Crystallographic Textures

G. E. Lloyd, N.-H. Schmidt, D. Mainprice and D. J. Prior
Department of Earth Sciences, The University, Leeds LS2 9JT, England; and Centre Geologique et Geophysique, U.S.T.L., 34060 Montpellier Cedex, France
Danish National Research Centre Riso, Postbox 49, DK-4000, Roskilde, Denmark
Laboratoire de Tectonophysique, U.S.T.L., 34060 Montpellier Cedex, France
Department of Geology, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, England

Abstract: To material scientists the term texture means the crystallographic orientation of grains in a polycrystal. In contrast, geologists use the term more generally to refer to the spatial arrangement or association of mineral grains in a rock. In this contribution we are concerned with the materials science definition. There are several established techniques available for the determination of crystallographic textures in rocks. It has also been realised that the scanning electron microscope (SEM) is applicable to the study of crystallographic textures via the electron channelling (EC) effect. This provides an image of mineral/rock microstructure (via orientation contrast), as well as a means of accurately indexing their crystal orientations (via electron channelling patterns, ECP). Both types of EC image result from the relationship between incident electron beam and crystal structure, and the subsequent modulation of the backscattered electron (BSE) emission signal according to Bragg's Law. It is a simple matter to switch between the two imaging modes. A related effect, electron backscattering, provides only the diffraction patterns, but has superior spatial resolution and pattern angles.

Due to crystal symmetry restrictions, there is only a limited range of ECP configurations possible for any mineral. Individual patterns can therefore be identified by comparison with the complete ‘ECP-map’. The location of an individual pattern within the map area is determined by spherical angles, the exact definition of which depends on the type of fabric diagram (e.g. inverse pole figure, pole figure or orientation distribution function). Originally, these angles were measured manually. A computer program (CHANNEL) has been developed which uses a digitisation approach to pattern recognition, derives the required fabric diagrams and also constructs ECP-maps from standard crystal data (i.e. unit cell parameters etc.).

The combination of SEM/EC and CHANNEL dramatically facilitates the study of crystal textures in minerals and rocks, making statistical crystallographic analysis from individual orientations a practicality. The following example applications are considered: (1) crystal structure representation of the Al2SiO5 polymorph system; (2) local crystal texture relationships (epitaxial nucleation) between andalusite and sillimanite grains; (3) bulk rock crystal textures of quartzites; and (4) physical properties (e.g. elastic constants and seismic velocities) determined from bulk rock texture.

Keywords: scanning electron microscopy • electron channelling • Al2SiO5 polymorphs • quartz • crystal fabrics • seismic properties

Mineralogical Magazine; September 1991 v. 55; no. 380; p. 331-345; DOI: 10.1180/minmag.1991.055.380.04
© 1991, The Mineralogical Society
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