| Tourmaline
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| Formula | Na(Mg,Fe,Li,Al) 3 Al 6 [Si 6 O 18 ](BO 3 ) 3 (O,OH,F) 4 | |
| Crystal System | Hexagonal (trigonal) | |
| Crystal Habit | striated hexagonal prisms | |
| Cleavage | {110} and {101} | very poor cleavages |
| Color/Pleochroism | blue, red, green, yellow, black, brown. (Schorl the most common tourmaline is black) | strongly pleochroic |
| Optic Sign | Uniaxial (-) | |
| 2V | 10 degrees | only occasionaly biaxial |
Refractive Indices omega = epsilon = |
1.631-1.698 1.610-1.675 |
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| Max Birefringence | 0.035 | cannot be used as a reliable guide to composition |
| Extinction | Longitudial sections show parallel extinction and are length fast. | |
| Unit Cell | a = 15.8-16.0 Angstroms |
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Physical Properties |
H = 7 G = 2.90-3.22 |
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| Distinguishing Features | In hand sample: columnar crystals with rounded triangular cross sections. Poor cleavage Deep color often masks interference colors |
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| Occurrence | Tourmaline is a characteristic mineral in granitic pegmatites. It is an accessory mineral in granite, granodiorite, and related felsic rocks. Tourmaline is also a common mineral in schist, gneiss, quartzite, and phyllite. Also found in metasomatically altered limestone and dolomite in contact metamorphic zones. | |
| Editors | Emma Anderson (2005). Data sources: Minerals in Thin Section Perkins, Dexter and Henke, Kevin 2000. Introduction to Opticial Mineralogy Nesse, William 1991. Introduction to Mineralogy Nesse, William 2000. | |
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Plane polarized image of tourmaline, which is the small green grain in the center of the image. Note the concentric zoning of colors. Also visible are biotite (tan), muscovite (colorless), quartz, and plagioclase. Click on the image to see a larger version with rollover to crossed polarized light. |
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Crossed polarized image of tourmaline |
| WWW Images |