Formula: Mn7(AsO4)2(OH)8
Reflected/transmitted colour: yellow, red to green
Pleochroism: α blood-red, β pale yellow, γ sea-green
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.755, β = 1.772, γ = 1.774
Birefringence δ: 0.019
2V angle: calculated: 36°
Dispersion: r > v, strong
Optical orientation: β = b, α ^ c = 51°
Optical axial plane:
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