Formula: Al2SiO5
Reflected/transmitted colour: colourless
Pleochroism: In thick sections coloured varieties may be pleochroic with α pale brown or pale yellow, β brown or greenish, γ dark brown or blue
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: α = 1.653-1.661, β = 1.657-1.662, γ = 1.672-1.683
Birefringence δ: 0.018-0.022
2Vγ angle: 21-30°
Dispersion: r > v, strong
Optical orientation: α = x, β = y, γ = z
Optical axial plane: (010), Bxa ┴ (001)
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