfactory EBSD datasets (depending on the quality of polishing, material characteristics and working conditions) can be acquired from a large
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