Fabric analysis and fabric development in ores

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Abstract
The preferred orientations of the constituent minerals of a polycrystalline ore fabric can be determined by means of an X-ray pole-figure goniometer. Several complete pole figures for different crystallography planes permit the calculation of a three-dimensional orientation distribution. These methods of measuring and calculation are quite precise. In practice, the measurement of preferred orientation in ores is often inhibited because of insurmountable restrictions. Metamorphosed ores sometimes reveal fabrics which may enable inferences to be drawn regarding the conditions under which these fabrics developed. Pole figures of these ores often show a preferred orientation with a more or less pronounced symmetry, indicating a flow deformation connected with the metamorphism. The interpretation of natural fabrics is generally only possible by making assumptions about the fabric before deformation, the state of stress and strain rate during deformation, the temperature conditions and the mechanism of flow. In order to check the reliability of these assumptions, it is necessary to perform deformation experiments on polycrystalline ores and single crystals of ore minerals. Deformation experiments at different confining pressures, temperatures and strain rates reveal the mechanism of flow, deformation features in the crystals and the development of preferred orientation. Theoretical derivations of preferred orientation, making use of the appropriate model, for example, the Taylor-Bishop-Hill model for dislocation flow, should be in agreement with the experimental data. These validated models may be used as feed-back for the interpretation of the usually much more complex fabrics developed in naturally deformed ores.