Raman spectroscopy as a tool for magnesium estimation in Mg‐calcite

Abstract

Despite their strong Raman scattering and importance in several applications in the geological and biological sciences, Mg‐calcites have not been thoroughly investigated by Raman spectroscopy. In this study, we investigated whether Raman spectra of carbonates are sensitive to the structural and chemical changes occurring when Mg2+ substitutes Ca2+ in the calcite lattice. Different carbonate samples with variable Mg content (from 0 to 20 mol% MgCO3) of biological and inorganic origin have been first beamed under a Raman spectrometer and then analysed by scanning electron microscopy‐energy dispersive spectrometer and electron microprobe analysis‐wavelength dispersive spectrometer microprobes to determine their chemistry. The biological samples (foraminifers, algae, barnacles and echinoderms) were collected from areas situated at different latitudes and water temperature and saturation, factors affecting the chemical and isotopic composition of shells in marine organisms. The Raman band positions are directly linked to the amount of magnesium present in the calcite lattice, and all peaks of Mg‐calcite spectra show a consistent and linear increase in Raman shifts according to their Mg content, which is a consequence of the decreased inter‐atomic distances following the substitution of Ca2+ with the smaller Mg2+ ion. This study demonstrates that Raman spectroscopy provides an innovative work perspective in marine biology, provenance studies and oil exploration when estimates of mineralogical and chemical changes are the focus of the study. Raman spectroscopy is even more attractive – for the mentioned fields of research – than many other methods because of the non‐destructive nature and its very short analytical time. Copyright © 2017 John Wiley & Sons, Ltd.

Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

“…In abiogenic aragonites with known fluid conditions, we found a clear dependence of Raman peak width on Ar , an observation that is not confounded by other factors, including temperature, Mg/Ca, pH, and [CO tors such as trace element variability to have some influence, our Raman results from abiogenic aragonites and the JCp-1 coral can be explained by Ar alone. Nevertheless, future investigations of controls on Raman spectra of corals skeleton will be useful, for instance in checking for positive correlations between ν 1 FWHM and wavenumber that would potentially indicate an effect of Mg/Ca (Bischoff et al, 1985;Perrin et al, 2016;Borromeo et al, 2017). We found no such relationships in either our Porites or Acropora analyses (p > 0.05, r 2 < 0.01 in both cases).…”

Raman spectroscopy of the eight natural carbonate minerals of calcite structure

“…[8] Near-infrared spectroscopy has been performed on all carbonate minerals of calcite structure. [9] However, to date, five natural carbonate minerals of calcite structure have been extensively studied by Raman spectroscopy, which include calcite, [1,[10][11][12][13][14][15] magnesite, [1,[12][13][14]16,17] siderite, [1,2,12,14,16] rhodochrosite, [12,14,18] and smithsonite, [12,14,19] and two synthetic carbonate compounds of calcite structure, CdCO 3 [12] and NiCO 3 , [12] have been studied to a lesser extent. It was noted by Rutt and Nicola [12] that synthetic CoCO 3 affords a poor Raman spectrum in which only two bands could be observed with certainty.…”

“…Alternatively however, Boulard et al [2] proposed that perhaps significant variation in the relative wavenumbers of phonons involving C-O bonds might be observed because Raman shifts might not be attributed to the slight length variation of the C-O bonds, but rather might be controlled by the bonding environment, that is, the potential variation around the CO 3 polyhedra. To date, Bischoff et al, [13] Rividi et al, [1] Boulard et al, [2] Perrin et al, [17] and Boromeo et al [18] have shown that the C-O phonons do shift in relative wavenumber with respect to divalent cations such as Ca 2+ , Mg 2+ , and Fe 2+ in the solid-solution series calcite-magnesite, calcite-siderite, and sideritemagnesite. With this in mind, Raman spectra are examined and interpreted for all eight naturally occurring carbonate minerals of calcite structure to investigate the effects on ionic radii and nearest neighbor (M-O) influences on phonon shifts for this group of minerals to facilitate a Raman spectroscopic model for the identification and discrimination of the calcite-group minerals.…”

Response to Referee Comment #1

“…While it remains theoretically possible for other fac- tors such as trace element variability to have some influence, our Raman results from abiogenic aragonites and the JCp-1 coral can be explained by Ar alone. Nevertheless, future investigations of controls on Raman spectra of corals skeleton will be useful, for instance in checking for positive correlations between ν 1 FWHM and wavenumber that would potentially indicate an effect of Mg/Ca (Bischoff et al, 1985;Perrin et al, 2016;Borromeo et al, 2017). We found no such relationships in either our Porites or Acropora analyses (p > 0.05, r 2 < 0.01 in both cases).…”