Manganese minerals

Birnessite/todorokite, uncharacterized iron and manganese hydroxides. Pacific Ocean

In this manganese nodule, collomorphic banding comprises intimate mixtures of manganese- and iron-bearing hydroxide minerals. Higher reflectance bands (white-grey) are bireflecting and anisotropic (in oil) and comprise birnessite and todorokite. Their presence was confirmed by X-ray diffraction. Lower reflectance bands (brown-grey) comprise poorly crystalline manganese and iron hydroxides and are less well polished. A detrital silicate grain (dark grey, bottom centre) is present. Medium grey areas are mainly resin and black areas are polishing pits.

Polished block, plane polarized light, x 80, air

Birnessite/todorokite, uncharacterized iron and manganese hydroxides. Pacific Ocean

A manganese nodule in which a radiolarian test (centre) lies within collomorphic banding of iron- and manganese-bearing hydroxides. Higher reflectance bands are mixtures of birnessite and todorokite, whereas lower reflectance bands are browner and less well polished and comprise uncharacterized iron and manganese hydroxides and oxides. Black areas are polishing pits. It is not possible to identify individual minerals within manganese nodules by reflected light microscopy alone.

Polished block, plane polarized light, x80, air

Magnetite, calcian rhodochrosite, limonite and chalcopyrite. Harlech Dome, Wales, Britain

Euhedral magnetite (brown, top left) crystals occur within a matrix of calcian rhodochrosite (light grey, faint bireflectance). Microconcretions of calcian rhodochrosite are replaced by fine-grained iron hydroxides/manganese oxides (blue-brown-grey, centre right) in a matrix of unreplaced calcian rhodochrosite. A veinlet of limonite (blue-grey, top) crosscuts the primary banding which is oriented left-right. A single crystal of chalcopyrite (yellow, centre bottom) illustrates the low reflectance of the other phases. Black areas are polishing pits.

Polished block, plane polarized light, x40, air

Calcian rhodochrosite, magnetite, limonite and psilomelane. Harlech Dome, Wales, Britain

The ore is essentially a banded pelite with magnetite-rich bands (left) and magnetite-poor bands (right). Magnetite (brown-grey, highest reflectance, left) is euhedral and occurs within a matrix of calcian rhodochrosite and silicates. Calcian rhodochrosite forms zoned microconcretions (centre) with browner core and light grey margins that are intergrown with unaltered calcian rhodochrosite (dark grey) and fine-grained quartz (black). A thin limonite (light grey, centre left) veinlet, lying parallel with the banding, is partially infilled with psilomelane showing bireflectance (top left).

Polished block, plane polarized light, x40, air

Magnetite, limonite, psilomelane and calcian rhodochrosite. Harlech Dome, Wales, Britain

Euhedral magnetite (brown, left) lies within a matrix of crystalline quartz (dark grey, featureless, centre left).,Microconcretions of calcian rhodochrosite (right) are replaced by fine-grained haematite and manganese oxides (white-grey, centre) and by limonite (blue-grey, top right). Calcian rhodochrosite (dark grey, bottom, centre) occurs as an outermost rim to the replaced cores. The radiating habit of the carbonate is seen by the orientation of the replacing limonite (bottom right). A veinlet comprises limonite (blue-grey, bottom left), which shows faint bireflectance, and psilomelane (top left), which also shows bireflectance and reflection pleochroism (grey to white). This veinlet cross-cuts the quartz matrix.

Polished block. plane polarized light, x 160, oil

Magnetite, limonite and psilomelane. Harlech Dome, Wales, Britain

Euhedral magnetite (pink-brown, top) is associated with minor amounts of replaced calcian rhodochrosite (blue-grey, bottom left, top left). Limonite (blue-grey, left centre) in a veinlet shows bireflectance (left). The centre of the veinlet carries curved laths of psilomelane showing strong bireflectance and reflection pleochroism (from white-grey to grey-white). The matrix is silicate.

Polished block, plane polarized light, x 160, oil