geology -Tourmaline

Jun 28th, 2015
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1. Relief - moderate 2. Colour - brown, blue, pink 3. RI - 1.610-1.655 4. Birefringence - 0.021-0.026 5. Interference colours - 2nd-3rd order 6. Extinction - straight

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TourmalineRing silicatestrigonal (+)Relief - moderateColour - brown, blue, pinkRI - 1.610-1.655Birefringence - 0.021-0.026Interference colours - 2nd-3rd orderExtinction - straightCommentsbrown, colour-zoned tourmalinePPLAplite, Megilligar Rockscolour zoned tourmaline with quartzPPLSt Austell Granitecolour zoned tourmaline with quartz; the image above under crossed-polarsXPLSt Austell GraniteAbove; brown tourmaline with epitaxial growth of acicular blue tourmaline (left) and the same field of view rotated through 90 to show the extreme pleochroism (right)< the same crystals under crossed polarsSt Austell Granitebrown primary tourmal with epitaxial growth of blue secondary tourmaline in PPL (left) and XPL (right)Luxullianite, St Austell GraniteTourmaline 'suns' in PPL (left) and XPL (right), the latter showing the straight extinction crossLuxullianite, St Austell Graniteprismatic crystal of tourmaline in PPL (left) and XPL (right).St Austell Granite

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