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, SiO 2 (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 SiO 2 (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 3 phase, which are of more direct relevance to the dissolution process but are still largely unknown.

Carbonation of calcium-magnesium pyroxenes: Physical-chemical controls and effects of reaction-driven fracturing

“…According to previous works (i.e., Daval et al, 2009a;Ruiz-Agudo et al, 2012;Wild et al, 2019), replacement processes taking place at the mineral-fluid interface result in strong compositional gradients and important pH differences between the bulk solution and the mineral-fluid interface.…”

“…Several studies have focused on the fundamental understanding of the mechanisms involving dissolution of silicate minerals and the subsequent precipitation of carbonate phases (e.g., Casey et al, 1988;Banfield et al, 1995;Lackner et al, 1995;Ruiz-Agudo et al, 2012;Ruiz-Agudo et al, 2016;Di Lorenzo and Prieto, 2017;Monasterio-Guillot et al, 2019). Studies on the dissolution of primary silicates have systematically reported the formation of an amorphous and hydrated silica-rich layer covering the mineral´s surface, the so-called "leached layer", surface altered layer (SALs) or amorphous silica-rich surface layer (ASSLs), which can passivate mineral dissolution (Casey et al, 1993;Daval et al, 2009a,b;Ruiz-Agudo et al, 2012;Hellmann et al, 2013;Ruiz-Agudo et al, 2016;Wild et al, 2019). Furthermore, most studies on the carbonation of primary silicates have reported an additional passivation effect caused by the formation of carbonate mineral coatings on the primary mineral surface, decreasing the porosity and the permeability of the starting material and limiting the progress of the reaction (O'Connor et al, 2005;;Daval et al, 2009a,b;Kelemen et al, 2011).…”

“…Furthermore, most studies on the carbonation of primary silicates have reported an additional passivation effect caused by the formation of carbonate mineral coatings on the primary mineral surface, decreasing the porosity and the permeability of the starting material and limiting the progress of the reaction (O'Connor et al, 2005;;Daval et al, 2009a,b;Kelemen et al, 2011). In general, passivation resulting in a reduction of permeability and porosity of the host material will take place due to precipitation of a more insoluble phase (Hellmann et al, 2012;Ruiz-Agudo et al, 2012;Wild et al, 2019). These effects play a critical role in GCS processes, controlling the dissolution rate of primary silicate materials and acting as a handicap for their effective CO 2 mineralization (McGrail et al, 2006;Stockmann et al, 2008;Daval et al, 2009a,b;Ruiz-Agudo et al, 2012;Wild et al, 2019).…”

“…In general, passivation resulting in a reduction of permeability and porosity of the host material will take place due to precipitation of a more insoluble phase (Hellmann et al, 2012;Ruiz-Agudo et al, 2012;Wild et al, 2019). These effects play a critical role in GCS processes, controlling the dissolution rate of primary silicate materials and acting as a handicap for their effective CO 2 mineralization (McGrail et al, 2006;Stockmann et al, 2008;Daval et al, 2009a,b;Ruiz-Agudo et al, 2012;Wild et al, 2019). However, the replacement of a mineral phase can involve precipitation of a secondary phase in a confined volume of the host material (dissolution pits, pores, and/or micro-fractures), and this can generate crystallization pressure and an associated stress (Scherer, 1999;2004) that can exceed the tensile strength of the host material resulting in fracturing of the parent phase, in turn increasing permeability and exposing fresh reactive surface (Emmanuel and Ague, 2011).…”

“…Despite the recent progress in the understanding of the mechanisms and kinetics of dissolution-carbonation of silicate minerals, the current knowledge regarding the effects of neutral to alkaline pH, carbonate speciation, secondary silicate formation ( Daval et al, 2009a;King et al, 2010;Sissmann et al, 2013;Monasterio-Guillot et al, 2019), with or without a passivation effect (i.e., the formation of an impervious layer of product phase on the parent phase) (Ruiz-Agudo et al, 2012;Ruiz-Agudo et al, 2016;Di Lorenzo et al, 2018;Wild et al, 2019), and its impact on porosity and permeability (Jamtveit et al, 2000;Kelemen et al, 2011;Royne and Jamtveit, 2015), is still far from complete. Moreover, most studies on silicate mineral carbonation have focused on olivine (King et all., 2010;Daval et al, 2011), wollastonite (Di Lorenzo et al, 2018), or thermally activated serpentine (O'Connor et al, 2005), while very little attention has been paid to other abundant primary silicates such as Mg-Ca-Fe pyroxenes (Stockmann et al, 2008(Stockmann et al, , 2013Bodor et al, 2013).…”