Formula: Na3(Mg,Fe)4(Al,Fe)(Si8O22)(OH,F)2
Reflected/transmitted colour: yellowish green, brownish green, gray-green, or gray-violet
Pleochroism: α deep blue, blue-green, green or indigo, β lavender-blue, green-blue, dark or olive-green, γ light yellowish green, greenish yellow, absorption variable
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.623-1.700, β = 1.631-1.706, γ = 1.635-1.710
Birefringence δ: 0.010-0.012
2Vα angle: 8-87° (optically +ve arfvedsonite with high 2V has been reported)
Dispersion: r < v, very strong
Optical orientation: α Λ z = 0-29°, γ = y (low Fe arfvedsonite β = y)
Optical axial plane: ┴ (010) [low Fe arfvedsonite (010)]
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