Optical Mineralogy

A fundamentals of optical mineralogy is that the majority minerals even dark colored minerals et al.  We use a polarizing microscope to examine them by transmitted light microscopy (Figure) . We look at small mineral grains (powdered samples) or specially prepared thin sections (0.03-mm-thick specimens of minerals or rocks mounted on glass slides) to work out properties that are otherwise not discernible. Minerals with metallic luster and a couple of others are termed opaque minerals. They don’t transmit light albeit they're thin-section thickness. For these minerals, transmitted light microscopy is of no use. Reflected light microscopy, a related technique, can reveal a number of an equivalent properties. It is an important technique for economic geologists who deal with metallic ores but is not used by most mineralogists or petrologists, so we discuss it only briefly in this text.

Most minerals can be identified when examined with a polarizing microscope, even if unidentifiable in hand specimen. For example, the magnesium to iron ratio of olivine,(Mg,Fe)2SiO4 , can be distinguished based on optical properties. The composition of plagioclase feldspar, CaAl2Si2O8 - NaAlSi3O8, can be similarly distinguished. Summarizes the optical properties used for mineral identification and gives the properties of some common minerals.We divide minerals into those that will not transmit light unless the sections are much thinner than normal thin sections (opaque minerals) and those that will (nonopaque minerals). Nonopaque minerals are further divided into those that are isotropic (having the same properties in all directions) and those that are anisotropic (having different optical properties in different directions). Finally, the anisotropic minerals are divided according to whether they are uniaxial or biaxial, and according to whether they have a positive or negative optic sign. The details of these and other diagnostic properties will be discussed later.

 When we use thin sections, distinguishing igneous, sedimentary, and metamorphic rocks is often easier than when we use hand specimens. More significantly, it is possible to identify minerals and distinguish among different types of igneous, sedimentary, and metamorphic rocks. The microscope allows us to ascertain textural relationships during a specimen that give clues about when and the way different minerals within the rock formed. Microscopic relationships between mineral grains allow us to determine the order in which minerals crystallized from magma, and we can identify minerals produced by alteration or weathering long after magma cooling. Similar observations are possible for sedimentary or metamorphic rocks. Only the microscope can give us such information, information that is essential if rocks are to be used to interpret geological processes and environments.

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