Characterization of the weathering status of feldspar minerals in sandy soils of Minnesota using SEM and EDX

Scanning electron microscopy was used to assess the weathering status of plagioclase and alkali feldspar grains isolated from sandy outwash soils located under three vegetation communities: mixed red and white pine, maple-basswood, and white cedar. Feldspars were selected for this study because they were the most common weatherable minerals in these sandy soils. A morphological weathering index, based on the percent of the surface of the feldspar grains covered with etch pits, was used to evaluate weathering intensity. Results revealed that significant pedogenic weathering has occurred in the surface and subsurface horizons of these soils compared to the parent material. Plagioclases were found to be much more weathered than alkali feldspars. Within the plagioclase solid solution series, the weathering index increased linearly with increasing Ca content as determined by energy dispersive X-ray analysis. Comparisons among the three sites showed that weathering environments under the pine and maple-basswood communities were relatively similar, being only slightly more intense under pine. The most intense weathering occurred under the white cedar community, particularly in the E horizon. Linear regression analyses showed that the weathering index for alkali feldspars was significantly related to soil pH (r² = 0.30, p < 0.001). In the case of plagioclases, multiple linear regression analyses showed that the weathering index was significantly dependent on both Ca (Ca + Na) molar ratio and pH (r2 = 0.83, p < 0.001), with 64% weight effect due to Ca (Ca + Na) ratio, and 19% due to pH.