Old Meadows Ferrihydrite
The Carboniferous coal measures of the north Pennines are layers of sandstones and siltstones with intervening seams of coal. The sandstones were deposited in rivers and deltas and are rich in iron, which was reduced to pyrites (iron sulphide), as the coals formed. When the mines were dug, they descended below the water table and required pumping out so that they could be worked. Now the pits have closed, the mines have flooded. The presence of water, oxygen and bacteria entering the mines oxidises the pyrites releasing sulphur and this causes the mine waters to become acidic; the iron is re-precipitated in the form of the bright orange mineral ferrihydrite (iron oxide pentahydrate). In the recent past waters pumped from the coal mines was drained into the local river and canal systems, attaining many of them bright orange, however in recent years, the Coal Authority have made substantial attempts to clean rivers and capture the precipitates in settling ponds. These ponds contain pure ochres composed of ferrihydrite in the form of finely particulate flakes with both crystalline and amorphous forms.
 Cornell, R. M. & Schwertmann, U., 2003, The iron oxides: Structure, Properties, Reactions, Occurrences and Uses., 2nd Edition., Wiley – VCH., 703 pp.