Grey limestone and iron oxide, close-up. This calcarerous sedimentary rock has an oolithic texture that is not easily visible. The grey colour comes from very fine iron sulphide (pyrite) crystals, produced in a reducing environment. Iron oxidation by oxygen-rich water may change the colour to reddish-orange, as seen here down centre. This specimen is part of the Geoscope geological park, located between Les Monts-Vert and Albaret-Sainte-Marie, central France.
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