Formula: (NH4,K)Al2(Si3Al)O10(OH)2
Reflected/transmitted colour: Nearly colourless
Pleochroism:
Optical isotropy/anisotropy: Biaxial
Optical sign: -
Refractive indices: α = 1.555-1.56, β = 1.575-1.587, γ = 1.581-1.595
Bireflectance/Birefringence δ: 0.026-0.035
2V angle: 28-30° (calculated), 28° (measured)
Dispersion:
Optical orientation:
Optical axial plane:
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