Measurement of preferred orientation of phyllosilicates in schists

Abstract

The transmission X-ray technique of measuring preferred orientation of phyllosilicates is applied to coarser-grained rocks than have heretofore been studied by the X-ray method. To do this, the X-ray pole-figure goniometer has been modified by adding an integrating device which moves the specimen back and forth across the X-ray path, causing the beam to scan a larger sample area. The new apparatus has been used to study mica orientations in two specimens of Moine schist that had previously been studied optically. Optical data contoured by the conventional Schmidt method indicate the existence of preferred-orientation submaxima in these specimens. However, neither the optical data contoured by the Kamb method nor the X-ray diagrams show these submaxima, which suggests that such submaxima in diagrams contoured by the Schmidt method do not have statistical significance. Principal preferred-orientation population densities and hypothetical “strains” calculated by the March method are similar for the Kamb-contoured optical data and the X-ray data. This indicates that the gap between optical and X-ray techniques of measuring preferred orientations of platy minerals has been closed.