NATURAL ETCHING RATES OF FELDSPAR AND HORNBLENDE

Analysis of the etch-pit size distributions (PSDs) observed on potassi um feldspar and hornblende grains in a soil catena in loess (age = 12, 500 y) reveals natural mineral etching rates. Rates estimated for horn blende (6 to 9 x 10(-15) mol/m(2)s) are based on consistent crystallog raphically controlled etch pits, while rates estimated for potassium f eldspar (2 x 10(-15) mol/m(2)s) are based on irregularly shaped pits. Although little difference in etching rate is observed between soil ho rizons, the highest etching rates generally occur in the upper B horiz ons where pH values are lowest. Decreasing soil drainage correlates wi th an increase in pit density, n(0), probably due to increased grain w etting, while decreased drainage correlates with a decrease in pit gro wth rate (G), probably due to increased dissolved solute concentration s. The PSD model predicts that etching rate is a function of n(0) and of G(4). Etching rates calculated for potassium feldspar do not vary w ith drainage, while those of hornblende decrease with decreasing drain age. Estimated etching rates are lower than bulk dissolution rates mea sured in the laboratory.