Tancoite

Formula: HNa2LiAl(PO4)2(OH)

Reflected/transmitted colour: Colourless

Pleochroism:

Optical isotropy/anisotropy: Biaxial

Optical sign: -

Refractive indices: α = 1.541, β = 1.563, γ = 1.564

Bireflectance/Birefringence δ: 0.023

2V angle: 24° (calculated), 23° (measured)

Dispersion: Weak, r < v

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

Optical axial plane:

 

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