Temperature dependence of amorphous magnesium carbonate structure studied by PDF and XAFS analyses

Abstract

Mineral trapping through the precipitation of carbonate minerals is a potential approach to reduce CO2 accumulation in the atmosphere. The temperature dependence of amorphous magnesium carbonate (AMC), a precursor of crystalline magnesium carbonate hydrates, was investigated using synchrotron X-ray scattering experiments with atomic pair distribution function (PDF) and X-ray absorption fine structure analysis. PDF analysis revealed that there were no substantial structural differences among the AMC samples synthesized at 20, 60, and 80 °C. In addition, the medium-range order of all three AMC samples was very similar to that of hydromagnesite. Stirring in aqueous solution at room temperature caused the AMC sample to hydrate immediately and form a three-dimensional hydrogen-bonding network. Consequently, it crystallized with the long-range structural order of nesquehonite. The Mg K-edge X-ray absorption near-edge structure spectrum of AMC prepared at 20 °C was very similar to that of nesquehonite, implying that the electronic structure and coordination geometry of Mg atoms in AMC synthesized at 20 °C are highly similar to those in nesquehonite. Therefore, the short-range order (coordination environment) around the Mg atoms was slightly modified with temperature, but the medium-range order of AMC remained unchanged between 20 and 80 °C.

Structural water in amorphous carbonate minerals: ab initio molecular dynamics simulations of X-ray pair distribution experiments

“…S4, ESI† of ref. 55) appear to be in qualitative agreement with our results and those published by Schmidt et al 15 and Jensen et al 17 The studies of AMC by Jensen et al 17,54 and Yamamoto et al 53 are the only ones we are aware of that include PDF data at multiple hydration levels. We have cut off the experimental difference curves in Fig.…”

“…3a (blue curves), but we have not included this data in Fig. 3 due to difficulties that we attribute to different normalizations of the data at different hydration levels, which complicates extraction of the difference signal (we attribute the oscillations in the difference signal in the AMC data of Yamamoto et al 53 similarly to normalization issues). Albéric et al 55 presented PDF data for ACC as it was dehydrated by heating in the range 1.4 > n > 0.4 that we did not include in Fig.…”

“…1 to aid comparison between amorphous and ordered systems. Comparisons to the published data have been made by carefully tabulating published PDF curves from studies that varied the hydration level in AMC 17,53,54 and ACC 15 and taking differences of them. The ACC simulation was performed with Q max = 22 Å À1 .…”

Structure of an amorphous calcium carbonate phase involved in the formation of Pinctada margaritifera shells

“…PC1 reproduces the prominent peaks #1, #3, and #4, which were reported in the G(r) of ACC ( 27 , 48 , 69 ). PC2 with its shift of peak #1 resembles the G(r) of amorphous magnesium carbonate, which features peaks at ∼2.1 Å and 3.8 Å ( 70 , 71 ). PC3 bears some characteristics of ACC G(r) ( 27 , 48 , 69 ) again, in particular peak #2 at 2.8 Å, which was not featured in PC1.…”

“…PC1 is associated with the periostracum, whereas PC2 is mostly found in the younger disc and PC3 in the older prisms. The first interatomic distance of the PC2 (prevalently Ca-O) is markedly shifted toward smaller distances with respect to PC1 and PC3, with a structure resembling the G(r) of ACC (27,48,69) and amorphous magnesium carbonate (70,71). The RMC modeling approach allowed us to model explicitly interatomic distances for the Ca-O, Ca-C, and Ca-Ca pairs.…”