PerspectiveMaterials Science

Why Mineral Interfaces Matter

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Science  28 Mar 2014:
Vol. 343, Issue 6178, pp. 1441-1442
DOI: 10.1126/science.1250884

Throughout Earth, rocks respond to changing physical and chemical conditions by converting one rock type to another. These conversions have conventionally been described in terms of solid-state mechanisms, in which new minerals nucleate and grow through exchange of elements by diffusion. The slow rates of solid-state diffusion suggested geological time scales for these processes. However, rocks in Earth's crust are not dry (1), and even very low concentrations of aqueous solutions can increase reaction rates substantially (2). In the presence of a fluid phase, mineral conversions turn out to proceed not via solid-state diffusion but through dissolution and recrystallization at the mineral-fluid interface (3). Well beyond mineralogy, these insights may prove useful in developing new methods of materials synthesis, for carbon removal from the atmosphere, and for safe nuclear waste storage.

Depending on the chemical composition of the aqueous solution, the dissolution of even a few monolayers of a mineral surface can supersaturate an interfacial layer of solution with respect to another solid phase. If this phase can nucleate and grow on the parent substrate, inside the diffusion distance in the fluid, a feedback between the dissolution rate and the growth rate couples the two processes. This feedback allows the parent solid phase to be replaced by a product phase with different composition while maintaining its original dimensions.

Such a “pseudomorphic” replacement was long thought to require a solid-state mechanism, especially given that crystallographic orientations from the parent can be transferred to the product phase. However, an interface-coupled dissolution-precipitation mechanism ( 4) can transfer crystallographic information via structural matching (epitaxy) when the product nucleates on the parent substrate. Depending on the degree of structural matching, a single parent crystal can be pseudomorphically replaced by a single crystal or a polycrystalline product through an interface-coupled mechanism.

For the complete replacement of one phase by another, surface coverage by the new phase should not armor the parent from further reaction; the product must have porosity and hence permeability to allow continued fluid access to the parent phase. The generation of porosity depends on the relative molar volumes of parent and product phases, as well as their relative solubility in the interfacial fluid. The product phase is more stable than the parent (and hence less soluble in the specific aqueous solution), meaning that more of the parent is dissolved than product precipitated within the same external dimensions; this generates porosity. Because of the close coupling between dissolution and precipitation, only the interfacial layer of fluid needs to be supersaturated with respect to the product phase.

Interfacial processes.

(A) Carbonated solution produces triangular dissolution etch-pits on the surface of brucite, Mg(OH)2, reflecting the trigonal symmetry of this mineral. Each step in this atomic force microscope image is a molecular monolayer (3). (B) Brucite dissolution is coupled to the precipitation of nanoparticles of a magnesium carbonate phase (13).

IMAGES: J. HÖVELMANN/UNIVERSITY OF MÜNSTER; REPRODUCED WITH PERMISSION FROM (3) (PANEL A) AND (13) (PANEL B)

The interaction of aqueous solutions with solids is ubiquitous throughout Earth, from chemical weathering at the surface to reactions deep in the crust (5). The dynamics of these large-scale processes ultimately depend on nanoscale mechanisms at the mineral-fluid interface, and the coupling of dissolution and precipitation forms the basis of our understanding of element transport in the crust. Understanding the nanoscale mechanisms of fluid-mineral interaction can be exploited to discover geo-inspired routes for the synthesis of functional materials.

For example, Reboul et al. have exploited the inheritance of morphology during a replacement process, which can be controlled in the laboratory by parameters such as fluid composition and temperature, to synthesize predefined architectures of porous coordination polymer (PCP) crystal networks (6). The method relies on pseudomorphically replacing a porous, three-dimensionally patterned aluminum (Al) oxide parent, which acts as both the metal source and the architecture-directing agent, by an identically shaped Al-PCP product. The local dissolution of the Al oxide by reaction with a solution containing organic ligands provides the metal ions that supersaturate the interfacial solution with respect to the Al-based PCP. An advantage of this method is the hierarchical nature of the porosity in the final product, combining the porosity of the original alumina architecture with the secondary porosity in the PCP due to the replacement mechanism. The method holds considerable promise for the design of porous coordination polymer architectures for use in catalysis, sensing, and adsorption separation (6).

Xia et al. (7) have used a similar strategy to synthesize three-dimensional ordered arrays of nanocrystals of the zeolite analcime (NaAlSi2O6.H2O) by pseudomorphic replacement of natural leucite (KAlSi2O6), which contains an inherent three-dimensional ordered pattern of nanosized lamellar twins. After reaction in NaCl solutions, these patterns are preserved in the resulting array of analcime nanocrystals, the size of which can be tuned by changing the pH of the solution. The same authors have also used this method to synthesize complex metal sulfides with low thermal stability (8). At low temperatures, traditional synthesis from the elements is far too slow compared to pseudomorphically replacing a preexisting precursor sulfide using an appropriate aqueous solution.

The same principles may apply to carbon capture and storage by mineral carbonation. The idea is that reaction of carbonated fluids with magnesium- and calcium-rich rocks supplies metal ions by dissolution, resulting in the precipitation of a metal carbonate. For example, dissolution of brucite, Mg(OH)2, a natural constituent of altered mantle rocks, is coupled to the precipitation of a magnesium carbonate phase (see the figure) (9). Understanding the chemical controls on this coupling is a necessary prerequisite to using this strategy of carbon capture and storage. During the dissolution of many silicate minerals by carbonated fluids, metal ions are preferentially released into solution. This results in the formation of silica-rich surface layers (“leached layers”) that may have a substantial effect on the reaction kinetics (10).

The differential extraction of metal ions from minerals by treatment with aqueous solutions (leaching) is routinely used in industrial processes (e.g., the production of titanium dioxide from ilmenite, FeTiO3). The mechanism of leaching has been the subject of debate, especially in relation to chemical weathering on Earth, with the consensus moving toward a dissolution-precipitation mechanism as opposed to diffusional exchange (11).

The same questions concern the mechanism of aqueous alteration of glass used to encapsulate radioactive nuclear waste. The currently accepted models, based on diffusional exchange of ions between the glass and the solution, are being challenged by new models based on coupled dissolution-precipitation mechanisms (12).

Understanding fluid-solid interaction mechanisms is important to diverse fields of Earth science, materials science, and chemistry. Knowledge of the chemical controls on the feedback mechanisms that couple dissolution and precipitation are a necessary prerequisite for predictive numerical modeling. Currently, this knowledge is limited. Studying mineral-fluid reactions in synthetic systems, as well as in Earth's natural laboratory, will continue to supply some of the answers.

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