Alleghanyite

Formula: Mn5(SiO4)2(OH)2

Reflected/transmitted colour: pink or buff in thick sections

Pleochroism: varieties of pink in thicker sections

Optical isotropy/anisotropy: biaxial

Optical sign: -

Refractive indices: a = 1.756, b = 1.780, g = 1.792

Birefringence δ: 0.036

2V angle: 72°

Dispersion: r > v

Optical orientation:

Optical axial plane: g = b

 

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