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symmetrical extinction

A Dictionary of Earth Sciences | 1999 | 386 words | Copyright

symmetrical extinction In optical mineralogy, the phenomenon which occurs when the vibration direction of the light bisects the angles between two sets of cleavages (as seen in basal sections of pyroxenes and amphiboles). A special form of symmetrical extinction may also occur in twinned crystals (see CRYSTAL TWINNING) of feldspar, and this may be used to determine their composition.

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AILSA ALLABY and MICHAEL ALLABY. "symmetrical extinction." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>.

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