Formula: (Fe,Mg)5Al(Si3Al)O10(OH,O)8
Reflected/transmitted colour:
Pleochroism:
Optical isotropy/anisotropy: Biaxial
Optical sign: -
Refractive indices: α = 1.6, β = 1.6, γ = 1.67
Bireflectance/Birefringence δ: 0.07
2V angle: 0° (calculated), 0-15° (measured)
Dispersion:
Optical orientation:
Optical axial plane:
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