Mobility of hydrous species in amorphous calcium/magnesium carbonates

Abstract

Amorphous calcium carbonate (ACC) is commonly found in many biological materials. As ACC readily crystallizes into calcite, stabilizers, such as anions, cations or macromolecules, often occur to avoid or delay unwanted crystallization. In biogenic ACC, magnesium is commonly present as one of the stabilizing agents. It is generally thought that the presence of mobile water in ACC is responsible for its limited stability and that the strong interaction of Mg2+ with water stabilizes the amorphous structure by retarding dehydration of ACC. To test this hypothesis, we studied the mobility of hydrous species in the model materials ACC, amorphous magnesium carbonate (AMC) and amorphous calcium/magnesium carbonate (ACMC), using quasi elastic neutron scattering (QENS) which is highly sensitive to the dynamics of H atoms. We discovered that hydrous species in the considered amorphous materials consist of water and hydroxide ions, as magnesium ions are incorporated in a ratio of 1 to about 0.6 with OH-. Surprisingly, we found that there is no evidence of translational diffusion of water and hydroxides when calcium is present in the samples, showing that hydrous species are highly static. However, we did observe diffusion of water in AMC with similar dynamics to that found for water in clays. Our results suggest that Mg2+-water interactions alone are not the only reason for the high stability of AMC and ACMC. The stabilizing effect of Mg ions, in addition to Mg-water binding, is likely to be caused by binding to hydroxide in amorphous calcium carbonates. In fact, the incorporation of hydroxides into the amorphous phase results in a mineral composition that is incompatible with any of the known Ca/Mg-carbonate crystal phases, requiring large scale phase separation to reach the composition of even the basic magnesium carbonate minerals artinite and hydromagnesite.

Shape-preserving amorphous-to-crystalline transformation of CaCO 3 revealed by in situ TEM

“…Yang et al (53) found that Mg-ACC actually consists of two types of carbonates, amorphous CaCO 3 and amorphous MgCO 3 . Using quasi-elastic neutron scattering, Jensen et al (54) recently observed that water diffuses in amorphous MgCO 3 with similar dynamics to that found for water in clays, although they did not report higher water mobility in Mg-ACC than in pure ACC. All these results are consistent with the conclusion that incorporation of Mg 2+ into ACC introduces relatively unstable sites in the ACC structure, increasing the possibility of ion rearrangement, thus enhancing the probability of structural transformations.…”

Nucleation in confinement generates long-range repulsion between rough calcite surfaces

“…Although there are several possible mechanisms by which Mg 2+ modifies CaCO 3 nucleation and affects the subsequent mineral growth (i.e. kinetic blocking of active growth sites due to Mg 2+ adsorption 68 , reduced thermodynamic stability and increased solubility due to Mg 2+ incorporation in the lattice 69,70 , or kinetic stabilization of metastable CaCO 3 polymorphs 71,72 ), there is a general agreement that Mg 2+ hinders the nucleation of calcite and reduces calcite growth rates. This is in line with our findings, with greatly delayed PFs in the presence of Mg 2+ .…”

Nanoscale Ion Dynamics Control on Amorphous Calcium Carbonate Crystallization: Precise Control of Calcite Crystal Sizes

“…This “crystallization of ACC by solid-state transformation” is interpreted here using a reduced activity for the water component. Past investigations of water mobility within ACC have shown deviations with respect to the behavior of bulk water, justifying this approach. , In this sense, the solid-state transformation is understood as a structural reorganization mediated by the presence of water.…”

“…Different structural environments for water in ACC have been reported that can be classified mainly as adsorbed and structural molecular water and as hydroxyl groups in more basic and Mg 2+ -bearing ACC. − Water in ACC has also been shown to be at least partially responsible for the kinetic persistence of its amorphous structure, via the formation of a hydrogen bond network of water molecules and carbonate ions . The diffusive dynamics of both the hydrous and ionic components of ACC have been studied using X-ray and neutron scattering techniques showing a highly dynamic environment compared with, for example, covalent or metal glasses. ,− The nanoscale dynamics have been related to the crystallization kinetics of ACC, showing how the presence of foreign ions can affect water dynamics, enhancing its kinetic persistence . X-ray photon-correlation studies of ACC and Mg 2+ -doped ACC have shown that there is a direct proportionality between the Mg 2+ concentration and the frequency of structural rearrangements undergone by structural entities within the amorphous material .…”