Formula: H(Ca,Fe,Mn,Mg)3(Al2BSi4O16)
Reflected/transmitted colour: colourless, or very pale shades of violet or yellow
Pleochroism: weak or absent, but in thick sections may be α light brown, β violet, γ light yellow or colourless
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.656-1.694, β = 1.660-1.701, γ = 1.668-1.706
Birefringence δ: 0.009-0.016
2Vα angle: 63-90°
Dispersion: r < v
Optical orientation: α ~ ┴ (-111)
Optical axial plane: ~ (111)
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