Fractionation of silicon isotopes in liquids: The importance of configurational disorder Romain Dupuis a,b, ⁎, Magali Benoit b , Elise Nardin a , Merlin Méheut a a GET, CNRS UMR 5563, IRD UR 154, Université Paul-Sabatier, Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, 31400 Toulouse, France b CEMES CNRS UPR8011, 29 rue Jeanne Marvig, 31055 Toulouse Cedex, France abstract article info Article history: Received 5 July 2014 Received in revised form 29 December 2014 Accepted 30 December 2014 Available online 9 January 2015 Editor: Carla M Koretsky Keywords: Silica precipitation Silicon isotopes Stable isotope fractionation Molecular dynamics First-principles calculations Dissolved silicon Silicon isotopes are a promising tool to assess low-temperature geochemical processes such as weathering or chert precipitation. However, their use is hampered by an insufficient understanding of the fractionation associated with elementary processes such as precipitation or dissolution. In particular, the respective contributions of kinetic and equilibrium processes remain to be determined. In this work, equilibrium fractionation factors for silicon isotopes have been calculated using first-principles methods for quartz, kaolinite, and dissolved silicic acid (H 4 SiO 4 and H 3 SiO 4 − ) at 300 K. The two liquid systems are treated both as realistically as possible, and as consistently with the solids as possible. They are first simulated by ab initio molecular dynamics, then individual snapshots are extracted from the trajec- tories and relaxed, giving inherent structures (IS). The fractionation properties of these IS are then calculated. A significant variability of the fractionation properties (σ = 0.4‰) is observed between the independent snapshots, emphasizing the importance of configurational disorder on the fractionation properties of solutions. Further- more, a correlation is observed between the fractionation properties of these snapshots and the mean Si–O dis- tances, consistent with calculations on minerals. This correlation is used to identify other parameters influencing the fractionation, such as the solvation layer. It is also used to reduce the number of configurations to be comput- ed, and therefore the computational cost. At 300 K, we find a fractionation factor of +2.1 ± 0.2‰ between quartz and H 4 SiO 4 , +0.4 ± 0.2‰ between ka- olinite and H 4 SiO 4 , and −1.6 ± 0.3‰ between H 3 SiO 4 − and H 4 SiO 4 . These calculated solid–solution fractionations show important disagreement with natural observations in low-temperature systems, arguing against isotopic equilibration during silicon precipitation in these environments. On the other hand, the large fractionation associated with the de-protonation of silicic acid suggests the importance of speciation, and in particular pH, for the fractionation of silicon isotopes at low temperature. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Due to analytical progress, silicon isotopes have recently emerged as a promising tool for estimating the impact of alteration on the long-term CO 2 budget (Opfergelt and Delmelle, 2012, and references therein), understanding soil formation (Ziegler et al., 2005), or constraining oceanic productivity (De La Rocha et al., 1998). Although numerous studies have measured silicon isotope compositions in natural samples, very few document quantitatively the fractionation re- lated to elementary processes. A quantitative understanding of the basic mechanisms causing isotopic fractionation is nevertheless es- sential to realize the full potential of this isotopic system, and can be attained through careful theoretical studies and laboratory experiments. Such experiments are difficult to realize, due in particular to the slow speed of Si precipitation, and to the complexity of alteration processes. So far, the identified causes for isotopic variability in these surface environments are inorganic precipitation of clays or silica (Li et al., 1995; Basile-Doelsch, et al., 2005; Georg et al., 2007; Geilert et al., 2014), organic precipitation of silica (de la Rocha et al., 1997; Hendry and Robinson, 2012), dissolution of silica (Demarest et al., 2009), or ad- sorption of silica onto oxides (Delstanche, et al., 2009). In all these sit- uations, the fractionation occurs between a solid and a liquid phase. In order to interpret these observations, and in particular to under- stand the relative contribution of kinetic and equilibrium processes, it is of primary importance to document fractionations related to the equilibria between minerals and liquids, and between different dis- solved species. The most common species of Si in solution at ambient conditions are silicic acid H 4 SiO 4 (aq) (denoted hereafter H4), and its associated base H 3 SiO 4 − (aq) (denoted hereafter H3). In this work, we will focus on the isotopic fractionation between these two species and between H 4 SiO 4 and two minerals, quartz and kaolinite. The calculated quartz/solution fractionation can be compared with the existing esti- mates of fractionation during precipitation of amorphous silica, whereas the kaolinite/solution fractionation is more relevant to the behavior of Si isotopes during clay precipitation. Chemical Geology 396 (2015) 239–254 ⁎ Corresponding author. E-mail address: rdupuisbelin@gmail.com (R. Dupuis). http://dx.doi.org/10.1016/j.chemgeo.2014.12.027 0009-2541/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo