Limonite, haematite and magnetite. Hamersley Basin, Australia
Two bands comprise a fine-grained intergrowth of magnetite (brown-grey) and limonite (blue-grey) with minor haematite (white, highest reflectance, centre left), especially on their margins. Lath-shaped coarse-grained haematite (blue-white, centre), 'specularite', grows our from the bands and lies within limonite (blue-greys), which shows distinct reflection pleochroism (blue-grey to brown-grey, centre) and bireflectance. The silicate gangue (left) shows red internal reflections due to finely disseminated haematite.
Polished block, plane polarized light, x 160, oil