Formula: Mg2FeO2(BO3)
Reflected/transmitted colour: polished section gray and blue, in oil much darker and blue colour more intense. Faint reddish brown internal reflections only along grain boundaries
Pleochroism:
Optical isotropy/anisotropy: polished section in air strong anisotropic, gray to black, 45° very dark gray, in oil moderate to strong, greatly reduced
Optical sign:
Refractive indices:
Birefringence δ/bireflectance: polished section in air strong bireflectance, different shades of gray to brownish gray, darker in oil, brownish tint is intensified
2V angle:
Dispersion:
Optical orientation:
Optical axial plane:
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