Does crystallographic anisotropy prevent the conventional treatment of aqueous mineral reactivity? A case study based on K-feldspar dissolution kinetics

Abstract

18Which conceptual framework should be preferred to develop mineral dissolution rate 19 laws, and how the aqueous mineral reactivity should be measured? For over 30 years, the 20 classical strategy to model solid dissolution over large space and time scales has relied on so-21 called kinetic rate laws derived from powder dissolution experiments. In the present study, we 22 provide detailed investigations of the dissolution kinetics of K-feldspar as a function of 23 surface orientation and chemical affinity which question the commonplace belief that 24 elementary mechanisms and resulting rate laws can be retrieved from conventional powder 25 dissolution experiments. Nanometer-scale surface measurements evidenced that K-feldspar 26 dissolution is an anisotropic process, where the face-specific dissolution rate satisfactorily 27 agrees with the periodic bond chain (PBC) theory. The chemical affinity of the reaction was 28 shown to impact differently the various faces of a single crystal, controlling the spontaneous 29 nucleation of etch pits which, in turn, drive the dissolution process. These results were used to 30 develop a simple numerical model which revealed that single crystal dissolution rates vary 31 with reaction progress. Overall, these results cast doubt on the conventional protocol which is 32 used to measure mineral dissolution rates and develop kinetic rate laws, because mineral 33 reactivity is intimately related to the morphology of dissolving crystals, which remains totally 34 uncontrolled in powder dissolution experiments. Beyond offering an interpretive framework 35 to understand the large discrepancies consistently reported between sources and across space 36 scales, the recognition of the anisotropy of crystal reactivity challenges the classical approach 37 for modelling dissolution and weathering, and may be drawn upon to develop alternative 38 treatments of aqueous mineral reactivity. 39 40 41 theory (TST) initially developed for elementary reactions in homogeneous media to overall 67 dissolution processes in heterogeneous media was proposed in the early 80's (Aagaard and 68 Helgeson , 1982), and subsequently coupled to surface complexation models (SCM), paving 69 the way to the SCM/TST framework (Schott et al., 2009). A fundamental and appealing 70 implication of applying this conceptual framework is that the complexity of heterogeneous 71 chemical reactions is boiled down to the isotropic dissolution of a given solid, justifying the 72 conventional measurement of bulk dissolution rates, and pushing to the background any 73 potential crystallographic control on the reaction rate and rate law. 74Arguably, the SCM/TST framework has succeeded in describing the dissolution of 75 materials with simple chemistry and fairly high symmetry space-groups, such as simple metal 76 oxides or hydroxides (e.g. Schott et al., 2009; Ohlin et al., 2010 and references therein). 77However, the detailed mechanisms and corresponding dissolution rate laws for more complex 78 materials such as silicat...

pH-dependent control of feldspar dissolution rate by altered surface layers

“…On the whole, orientation seems to be the only passivated orientation in an undersaturated fluid, while was the only non-passivated orientation amongst those tested in a saturated fluid. Consistent with the mechanism leading to passivation proposed above, the the fact that the face is among the slowest dissolving faces of feldspars (Zhang and Lüttge, 2009;Pollet-Villard et al, 2016) may explain why it was the sole face that was systematically passivated at pH 2.5.…”

Olivine dissolution rates: A critical review

“…First, as noted in section 3.4 below, the surface area normalized dissolution rate of a mineral can depend strongly on its crystallographic orientation (cf. Bandstra and Brantley, 2008;Daval et al, 2013;Pollet-Villard et al, 2016). This undermines the accuracy of using a single surface area to quantify the rates of a mineral grain consisting of numerous different crystallographic faces, in part as the relative proportion of each surface can change during the dissolution of the mineral.…”

Temporal Evolution of Calcite Surface Dissolution Kinetics

“…For example, from K-feldspar surface rate measurements, Pollet-Villard et al [34] found that despite increased etch pit distribution, surface retreat was a linear function of time. Furthermore, these authors reached the conclusion that anisotropic mineral reactivity lead to a continuous modification of the dissolution rate as a function of reaction time.…”

“…Furthermore, these authors reached the conclusion that anisotropic mineral reactivity lead to a continuous modification of the dissolution rate as a function of reaction time. Pollet-Villard et al [34] also concluded that because of a significant variation in (intrinsic) surface reactivity, it was inappropriate to use the "mean rate" approach to quantify the reactivity of crystalline materials. In contrast, some researchers have reported an intrinsic decrease of long-term dissolution rate of the considered surface due to the development of pit walls with lower surface energy [35,36].…”