Formula: CaAl2(Si2O7)(OH)2.H2O
Reflected/transmitted colour: colourless to bluish green
Pleochroism: usually non-pleochroic
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: α = 1.663-1.665, β = 1.672-1.675, γ = 1.682-1.686
Birefringence δ: 0.019-0.021
2Vγ angle: 76-87°
Dispersion: r > v variable, commonly very strong
Optical orientation: γ = y
Optical axial plane: (100)
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