Gibbsite

Formula: Al(OH)3

Reflected/transmitted colour: colourless to pale brown

Pleochroism: -

Optical isotropy/anisotropy: biaxial

Optical sign: +

Refractive indices: α = 1.56-1.58, β = 1.56-1.58, γ = 1.58-1.60

Birefringence δ: ~ 0.02

2Vγ angle: 0-40°

Dispersion: r > v, strong

Optical orientation: α = y, β Λ x = 25°, γ Λ z = ~ 21°

Optical axial plane: (010)

 

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