DISTRIBUTION AND CHARACTERIZATION OF SHORT-RANGE-ORDER MINERALS IN SPODOSOLS FROM THE WASHINGTON CASCADES

Short-range-order minerals were examined in a subalpine Spodosol formed from mixed tephra-andesitic glacial drift in the Cascade Range of Washington State. The short-range-order mineral fraction was examined using selective dissolution, IR spectroscopy, scanning and transmission electron microscopy, and potentiometric titrations. Short-range-order minerals represented less than 5% of the inorganic, smaller than 2-mu-m fraction in the E horizon, and from 20 to 50% in the B and C horizons. Thick surface coatings were found on the sand-sized grains of the B horizons, but these coatings were absent in the E horizon. The coatings in the Bhs horizon were dominated by humic substances, ferrihydrite, and geothite; in the Bs horizons, imogolite, ferrihydrite, and geothite were the primary constituents. Concentrations of both ferrihydrite and imogolite decreased with depth. The variable charge surfaces of the short-range-order minerals strongly influenced the surface chemistry in the B horizons. The point of zero salt effect (PZSE) occurred at pH 3.6 and 4.7 for the Bhs and Bs horizons, respectively. There was no unique PZSE in the E horizon because of the absence of variable charged substances. Short-range-order minerals play important roles in ecosystem processes through their influence on acid/base buffering, sorption, and regulation of aqueous ion activities.