Iron formations and bauxite

Limonite, haematite and magnetite. Hamersley Basin, Australia

The main band comprises sub-bands of magnetite (brown-grey) and haematite (blue-white) within limonite (blue-grey). Coarse-grained haematite (blue-white, bottom right), 'specularite', is associated with coarse-grained limonite (blue-grey), showing bireflectance (bottom left), and quartz (black, bottom centre).

Polished block, plane polarized light, x 160, oil

Limonite, haematite and magnetite. Hamersley Basin, Australia

Two curved bands (brown-grey) comprise a fine-grained intergrowth of magnetite, haematite and limonite which cannot be distinguished from each other at this magnification. Coarse-grained, characteristically lath-shaped haematite (blue-white, centre) grows between the bands within limonite (blue-grey, top right), which is the main phase. Limonite shows faint reflection pleochroism (grey-blue to grey-brown) and bireflectance. Dark grey areas are silicates.

Polished block, plane polarized light, x 80, air

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

Haematite. Little Broken Hill, Australia

A highly metamorphosed iron formation. Coarsely crystalline haematite (white) has totally replaced magnetite and so is martite. The poorly polished cores (centre top) show less complete replacement than the well polished rims. The matrix comprises a mosaic of equigranular garnet (light grey, centre bottom) and quartz (dark grey) with characteristic 1200 angles between adjacent crystals. Black areas are polishing pits.

Polished block, plane polarized light, x40, air

Gibbsite, boehmite and haematite. Gove, Northern Territories, Australia

A pisolitic bauxite in which an angular fragment of an earlier pisolith has been extensively haematitized (blue-white, centre). Haematite is the only mineral that can be identified by reflected light microscopy in this section. The matrix, which is light red-brown due to finely disseminated iron minerals, comprises gibbsite and boehmite which were identified by X-ray diffraction.

Polished block, plane polarized light, x40, air

Haematite, gibbsite, boehmite and TiO₂ mineral. Gove, Northern Territories, Australia

An angular haematitized clast comprises banded haematite (white, centre left) with variations in reflectance that are related to grain size and crystallinity of the haematite. The dark brown-grey matrix is a mixture of gibbsite and boehmite. All three minerals were confirmed by X-ray diffraction. A single rounded grain of a TiO₂ mineral (white, highest reflectance, centre left) lies close to a polishing pit (black).

Polished block, plane polarized light, x 40, air