Internal consistency in aqueous geochemical data revisited: Applications to the aluminum system
Section snippets
Background and motivation
Ongoing research in the fields of equilibrium and kinetic geochemistry and their applications to many geologic processes, including hydrothermal crustal alteration (Seyfried et al., 2011), equilibrium isotope fractionation (Syverson et al., 2013), geologic CO2 utilization (Randolph and Saar, 2011) and storage (DePaolo et al., 2013), soil formation (Brantley and White, 2009), and others, have illustrated the importance of accurate calculations of equilibrium solubilities of rock-forming
Objectives and strategy
Aqueous geochemists require a database that provides an internally consistent means of calculating aluminum mineral and aqueous species thermodynamic properties over the wide range of crustal T and P, including not only the steam saturation curve, but also at the extreme conditions present in phase separation and hydrothermal alteration systems. In this study, we begin to develop this data set by taking advantage of both new and existing calorimetric measurements, phase equilibria, and aqueous
Methods
The standard states adopted in this study are a unit activity of an aqueous species in a hypothetical one molal solution referenced to infinite dilution, unit activity of pure minerals, and unit activity of pure liquid H2O at all temperatures and pressures. Mineral abbreviations utilized in this study follow the recommendations of Whitney and Evans (2010) (Table 1).
Anchor minerals: quartz, kaolinite, K-feldspar, and andalusite
Holland and Powell (1990) have shown that, for very large data sets, the relative size of the reaction matrix, R, and number of phases included in the vector g will reach a condition in which anchoring the matrix solution with input properties becomes unnecessary. However, the size of these data sets are much larger than the one presented here (e.g, >123 end-member phases (Holland and Powell, 1990)) and we therefore must employ anchor phases, as in their early studies (Powell and Holland, 1985,
Discussion
At its most basic level, the aluminum problem is truly a kaolinite problem, due to the improper reference state adopted by Helgeson et al. (1978) through Reaction (1) and the data shown in Fig. 1. By deriving and adopting thermodynamic data for 12 key Al minerals, including kaolinite, we have attempted to correct this longstanding problem. The we derive for kaolinite is 2.3 kcal/mol more negative than that reported by Helgeson et al. (1978), and, compared to the data provided here, the
Conclusion
Ongoing aqueous geochemical research has shown the critical significance of accurate calculations of rock-forming mineral solubilities, which, in turn, form the foundation of models of kinetically controlled mineral–fluid reactions. These calculations require a thermodynamic data set that is internally consistent with respect to both minerals and aqueous species. In this contribution, we have exhibited a method for deriving internally consistent thermodynamic data with estimates of
Disclaimer
Dr. Saar has a royalty and equity interest in, and serves as the Chief Scientific Officer for, Heat Mining Company, LLC, a company which may commercially benefit from the results of this research. Dr. Saar and the University of Minnesota have financial interests arising from the rights to receive royalty income under the terms of a license agreement with Heat Mining Company LLC. These relationships have been reviewed and managed by the University of Minnesota in accordance with its conflict of
Acknowledgments
We gratefully acknowledge support from the Department of Energy (DOE) Geothermal Technologies Program under Grant Number EE0002764 for this contribution and related research. W.E.S. wishes to acknowledge funding from NSF MGG-OCE under grant numbers 0751771 and 0813861. M.O.S. also thanks the George and Orpha Gibson endowment for its generous support of the Hydrogeology and Geofluids Research Group. The authors also thank the anonymous reviewers, Associate Editor Gleb Pokrovski, and Executive
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