Earth Materials Introduction to Mineralogy and Petrology

The study of rocks and minerals was revolutionized in 1850 when Henry Sorby invented the petrographic microscope, with which he examined extremely thin (30 μm) slices of rock using polarized light. The petrographic microscope is still the most useful instrument for identifying minerals and studying the texture of rocks. It is used by mining and petroleum geologists, environmental scientists, and even forensic scientists. In this chapter, we describe how minerals can be identified using the microscope. The microscope uses polarized light, and we describe how polarized light interacts with the crystal structure of minerals. When light passes through a mineral, it is slowed down, and we measure this slowing using a property called the refractive index of the mineral, which provides one of the most useful identifying properties of minerals. When polarized light passes through minerals that do not belong to the isometric crystal system, different refractive indexes can be measured in different crystallographic directions. This variation of light velocity with direction in a crystal results in numerous optical effects, all of which provide additional properties that can be used to identify minerals. One of these is the production of interference colors, which gives minerals their spectacular colors under crossed polarized light in the microscope. By using special lenses and filters, it is possible to produce interference figures, which allow you to determine which crystal system a mineral belongs to and to give parameters that can be used in mineral identification. By the end of this chapter, you should be able to place a thin section of rock on the microscope stage and identify any of the common rock-forming minerals from their optical properties. Once minerals have been identified, their abundance can be determined under the microscope, and we will have taken the first step toward classifying rocks.