Formula: K(Mg,Fe)6(Si,Al)8O20(OH)4 . nH2O
Reflected/transmitted colour: Brown
Pleochroism: Light brown (x), brown (y), brown (z)
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: β = 1.56-1.562, γ = 1.565-1.567
Bireflectance/Birefringence δ:
2V angle: 10° (measured)
Dispersion:
Optical orientation:
Optical axial plane:
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