Some examples of the effect of crystallographic orientation on the cathodoluminescence colors of quartz

Study of metamorphic and igneous rocks from onshore and offshore Norway sho ws that quartz cathodoluminescence (CL) colors are not uniform in the exami ned lithologies, Neighboring quartz crystals in thin sections from granites and granodiorites may vary in color from blue or violet to brown, and neig hboring quartz crystals in thin sections from phyllites range in color from yellow through brown. to violet. Similarly, quartz CL colors in examined m igmatites and gneisses vary between yellowish brown and violet brown in ind ividual samples, although color variations are less striking than in the gr anites, granodiorites, and phyllites. Comparison of c-axis orientations and CL colors clearly indicates that the observed color variations are a funct ion of crystallographic orientation in all the mentioned lithologies. This mag be due to selective absorption of light along different crystallo graph ic directions, a phenomenon well known from spectroscopy. The observed colo r variations have important consequences for provenance studies, because va riations in CL colors may be at least partly due to different crystallograp hic orientations. On the other hand, the crystallographic dependence of qua rtz CL may enable the provenance of quartz grains in a sandstone to be more precisely determined. A prom ising observation in this respect is the diff erent relationship between c-axis orientation and CL colors of quartz grain s in phyllites and in higher-grade metamorphic rocks. In phyllites CL is ye llowish when the c axes of the quartz grains are parallel to the plane of t he thin section, whereas yellowish brown CL in quartz from the studied migm atites and gneisses occurs in crystals with the c axes at right angles to t he plane of the thin section.