Formula: (Ba,K)(Fe,Mg)3(Si,Al,Fe)4O10(S,OH)2
Reflected/transmitted colour: brownish green
Pleochroism: b = green, g = brown
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: a = 1.85, b = n.d., g = >1.88
Birefringence δ:
2V angle: 8-16°
Dispersion: strong
Optical orientation:
Optical axial plane:
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