Magnetite

Formula: FeFe2O4

Reflected/transmitted colour: variable, darker varieties almost opaque, polished section gray, commonly with a brownish tint, substitution by Ti may cause a brownish pink tint, Mn a more yellowish green tint, Al and Mg a yellowish tint and Cr a darker gray tint

Pleochroism: -

Optical isotropy/anisotropy: isotropic, polished section isotropic, a slight anomalous anisotropy due to internal tension by zoning or deformation may be present, distinct anisotropy caused by twinning has been reported

Optical sign:

Refractive indices: n  = 2.42

Birefringence δ/bireflectance:

2V angle:

Dispersion: moderate

Optical orientation:

Optical axial plane:

 

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