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UW-Madison GEOSCI 777 - The Application of Electron Backscatter Diffraction and Orientation Contrast Imaging

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American Mineralogist, Volume 84, pages 1741–1759, 19990003-004X/99/1112–1741$05.00 1741The application of electron backscatter diffraction and orientation contrast imaging in theSEM to textural problems in rocksDAVID J. PRIOR,1,* ALAN P. BOYLE,1 FRANK BRENKER,2 MICHAEL C. CHEADLE,1 AUSTIN DAY,3GLORIA LOPEZ,4 LUCA PERUZZO,6 GRAHAM J. POTTS,1 STEVE REDDY,5 RICHARD SPIESS,6NICK E. TIMMS,7 PAT TRIMBY,8 JOHN WHEELER,1 AND LENA ZETTERSTRÖM91Department of Earth Sciences, Liverpool University, L69 3BX, U.K.2Institut für Mineralogie und Geochemie, Universität zu Köln, Zülpicher Strasse 49b, 50674 Köln, Germany3HKL technology, Blåkildevej 17k, 9500 Hobro, Denmark4Departamento de Geología, Universidad de Chile, Santiago, Chile5Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, Perth, WA 6102, Australia6Department of Mineralogy and Petrology, University of Padua, Italy7Department of Geology, James Cook University, Townsville, Australia8Department of Geology, Utrecht University, Utrecht, Netherlands9Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, SwedenABSTRACTIn a scanning electron microscope (SEM) an electron beam sets up an omni-directional source ofscattered electrons within a specimen. Diffraction of these electrons will occur simultaneously on alllattice planes in the sample and the backscattered electrons (BSE), which escape from the specimen,will form a diffraction pattern that can be imaged on a phosphor screen. This is the basis of electronbackscatter diffraction (EBSD). Similar diffraction effects cause individual grains of different orien-tations to give different total BSE. SEM images that exploit this effect will show orientation contrast(OC). EBSD and OC imaging are SEM-based crystallographic tools.EBSD enables measurement of the crystallographic orientation of individual rock-forming min-erals as small as 1 µm, and the calculation of misorientation axes and angles between any two datapoints. OC images enable mapping of all misorientation boundaries in a specimen and thus provide alocation map for EBSD analyses. EBSD coupled to OC imaging in the SEM enables complete speci-men microtextures and mesotextures to be determined. EBSD and OC imaging can be applied to anymineral at a range of scales and enable us to expand the microstructural approach, so successful instudies of quartz rocks, for example, to the full range of rock-forming minerals. Automated EBSDanalysis of rocks remains problematic, although continuing technical developments are enablingprogress in this area.EBSD and OC are important new tools for petrologists and petrographers. Present and futureapplications of EBSD and OC imaging include phase identification, studying deformation mecha-nisms, constraining dislocation slip systems, empirical quantification of microstructures, studyingmetamorphic processes, studying magmatic processes, and constraining geochemical microsampling.In all these cases, quantitative crystallographic orientation data enable more rigorous testing of mod-els to explain observed microstructures.INTRODUCTIONA complete petrographic description of any rock comprisesobservations at the centimetre to angstrom scale, against whichpetrogenetic models are tested. Such data underpin many as-pects of earth sciences. Petrographic observations include (1)spatial distribution, size and shape of phases; (2) the composi-tion of phases and the variation of composition within phases;(3) the crystallographic orientations of phases and variation oforientation within phases; (4) the geometry and structure ofsubgrain boundaries; (5) the geometry and structure of bound-aries between grains of the same phase; and (6) the geometryand structure of boundaries between different phases; and thegeometry and structure of triple junctions.By virtue of widespread availability, low cost, and versatil-ity, transmitted and reflected light microscopy have served, andwill continue to serve, as the mainstay of petrographic datacollection. However, some data are inaccessible using lightoptics and, increasingly, more demanding scientific objectivesnecessitate that certain data be collected with greater thorough-ness and precision than is possible with light microscopy.*E-mail: [email protected] ET AL.: APPLICATION OF ELECTRON BACKSCATTER DIFFRACTION1742BSE imaging (Lloyd 1985) together with X-ray microanaly-sis (Goldstein et al. 1992) in the SEM provide (1) images ofrock textures with sub-micrometer spatial resolution, based oncontrasts between materials of different composition (atomicnumber or Z-contrast) and (2) qualitative and quantitative mi-crochemical data with micrometer resolution. With modernhardware and software developments, these techniques havebecome commonplace in the analysis of rock textures.OC imaging using forescatter detectors (Day 1993; Prior etal. 1996) and EBSD (Dingley 1984; Randle 1992; Wikinsonand Hirsch 1997; Humphreys 1999) in the SEM provide: (1)images of rock textures with sub-micrometer spatial resolu-tion, based on signal contrasts between materials with differ-ent crystallographic orientations or different crystallographicstructures and (2) qualitative and quantitative crystallographicorientation data with micrometer resolution.OC imaging and EBSD are relatively new techniques to thepetrologist. They are of fundamental importance as many mod-els that seek to explain rock microstructures have implicationsfor the distribution of crystallographic orientations and are thustestable using SEM-based crystallographic data. This paper aimsto review (1) the principles behind EBSD and OC imaging; (2)analytical procedures appropriate to rock specimens; and (3)some applications to hard-rock petrology and petrography.ELECTRON BACKSCATTER DIFFRACTIONIn the literature, EBSD is also called electron backscatterpatterns (EBSP) and backscatter kikuchi diffraction. In thispaper, EBSD refers to the diffraction technique and EBSP re-fers to individual diffraction patterns. The same diffraction pro-cess is exploited in transmission electron microscopy givingrise to kikuchi lines (Nishikawa and Kikuchi 1928; Loretto1994), and in electron channeling in the SEM (Joy 1974; Lloyd1987, 1994). A direct comparison of EBSD and electron chan-neling is given by Randle (1992) and Wilkinson and Hirsch(1997), and the reciprocity theorem linking the two techniquesis detailed by Reimer et al. (1986). A comparison of EBSD andother


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