Formula: Na2(Fe,Mg)3Fe2(Si8O22)(OH)2
Reflected/transmitted colour: dark yellow green
Pleochroism: α blue indigo, β yellow-brown, yellow-green, γ dark blue
Optical isotropy/anisotropy: biaxial
Optical sign: - or +
Refractive indices: α = 1.650-1.701, β = 1.655-1.711, γ = 1.670-1.717 (lower values refer to Mg-riebeckite)
Birefringence δ: 0.006-0.016
2Vα angle: 40-100°
Dispersion: r < v or r > v
Optical orientation: α Λ z = 0-30°, β Λ x = 17-35°, γ = y
Optical axial plane: ┴ (010)
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