Formula: Na(Ca,Na)(Mg,Fe,Fe,Mn)5(Si8O22)(OH,F)2
Reflected/transmitted colour: colourless, pale yellow, violet
Pleochroism: very variable, usually in pale yellow orange and reddish tints, sometimes violet and blue, anomalous interference colours shown by some varieties, absorption variable but generally β > γ > α
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.604-1.690, β = 1.616-1.700, γ = 1.622-1.712
Birefringence δ: 0.015-0.029 (maximum for Ferri-rich)
2Vα angle: 55-87°
Dispersion: r < v, strong
Optical orientation: α Λ x = 1-26°, β = y, γ Λ z = 5-55°
Optical axial plane: (010)
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