Althausite

Formula: Mg2(PO4)(OH,F,O)

Reflected/transmitted colour:

Pleochroism:

Optical isotropy/anisotropy: biaxial

Optical sign: +

Refractive indices: α = 1.588, β = 1.592, γ = 1.598

Birefringence δ: 0.010

2V angle: measured: 70° , calculated: 80°

Dispersion:

Optical orientation: α = b, β =c, γ = a

Optical axial plane:

 

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