Physical Properties of Interfacial Layers Developed on Weathered Silicates: A Case Study Based on Labradorite Feldspar

Abstract

Amorphous silica-rich surface layers (ASSLs) formed at the interface between silicate materials and reacting fluids are known to strongly influence, at least in some cases, the dissolution rates of silicate phases including soil minerals, glasses, and cements. However, the factors governing the formation of these ASSLs remain largely unknown. Here, we outline a novel approach that uses recent developments in vertical scanning interferometry (VSI) and in-situ synchrotron-based X-ray reflectivity (XRR) to directly follow the development of ASSLs, and the evolution of their physical properties, on a model silicate (labradorite feldspar). Our approach enabled independently probing the reactivities of the outer (bulk fluid/ASSL) interface and of the inner (ASSL/pristine mineral) interface in-situ, providing a detailed picture of the temporal evolution of the fluid-mineral interface. We investigated the effects of pH, SiO2(aq) concentration, crystallographic orientation, and temperature on the layer thickness, density, and reactivity as well as on the dissolution rate of the primary mineral. The dissolution rate of labradorite crystals increased with temperature, according to an apparent activation energy of ~57 kJ mol-1 and showed no significant difference between crystallographic faces. Both labradorite and ASSL dissolution rates decreased as circum-neutral pH conditions were approached. High SiO2(aq) concentrations resulted in (i) decreased apparent dissolution rates, while far-from-equilibrium conditions with respect to labradorite were maintained in the bulk fluid, and (ii) an increasing ASSL density when combined with low temperature and close-to-neutral pH. Our results highlight the importance of ASSLs and their complex impact on the dissolution process. In particular, our results provide evidence of a discrepancy between bulk fluid conditions, generally probed and reported, and those actually operating at the interface with the dissolving primary phase, which are of more direct relevance to the dissolution process but are still largelyunknown.