What is Optical Mineralogy?
Optical mineralogy is a science dealing with the study of minerals and their optical properties. Geologists study rocks and minerals using a polarized light microscope. This type of microscope is also known as a petrographic microscope, as the word “petro” means rock. The rock and mineral specimens are viewed using thin sections, so thin that light can pass through them. This gives rise to transmitted polarized light microscopy as a means to examine petrographic specimens for the study of optical mineralogy. The mineralogical composition of geological petrographic specimens are studied by the geology student in optical mineralogy. The evolution and origin of rocks and minerals can often be determined by knowing more about their compositions. The study of optical mineralogy involves properties of rocks and minerals such a the refractive index, extinction angle, interference of light, cleavage planes, shapes of the grains, relief patterns, color (pleochroism), absorption scheme, twinning, birefringence, optical indicatrix, and interference figures. Pleochroism refers to the ability of mineral thin sections under transmitted polarized light to display an array of colors when viewed under cross polarization conditions in a polarized light microscope. If the reader is interested in the subject, it is suggested to obtain a good book related to optical mineralogy. Local colleges may also teach courses on optical mineralogy as a part of their geology degree curriculum. In any case, practical applications will involve the use of the polarized light microscope. These microscopes have a Bertrand lens, polarizer, analyzer, quartz wedge compensator plate, mica plate, gypsum plate, rotating stage, centerable eyepieces, and often a centerable stage. They tend to be rather expensive and complex instruments. One site that we have found that provides reasonably priced geological microscopes is www.polarizingmicroscopes.com. If your study of optical mineralogy requires the purchase of a petrographic microscope, it is recommended to contact them as they carry a wide selection of polarizing microscopy products.