Mechanism of diopside dissolution from hydrogen depth profiling

Abstract

The basic processes involved at the reaction interface during the aqueous dissolution of crystalline silicates are a matter of controversy. In particular, the existence of a hydrated or protonated surficial layer which has been invoked to explain dissolution kinetics has not been demonstrated yet. By using a resonant nuclear reaction, which allows hydrogen depth profiling, we present, in the case of diopside, the first direct evidence that dissolution proceeds via a surficial hydration over thicknesses of about 1,000 Å. This evidence is combined with X-ray photoelectron spectrometry and secondary ion mass spectrometry data, to show that approximately congruent leaching at energetic portions of the mineral surface allows permeation of the resulting porous structure by water, much of which is strongly bonded to the silicate surface. Thus, we suggest that the migration of molecular water into the surface could be a key step in the dissolution kinetics of diopside and probably of other cationic silicate minerals.