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| Formula | (Mg,Fe)4(Al,Fe3+)6(SiO4)4BO4(O,OH)2 |
| | Optic class & sign | Biaxial negative |
| | Optical orientation | X = c, Y = a, Z = b |
| | Optical plane | (100) |
| | Relief | High |
| | Refractive indices | nx = 1.660 -1.682
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ny = 1.673 -1.696
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nz = 1.674 -1.699
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| n increases with Fe |
| | Birefringence (max.) | 0.012 - 0.017 |
| | | Δn increases with Fe |
| | Optic Angle
| 2Vx
= 3 - 48° |
| | 2Vz
|
| | Sign of elongation | Length-fast, l (-) |
| | Interference figure | |
| | Colour / pleochroism | X: colourless to pale brown or green, Y: pale brownish yellow, pale yellowish green, Z: pale brownish, yellowish green, yellow-brown; green; absorption Z > Y > X |
| | Zoning | |
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| Form | Habit | Elongate-prismatic, acicular |
| | | Surface | Typically subhedral, with well-developed prism faces |
| | Cleavage | 2 sets {110} |
| | Twinning | None |
| | Extinction | Straight to prism faces and cleavage traces in prismatic sections; in sections orthogonal to c, symmetrical to {110} faces and cleavage traces, straight to {100} and {010} faces |
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| Reaction textures | |
| | Alteration / decomposition | |
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| Occurence | Ign | Pegmatites |
| | | Met | B-bearing Mg-Al-rich granulites and schists |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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| Distinctive properties | High n, low Δn, extinction; restricted occurrence, association with other boron silicates |
| | Additional comments | |
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