Formula: (Na,K)(Mg,Fe,Al)6(Si,Al)8O20(OH,F)4
Reflected/transmitted colour: Pale to dark brown
Pleochroism: Pale brown (x), dark brown (y,z)
Optical isotropy/anisotropy: Biaxial
Optical sign: -
Refractive indices: α = 1.544, β = 1.608, γ = 1.608
Bireflectance/Birefringence δ: 0.064
2V angle: 0° (calculated), 0-5° (measured)
Dispersion:
Optical orientation:
Optical axial plane:
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