Durangite

Formula: NaAl(AsO4)F

Reflected/transmitted colour:

Pleochroism:

Optical isotropy/anisotropy: Biaxial

Optical sign: -

Refractive indices: α = 1.634, β = 1.673, γ = 1.685

Bireflectance/Birefringence δ: 0.051

2V angle: 56 (calculated), 45 (measured)

Dispersion:

Optical orientation:

Optical axial plane:

 

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