Formula: K1.5-1.0Al4(Si6.5-7.0Al1.5-1.0O20)(OH)4
Reflected/transmitted colour: colourless
Pleochroism: -
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.54-1.57, β = 1.57-1.61, γ = 1.57-1.61
Birefringence δ: ~ 0.03
2Vα angle: generally les than 10°
Dispersion: -
Optical orientation: α ~ ┴ (001)
Optical axial plane:
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