The petrographic, or polarizing, microscope, a compound instrument using plane-polarized transmitted light, has been applied for over a hundred years as a standard tool in the examination of rock textures and optical properties of minerals. Textures and most optical properties are normally observed in thin sections about 30 μm thick mounted in balsam or synthetic resin, but minerals may also be examined as individual grains immersed in liquids of known refractive index. Microscopes using reflected light are also available for the examination of opaque minerals. Polarization of light in modern microscopes is achieved by a synthetic material made of a plastic film containing a mass of fine-grained prismatic crystals of herapathite (a complex sulfate and iodide of quinine) with a parallel orientation. Early instruments used nicol prisms made of two elements of calcite cleavage rhombs cemented together in a particular optical orientation by balsam to achieve the same effect.
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Bibliography
Gay, P., 1967, An Introduction to Crystal Optics. London: Longmans Green.
Hartshorne, N. H. and A. Stuart, 1969, Practical Optical Crystallography, 2nd edn. London: Edward Arnold.
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© 1989 Van Nostrand Reinhold
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Chadwick, B. (1989). Petrographic microscope . In: Petrology. Encyclopedia of Earth Science. Springer, Boston, MA. https://doi.org/10.1007/0-387-30845-8_179
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DOI: https://doi.org/10.1007/0-387-30845-8_179
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