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

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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, x 40, air

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