Formula: (CaxNa1-x)(Si3-xAl1+x)4O8 (x = 0.3 to 0.5)
Reflected/transmitted colour: colourless
Pleochroism:
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: a = 1.545-1.556, b = 1.548-1.558, g = 1.552-1.563
Birefringence δ: 0.007-0.018
2V angle: 78-90° (low), 73-80° (high)
Dispersion: r > v, weak
Optical orientation:
Optical axial plane:
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