Distribution changes of Mn2+ and Fe3+ on weathered marble surfaces measured by EPR spectroscopy

Abstract

The behavior of Mn2+ and Fe3+ on weathered marble surfaces was studied by electron paramagnetic resonance spectroscopy (EPR). The paper is concentrated in Mn2+ and Fe3+ replacing Ca2+ in the CaCO3 of marble. Nineteen marble samples of different types and exposed to different environmental conditions were analyzed: Samples of various grain sizes and manganese concentrations, samples from quarry fronts, excavations and monuments exposed in rural or polluted environments. Mn2+ does not show a systematic behavior but Fe3+ decreases up to 100% on the surface, compared to the marble bulk. The depletion starts at a depth around 4 mm from the outer surface. The phenomenon is explained by dissolution and re-crystallization of calcite together with microbe-mediated reduction of Fe3+ to Fe2+. Higher decrease is observed on samples of low-grade metamorphosis, on excavated samples and samples exposed to heavily polluted urban environment. Unexpectedly, this depletion extends deeper than the actual weathering depth (measured by SEM), i.e. the decrease in Fe3+ concentration extends beyond the patina–marble interface, up to a few millimeters into the healthy marble. This additional depletion may be explained by solid-state diffusion of iron. Such a hypothesis has already been reported in literature for other ions in calcite crystals.