Metacinnabar

Formula: HgS

Reflected/transmitted colour: opaque, polished section  in air grayish white, in oil much darker with a brownish gray tint

Pleochroism: occasionally weak

Optical isotropy/anisotropy: polished section isotropic, often shows anomalous anisotropy, then a very weak bireflectance may be visible at grain boundaries

Optical sign:

Refractive indices:

Birefringence δ/bireflectance: polished section usually not present

2V angle:

Dispersion:

Optical orientation:

Optical axial plane:

 

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