THE ROLE OF INTRAGRANULAR MICROTEXTURES AND MICROSTRUCTURES IN CHEMICAL AND MECHANICAL WEATHERING: DIRECT COMPARISONS OF EXPERIMENTALLY AND NATURALLY WEATHERED ALKALI FELDSPARS - IMPLICATIONS FOR HYDROLOGIC CONTROL OF DIFFERENCES IN ABSOLUTE RATES

Abstract

Electron microscopic observations of alkali feldspars from soils show that intragranular microtextures, such as exsolution lamellae, and microstructures, primarily dislocations, are both highly significant determinants of the weathering behaviour of these minerals. In particular, strained structure around intersecting edge dislocations in the plane of exsolution lamellae, ~(601), dissolves at a rate which is orders of magnitude greater than unstrained feldspar, producing a mesh of intersecting etch tubes extending >5 x 10-3 cm into the crystal. As a result, dissolution at dislocations is the major source of solutes during initial stages of chemical weathering in the field. With progressive chemical weathering, the most highly reactive feldspar is consumed by growth and coalescence of etch tubes, but outer parts of the grain are physically weakened, leading to mechanical flaking that increases available surface area and exposes further reactive sites. In contrast, previous dissolution experiments, and microscopy of reacted surfaces, have shown little or no correlation between dissolution rate and dislocation density and few visible signs of dissolution at particularly reactive sites.To resolve the apparent discrepancy between field and laboratory behaviour we have carried out flow-through dissolution experiments using pH 2 HCl at 25°C on three alkali feldspars with carefully characterized intragranular microtextures and microstructures. These alkali feldspars were: (1) Eifel sanidine, an alkali feldspar that has no microtextures at the TEM scale and a low dislocation density (<106 cm-2), (2) unweathered alkali feldspars from the Shep Granite, which have a mainly coarse exsolution microtextures and higher dislocation density (>2-3 x 108 cm-2), and (3) naturally weathered alkali feldspars, also from the Shap Granite, which have the same microtextures as unweathered Shap Granite alkali feldspars but, because they have been weathered, have a lower density of dissolution reactive dislocations exposed on grain surfaces (<2-3 x 108 cm-2). Results from the experiments are ambiguous. If the rate data are normalised to the powder's initial BET surface area, dissolution rates increase with dislocation density. Normalisation to the powder's BET surface area as it is inferred to have changed during the experiments, yields no correlation with dislocation density. SEM and AFM images of reacted grain surfaces show that dislocation outcrops and albite exsolution lamellae have both etched more rapidly than tweed orthoclase, but dissolution at these sites makes a quantitatively insignificant contribution to the overall rate of laboratory dissolution of the feldspar powders. Major differences in the importance of dislocations to rates of early chemical weathering in field and laboratory contexts probably result from corresponding contrasts in the saturation state of ambient solutions. Observations of naturally weathered alkali feldspars show that microtextures and microstructures have the greatest impact on mineral weathering rates during advanced stages of dissolution when grain surfaces start to disintegrate. Hundreds of years of dissolution under the laboratory conditions used here would be required to reach this stage.