instrumental techniques applied to mineralogy and geochemistry

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8 Elisabetta Mariani et al Introduction The analysis of 1) crystallographic orientations and misorientations and 2) the geometry and structure of subgrain and grain boundaries is fundamental to a comprehensive petrographic study of any rock sample. In the past quantitative information on 1) and 2) was gathered using the optical microscope combined with the universal stage. However such measurements were time consuming and limited to a few crystal symmetries. More recently new techniques have been developed which provide high resolution (few 10s of nm) qualitative and quantitative 2D and 3D microstructural data. These include computer-integrated polarization microscopy (CIP) (van Daalen et al. 1999), electron backscatter diffraction (EBSD) (Prior et al. 1999), serial sectioning using a focused-ion-beam (FIB) system in the SEM (Groeber et al. 2006) and synchrotron X-Ray tomography (Mikulik et al. 2003). Of these techniques EBSD is the more widely used in the Earth Sciences. It is fully automated, fast and allows collection of accurate, reproducible and statistically meaningful crystallographic orientation data of minerals belonging to any of the seven crystal systems (from cubic to triclinic). EBSD is a very important tool to the mineralogist and petrologist as it allows testing of microstructural models based on the distribution of crystallographic orientations (Prior et al. 1999). In this short contribution we review the basic principles of EBSD, analytical procedures that are specific to mineral and rock samples and some applications to mineralogy and petrology. Recent progress on in-situ heating and deformation experiments is also addressed. Basic principles of EBSD High energy electrons from an electron beam interact with the target specimen in many different ways. Broadly we may distinguish between 1) elastic interactions and 2) inelastic interactions. In 1) the scattered electrons do not loose significant energy compared to the primary electrons, whilst in 2) considerable energy is lost due to the activation of a variety of physical processes in the specimen. A thorough review of specimen-beam interactions is given in the software package MATTER (www.matter.org.uk). The incident electron beam diameter is always larger than the atomic spacing, thus, by interaction with a population of nuclei in the specimen, incident
Electron backscatter diffraction (EBSD) in the SEM: applications to microstructures in minerals and rocks and recent technological advancements 9 primary electrons will be scattered within the sample in all directions. High energy electrons which exit the specimen via the surface of incidence after one or more scattering events are backscatter electrons. Of these, those that satisfy the Bragg equation for diffraction describe conical trajectories for each lattice plane. Such diffraction cones approximate planes and may be imaged on a phosphor screen as sub-parallel diffraction lines (bands). A network of diffraction lines forms an electron backscatter diffraction pattern (EBSP) or Kikuchi pattern. Intersecting bands result in bright spots on the EBSP which correspond to zone axes. Thus elements of symmetry can be recognized in EBSPs. Kikuchi patterns may be imaged in the transmission electron microscope (TEM) as well as in the SEM (e.g. by electron channelling (Lloyd et al. 1987). In this contribution we focus on EBSD in the SEM (Randle 1992 and Prior et al. 1999). The resolution of EBSD is a function of the accelerating voltage, which controls the depth of penetration of the electrons in the specimen (activation volume). The smaller the activation volume, the higher the resolution. Also, an angle of incidence of 70° between the electron beam and the specimen normal results in a statistically higher number of BSE emitted from within few tens of nm of the sample surface and thus in a clearer EBSD signal. In a field emission (FE) SEM, at 70° tilt angle and 20 kV accelerating voltage the resolution of EBSD is < 1 μm and sometimes as low as 30-100 nm. The quality of EBSPs is controlled by the beam current (or spot size). A large spot size is required to obtain sharp EBSPs, however this reduces special resolution. Thorough descriptions of the principles of EBSD are given by Randle (1992) and Prior et al. 1999. Problems with resolution and quality of EBSPs mainly due to charging in nonconductive materials such as rocks and minerals have been largely overcome by coating with a very thin layer of carbon the specimen surface. Using EBSD Sample preparation The surface of interest must be smooth to avoid shadowing caused by topography. This can be achieved by mechanical polishing. The amorphous layer produced during mechanical polishing may be removed using chemo-mechanical polishing. The latter results in a surface of pristine lattice that is required for orientation analyses by EBSD. A large number of rock forming minerals can be chemo-mechanically polished
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