Natural etching rates of feldspar and hornblende

Analysis of the etch-pit size distributions (PSDs) observed on potassium feldspar and hornblende grains in a soil catena in loess (age = 12,500 y) reveals natural mineral etching rates. Rates estimated for hornblende (6 to 9×10^–15mol/m²s) are based on consistent crystallographically controlled etch pits, while rates estimated for potassium feldspar (2×10^–15mol/m²s) are based on irregularly shaped pits. Although little difference in etching rate is observed between soil horizons, the highest etching rates generally occur in the upper B horizons where pH values are lowest. Decreasing soil drainage correlates with an increase in pit density,n°, probably due to increased grain wetting, while decreased drainage correlates with a decrease in pit growth rate (G), probably due to increased dissolved solute concentrations. The PSD model predicts that etching rate is a function ofn° and ofG⁴. Etching rates calculated for potassium feldspar do not vary with drainage, while those of hornblende decrease with decreasing drainage. Estimated etching rates are lower than bulk dissolution rates measured in the laboratory.