Formula: (K,Na,Ca)4(Fe,Fe,Mn)2Si8O20(OH, F)
Reflected/transmitted colour:
Pleochroism:
Optical isotropy/anisotropy: Biaxial
Optical sign: +
Refractive indices: α = 1.541, β = 1.56, γ = 1.567
Bireflectance/Birefringence δ: 0.026
2V angle: 60° (calculated), 84° (measured)
Dispersion:
Optical orientation:
Optical axial plane:
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