Petrology Thin Section Muscovite in Granite LengthSlow

>> YOUR LINK HERE: ___ http://youtube.com/watch?v=QwK6lKWfMQQ

Note the muscovite near the center of the field of view. The vivid interference colors of muscovite are evident when switched to XPL, at 00:12. At 00:26, the microscope is switched to 200x magnification. At 00:35, a length-fast gypsum plate is inserted from the SE over the length of the crystal (traces of cleavage), causing the interference colors to subtract from 2nd-order blue and green to 1st-order gray. This demonstrates that the crystal is length-slow, opposite that of the gypsum plate. At 00:43, a length-fast gypsum plate is inserted from the SE over the width of the crystal (perpendicular to traces of cleavage). In this orientation, the gypsum plate causes the interference colors to add from 2nd-order blue and green to 3rd-order green, demonstrating that the width of the crystal is the fast direction. Muscovite is indeed length-slow. Also, notice that muscovite has parallel extinction; it goes extinct when it's length (cleavage) is parallel to the polarizer and the analyzer. At low power, the field also includes biotite, chlorite, and essential minerals of a granite. • This video was produced during the spring of 2020, at the heart of COVID, as part of my effort to teach petrology online.

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