Nucleation of metastable aragonite CaCO ₃ in seawater

Abstract

Predicting the conditions in which a compound adopts a metastable structure when it crystallizes out of solution is an unsolved and fundamental problem in materials synthesis, and one which, if understood and harnessed, could enable the rational design of synthesis pathways toward or away from metastable structures. Crystallization of metastable phases is particularly accessible via low-temperature solution-based routes, such as chimie douce and hydrothermal synthesis, but although the chemistry of the solution plays a crucial role in governing which polymorph forms, how it does so is poorly understood. Here, we demonstrate an ab initio technique to quantify thermodynamic parameters of surfaces and bulks in equilibrium with an aqueous environment, enabling the calculation of nucleation barriers of competing polymorphs as a function of solution chemistry, thereby predicting the solution conditions governing polymorph selection. We apply this approach to resolve the long-standing “calcite–aragonite problem”––the observation that calcium carbonate precipitates as the metastable aragonite polymorph in marine environments, rather than the stable phase calcite––which is of tremendous relevance to biomineralization, carbon sequestration, paleogeochemistry, and the vulnerability of marine life to ocean acidification. We identify a direct relationship between the calcite surface energy and solution Mg–Ca ion concentrations, showing that the calcite nucleation barrier surpasses that of metastable aragonite in solutions with Mg:Ca ratios consistent with modern seawater, allowing aragonite to dominate the kinetics of nucleation. Our ability to quantify how solution parameters distinguish between polymorphs marks an important step toward the ab initio prediction of materials synthesis pathways in solution.

Experimental diagenesis: insights into aragonite to calcite transformation of <i>Arctica islandica</i> shells by hydrothermal treatment

“…We discussed in the previous section that the thermodynamic potential (supersaturation) for the formation of calcite from a fluid which is able to dissolve aragonite is smaller than the critical supersaturation required to obtain a discernible nucleation rate for calcite in normal laboratory experiments. The presence of magnesium in the solution further inhibits calcite nucleation, as do high temperatures between 70 and 160 • C ( Kitano et al, 1962;Taft, 1967;Kitano et al, 1972;Katz, 1973;Berner, 1975;Morse et al, 1997;Choudens-Sánchez, 2009;Radha et al, 2010;Perdikouri et al, 2011Perdikouri et al, , 2013Balthasar and Cusack, 2015;Sun et al, 2015), which is supported by the lack of calcite formation in our experiments between 100 and 150 • C (Table 1, Fig. A11).…”

“…To obtain a significant number of supercritical nuclei a critical supersaturation needs to be reached (Morse et al, 2007;Gebauer et al, 2008;Nindiyasari et al, 2014;Sun et al, 2015). Reported values for critical supersaturation levels crit required for calcite nucleation in various conditions range from the order of 3.7 (Lebron and Suarez, 1996;Zeppenfeld, 2003) to the order of 30 (Morse et al, 2007;Gebauer et al, 2008) or even several hundreds in, for example, hydrogel matrices (Nindiyasari et al, 2014).…”

“…Reported values for critical supersaturation levels crit required for calcite nucleation in various conditions range from the order of 3.7 (Lebron and Suarez, 1996;Zeppenfeld, 2003) to the order of 30 (Morse et al, 2007;Gebauer et al, 2008) or even several hundreds in, for example, hydrogel matrices (Nindiyasari et al, 2014). The density functional theory study of Sun et al (2015) arrives at crit = 5 for systems free of inhibitors such as Mg, and crit = 35 for modern seawater. Accordingly, the supersaturation produced by the dissolution of aragonite is very small compared to supersaturation levels typically required for the nucleation of calcite.…”

“…replacement reaction in our 100-150 • C treatments is related to inhibition of calcite nucleation (Sun et al, 2015), a mechanism that has rarely been rigorously explored.…”

“…There is no sharp tipping point but rather a gradual change of fraction of the precipitating phases (Ogino et al, 1987;Balthasar and Cusack, 2015). Furthermore, inhibitors of calcite nucleation and/or growth decrease the temperature of this regime shift in precipitation even further; in marine and diagenetic environments the most important inorganic inhibitor is Mg 2+ (Kitano et al, 1972;Katz, 1973;Berner, 1975;Morse et al, 1997;Choudens-Sanchez, 2009;Radha et al, 2010;Balthasar and Cusack, 2015;Sun et al, 2015).…”