Dissolution and Stability of Biogenic Opal

Abstract

AbstractBiogenic opal isolated from leaves of seven deciduous trees, one grass species, and surface horizons of forest and grassland soils, was examined for dissolution in hot and cold distilled water. In cold water, silica dissolution was torpid and Si concentrations remained nearly constant through a 4‐week equilibration period; Si values for all but one opal specimen were between < 0.5 and 2 µg Si/ml. Dissolution over a 5‐hour period in distilled water at 90°C yielded the following Si concentrations (µg Si/ml): 4 to 5 for tree‐leaf isolates; 1 to 8 for soil isolates, 83 for the grass isolate, 0.5 for quartz, and 110 for silica gel. Opal dissolution is greater for 2–5 µm soil isolates than 5–20 µm isolates. Biogenic opal isolated from soils and deciduous tree leaves is much more stable than previously considered and exhibited Si dissolution similar to that reported for geologic opal. Further work is needed to clarify differences in opal solubilities between grass and forest origin.