Formula: Na2(Mg,Fe)3(Al,Fe)2Si8O22(OH)2
Reflected/transmitted colour: lavender-blue to colourless
Pleochroism: α colourless, yellow, β blue, γ lavender-blue
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.630-1.665, β = 1.650-1.670, γ = 1.652-1.680
Birefringence δ: 0.008-0.020
2Vα angle: 0-90°
Dispersion: r < v or r > v
Optical orientation: β Λ z = 0-20°, γ = y
Optical axial plane: ┴ (010)
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