Democracy Essay

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Features in Thin Section - Plane Polarized Light Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay First-time Visitors: Please visit Site Map and Disclaimer. Use "Back" to return here. The Cardinal Rule of Thin Sections What you see in thin section is a random cross-section through a grain whose properties vary in three dimensions. Therefore, always view as many grains of a mineral as possible to get an idea of the total range of properties you observe. Also, bear in mind that you may not get a grain that displays some property to optimum effect. Color Color in thin section tends to be more consistent than in hand specimen. Most major rock forming minerals are colorless (A). Some have distinctive colors (B). Some minerals like heamatite (C) which appear opaque in hand specimen are transparent on thin edges in thin section. The most common truly opaque minerals (D) are metallic oxides (magnetite, ilmenite) and sulfides (pyrite). Pleochroism Colored minerals often show different colors in thin section depending on how the grain is oriented relative to the polarizer directions. Top: Most minerals change from lighter to darker as the stage is rotated. Middle: Some minerals change color entirely as the grain is rotated. Bottom: In a few cases the color change is so extreme that the mineral is, in effect, a natural polarizer. Thin slices of tourmaline were often used as polarizing filters before good synthetic filters became available. Relief Relief is the contrast between a mineral and its surroundings due to difference in refractive index. The four grains shown here show increasing relief clockwise from left. Relief is positive when the grain has higher refractive index than its surroundings, negative if lower. Negative relief compared to quartz, feldspar and normal slide mounting media is relatively rare. A few

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