Tokkoite

Formula: K2Ca4Si7O17(O,OH,F)4

Reflected/transmitted colour: Colourless

Pleochroism:

Optical isotropy/anisotropy: Biaxial

Optical sign: +

Refractive indices: α =1.570, β = n.d., γ = 1.577

Bireflectance/Birefringence δ: 0.007

2V angle: 38° (measured)

Dispersion: Weak, r < v

Optical orientation: Z c = 0-15°

Optical axial plane:

 

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