Formula: (NH4)AlSi3O8 . 0.5H2O
Reflected/transmitted colour: colourless
Pleochroism:
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: a = 1.530, b = 1.531, g = 1.534
Birefringence δ: 0.004
2V angle:
Dispersion:
Optical orientation: g = b, a Ù a = 4°, b Ù c = 19°
Optical axial plane:
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