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Petrography
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===Microscopic characteristics=== [[File:LvMS-Lvm.jpg|thumb|Photomicrograph of a [[volcanic]] [[sand grain]]; upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter.]] {{main|Optical mineralogy}} When dealing with unfamiliar types or with rocks so fine grained that their component minerals cannot be determined with the aid of a hand lens, a microscope is used. Characteristics observed under the microscope include colour, colour variation under plane [[polarised light]] ([[pleochroism]], produced by the lower [[Nicol prism]], or more recently [[Polarizer#Absorptive polarizers|polarising films]]), fracture characteristics of the grains, refractive index (in comparison to the mounting adhesive, typically [[Canada balsam]]), and optical symmetry ([[Birefringence|birefringent]] or [[isotropy|isotropic]]). ''In toto'', these characteristics are sufficient to identify the mineral, and often to quite tightly estimate its major element composition. The process of identifying minerals under the microscope is fairly subtle, but also mechanistic β it would be possible to develop an [[identification key]] that would allow a computer to do it. The more difficult and skilful part of optical petrography is identifying the interrelationships between grains and relating them to features seen in hand-sized specimen, at outcrop, or in mapping.
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