Black Weathering of Bentheim and Obernkirchen Sandstone

Black weathering of sandstone in monuments is widespread. Some objects owe their name to it, like the Porta Nigra in Trier (Germany). Other than the black gypsum crusts common on limestone, the black weathering layer on sandstone is rather thin and well adherent. Formation of such layers on Bentheim and Obernkirchen sandstone, both widely used...