Magnesian calcites are important components of sediments and biominerals. As such, they are important carbon reservoirs in the subducted oceanic lithosphere. Although Raman spectra of calcite, dolomite, and magnesite are well known, those of Mg calcites deserve further investigation. Nineteen syntheses of Mg calcites covering the range 0-50 mol% MgCO3 have been carried out at high pressure and temperature (1-1.5 GPa, 1000-1100°C). The crystalline run products have been studied by µ-Raman spectroscopy. For all lattice and internal modes (L, T, ν1, ν4, 2ν2) but ν3, the wavenumbers align along the calcite–dolomite line and not along calcite–magnesite. The compositional dependence is strong and regression curves with high correlation coefficients of have been determined.
Concerning the full
width half maximum (FWHM), all modes display regular data alignments
parabolas that depart from the calcite–dolomite or calcite–magnesite
FWHM at low Mg contents are in good
agreement with the previous data of Bischoff et al (1985) obtained in
0-25 mol% MgCO3. However, contrary to previous conclusions,
data dispersion and high correlation coefficients allow using Raman
properties of Mg calcites (both shifts and FWHM) to determine the Mg
abiotic calcites. Application to biogenic calcites is not yet
crystallite size, organic matter and variations of compositions at
are additional parameters that affect Raman properties.
A comparison with Raman
data obtained on synthetic magnesian amorphous calcium carbonate (Mg
ACC) by Wang et
al. (2012) show
that the wavenumber position of the ACC ν1 mode
is systematically shifted towards lower values than Mg Calcites, and
FWHM are higher than those of their crystalline counterparts. The FWHM
parameters of crystalline and amorphous materials do not overlap, which
a clear-cut distinction between crystalline and amorphous materials.
In the synthetic Mg calcites, the shift and FWHM of Raman bands as a function of Mg can be interpreted in terms of changes of metal-O bond lengths resulting from the replacement of calcium by magnesium. In addition, the facts that the wavenumber of Mg calcites are close to the calcite–dolomite line, that the FWHM of the T, L, and ν4 modes reach a maximum around 30±5 mol% MgCO3, and that a peak specific to dolomite at 880 cm-1 is observed in high-Mg calcites indicate that dolomite-like ordering is present above 20 mol% MgCO3. Mg atom clustering in cation layers combined with cluster ordering in successive cation basal layers may account for the progressive ordering observed in synthetic Mg calcites.
Last updated 22Dec-2015