Reactions at the mineral-fluid interface control the rates of almost all geochemical reactions. These reactions determine the rates of geochemical mass transport at both low and high temperatures and are thus important in influencing a wide range of geological phenomena varying from metamorphism and diagenesis to levels of CO2 in the atmosphere and the mobility of chemical waste in the environment. Our aim is to understand how these reactions take place and how they control the speeds of mineral dissolution and precipitation. This work combines experiment and theory.

Experimentally, it involves state-of-the-art measurements of single crystal and powder dissolution rates in fluids of well-controlled pH, ionic strength and temperature, combined with determinations of proton penetration depths by ERDA, measurements of mineral surface properties by atomic force microscopy (AFM), X-ray reflectivity and measurements of the coordination states of ions on freshly exposed surfaces by X-ray absorption spectroscopy (REFLEXAFS), and in situ measurements of surface roughening and hydrated growth by synchroton glancing incidence X-ray reflection (GIXR) studies. In parallel with these experiments on carbonates, we have measured dissolution rates from carefully-orientated, cut and polished MgO and olivine single-crystal surfaces and are monitoring the changes in surface roughening during dissolution using small angle X-ray reflectivity.

Oxford is fortunate in possessing one of the world’s best installations of different ion microprobes. A new development on the Oxford Scanning Proton Microprobe allows measurement of proton penetration in the leached mineral surfaces by 16O- Elastic Recoil Detection Analysis (ERDA) and we are using this technique in collaboration with members of the SPM group in Physics to measure the nature of proton-cation exchange during dissolution. Recently, in collaboration with Xianyu Xue and Masami Kanzaki at the Institute for the Study of the Earth’s Interior at Misasa in Japan, I have used MAS NMR to characterise changes in silicate surfaces which occur as a result of this proton-cation exchange – watch this space!

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AFMFig1a
Atomic Force Micrograph image of MgO (100) face etched in deionised water showing pyramidal etch pits. Dissolution of the MgO surfaces is controlled by the chemisorption of H2O molecules onto (h10) surfaces.

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Synchrotron X-ray studies of the weathering mechanisms of silicate minerals. Simultaneous EXAFS (Extended X-ray Asorption Fine Structure) and Glancing Incidence X-ray Reflectivity (GIXR) allow measurements of changes in the thickness and atomic arrangements in silicate surface and in solution boundary layers as thin as 2 nm.

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STM images: (a) of a monolayer of adenine assembled on diline and triline domains, (b) illustrating structural features of the adenine ordering. (c) is a high-contrast STM image exemplifying details of the surface ordering shown in (b). (d) The grey and black profiles are relative heights and are drawn from where the arrows G and H point in image (b), respectively. The black profile in (e) is drawn from where arrow F points in image (b), whereas the grey profile in (e) is taken from where arrow D points on the clean surface in the previous Fig. 1(b). Imaging parameters: (a) [95×95 nm2; Vs = 1.8 V; It = 0.04 nA]. (b) [51.4×19.7 nm2; Vs = 1.8 V; It = 0.05 nA]. (c) [19.6×19.6 nm2; Vs = 1.8 V; It = 0.05 nA].

magritte
Ceci n’est pas une pomme.