Formula: Al4(Si4O10)(OH)8
Reflected/transmitted colour: colourless
Pleochroism: very slight, least absorption for vibration directions perpendicular to cleavage
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.553-1.565, β = 1.559-1.569, γ = 1.560-1.570
Birefringence δ: ~ 0.006
2Vα angle: 24-50°
Dispersion: r > v, weak
Optical orientation: β Λ x = 1-3°, γ = y
Optical axial plane: ┴ (010)
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