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Na(Mg,Fe,Mn,Li,Al)3Al6[Si6O18](BO3)3(OH,F)4
Crystal system: Trigonal system
Refractive index: 1.610-1.675
Birefringence: 0.017-0.035
Cleavage: very poor
Twinning: rare
Colour in PPL: Fe-rich blue to yellow, Mg-rich yellow-pale yellow, Li-rich colourless-other
Pleochroism: Fe-rich blue to yellow, Mg-rich yellow-pale yellow, Li-rich colourless-other
Relief: moderate
Distinction: marked pleochroism, maximum absorption when crystal elongation is perpendicular to the vibration plane of the
polarizer (biotite and amphiboles parallel); straight extinction, relatively high releif, and moderate birefringence are
adiagnostic
Occurrence: Granites, higher grade metamorphic rocks, and often as detrital grains
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Tourmaline - Euhedral - Plain polarised light - Sangalan Tepe - Iran
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