Formula: (Fe,Mg)2FeO2(BO3)
Reflected/transmitted colour: polished section in air gray to brown or blue, darker in oil, but the colours are intensified. Internal reflections are only present when Fe2+ content is strongly reduced.
Pleochroism:
Optical isotropy/anisotropy: polished section in air very strong anisotropic, blue, pink, reddish brown, 45° fiery orange, in oil slightly reduced, 45° pinkish gray
Optical sign:
Refractive indices:
Birefringence δ/bireflectance: polished section in air very strong bireflectance, blue or blue-gray and brown or pinkish brown, in oil slightly intensified but darker colours
2V angle:
Dispersion:
Optical orientation:
Optical axial plane:
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