Formula: (Mg,Fe,Fe,Al)8O2[(Al,Si)6O18]
Reflected/transmitted colour: colourless to blue (sometimes pale pink)
Pleochroism: α colourless, pale reddish or pink, yellowish green, pale yellow, greenish gray, grayish green, β sky-blue, pale blue, lavender-blue, bluish green, gray-blue, γ blue, sapphire-blue, dark blue, greenish blue, blue-green, pale green
Optical isotropy/anisotropy: biaxial
Optical sign: - or +
Refractive indices: α = 1.701-1.731, β = 1.703-1.741, γ 1.705-1.745
Birefringence δ: 0.005-0.009
2Vα angle: 47-114°
Dispersion: r < v, strong
Optical orientation: β = y, γ Λ z = 6-9°
Optical axial plane: (010)
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