The Crystallization of Amorphous Calcium Carbonate is Kinetically Governed by Ion Impurities and Water

Abstract

Many organisms use amorphous calcium carbonate (ACC) and control its stability by various additives and water; however, the underlying mechanisms are yet unclear. Here, the effect of water and inorganic additives commonly found in biology on the dynamics of the structure of ACC during crystallization and on the energetics of this process is studied. Total X‐ray scattering and pair distribution function analysis show that the short‐ and medium‐range order of all studied ACC samples are similar; however, the use of in situ methodologies allow the observation of small structural modifications that are otherwise easily overlooked. Isothermal calorimetric coupled with microgravimetric measurements show that the presence of Mg2+ and of PO4 3− in ACC retards the crystallization whereas increased water content accelerates the transformation. The enthalpy of ACC with respect to calcite appears, however, independent of the additive concentration but decreases with water content. Surprisingly, the enthalpic contribution of water is compensated for by an equal and opposite entropic term leading to a net independence of ACC thermodynamic stability on its hydration level. Together, these results point toward a kinetic stabilization effect of inorganic additives and water, and may contribute to the understanding of the biological control of mineral stability.

Water: How Does It Influence the CaCO3 Formation?

“…[68] This thermodynamic argument has often been used to describe solid-state transformations of biogenic ACCu nder ambient conditions. [66,85] These more recent experimental results are supported by MD simulations that suggest the dehydration to be thermodynamically unfavorable,a sshown in Figure 6A. [66,85] These more recent experimental results are supported by MD simulations that suggest the dehydration to be thermodynamically unfavorable,a sshown in Figure 6A.…”

“…[45,60,64,65] This growth or agglomeration reduces their surface-to-volume ratio such that, at some point, the free energy of ACC particles exceeds that of the crystalline polymorphs. [66][67][68][69] Despite their metastability,A CC particles can grow into particles with diameters reaching up to several mmw ithin only af ew seconds to minutes. Forexample,the enthalpy of ACCparticles with diameters above 45 nm is 3to15AE 3kJmol À1 higher than that of calcite,i ndicating that ACCp articles of these sizes are metastable.…”

“…[60] The higher free energy of the ACCp hase is supported by calorimetric measurements that neglect the small entropy contribution. [27,54,55,[70][71][72] During the fast growth of ACC, significant amounts of water are incorporated into the particles, [55] as shown in Figure 4A.This incorporated water decreases the enthalpy of ACCp articles, [66,73,74] rendering their growth thermodynamically favorable.H owever, when the size of the growing ACCp articles exceeds ac ertain value,they transform into one of the more stable crystalline forms. [66][67][68][69] Despite their metastability,A CC particles can grow into particles with diameters reaching up to several mmw ithin only af ew seconds to minutes.…”

“…[7] Reproducedw ith permission from the American Chemical Society, [73,83] John Wiley & Sons, [96] and the American Association for the Advancement of Science. [66,85,87] Structural changes in ACCc ould also be monitored with PDF when the dehydration is induced with uniaxial pressures as high as 12 GPa. [74] This observation suggests that the majority of water contained in ACCwith n below 0.8 is rigid.…”

“…[57,61,66,93] Different ACCc rystallization mechanisms have been proposed, as schematically summarized in Figure 2. [57,61,66,93] Different ACCc rystallization mechanisms have been proposed, as schematically summarized in Figure 2.…”