In situ synchrotron x-ray reflectivity study of the oligoclase feldspar mineral-fluid interface

Feldspar minerals are the most abundant minerals in the earth's crust. Ther efore, alteration reactions involving feldspars and aqueous fluids must be accounted for in a wide variety of geochemical processes, especially weathe ring, diagenesis, and trace element transport during fluid flow. The work p resented here is, to the best of our knowledge, the first direct in situ me asurement of the cation exchange process at the feldspar surface. It is kno wn that incongruent dissolution occurs under acidic conditions and this is ascribed to an initial step of cation-proton exchange at the near surface o f the mineral. Cation depletion has been confirmed in other studies by ex s itu analysis of reacted feldspar grains, and indirect chemical methods have been used to infer that this exchange reaction is rapid. We have used spec ular x-ray reflectivity both in situ and ex situ to monitor the structural changes that occur on a polished oligoclase feldspar (001) surface in conta ct with dilute aqueous solutions. Starting material was Ab(74) with an r.m. s. surface roughness (sigma) of 6.1 Angstrom. Upon addition of distilled de ionised water into the reaction cell the reflectivity of the surface change d immediately to produce a reflectivity profile consistent with the formati on of a leached layer 32.4 Angstrom thick. The model electron density (rho) of this leached layer is consistent with complete removal of Na and Ca fro m this layer (rho(layer) = 0.79 rho(ottig)). Changes to this surface in dis tilled water were monitored for 19.5 h, during which time the leached layer /fluid surface roughness and layer thickness did not change. This confirms previous studies which concluded that the formation of a leached layer is r apid and that the layer itself stabilises quickly. Upon acidification of th e reactant fluid to pH 3.1, the layer thickness did not change. Acidificati on did cause the leached layer/fluid surface to roughen to sigma = 7.1 Angs trom. Leached layer thickness was corroborated ex situ by XPS depth profili ng of the same sample after the x-ray reflectivity experiments. Na depletio n measured by X-ray photoelectron spectroscopy analysis is significant to b etween 25 and 30 Angstrom depth. This demonstrates the broad agreement betw een the techniques and shows that in situ studies are now possible. Along w ith information about layer thickness and the roughness of the leached laye r/fluid surface, the x-ray data have also been used to constrain the roughn ess at the interface between the leached layer and the unreacted bulk felds par. This interface is approximately 15 Angstrom rough, which may suggest t hat cation leaching from the surface is not uniform. Models of the feldspar dissolution process must account for the three-dimensional nature of this reaction volume. Copyright (C) 1999 Elsevier Science Ltd.