IMAGING BIREFRINGENT MINERALS WITHOUT EXTINCTION USING CIRCULARLY POLARIZED LIGHT

Linear cross-polarized optical images reveal the birefringence of mineral grains and are very useful in petrography. However, if the vibration directions of the crystal are not at 45 to the polarizers, then the intensity of the interference colors is reduced, and in some orientations, to zero (extinction). This reduction in intensity, and extinction, can be eliminated if circular polarizers are used in place of linear polarizers. The interference colors of colorless minerals then correspond exactly with those on the Michel-Levy chart. Similarly, isogyres are eliminated from conoscopic images. This simple, inexpensive technique can be used with ordinary petrographic microscopes and scanners. It can be very useful for quantification of textures (microstructures) from single images.