Back-scattered electron image	Ca x-ray map
Walnut Fm limestone before exposure
(Cream colored rock contains a great number of cavities and fossils. There are two different calcite species. A higher-Mg calcite, probably biogenic, dominates and was replaced by a low-Mg calcite. Where the latter grew into cavities, it forms euhedral trigonal crystals.)
Walnut Fm limestone after exposure.
The limestone was roughened – the 1 micron polish had been degraded by corrosion. There were partings along many grain boundaries between individual grains of higher-Mg calcite. These areas were free from sulfur according to the S x-ray image. Various other regions, however, contained significant amounts of sulfur. Sulfur was dominantly concentrated in regions that were initially composed of coarse-grained low-Mg calcite.
Interpretation:
The overall reaction of calcium carbonate in the limestone with atmospheric sulfur dioxide in an oxidizing and hydrous environment is expected to be:
CaCO3 (s) + SO2 (g) + 2 H2O (g) + 0.5 O2 (g) « CaSO4.2H2O (s) + CO2 (g)
The most stable calcium sulfate is gypsum.
Ca dissolved from the calcite grain boundaries has been transported from the site of dissolution (no S x-rays were derived from such crazed regions). Sulfur dioxide from the atmosphere and water (either from the atmosphere or from water adsorbed on the minerals in the limestone) form a dilute sulfuric acid solution. The sulfuric acid dissolved calcite along grain boundaries and the dilute calcium sulfate migrated through the rock to precipitate gypsum elsewhere in the stone.

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Last modified: Feb 2, 1999