Imaging feldspar reactivity with X-ray reflection interface microscopy

By Steve McGregorMay 1, 2010

The mineral-fluid interface is the principal site of low-temperature geochemical processes at and near Earth’s surface, and exerts a powerful influence on natural geochemical cycles. The dissolution of silicate minerals (e.g., feldspars) is a process that buffers atmospheric CO2, and has been widely studied through traditional approaches (batch dissolution, etc.).

Our research explored the sensitivity of the orthoclase dissolution process to elevated salinity at pOH = 2 and 85°C.  In-situ X-ray reflectivity (XR) measurements of orthoclase (001) show that the addition of 2 m NaCl increases the overall dissolution rate by a factor of ~5, and leads to a rougher surface than found in the absence of NaCl. Ex-situ XRIM images of reacted orthoclase surfaces show that orthoclase initially dissolves inhomogeneously at 1 m NaCl, leading to micron-scale regions with locally increased roughness, providing information on the lateral variation of orthoclase reactivity.  The combined XR and XRIM observations suggest that the increase in dissolution rate is associated with an increase in the intrinsic reactivity of the terrace regions on the surface. 

These observations demonstrate, for the first time, a new capability to image interfacial reactivity using X-ray microscopy.  This opens up the potential for real-time imaging of interfacial processes through in-situ studies and complements existing interfacial imaging approaches (e.g., atomic force microscopy).

Citation Information: P. Fenter,  S.S. Lee, C. Park, J. G. Catalano, Z. Zhang, N. C. Sturchio, “Probing Interfacial Reactions with X-ray Reflectivity and X-ray Reflection Interface Microscopy: Influence of NaCl on the Dissolution of Orthoclase at pOH 2 and 85 °C”, Cosmochim. Acta, 74 (2010).