Formula: (K,Na)3(Fe,Mn)6(Nb,Ti)2Si8(O,OH,F)31
Reflected/transmitted colour: Brownish yellow to red-orange
Pleochroism: Strong, brownish yellow (X=Y) red-orange (Z)
Optical isotropy/anisotropy: Biaxial
Optical sign: -
Refractive indices: α = 1.724, β = 1.760, γ = 1.772
Bireflectance/Birefringence δ: 0.048
2V angle: 58° (calculated), 60° (measured)
Dispersion:
Optical orientation: Y = a, X Ù b = 13°, Z ┴ {001}
Optical axial plane:
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