In-Situ Observation of Oxide and Silicate Mineral Dissolution by Hydrothermal Scanning Force Microscopy: Initial Results for Hematite and Albite

Abstract

The dissolution and growth of oxide and silicate minerals is of interest to the geochemical community in fields ranging from radioactive waste storage, enhanced oil recovery and the influence of weathering on global climate. Recently, application of scanning force microscopy (SFM) has revealed new details of dissolution and growth mechanisms for ionic solids such as calcite (Gratz et al., 1993), but the dissolution rates of most oxide and silicate minerals are lower than the range accessible to in situ SFM imaging at room temperature (about 10 6 to 10 -9 moles m -2 s 1; Dove and Chermak, 1994, Stumm and Morgan, 1996). The vapour pressure of water imposes a fundamental temperature limit on currently available SFM fluid cell temperatures. Here, we present initial results from an SFM of our design capable of imaging in aqueous solution up to 150~ We report initial observations on the dissolution of hematite and albite in aqueous solution at 100~ ~ and 6 bar. Albite dissolution is compared to the behaviour of p e r i c l a s e ( M g O ) at r o o m t e m p e r a t u r e . Multicomponent oxides such as albite probably do not dissolve by a step-motion mechanism such as observed for calcite, gypsum, and brucite by SFM (e.g. Jordan and Rammensee, 1996). Rather, surface alteration and roughening occurs. Hydrothermal AFM provides real-time, in situ imaging access to reactions of silicate minerals with hydrothermal solutions. thermocouple ports. Flow is controlled by a mass flow controller. A 600 ml Ti bomb serves as a solution source. A Digital Instruments (DI) optical head was mounted on a custom base; electronics were developed to couple our SFM to a DI controller.