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
along
parabolas that depart from the calcite–dolomite or calcite–magnesite
lines. The
FWHM at low Mg contents are in good
agreement with the previous data of Bischoff et al (1985) obtained in
the range
0-25 mol% MgCO3. However, contrary to previous conclusions,
the limited
data dispersion and high correlation coefficients allow using Raman
spectral
properties of Mg calcites (both shifts and FWHM) to determine the Mg
content of
abiotic calcites. Application to biogenic calcites is not yet
recommended as
crystallite size, organic matter and variations of compositions at
small scales
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
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 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.