Structural Characterization of Mg-Stabilized Amorphous Calcium Carbonate by Mg-25 Solid-State NMR Spectroscopy

Abstract

Biogenic amorphous calcium carbonates (ACCs) play a crucial role in the mineralization process of calcareous tissue. Most biogenic ACCs contain Mg ions, but the coordination environment of Mg, which may influence the kinetics of the phase transformation of an ACC, remains poorly understood. We demonstrate that Mg-25 solid-state NMR can be used to probe the coordination shells of Mg in synthetic ACCs. The variation in Mg-25 chemical shifts suggests that Mg−O bond lengths increase as Mg content increases. On the basis of the Van Vleck second moments obtained from the double-resonance NMR experiments, we infer that the average number of carbonates surrounding the central Mg ion is in the range of 4−4.5 and that there is at least one water molecule coordinated to each Mg ion for the synthetic Mg-ACC samples. We suggest that the stability of Mg-ACC is owing to the structural water bound to Mg ions, which increases considerably the activation energy associated with the dehydration of Mg-ACC.

Water: How Does It Influence the CaCO3 Formation?

“…As aresult of this correlation, the stability of ACCparticles sequentially increases when functionalized with valine,a sparagine,g luta-mic acid, and aspartic acid due to their increasing affinity towards the CaCO 3 crystal surface,asshown in Figure 9B. [66,136,161,163] Forexample,pure ACCt ransforms into calcite at 160 [161] Similar increases in the critical crystallization temperature have been reported for ACCp articles functionalized with phosphate [163] and aspartic acid. These additives do not significantly change the structure or overall degree of hydration of ACC.…”

“…[66,143,144,151] Additives such as Mg 2+ influence the crystallization kinetics of ACCi nt he presence of bulk water. [139,142,143,158,159] Thei ncorporation of Mg 2+ ions into ACCa lso delays its solid-state transformation:T he strong interactions between Mg 2+ and water molecules reduce the mobility of water, [148,154,160] thereby increasing the fraction of rigid water contained in Mg-ACC compared to that contained in ACC. [139,141] By contrast, when the Mg 2+ /Ca 2+ ratio is low,M g-ACC preferentially transforms into Mgcalcite.…”

Mobility of hydrous species in amorphous calcium/magnesium carbonates

“…The composition of AMC has been previously reported as MgCO 3 ÁnH 2 O. 29,[31][32][33] However, performing FTIR on dehydrated samples indicated the presence of hydroxide ions in the magnesium containing samples and this was also confirmed by the mass loss seen from TGA analysis, which showed a mass loss too low to be consistent with a pure carbonate sample. The high levels of OH À incorporation with magnesium in ACMC/AMC is consistent with the lower solubility of Mg(OH) 2 34 than MgCO 3 .…”

“…16 This is not surprising as CaCO 3 38 is less soluble than Ca(OH) 2 . 39 To the best of our knowledge only a handful of AMC studies have been published, 29,[31][32][33] and there the OH À content of the material has not been reported. For this reason, it is difficult to say if the incorporation of hydroxide is a general feature or if it is only a feature of our synthesis.…”

Transformation process of amorphous magnesium carbonate in aqueous solution

“…In addition, thermogravimetry-differential thermal analysis (TG-DTA, Rigaku Thermo plus EVO TG 8120) was performed by heating the samples on an alumina pan in flowing N 2 gas (100 ml/min) with alumina powder as a standard material to determine the concentrations of CO 2 and H 2 O constituents in the samples. The heating conditions were followed the scheme suggested by Lin et al (2015): raising T at 10°C/min to 100°C, keeping T at 100°C for 20 minutes; and raising T again at 10°C/min to 1400°C.…”