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

The surfaces of sanidine grains (KAlSi3O8), leached in acid solutions (pH 1 ) in the presence of amorphous silica powder, have been examined by SIMS, X PS, Fourier transform infrared spectroscopy (FTIR), SEM, transmission elect ron microscopy (TEM) and electron probe microanalysis (EPMA). Results revea l a layer at the mineral's surface that is depleted in Na, K and Al in comp arison to the bulk material. The purpose of this study was to perform a det ailed characterization of the altered surface layer and to decide whether t his 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 hydra ted layer has been observed to increase with elevated reaction times and te mperatures. The amount of dissolved silicon plays an important role in the lifetime of this layer. Diffusion coefficients calculated from the K/Si SIM S depth profiles yielded realistic numbers for the out-diffusion of K at th e fresh sanidine/altered layer boundary. Fourier transform infrared spectro scopy analyses reveal that a large part of the H signal observed in the SIM S profiles is adsorbed water, although some hydrogen is preferentially boun d to AlO. The amount of adsorbed H2O, together with the increasing number o f etch pits, supports the hypothesis that penetration of molecular water is a key step in the dissolution process of feldspar minerals. Transmission e lectron microscopy allows imaging of the contact region between the crystal line sanidine and the amorphous surface layer: the observations strongly su pport the hypothesis of an Na, K, Al-depleted layer, Secondary ion mass spe ctrometry measurements were also carried out on sanidine samples leached in the presence of isotopically enriched, amorphous silica powder. This allow ed confirmation that the Na, K, Al-depleted layer was at least partly forme d by leaching but also showed an exchange of silicon between solution and m ineral. (C) 1999 John Wiley & Sons, Ltd.