Formula: 3Al2O3.2SiO2
Reflected/transmitted colour: colourless or pinkish
Pleochroism: α = β colourless, γ pinkish
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: α = 1.630-1.670, β = 1.636-1.675, γ = 1.640-1.690
Birefringence δ: 0.010-0.029
2Vγ angle: 45-61°
Dispersion: r > v
Optical orientation: α = x, β = y, γ = z
Optical axial plane: (010)
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