Brewsterite

Formula: (Sr,Ba,Ca)Al2Si6O16 . 5H2O

Reflected/transmitted colour: colourless

Pleochroism:

Optical isotropy/anisotropy: biaxial

Optical sign: +

Refractive indices: a = 1.510, b = 1.512, g = 1.523

Birefringence δ: 0.013

2V angle: 65°

Dispersion: g ┴ (010), a Ù c = 19-34° in various sectors of the crystal

Optical orientation:

Optical axial plane:

 

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