Formula: (Ca,Mn,Fe)3Al2BSi4O15(OH)
Reflected/transmitted colour: Orange to red
Pleochroism: In thick sections weak, light brown (X), violet (Y),, light yellow or colourless (Z)
Optical isotropy/anisotropy: Biaxial
Optical sign: -
Refractive indices: α = 1.690, β = 1.698, γ = 1.705
Bireflectance/Birefringence δ: 0015
2V angle: 62° (calculated), 80-84° (measured)
Dispersion:
Optical orientation:
Optical axial plane:
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