Isotopic tracer evidence for the amorphous calcium carbonate to calcite transformation by dissolution–reprecipitation

Abstract

Please cite this article as: Giuffre, A.J., Gagnon, A.C., De Yoreo, J.J., Dove, P.M., Isotopic tracer evidence for the amorphous calcium carbonate to calcite transformation by dissolution-reprecipitation, Geochimica et Cosmochimica Acta (2015), doi: http://dx. AbstractObservations that some biogenic and sedimentary calcites grow from amorphous calcium carbonate (ACC) raise the question of how this mineralization process influences composition. However, the detailed pathway and geochemical consequences of the ACC to calcite transformation are poorly constrained. This experimental study investigated the formation of calcite from ACC by using magnesium and calcium stable isotope labeling to directly probe the transformation pathway and controls on composition. Four processes were considered: dissolution-reprecipitation, solid state transformation, and combinations of these end-members. To distinguish between these scenarios, ACC was synthesized from natural isotope abundance solutions and subsequently transferred to spiked solutions that were enriched in 43 Ca and 25 Mg for the transformation to calcite.Isotope measurements by NanoSIMS determined the 43 Ca/ 40 Ca, and 25 Mg/ 24 Mg ratios of the resulting calcite crystals.Analysis of the data shows the transformation is best explained by a dissolutionreprecipitation process. We find that when a small amount of ACC is transferred, the isotopic signals in the resulting calcite are largely replaced by the composition of the surrounding spiked solution. When larger amounts of ACC are transferred, calcite compositions reflect a mixture between the ACC and initial solution end-member.Comparisons of the measurements to the predictions of a simple mixing model indicate that calcite compositions 1) are sensitive to relative amounts of ACC and the surrounding solution reservoir and 2) are primarily governed by the conditions at the time of ACC transformation rather than the initial ACC formation. Shifts in calcite composition over the duration of the transformation period reflect the progressive evolution of the local solution conditions. This dependence suggests the extent to which there is water available would change the end point composition on the mixing line. While these findings have significant geochemical implications, the question remains whether this transformation pathway is generally followed when biomineralization involves ACC or is particular to these inorganic experiments. Insights from this study nonetheless suggest that some types of compositional variability, such as 'vital effects', may be explained inpart by a co-evolution of reservoir and products over the duration of the transformation.3

Chemical and physical controls on the transformation of amorphous calcium carbonate into crystalline CaCO3 polymorphs

“…Figure 4A suggests the latter process is predominant with fields of polymorphs correlated with output solution composition. The isotope tracer study of the ACC to calcite transformation is consistent with this interpretation (Giuffre et al, 2015).…”

Water: How Does It Influence the CaCO3 Formation?

“…Indeed, the lifetime of Mg-ACC particles increases with increasing Mg 2+ mole fraction in solution which can lead to an increased concentration of Mg 2+ in Mg-ACC,a ss hown in Figure 8B. [106] In this case,the observed delay in the crystallization of Mg-ACC must be related to its retarded [144] B) Influence of the Mg 2+ /Ca 2+ molar ratio in aqueouss olutionso nthe kinetic stability of Mg-ACC particles against transformation into different crystalline CaCO 3 polymorphs. [144] However,t he lifetime of ACCi ss olely related to the dissolution and crystallization kinetics and hence,i ti si n af irst approximation independent of the thermodynamic stability of these particles.I ndeed, the lifetime of AMC is longer than that of ACC, despite its lower thermodynamic stability.…”

“…[47,61,106] Thed issolution kinetics influences the local degree of supersaturation with respect to one of the crystalline CaCO 3 polymorphs.H ence,i ta lso influences the kinetics of nucleation and growth of new crystals. [47,61,106] Thed issolution kinetics influences the local degree of supersaturation with respect to one of the crystalline CaCO 3 polymorphs.H ence,i ta lso influences the kinetics of nucleation and growth of new crystals.…”

Induction time of a polymorphic transformation

“…There is increasing evidence that dissolution-reprecipitation is the relevant transformation mechanism between polymorphs that are not topotactically-related. 17,18 Our analysis is conducted within the framework of classical nucleation and crystal growth theories, which suggests that recently proposed 'nonclassical' models of crystal nucleation and growth [19][20][21] may not be necessary to rationalize Ostwald Rules of Stages. We will also discuss how prenucleation clusters, dense liquid phases, and amorphous precursors can be reconciled within a classical nucleation and crystal growth framework.…”