Formula: KFe3AlSi3O10(OH,F)2
Reflected/transmitted colour: brown
Pleochroism: a = brown, b = g = dark brown
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: a = 1.624, b = 1.672, g = 1.672
Birefringence δ: 0.048
2V angle: 0-5°
Dispersion: r < v, weak
Optical orientation: b = b
Optical axial plane:
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