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Published November 2016 | public
Journal Article

Raman characterization of synthetic magnesian calcites

Abstract

Magnesian calcites are important components of sediments and biominerals. Although Raman spectra of calcite, dolomite, and magnesite are well known, those of magnesian calcites deserve further investigation. Nineteen syntheses of magnesian calcites covering the range 0–50 mol% MgCO_3 have been carried out at high pressure and temperature (1–1.5 GPa, 1000–1100 °C). The crystalline run products have been characterized by μ-Raman spectroscopy. For all lattice and internal modes (L, T, ν_1, ν_4, 2ν_2) but ν_3, wavenumbers align closer to the calcite– dolomite line than the calcite–magnesite line. The compositional dependence is strong and regression curves with high correlation coefficients have been determined. Full-width at half maximum (FWHM) plot along parabolas that depart from the calcite–dolomite or calcite–magnesite lines. The limited data dispersion of both shifts and FWHM allow using Raman spectral properties of magnesian calcites to determine the Mg content of abiotic calcites. A comparison with Raman data from the literature obtained on synthetic magnesian amorphous calcium carbonate (Mg ACC) shows that the wavenumber position of the ACC ν1 mode is systematically shifted toward lower values, and that their FWHM are higher than those of their crystalline counterparts. The FWHM parameters of crystalline and amorphous materials do not overlap, which allows a clear-cut distinction between crystalline and amorphous materials. In synthetic magnesian calcites, the shift and FWHM of Raman bands as a function of magnesium can be interpreted in terms of changes of metal-O bond lengths resulting from the replacement of calcium by magnesium. The facts that the wavenumber of magnesian calcites are close to the calcite–dolomite line (not calcite-magnesite), that the FWHM of the T, L, and ν4 modes reach a maximum around 30 ±5 mol% MgCO_3, and that a peak specific to dolomite at 880 cm^(−1) is observed in high-magnesian calcites indicate that dolomite-like ordering is present abov ~10 mol% MgCO_3. Mg atom clustering in cation layers combined with ordering in successive cation basal layers may account for the progressive ordering observed in synthetic magnesian calcites.

Additional Information

© 2016 Mineralogical Society of America. Manuscript received February 8, 2016; Manuscript accepted June 3, 2016; First Published on November 01, 2016. This work has been supported by the Centre National de la Recherche Scientifique (CNRS), by Institut National des Sciences de l'Univers (INSU) through grant INTERRVIE 2013, by the Agence National pour la Recherche (ANR) through ANR CoRo 2011–2015, by the Centre Interdisciplinaire de Nanosciences de Marseille (CINaM), and by the European Union COST action TD0903. This work has been partly carried out within the framework of the ICoME2 Labex (ANR-11-LABX-0053) and the A*MIDEX projects (ANR-11-IDEX-0001-02) cofunded by the French program "Investissements d'Avenir," which is managed by the ANR, the French National Research Agency. Observations were made on a FESEM financed by the European Fund for Regional Development (EFRD). We thank F. Bedu and I. Ozerov for their assistance on this instrument and B. Devouard for providing the OCN calcite. We thank J.L. Devidal and N. Bolfan-Casanova (LMV, Clermond-Fd) for their assistance with electron microprobe and μ-Raman, respectively. D.V. benefited from a financial support by E.M. Stolper for a three month stay at Caltech in 2014, where and when most of the μ-Raman analyses were made. Reviews by R.L. Frost and an anonymous reviewer as well as editorial handling by R. Stalder and K. Putirka are gratefully acknowledged. This is contribution ANR CoRo number 09.

Additional details

Created:
August 22, 2023
Modified:
October 23, 2023