Formula: Ca2(Mg,Al)6(Si,Al,B)6O20
Reflected/transmitted colour:
Pleochroism: Strong, pale yellow, yellow-green to blue green (X), almost colourless, pale yellow, blue, blue-green (Y), light to dark blue (Z)
Optical isotropy/anisotropy: Biaxial
Optical sign: +
Refractive indices: α = 1.700-1.738, β = 1.703-1.741, γ = 1.706-1.743
Bireflectance/Birefringence δ: 0.005
2V angle: 80° (calculated), 78-90° (measured)
Dispersion: Strong to extremely strong, r > v, may produce abnormal interference colours
Optical orientation:
Optical axial plane:
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