Formula: Mn9(Si, Al)10O24(OH)8
Reflected/transmitted colour: yellow
Pleochroism: a = colourless, b = pale yellow, g = canary-yellow
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: a = 1.698, b = 1.704, g = 1.720
Birefringence δ: 0.02
2V angle: 65.5°
Dispersion:
Optical orientation: a Ù {010} = 30°
Optical axial plane:
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