Elsevier

Geochimica et Cosmochimica Acta

Volume 198, 1 February 2017, Pages 439-465
Geochimica et Cosmochimica Acta

Stable mineral recrystallization in low temperature aqueous systems: A critical review

https://doi.org/10.1016/j.gca.2016.11.013Get rights and content

Abstract

Minerals may undergo recrystallization reactions in low temperature (<100 °C) aqueous systems, during which they exchange isotopes and trace elements with the dissolved reservoir without undergoing overt structural, bulk compositional, or morphological changes. These interfacial reactions, which are often referred to in the literature as “atom exchange” and herein as “stable mineral recrystallization”, have important implications for the use of isotopic and elemental proxies to interpret past temperatures, oxidation states, and aqueous chemistries on Earth. The reactions are also significant for modern environments, including engineered systems, as they imply that mineral lattices may be substantially more open to exchanging toxic elements and radionuclides with coexisting solutions than previously thought. To date, observations of stable mineral recrystallization are distributed among several disciplines, and no work has attempted to review their findings comprehensively. Accordingly, this review article presents laboratory evidence for stable mineral recrystallization, describes data collection and interpretation strategies, summarizes similar recrystallization systematics observed in multiple studies, explores the potential occurrence of stable mineral recrystallization in natural systems, and discusses possible mechanisms by which stable mineral recrystallization occurs. The review focuses primarily on carbonates, sulfates, and iron oxides because these minerals have been studied most extensively to date. The review concludes by presenting key questions that should be addressed in this field to further understand and account for stable mineral recrystallization in natural and engineered aqueous systems at low temperatures.

Introduction

Minerals are fundamentally important to the fields of geoscience and environmental geochemistry. A mineral’s elemental and isotopic compositions are often viewed as reflecting the physical and/or geochemical conditions under which it formed. Accordingly, minerals such as carbonates serve as important geochemical proxy archives that are used to constrain temperatures, oxidation states, and aqueous chemistries (e.g., seawater chemical evolution) in a range of past environments. In modern environments, minerals can dictate the bioavailabilities of biological nutrients and inorganic environmental contaminants through sorption, co-precipitation, and structural incorporation reactions (e.g., Cooper et al., 2000, Cooper et al., 2005). Consequently, minerals are used to prevent and mitigate groundwater contamination by immobilizing toxic or radioactive elements. A classic case of using minerals to prevent groundwater contamination is the employment of barite (BaSO4) in radioactive waste repositories to immobilize radium (Ra) through the formation of a solid solution (RaxBa1-xSO4), which is substantially less soluble than RaSO4 (Bosbach et al., 2010, Curti et al., 2010, Klinkenberg et al., 2014, Brandt et al., 2015).

The effective use of minerals in both fields requires an understanding of how a mineral’s elemental or isotopic composition changes over time. A mineral that preserves its original composition over long time scales is characterized as a “high fidelity” proxy archive; in contrast, “low fidelity” proxy archives are those that have been altered to a significant extent due to post-formational processes. Because low fidelity archives reflect the conditions in a multitude of environments, interpreting isotopic and/or elemental compositions in terms of formational environment can result in incorrect conclusions regarding paleoclimate and paleoenvironment. Likewise, a mineral with low fidelity is relatively ineffective at permanently immobilizing toxic and radioactive elements.

In general, the post-formational changes that alter a mineral’s elemental and isotopic compositions in the environment occur in the presence of water (Putnis, 2009, Putnis, 2014, Prieto et al., 2013). Solid-state diffusion of elements within a mineral is generally reasoned to be too slow to be important in low temperature systems (T < 100 °C), even over geological time scales (e.g., Urey, 1948, Fisler and Cygan, 1999, Cole and Chakraborty, 2001). Instead, low temperature mineral transformations are thought to occur primarily by dissolution and precipitation reactions (e.g., Cole and Chakraborty, 2001, Putnis, 2009). These reactions can involve a mineral transforming from one phase to another, a mineral maintaining its structure and morphology but altering its elemental composition, or a mineral maintaining its structure and elemental composition but reforming into more stable particles through aggregation and growth (e.g., Ostwald ripening). Collectively, these reactions can lead to element- or nuclide-specific repartitioning between the aqueous and solid phases. While there are many different mineral growth, replacement, and transformation mechanisms that can cause these changes, they are generally thought to occur when a system is at disequilibrium (Putnis, 2009, Yardley, 2009, Putnis and John, 2010, Putnis and Ruiz-Agudo, 2013).

In recent years, however, this notion has been challenged. Mounting evidence from laboratory studies suggests that a mineral’s elemental and/or isotopic composition can be significantly altered in low-temperature aqueous environments at rapid rates even when a system is at (or near) equilibrium. Using isotopic and elemental tracers, researchers found that extensive isotopic exchange occurs between mineral and aqueous ions over experimental time scales – without any overt changes in the mineral structure, elemental composition, or grain size – that exceed rates expected for solid-state diffusion (Stipp et al., 1992, Handler et al., 2009, Handler et al., 2014, Curti et al., 2010, Frierdich and Catalano, 2012a, Avrahamov et al., 2013, Lestini et al., 2013). In many cases, investigations have concluded that the mineral has undergone complete exchange with the aqueous solution, as evidenced by the solid and solution having virtually identical isotopic compositions at the end of the experiment (Zachara et al., 1991, Handler et al., 2009, Handler et al., 2014, Bosbach et al., 2010, Avrahamov et al., 2013, Joshi and Gorski, 2016). This suggests that atoms initially deep within the bulk mineral lattice are capable of exchanging with atoms in solution. The effect that these reactions can have on the isotopic and elemental compositions of minerals can be profound, particularly when the aqueous concentration of the element of interest is sizeable compared to the concentration in the solid phase (i.e., the leverage to alter the solid is significant; see Fantle and Higgins (2014) for more detail).

Translating the observations of this phenomenon from laboratory studies to natural systems is not straightforward. Analyses of minerals and their surrounding pore fluids collected from field sites indicate that materials such as deep sea sediments and carbonates undergo significant post-depositional isotopic and elemental exchange with pore fluids at low temperatures (e.g., Killingley, 1983, Mozeto et al., 1984, Richter and DePaolo, 1987, Spivack and Edmond, 1987, Schrag et al., 1992, Schrag et al., 1995, Richter and Liang, 1993, Fantle and DePaolo, 2007, Fantle et al., 2010, Fantle, 2015, Turchyn and DePaolo, 2011). Determining the extent to which this post-depositional exchange is coupled to structural and/or morphological changes in the mineral is obviously challenging, however, since the system is open and the original specimen is not well characterized. Regardless of these complications, simply recognizing that a mineral may undergo significant compositional changes without overt accompanying morphological alterations has important implications for how data from natural samples is interpreted. For instance, the primary method for recognizing post-depositional alteration in biogenic calcium carbonate is visual evidence for morphological changes (e.g., test fragmentation, micro- and ultrastructural alteration, and/or assemblage change) or the presence of secondary mineralization products (e.g., micritic cements and euhedral calcite). Clearly, the exchange reactions typically associated with the process described here would be difficult to identify using such indicators, leading to potentially erroneous conclusions regarding the interpreted conditions under which a mineral formed.

In the literature, interfacial exchange reactions between a mineral and aqueous ions have been described using various terminologies: isotope exchange, atom exchange, recrystallization, and neomorphism (e.g., Nakamura et al., 2005, Pedersen et al., 2005, Poulson, 2005, Sexton et al., 2006, Handler et al., 2009, Handler et al., 2014). Both isotope exchange and atom exchange have been used to describe the exchange of atoms between two components in a given system. The distinction between these two terms is that isotope exchange describes only two atoms of the same element exchanging (e.g., aqueous Ba2+ exchanging with structural Ba2+ in BaSO4), while atom exchange includes isotope exchange as well as elemental substitution reactions (e.g., the replacement of structural Ba2+ by aqueous Ra2+ in BaSO4). While the terms “isotope exchange” and “atom exchange” accurately describe the phenomenon discussed in this review, the usage of these terms is not unique to this phenomenon. For example, both terms have been used to characterize exchange between components of a single phase (e.g., exchange between two aqueous complexes) (Cole and Chakraborty, 2001). Additionally, when such exchange reactions involve a mineral phase, these terms cannot be used to differentiate between exchange coupled to a mineral transformation and exchange that occurs in the absence of a mineral transformation.

The term recrystallization has been defined in different ways over time, due in part to the term being used by multiple sub-disciplines within the geosciences and metallurgy (e.g., Folk, 1965, Spry, 1969, Yund et al., 1991). Recrystallization was first defined by Sorby (1880) as the reorganization of a mineral or metallic constituent already present in a system. This definition includes the phenomenon of interest in this review, but also includes other processes, such as mineral replacement reactions, secondary mineralization transformations, and atom exchange reactions coupled to structural and/or morphological changes in the mineral. In the geosciences, recrystallization was later defined more narrowly to refer to processes in which the mineral structure “remains identical” through a reaction (Folk, 1965, Spry, 1969, Bathurst, 1975). This definition included processes that were coupled to changes in the mineral morphology and/or trace element compositions (e.g., the recrystallization of a calcite with a high Mg content to a calcite with a low Mg content). More recently, recrystallization has been used to describe (1) the growth of new grains of a mineral with the same composition (Yund et al., 1991) and (2) the growth of new mineral phases through dissolution and precipitation reactions (Yund et al., 1991, Putnis, 2009). With specific reference to carbonate diagenesis, the term “recrystallization” has been categorized as a catch-all term that encompasses “all forms of diagenetic alteration of biogenic calcite”, mainly due to its less strictly defined usage in the literature (Sexton et al., 2006). From these studies, it is apparent that the term recrystallization could be used to describe the phenomenon of interest in this review, but the term would be too general and may be misinterpreted and/or misunderstood.

The term “neomorphism” faces the same issue of inconsistent usage as the term recrystallization. Neomorphism was recently used to describe the replacement of biogenic calcite by so-called inorganic calcite (i.e., replacement accompanied only by a change in crystal “form”; Sexton et al., 2006), consistent with the phenomenon of interest in this review. However, Folk (1965) originally used the term to describe diagenetic mineral transformations of various types, including recrystallization and inversion. In fact, Folk (1965) described “neomorphism” as a “comprehensive term of ignorance (p. 21)” that refers to “all transformations between one mineral and itself or a polymorph” no matter if the affected crystals change in size and/or shape. Consequently, the use of neomorphism to describe this phenomenon could also lead to confusion or misinterpretation.

Given both the ambiguities in current usage and the original definitions of the terms “recrystallization” and “neomorphism” in the sedimentary diagenesis literature (e.g., Folk, 1965, Bathurst, 1975), we suggest the term “stable mineral recrystallization” to describe the phenomenon emphasized in this review. We define stable mineral recrystallization as a reaction in which a “stable mineral” extensively exchanges atoms with ions in solution under apparent chemical equilibrium conditions with no overt changes in mineral structure, morphology, or grain size. In this case, “extensively exchanges” means that the number of atoms participating in mineral-fluid exchange exceeds the number of atoms initially present at the mineral-fluid interface. The term “overt” refers generally to those changes that are observable at the macroscopic or microscopic level. This term is more of a loose descriptor than a solid constraint, as the description of a change as overt depends heavily on the spatial scale (and precision) at which the observation is made. The appropriateness of this term may therefore diminish in the future, as high spatial resolution techniques, such as transmission electron and/or atomic force microscopy, are employed to characterize stable mineral recrystallization.

According to our definition, a “stable mineral” is the most thermodynamically stable, and therefore least soluble, solid polymorph for a given chemical composition at ambient conditions (i.e., temperature, pressure, and solution composition). For example, calcite (CaCO3) is considered a “stable mineral” under most environmental conditions because it is the least soluble solid that contains Ca2+ and CO32− (Tai and Chen, 1998). Aragonite, a CaCO3(s) polymorph, is more soluble than calcite under most environmentally relevant conditions (Morse et al., 1980, Plummer and Busenberg, 1982, Mucci, 1983), and therefore would not be considered a “stable mineral”, as it is expected to transform to calcite in aqueous systems (Bischoff and Fyfe, 1968, Budd, 1988, Putnis, 2009). Of course, such “stable minerals” can exist in a range of forms that have varying surface energies and imperfections (e.g., heterogeneities, crystal defects, and elemental impurities) that influence their solubilities and hence thermodynamic stabilities (e.g., Fyfe and Bischoff, 1965, Nordstrom et al., 1990, Navrotsky et al., 2008). These deviations from ideality will affect local surface energies of the mineral and likely if and how a mineral recrystallizes. For the purpose of this review, we focus on low temperature (T < 100 °C) aqueous systems, but note that stable mineral recrystallization could and may occur at higher temperatures.

Observations of low-temperature stable mineral recrystallization are dispersed among several fields and – to the best of our knowledge – have not been discussed collectively to date. Therefore, the purpose of this review article is to summarize and evaluate critically the observations of stable mineral recrystallization from laboratory experiments and discuss the commonalities among, and differences between, previous studies. Accordingly, we discuss the influence of mineralogy, aqueous chemistry, and trace elemental chemistry of minerals on stable mineral recrystallization. In the review, we focus on data collected for sulfates, carbonates, and iron oxides, as these minerals have been the most extensively studied to date.

The review of existing work is divided into two sections. The first section focuses on laboratory-based studies, in which stable isotopes, radioactive nuclides, and/or trace elements are used as probes of stable mineral recrystallization over short time scales. We focus our discussion on studies in which evidence indicates that minerals extensively recrystallized, while exhibiting no overt structural changes over the course of the reaction. The second section focuses on field-based studies, in which the spatial distribution of isotopes and/or trace elements in marine sediments clearly indicate post-depositional exchange between the bulk solid and coexisting pore fluids over long time scales. For the field-based examples discussed, we explore where in the surface Earth system stable mineral recrystallization may be relevant and question if the reaction observed in marine carbonates could be due in part to a long-term expression of stable mineral recrystallization, as observed over short time scales in the laboratory.

To these ends, we first discuss how stable mineral recrystallization data have been interpreted in the literature. Multiple models have been developed to quantify rates and extents of reaction; we explore how each model can yield substantially different results with respect to the extent of mineral exchange, depending on the relative pool sizes of the aqueous and mineral phases. We then summarize the available data from laboratory and field studies and, using these data, address the possible mechanism(s) of stable mineral recrystallization. We conclude by presenting outstanding questions whose answers are required to further our understanding of stable mineral recrystallization and quantify its relevance to natural systems.

Section snippets

Summary of experimental data collection protocol and data interpretation

In laboratory studies, stable mineral recrystallization is most commonly examined by exposing a mineral to an aqueous solution that is ostensibly saturated with respect to the mineral phase. To generate an initial isotopic disequilibrium between the mineral and the solution, the solution or mineral is enriched with either a stable or radioactive isotope tracer of the target element. Both stable and radioactive isotope tracers are highly attractive in that small amounts of tracer can be added to

Potential for stable mineral recrystallization in natural systems

What makes the recent observations of stable mineral recrystallization in the laboratory so intriguing is the possibility that the same type of reactions may occur in natural systems over a range of time scales. If stable mineral recrystallization occurs in the environment, the implications with respect to interpreting and understanding diagenetic alteration of geological proxies would be manifold. Most importantly, stable mineral recrystallization, as it is currently understood, does not

Mechanistic insights regarding stable mineral recrystallization

Despite extensive observations of stable mineral recrystallization in low temperature aqueous experiments, the molecular-scale mechanism(s) by which this process occurs remains poorly understood. The primary reason for this lack of understanding is that stable mineral recrystallization occurs under apparent equilibrium conditions, in which the solution chemistry remains stable and the mineral does not appear to undergo any major morphological or structural changes. Thus, there is no overt

Outlook

Experimental observations of stable mineral recrystallization have generated fascinating findings and questions. The collective result that small particles (i.e., nm to μm scale) can undergo rapid and extensive stable mineral recrystallization in the presence of a saturated solution indicates that minerals can be chemically dynamic at chemical equilibrium in low temperature aqueous environments. Such experimental findings are especially significant given the suggestion that stable mineral

Acknowledgements

This work was supported by U.S. National Science Foundation Grants EAR-1451593 and EAR-OCE-1154839. The authors thank E. Curti and R. Handler for generously providing original datasets from their studies, and E. Curti, P. Joshi, and M. Scherer for providing feedback and conversations during the writing of this manuscript. The authors also thank two anonymous reviewers, Andrew Frierdich, and associate editor Jeffery Catalano for constructive comments that greatly helped to revise the original

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