Probing the surface properties of weathered silicate minerals to better understand their reactivity

Abstract

While we expect conventional reactive transport simulations to provide reliable estimations of the evolution of fluid-rock interactions over time scales of centuries and even more, recent experimental studies showed that they could hardly be satisfactorily used on simplified systems (e.g. batch experiments on single minerals), on time scales of weeks [1]. As emphasized elsewhere [1, 2], the reasons for such inconsistencies have to be sought in the nature of the rate laws used in the geochemical codes, which heavily rely on our description of the fundamental mechanisms involved during water-mineral reactions. In that respect, the present ongoing work aims at gathering some of our recent findings in the dissolution kinetics of a series of Al-free silicates, in relation to the physicochemical properties of their surfaces after/during hydrothermal weathering. A first still unresolved issue that we are addressing is the effect of ubiquitous silica-rich layers which form on silicate minerals. While µm-thick silica coatings formed on the surface of wollastonite crystals without significantly affecting their dissolution rate, we observed that nm-thick silica coatings fully passivate the surface of olivine crystals [1, 3]. We will show how the use of microscopic (STEM, HTEM) [3] and spectroscopic (ToF-SIMS, XPS) techniques helped us to unravel these paradoxical properties, and which chemical parameters could influence the textural features of the layers. A different (or supplementary) mechanism possibly responsible for unexpected decreases of silicate dissolution rate at "far-from-equilibrium" conditions (e.g. diopside, [4]) was proposed to arise from the surface topography of the dissolving crystals and the occurrence (or absence) of etch pits [5]. We will show how the in situ monitoring of the dissolving surface of diopside as a function of fluid saturation state in a HAFM flow-cell (e.g. [6]) is allowing us to address this question. [1] Daval et al (2010) Proceed WRI-13, 1, 713-716 [2] Zhu (2009) Rev Mineral Geochem, 70, 533-569 [3] Daval et al (2009) Am Mineral, 94, 1707-1726 [4] Daval el al (2010) Geochim Cosmochim Ac, 74, 2615-2633 [5] Arvidson & Luttge (2010) Chem Geol, 269, 79-88 [6] Saldi et al (2009) Geochim Cosmochim Ac, 73, 5646-565