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