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

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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

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