Vantasselite

Formula: Al4(PO4)3(OH)3.9H2O

Reflected/transmitted colour:

Pleochroism:

Optical isotropy/anisotropy: Biaxial

Optical sign: -

Refractive indices: α = 1.511, β = 1.56, γ = 1.578

Bireflectance/Birefringence δ: 0.067

2V angle: 60° (calculated), 61° (measured)

Dispersion:

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

Optical axial plane:

 

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