The role of humic substances in the formation of marble patinas under soil burial conditions

Abstract

The present work aim to study the effect of burial on the photoluminescnece (PL) spectra of white, crystalline marble surfaces and the physicochemical processes that take place at the marble-soil interface. The PL was studied by an argon ion laser beam, focused through a microscope objective onto the sample, offering a spatial resolution of 3 mu m. Long-buried (time scale of 1,000 years) surfaces show a red (at 610 nm) emission due to Mn2+, which is also shown on fresh marble spectra and an additional broadband blue-green (380-530 nm) one. Electron paramagnetic resonance (EPR) spectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) indicate that the latter emission originates from humate complexes. The complexes are most probably Ca-humates, the humic substances found in the soil and the divalent calcium cations released by the dissolution of marble calcite. Finally, the examination of recently (time scale of 50 years) buried surfaces shows that the blue-green emission and consequently the presence of humates in marble patinas are not affected by the soil organic matter content. Soil acidity however, is a critical factor, with a total absence of the blue-green emission at pH values lower than 6.