PRECIPITATION BY WETTING-DRYING MECHANISMS OBSERVED ON CALCITE AND GYPSUM SURFACES WITH ATOMIC FORCE MICROSCOPY AND SCANNING ELECTRON MICROSCOPY

Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to study calcite and gypsum nucleation, precipitation and growth. This study is aimed at improving the understanding of mechanisms and rates of nucleation, precipitation and growth resulting from wetting and drying cycles in natural soils under conditions of variably acidic environments. In the experiments with calcite, a solution saturated with calcium carbonate at pH=4 (adjusted with HCl) and doped with KH2PO4 was repeatedly evaporated on a freshly cleaved calcite crystal. Evaporation on glass surfaces was used for comparison. In the experiments with gypsum, a solution saturated with calcium sulfate at pH=3 (adjusted with H2SO4) was repeatedly evaporated on freshly cleaved gypsum. Up to 15 cycles of wetting and drying were performed. Freshly cleaved surfaces showed relatively flat, wide terraces with steps only one or few monolayers high. Exposure to distilled water enhanced resulted in the development of shallow etch pits. Exposure to a diluted hydrochloric or sulfuric acids promoted deeper etch pits with less regular shapes. Nucleation and precipitation from saturated aqueous solutions starts along surface defects, cleavage planes, on microparticles, or in the center of etch pits. A uniform coating forms with an increasing number of wetting-drying cycles. The morphology of crystals precipitated on glass differs from these precipitated on calcite or gypsum. Numerous phases were identified among precipitated crystals. For example, in the experiment with phosphate-doped calcium carbonate precipitation of KCa2H7(PO4)4 x 2H2O, sylvite KCl; brushite CaHPO4 x 2H2O, monetite CaHPO4, and vaterite CaCO3 was determined with X-ray diffraction. Results of this study suggest that crystal nucleation, precipitation and growth depends strongly not only on the chemistry of the solution but on the character of the substrate and number of wetting drying cycles as well.