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| Formula | Al2SiO4(F,OH)2 |
| | Optic class & sign | Biaxial positive |
| | Optical orientation | X = a, Y = b, Z = c |
| | Optical plane | (010) |
| | Relief | Moderate-positive |
| | Refractive indices | nx = 1.606 -1.634
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|
ny = 1.609 -1.637
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|
nz = 1.616 -1.644
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|
| n increases with OH substituting for F |
| | Birefringence (max.) | 0.008 - 0.011 |
| | | |
| | Optic Angle
| 2Vx
|
| | 2Vz
= 48 - 68° |
| | Sign of elongation | Length-slow, l(+); length-fast, l(-), to cleavage traces |
| | Interference figure | |
| | Colour / pleochroism | Colourless |
| | Zoning | |
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|
| Form | Habit | Prismatic, short-columnar to acicular, granular; cross-sections near-square to 8-sided |
| | | Surface | Euhedral to anhedral |
| | Cleavage | (001) |
| | Twinning | Rare |
| | Extinction | Straight to traces of prism faces and cleavage; symmetrical to prism faces in sections orthogonal to c |
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| Reaction textures | |
| | Alteration / decomposition | Sericite, illite, kaolinite, fluorite; relatively stable in the weathering environment |
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| Occurence | Ign | Quartz-topaz rocks |
| | | Met | Al-rich schists, F-bearing meta-bauxites |
| | | Sed | Detrital in sediments and sedimentary rocks |
| | | Hyd | Greisen; veins and cavities in granite, rhyolite and granitic pegmatite |
| | | Other | |
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|
| Distinctive properties | Moderate relief, low birefringence, single cleavage |
| | Additional comments | |
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