FT-Raman and Infrared Spectroscopic Study of Aragonite-Strontianite (Ca,Sr, $03) Solid Solution’ J.M
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SPECTROCHIMICA ACTA PART A Spectrochiica Acta Part A 53 (1997) 2347-2362 FT-Raman and infrared spectroscopic study of aragonite-strontianite (Ca,Sr, _ $03) solid solution’ J.M. Alia a**, Y. Diaz de Mera a, H.G.M. Edwards b, P. GonzsilezMartin c, S. L6pez And&s ’ a E.U.I.T.A., Departamento de Quimica Fisica, Universidad de Castilla-La Mancha, Ronda de Calatrava 5. 13071 Ciudad Real, Spain b Chemistry and Chemical Technology, University of Bradford, Richmond Road, Bradford, West Yorkshire BD7 IDP, UK ’ Dpto. de Cristalografia y Mineralogia, Facultad de CC. Geoldgicas, Universidad Complutense, 28040 Madrid, Spain Received 15 May 1997; accepted 7 July 1997 Abstract Synthetic aragonite-strontianite solid-solution samples have been studied using dispersive IR and FT-Raman spectroscopy. In addition to the end-members, nine samples over a range of composition from Ca,.,Sr,,,CO, to Ca,,,Sr,,,CO, were analysed. Carbonate anion internal modes are examined in detail by means of band-shape analysis and component fitting procedures. Positional disorder induced by the random cationic substitution results in strong increase of the halfwidth in several vibrational bands. Results obtained for the doubly degenerate modes (antisymmet- ric stretching and bending, v3 and v4) reveal the presence of three components both in the IR antisymmetric stretching band as well as in the Raman antisymmetric bending band. These observations are interpreted in terms of an overtone 2v, in possible Fermi resonance with the corresponding vg fundamental in the IR spectra, and the presence of Davidov (factor group) splitting in the FT-Raman v4 band. Lattice modes in the FT-Raman spectra demonstrate weaker cohesion between the cation/carbonate/cation layers in aragonite (synth) than in strontianite (synth). 0 1997 Elsevier Science B.V. Keywords: FT-Raman; FTIR; Aragonite; Stronitianite; Fundamental modes 1. Introduction In nature, the most important calcium carbon- ate polymorphs (calcite, rhombohedral, and arag- * Corresponding author. onite, orthorhombic) are rarely encountered as ’ Dedicated to the memory of Lucienne Couture, in the chemically pure substances. Both crystal forms fiftieth anniversary of the publication of her extraordinary work ‘Etude des spectres de vibrations de monocristaux hold cationic changes that, in the case of arago- ioniques’, Annales de Physique. 12eme S&e, t. 2 (Janvier- nite substituted by strontium, can constitute an Fevrier 1947) 5-94. authentic solid solution. This solid solution has 1386-1425/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PIIS1386-1425(97)00175-3 2348 .I.M. ,4/h rt ul. Spectrochinlicu Acta Part A 53 (1497) 23317 ~17ih2 several interesting aspects. From the geochemical Kerr [14] who reported the IR spectra of a dozen point of view, the introduction of small quantities of simple and double carbonates, Scheetz and ( Y 0.6 mol%) of strontium ion into the aragonite White [15] published a detailed vibrational (Ra- lattice increases the stability of the solid. In the man and IR) study of alkaline-earth double car- same way, the stability of strontianite (SrCO,) is bonates of natural origin that does not include strengthened by the presence of ca. 12.5 mol% of species containing strontium ion. Moreover. only calcium ion [l]. Moreover, it seems energetically one of the minerals studied, alstonite [Ca- easier to introduce calcium ion into the strontian- Ba(CO,),], is orthorhombic and could be consid- ite lattice than vice versa. These results from ered structurally similar to the samples included synthetic samples are confirmed by the usual com- in the present work. Gil et al. [16] studied several position of the corresponding minerals: the stron- alkaline-earth simple carbonates with a high con- tium content in natural aragonite does not exceed tent Fe’+ by means of FT-IR spectroscopy. l--2 mol% [2], whereas strontianite can contain up Biittcher et al. [17- 191 have reported an interest- to 25-30 mol% of calcium ion [3]. It is interesting ing series of papers studying the changes pro- to note that marine aragonite, with a strontium moted in the FT-IR spectra of the rhombohedral content of approximately 1 mol% [4], comes from carbonates, MgCO, and CaCO,, by the presence sea-water with calcium/strontium molar ratios of of other bivalent metallic ions such as Fe’-‘. z 1200/l [5]. Mn2+ or CO*+, which, in some cases (e.g. Casey et al. [6] studied the thermochemistry of FeCO,-MgCO,) can form solid-solutions. These the aragonite-strontianite solid solution and ob- samples occur naturally and their vibrational tained an excess entropy of mixing which was study has been applied to geothermometry. The very small or zero, supporting the existence of a same group have published lately [20] and Raman regular solution. The value of such thermody- and FT-IR spectra of synthetic x-Na,Ca(CO,),. namic predictions goes beyond an academic inter- To the best of our knowledge, there is no refer- est; for example, in the Chernobyl nuclear ence of similar studies for orthorhombic carbon- accident some 37.50 km2 were contaminated by ates and, moreover, in only one case [12] has radioactive material [7] comprising “Sr’+. Up to FT-Raman spectroscopy been employed. 70% of this metallic ion could have been fixed in In the present work, a series of synthetic sam- the ground after reaction with the solid carbon- ples of the solid-solution aragonite-strontianite, ates. On the other hand, the Sr:Ca ratio in obtained by co-precipitation, is studied by means coralline aragonite is often employed to determine of IR (dispersive) and FT-Raman spectroscopy. the sea surface temperature in paleoclimate recon- Besides calcium carbonate and strontium carbon- structions [8]. Several authors have documented ate, nine samples whose range of composition an inverse relationship between the temperature extends from Ca,,,Sr,,,CO, to CaO.,Sr,,,CO, are of the marine environment and the strontium analysed. The aim of this research is twofold: to uptake by coralline aragonite [5,9]. assess spectroscopically the existence of the solid There have been numerous vibrational spectro- solution and to provide some data about the scopic investigations of pure carbonates. After the effect of the cationic positional disorder on the comprehensive and often cited review of White vibrational dynamics of the carbonate anion. [lo], several collections of spectra of simple car- bonates have been published. Herman et al. [l l] have used a laser microprobe to obtain analyti- 2. Experimental cally the Raman spectra of natural carbonates. Griffith et al. [12] have recently published the first 2.1. Solids compilation of FT-Raman spectra of minerals, including several carbonates. However, papers Samples were prepared by the method of Plum- dealing with double or mixed carbonates are more mer and Busenberg [l]. Briefly, for the synthesis scarce [13]. From the seminal work of Adler and of aragonites (lattice calcium ion mole fraction J.M. Alia et al. /Spectrochimica Acta Part A 53 (1997) 2347-2362 2349 1) was carried out in a similar way, but the initial SrCl, concentration was reduced to 0.05 M, with the corresponding amounts of CaCl, and ,f-/‘4 Na,CO,. Precipitates were separated from the /I’ mother liquor through 0.65 urn Millipore filters, ./- and washed repeatedly with deionized water until _* the elimination of Cl - ion was confirmed (no , l reaction with AgNO,). Afterwards, solids were dried by heating at 110°C for 48 h. The chemical composition of the solids was determined by dissolution in 20% HCl at 60°C 0 1 0.2 %4 0.6 0.8 and atomic absorption spectrophotometric analy- Mole fraction of S?+ sis. Maximum error over the theoretical stoi- Fig. 1. Spacing for the [I 1 l] lattice plane in several samples of chiometry was always less than 5%. The purity of the synthetic aragonite-strontianite solid-solution. the samples was confirmed by powder X-ray dif- fraction, scanning at 0.02 s-l (28) using Cu-K, between 0.5 and 1) different quantities of SrCl, radiation and silicon as an internal standard. A were added to the base solution containing NaCl single-phase solid having the aragonite structure 0.43 M, MgCl, 0.078 M and CaCl, 0.10 M in was always obtained in the synthesis. The spacing order to obtain Sr2+ mole fractions of 0.1, 0.2, for the [l 1 I] lattice plane changes linearly with the 0.3, 0.4 and 0.5, respectively. The solution was strontium content, as can be observed in Fig. 1, in heated to 76 f 0.5”C and the stoichiometric good agreement with the results reported by amount of carbonate as Na,CO, (0.75 M) was Casey et al. [6]. added dropwise with vigorous stirring. On com- pletion of the precipitation, the suspension was 2.2. Vibrational spectra stirred for 1 h at the same temperature. The presence of NaCl and MgCl,, as well as the FT-Raman spectra were excited at 1064 nm moderately high temperature of reaction, are using an Nd:YAG laser and a Bruker IFS66 needed to avoid the precipitation of the calcite optical bench with an FRA 106 Raman accessory. phase [21]. The preparation of strontianites (lat- Laser power was set at 70-80 mW and 1000 scans tice strontium ion mole fraction between 0.5 and were accumulated with a resolution of 2 cm ’ . Table 1 Factor group analysis of the carbonate ion in aragonite-type structures (Pmcn, 02) Free symmetry Site symmetry Factor group Activity D3h g Inactive Infrared 7320 J.M. Alia et al. /Spectrochimicu Acta Part A 53 (1997) 2347-2362 1 Fig. 3. Wavenumber of the carbonate anion symmetric stretch- ing IR band plotted against the cation polarizing power, P [eV (A)-“2].