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

Abstract

Feldspar minerals are the most abundant minerals in the earth’s crust. Therefore, alteration reactions involving feldspars and aqueous fluids must be accounted for in a wide variety of geochemical processes, especially weathering, diagenesis, and trace element transport during fluid flow. The work presented here is, to the best of our knowledge, the first direct in situ measurement of the cation exchange process at the feldspar surface. It is known that incongruent dissolution occurs under acidic conditions and this is ascribed to an initial step of cation–proton exchange at the near surface of the mineral. Cation depletion has been confirmed in other studies by ex situ analysis of reacted feldspar grains, and indirect chemical methods have been used to infer that this exchange reaction is rapid. We have used specular x-ray reflectivity both in situ and ex situ to monitor the structural changes that occur on a polished oligoclase feldspar (001) surface in contact with dilute aqueous solutions. Starting material was Ab 74 with an r.m.s. surface roughness (σ) of 6.1 Å. Upon addition of distilled deionised water into the reaction cell the reflectivity of the surface changed immediately to produce a reflectivity profile consistent with the formation of a leached layer 32.4 Å thick. The model electron density (ρ) of this leached layer is consistent with complete removal of Na and Ca from this layer (ρ layer = 0.79 ρ olig). Changes to this surface in distilled 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 rapid and that the layer itself stabilises quickly. Upon acidification of the reactant fluid to pH 3.1, the layer thickness did not change. Acidification did cause the leached layer/fluid surface to roughen to σ = 7.1 Å. Leached layer thickness was corroborated ex situ by XPS depth profiling of the same sample after the x-ray reflectivity experiments. Na depletion measured by X-ray photoelectron spectroscopy analysis is significant to between 25 and 30 Å depth. This demonstrates the broad agreement between the techniques and shows that in situ studies are now possible. Along with information about layer thickness and the roughness of the leached layer/fluid surface, the x-ray data have also been used to constrain the roughness at the interface between the leached layer and the unreacted bulk feldspar. This interface is approximately 15 Å rough, which may suggest that 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.