Berthierite

Formula: FeSb2S4

Reflected/transmitted colour: polished section white-gray with a pinkish or brownish tint

Pleochroism: marked

Optical isotropy/anisotropy: polished section very strong anisotropy,  about same as for stibnite, but colours are more vivid. Sharp extinction

Optical sign:

Refractive indices:

Birefringence δ/bireflectance: polished section strong and characteristic //α brownish pink, //β gray-white, //γ white

2V angle:

Dispersion:

Optical orientation:

Optical axial plane:

 

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