Bismutocolumbite

Formula: Bi(Nb,Ta)O4

Reflected/transmitted colour: Brown

Pleochroism:

Optical isotropy/anisotropy: Biaxial

Optical sign: +

Refractive indices: α = 2.38, β = 2.42, γ = 2.47

Bireflectance/Birefringence δ: 0.09

2V angle: 86° (calculated), 85° (measured)

Dispersion:

Optical orientation: X = a, Y = c, Z = b

Optical axial plane:

 

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