Cosalite

Formula: Pb2Bi2S5

Reflected/transmitted colour: white, with only a trace of cream

Pleochroism: weak

Optical isotropy/anisotropy: weak anisotropy in air, distinct in oil, pinkish yellow, bluish or violet-gray. Se-cosalite more strongly anisotropic

Optical sign:

Refractive indices:

Bireflectance/Birefringence δ: weak in air, more distinct in oil; parallel elongation creamy, lighter, perpendicular elongation darker, greenish

2V angle:

Dispersion:

Optical orientation:

Optical axial plane:

 

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