Formula: Al6(Al,Ta)[(Si,As,Sb)O4]3(BO3)(O,OH)3
Reflected/transmitted colour:
Pleochroism: Colourless (x and y), yellow (z)
Optical isotropy/anisotropy: Biaxial
Optical sign: -
Refractive indices: α = 1.705-1.746, β = 1.728-1.759, γ = 1.73-1.761
Bireflectance/Birefringence δ: 0.015-0.025
2V angle: 32-42° (calculated), 20-55° (measured)
Dispersion: r < v
Optical orientation:
Optical axial plane:
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