Dissolution study of potassium feldspars using hydrothermally treated sanidine as an example

Abstract

The surfaces of sanidine grains (KAlSi3O8 ), leached in acid solutions (pH 1) in the presence of amorphous silica powder, have been examined by SIMS, XPS, Fourier transform infrared spectroscopy (FTIR), SEM, transmission electron microscopy (TEM) and electron probe microanalysis (EPMA). Results reveal a layer at the mineral's surface that is depleted in Na, K and Al in comparison to the bulk material. The purpose of this study was to perform a detailed characterization of the altered surface layer and to decide whether this layer was formed by actual leaching of the mineral's surface or/and by precipitation of silica on the surface. The thickness of the residual hydrated layer has been observed to increase with elevated reaction times and temperatures. The amount of dissolved silicon plays an important role in the lifetime of this layer. Diffusion coefficients calculated from the K/Si SIMS depth profiles yielded realistic numbers for the out-diffusion of K at the fresh sanidine/altered layer boundary. Fourier transform infrared spectroscopy analyses reveal that a large part of the H signal observed in the SIMS profiles is adsorbed water, although some hydrogen is preferentially bound to AlO. The amount of adsorbed H2O, together with the increasing number of etch pits, supports the hypothesis that penetration of molecular water is a key step in the dissolution process of feldspar minerals. Transmission electron microscopy allows imaging of the contact region between the crystalline sanidine and the amorphous surface layer: the observations strongly support the hypothesis of an Na, K, Al-depleted layer. Secondary ion mass spectrometry measurements were also carried out on sanidine samples leached in the presence of isotopically enriched, amorphous silica powder. This allowed confirmation that the Na, K, Al-depleted layer was at least partly formed by leaching but also showed an exchange of silicon between solution and mineral. © 1999 John Wiley & Sons, Ltd.