A LABORATORY SIMULATION OF DEGRADATION OF LEINSTER GRANITE BY DRY AND WET DEPOSITION PROCESSES

The accelerating effect of pollutant gases and acids on the degradation of `fresh' Leinster granite has been simulated in laboratory-based dry deposition and wet deposition exposure chambers. Granite degradation has been followed by weight change, stone extract and sectional analyses, pH change and cation content of the run-off solution. General deterioration of the granite structure is revealed by petrographic analyses to occur by a series of mineralogical tranformations, of which kaolinization and seritization are the most clearly visible. Microcracking, predominantly in the alkali/microcline and plagioclase feldspar crystals, is evident on the exposed side, especially in simulated acid rain solutions but not in deionized water. Ion exchange of H+ for Ca2+, allied with mineralogical transformation and salt crystallization in pores, evidently promote the cracking. Calcite and gypsum deposition in intergranular regions and cracks are also observed and there is evidence of the removal and oxidation of iron species on the mineral surface.