Formula: Na2(Mg,Fe)3Al2(Si8O22)(OH)2
Reflected/transmitted colour: lavender-blue to colourless
Pleochroism: α colourless, yellow, β lavender-blue, γ blue
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.594-1.630, β = 1.612-1.650, γ = 1.618-1.652
Birefringence δ: 0.020-0.023
2Vα angle: 0-50°
Dispersion: r < v
Optical orientation: α Λ x = 0-10°, β = y, γ Λ z = 4-14°
Optical axial plane: (010)
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