Formula: Al2(SiO4)(OH,F)2
Reflected/transmitted colour: colourless
Pleochroism: coloured varieties may show pleochroism in thick sections, e.g. α yellow, γ pink
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: α = 1.606-1.634, β = 1.609-1.637, γ = 1.616-1.644
Birefringence δ: 0.008-0.011
2Vγ angle: 48-68°
Dispersion: r > v
Optical orientation: α = x, β = y, γ = z
Optical axial plane: (010)
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