Some Examples of the Effect of Crystallographic Orientation on the Cathodoluminescence Colors of Quartz

Abstract

ABSTRACT Study of metamorphic and igneous rocks from onshore and offshore Norway shows that quartz cathodoluminescence (CL) colors are not uniform in the examined lithologies. Neighboring quartz crystals in thin sections from granites and granodiorites may vary in color from blue or violet to brown, and neighboring quartz crystals in thin sections from phyllites range in color from yellow through brown to violet. Similarly, quartz CL colors in examined migmatites and gneisses vary between yellowish brown and violet brown in individual samples, although color variations are less striking than in the granites, granodiorites, and phyllites. Comparison of c-axis orientations and CL colors clearly indicates that the observed color variations are a function of crystallographic orientation in all the mentioned lithologies. This may be due to selective absorption of light along different crystallographic directions, a phenomenon well known from spectroscopy. The observed color variations have important consequences for provenance studies, because variations in CL colors may be at least partly due to different crystallographic orientations. On the other hand, the crystallographic dependence of quartz CL may enable the provenance of quartz grains in a sandstone to be more precisely determined. A promising observation in this respect is the different relationship between c-axis orientation and CL colors of quartz grains in phyllites and in higher-grade metamorphic rocks. In phyllites CL is yellowish when the c axes of the quartz grains are parallel to the plane of the thin section, whereas yellowish brown CL in quartz from the studied migmatites and gneisses occurs in crystals with the c axes at right angles to the plane of the thin section.