Fool's gold in concretion

Fool's gold in concretion -You can see some large oval discs embedded in the black sedimentary rocks. These are concretions composed of iron oxide, clay and other materials. This used to be a bay area with weak wave action in the past. The black colour of the sedimentary rock indicates that the water which precipitated it contained huge amount of sludge. Microorganisms consumed most oxygen in the sludge to degrade the organic materials while the weak water current could not replenish the oxygen in the water. This encouraged sulphur and iron containing compounds to react and produce pyrite, an iron sulphide crystal which is gold in colour. The pyrite accumulated around a foreign solid object and eventually a rounded and symmetrical concretion was formed. As pyrite is gold in colour and is frequently associated with gold and copper minerals, the discovery of pyrite alwayls leads to disappointment to people who think they found gold. Its name, fool¡¦s gold, was thus coined.