Formula: C
Reflected/transmitted colour: opaque, transparent in very thin flakes, deep blue, polished section brownish to blackish
Pleochroism: strong
Optical isotropy/anisotropy: uniaxial, polished section very strong anisotropic, straw-yellow to dark brown or violet-gray. The anisotropy is strongest for yellow light. Basal sections appear isotropic
Optical sign: -
Refractive indices: w = 1.93-2.07 (red), ε not determined
Birefringence δ/bireflectance: extreme, polished section extremely strong and characteristic, O brownish, E much darker, almost completely black (in air bluish gray)
2V angle:
Dispersion:
Optical orientation:
Optical axial plane:
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