ETCH-PIT SIZE AND SHAPE DISTRIBUTION ON ORTHOCLASE AND PYRIBOLES IN A LOESS CATENA

Etch pits on 50-100 mum orthoclase and pyribole grains from the E, B, and C horizons of a catena or drainage sequence of well-drained, somewhat poorly drained, and poorly drained loessal soils were analyzed using SEM and digital image analysis. The objective was to determine the influence of soil drainage conditions and the associated geochemical environments on dissolution of primary silicates as determined from the distribution of etch pits on the grain surfaces. Etch-pit size frequency distributions indicated that the poorly drained soil contained the smallest and most numerous etch pits for both minerals. This indicated low growth rates for etch pits in the poorly drained soil. Analysis of etch-pit shapes indicated that etch pits on pyriboles were more elongated and irregular in the poorly drained soil, possibly resulting from a greater growth rate; whereas etch pits on orthoclase grains were more elongated and irregular in the well-drained soil. The larger, more elongated and irregularly shaped etch pits on orthoclase from the well-drained soil, indicative of a greater pit growth rate, is consistent with a flushing model of mineral weathering in soils. The etch-pit size distribution on pyriboles is consistent with the flushing concept, but the shape data indicates that other mechanisms of weathering may be involved.