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Issue 9, 2021

Effect of temperature on the transformation of amorphous calcium magnesium carbonate with near-dolomite stoichiometry into high Mg-calcite

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Abstract

High Mg-calcite (HMC) is thermodynamically unstable under ambient conditions, yet it has been found in many biogenic and abiotic depositional settings with Mg contents up to 50 mol%. The elevated Mg content of HMC was frequently attributed to an amorphous calcium magnesium carbonate (ACMC) precursor, but the effects of transformation conditions on the Mg content of HMC are still debated and far from being quantified. Therefore, the transformation of ACMC with near-dolomite stoichiometry (∼48 mol% Mg) into HMC has been studied in a MgCl2–NaHCO3 buffered solution at moderate pH (7.6) and temperatures from 10 to 80 °C. The obtained chemical data show that the apparent solubility of ACMC (KACMC) is lower at higher temperature and the relative increase of KACMC as a function of the Mg content is similar between 10 and 80 °C which can be assessed by the equation: log(KACMC) = 0.01629 * [Mg]ACMC − 0.0001096 T2 + 0.0545 T −12.919, where T is reported in Kelvin and [Mg]ACMC refers to the amount of Mg in ACMC in mol%. The Mg content of the final HMC increases from 5 to 40 mol% with increasing temperature, thus is significantly lower compared to the Mg content of the ACMC precursor. Our findings argue for a dissolution and re-precipitation process during ACMC transformation, where at higher temperatures the Mg incorporation into HMC is enhanced due to (i) the reduced solvation energy barrier of aqueous Mg2+ and (ii) the high prevailing molar Mg2+/Ca2+ ratio of the solution after its reaction with the ACMC. Notably, the Mg2+/Ca2+ ratio of the latter solution is controlled by the temperature-dependent solubility of ACMC. Finally, the findings indicate that the nanocrystalline HMCs formed via transformation of ACMC are physically defective from rapid crystal growth and have higher solubilities compared to well-crystallized Mg-bearing calcites.

Graphical abstract: Effect of temperature on the transformation of amorphous calcium magnesium carbonate with near-dolomite stoichiometry into high Mg-calcite

Supplementary files

Article information


Submitted
18 Nov 2020
Accepted
05 Feb 2021
First published
08 Feb 2021

This article is Open Access

CrystEngComm, 2021,23, 1969-1981
Article type
Paper

Effect of temperature on the transformation of amorphous calcium magnesium carbonate with near-dolomite stoichiometry into high Mg-calcite

B. Purgstaller, V. Mavromatis, K. E. Goetschl, F. R. Steindl and M. Dietzel, CrystEngComm, 2021, 23, 1969 DOI: 10.1039/D0CE01679A

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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