The Use of X-Rays in Petrofabric Studies

Abstract

Since the observation by Trener in 1906 that in a foliated metamorphic rock from Tonale a majority of the quartz grains were oriented with their optic axes approximately normal to the foliation, the examination of such ‘patterns of preferred orientation’ has played an important part in the study of rock fabrics. At first the only available procedure involved the use of a compensator above the section and the measurement of extinction angles against directions of determined optical character in a large number of grains. Such observations did not provide a complete determination of the rotational attitude of the indicatrix of each grain (Sander, 1950, p. 20), but in 1925, W. Schmidt, following a suggestion made by F. Becke, published the first fabric diagrams of complete orientations determined by using a universal stage. This optical method at once became, and still remains, a most valuable petrofabric technique, but there are inherent limitations. For a uniaxial mineral, such as quartz, only the attitude of the principal axis, and not the complete rotational attitude, can be determined unless there are visible morphological features such as cleavage or twinning. No results can be obtained for isotropic minerals; for opaque minerals, too, satisfactory optical procedures are virtually lacking. Moreover, perhaps in some ways the most important restriction, there is a limit to grain-size below which U-stage techniques become impracticable. All these difficulties can be overcome by the use of X-rays, for which the only important restriction in petrofabric work is that they cannot be used to study patterns of preferred orientation by shape.