Formula: Na(Fe,Mn)3Al6B3Si6(O,OH)30(OH,F)
Reflected/transmitted colour: blue-yellow
Pleochroism: variable in intensity, normally very strong, absorption always w > ε
Optical isotropy/anisotropy: uniaxial
Optical sign: -
Refractive indices: ε = 1.635-1.650, w = 1.660-1.671
Birefringence δ: 0.025-0.035
2V angle:
Dispersion: weak
Optical orientation:
Optical axial plane:
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