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| Formula | (Mg,Fe)Al3O2BO3SiO4 |
| | Optic class & sign | Biaxial negative |
| | Optical orientation | X = a, Y = c, Z = b |
| | Optical plane | (001) |
| | Relief | Moderate-positive |
| | Refractive indices | nx = 1.590 -1.602
|
|
ny = 1.618 -1.636
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nz = 1.623 -1.639
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|
| - |
| | Birefringence (max.) | 0.033 - 0.037 |
| | | - |
| | Optic Angle
| 2Vx
= 27 - 32° |
| | 2Vz
|
| | Sign of elongation | Length-slow, l(+), or length-fast, l(-) |
| | Interference figure | Strong dispersion |
| | Colour / pleochroism | Deep blue, blue-green, green; Y: colourless; strong pleochroism Z>X>Y |
| | Zoning | |
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|
| Form | Habit | Prismatic, acicular |
| | | Surface | Typically anhedral |
| | Cleavage | (100), (010), intersecting at 90° |
| | Twinning | |
| | Extinction | Straight to cleavage traces |
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| Reaction textures | |
| | Alteration / decomposition | |
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|
| Occurence | Ign | Pegmatites, aplites |
| | | Met | High-grade, low-pressure gneisses and granulites |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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|
| Distinctive properties | Relief, birefringence, colour and pleochroism, association with other B minerals |
| | Additional comments | |
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