Kyanite

Formula: Al₂SiO₅

Reflected/transmitted colour: colourless to pale blue

Pleochroism: weak, in thick sections α colourless, β violet-blue, γ cobalt-blue

Optical isotropy/anisotropy: biaxial

Optical sign: -

Refractive indices: α = 1.710-1.718, β = 1.719-1.724, γ = 1.724-1.734

Birefringence δ: 0.012-0.016

2V angle: 78-83°

Dispersion: r > v, weak

Optical orientation: γ' Λ z on (100) = 27-32°, on (010) = 5-8°, α' Λ x on (001) = 0-3°, Bx_a nearly _┴ (100)

Optical axial plane: almost _┴ (100) and inclined approx. 30° to (010) and z-axis