Lithofacies and petrophysical properties of Portland Base Bed and Portland Whit Bed limestone as related to durability
T.G. Nijland, C.W. Dubelaar, R.P.J. van Hees, T.J.M. van der Linden
Summary: 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 in the Netherlands, has been investigated by microscopy and whole rock chemistry. Samples were obtained from several monuments in the Netherlands, amongst them the Old and New Church (Delft), St. John's cathedral ('s Hertogenbosch) and St. Plechelmus' basilica (Oldenzaal). Microscopically, the layers are composed of algae and fungi, gypsum, airborne particles such as fly ash, and iron (hydr)oxides, present on the surface and in directly adjacent pores. Gypsum is present in all samples, algae are not, but typically occur in the most blackish layers. Black layers show significant increases in loss on ignition (LOI), total and organic carbon, total sulfur and iron, as well as Pb, Cu, Zn and Sn. Formation of thin black layers is evidently not due to a single process, but involves formation of gypsum, deposition of airborne material, microbiotic activity and dissolution and redeposition of Fe-(hydr)oxides.
Key Words: Natural Stone, Bentheim Sandstone, Obernkirchen Sandstone, Black Weathering, Algae, Gypsum, Microscopy
In: Heron 48(3), 179-195, 2003