Formula: (Mg,Fe,Mn)7(Si8O22)(OH,F)2
Reflected/transmitted colour: colourless to pale green
Pleochroism: α = β very pale yellow or brown, γ pale brown
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.630-1.696, β = 1.644-1.709, γ = 1.652-1.730
Birefringence δ: 0.020-0.045
2Vγ angle: 65-96°
Dispersion: r < v or r > v
Optical orientation: β = y, γ Λ z = 10-15°
Optical axial plane: (010)
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