Tilasite

Formula: CaMg(AsO4)F

Reflected/transmitted colour: Colourless to pale green

Pleochroism:

Optical isotropy/anisotropy: Biaxial

Optical sign: -

Refractive indices: α = 1.640, β = 1.660, γ = 1.675

Bireflectance/Birefringence δ: 0.035

2V angle: 80° (calculated), 82°44' (measured)

Dispersion: Slight, r < v

Optical orientation: Z = b, X ⋀ c = ~ 30°

Optical axial plane:

 

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