Formula: K(Cr,Ti,Fe,Mg)12O19
Reflected/transmitted colour: RL gray to grayish white
Pleochroism: RL weak
Optical isotropy/anisotropy: Uniaxial
Optical sign: ?
Refractive indices: n = 2.22 and n = 2.3 from Gladstone-Dale relationship
Bireflectance/Birefringence δ:
2V angle:
Dispersion:
Optical orientation:
Optical axial plane:
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