Project

For decades, the isotopic composition of minerals is explored in order to have access to the dynamics of a natural system or to make paleoenvironmental reconstructions. The interpretation of isotopic measurements done on natural samples is based on the knowledge of isotopic fractionation factors when the components of the system of interest (minerals, solutions) are in equilibrium. These equilibrium constants are traditionally determined from a delicate experimental approach. A theoretical alternative approach has been developed recently. It is based on the calculation of vibrational properties by molecular modeling and has been successfully applied to many minerals. On the other hand, the accurate calculation of the isotopic properties of an ion in solution is the technical barrier to overcome in order to rigourously compare the theoretical and experimental data. Beyond the methodological aspect, this project focused on the iron-bearing minerals that play a key role in the fate of inorganic and organic pollutants.

Modeling of Fe-bearing Minerals    Goethite (α-FeOOH) is a common mineral of soil and sediments, whose identification and characterization is often performed by means of infrared spectroscopy (IR). During this study (Blanchard et al. 2014), the temperature dependence of the IR spectrum of a synthetic sample was measured. This approach provides information on the anharmonicity and the origin of the broadening of the absorption bands. Furthermore the IR spectra of pure and Al-substituted goethite were modeled from DFT calculations. This allowed to assign the main absorption bands, to estimate the effect of the particles shape on the IR spectrum, as well as the effect of the Fe-Al substitution on structural and vibrational properties. In addition, the ab initio modelling of Al K-edge X-ray absorption spectra allowed to explain the observed trends in the spectra depending on the Al content. This is due to the distortion of the Al lattice site as well as the chemical environment of Al.     Regarding isotopic properties, our method is based on the DFT calculation of vibrational densities of states. The vibrational properties of the iron atoms in the crystal structure can also be measured by Mössbauer spectroscopy or Nuclear Resonant Inelastic X-ray Scattering (NRIXS). The comparison of results from different experimental and theoretical techniques, allows to better constrain isotope fractionation factors and to improve the treatment procedure of NRIXS data. With this in mind, we started a collaboration with N. Dauphas (Chicago) and Mr. Roskosz (Lille) to determine the isotope fractionation factor of iron in goethite.     The same modeling techniques were used to investigate the incorporation mechanism of sulfate groups in several calcium carbonates and to determine the associated sulfur isotope fractionations (Balan et al. 2014). This question is particularly important since the carbonate-associated sulfate (CAS) is considered as an efficient proxy of the sulfur isotope composition of ancient oceans. Modeling of Aqueous Fluids     Along with determining the isotope fractionation of minerals, for which the theoretical approach is well defined, we undertook a methodological study of the modeling of isotopic fractionation in liquid phase depending on the degree of approximation of the calculations. For treating a liquid phase, approximations are usually needed and may include the use of molecular clusters of finite sizes or the use of relaxed configurations from molecular dynamics simulations. This work began with the calculation of the equilibrium fractionation of H and O isotopes in pure water (Pinilla et al. 2014) and continued with the modeling of a solvated ion. We focused on magnesium for which isotopic measurements in sedimentary carbonate environments have applications in paleoclimatic reconstruction. The approaches usually found in literature were compared and discussed with respect to a more sophisticated method based on path integrals calculations. Modeling of Solid-Liquid Interfaces      An experimental study showed an unprecedented bimodal distribution of As5+ at the surface of hematite, with simultaneous adsorption of inner-sphere and outer-sphere complexes. We used DFT modeling to perform a detailed analysis of structural and electronic properties of these two types of complexes. We were able to discuss the stabilization mechanisms involved, from the geometry of adsorption complexes energetically most favorable (Blanchard et al., 2012).    Using this approach, molecular modeling provides a general theoretical framework for interpreting the experimental data derived from the different analytical techniques (e.g. diffraction techniques, vibrational spectroscopies, X-ray absorption spectroscopies, isotopic analysis). Beyond the results relevant to environmental mineralogy, the substantial methodological work conducted in this project (improvement of the modeling of mineral phases containing transition elements and theoretical determination of the isotopic properties of liquid solutions) could find a wide range of applications in Earth Sciences, Material Sciences and Physics.

People involved

Communications

1. Presentation of the proposal. ANR kick-off meeting, 13 dec. 2011, Paris
2. Blanchard M.: Complementarity of computational molecular modelling and experimental techniques to study trace elements geochemistry. keynote presentation, Goldschmidt, Montreal, Canada, 24-29 June 2012.
3. Pinilla C., Blanchard M., Ferlat G., Balan E., Vuilleumier R. & Mauri F.: Equilibrium isotope fractionation factors in liquids from path integral molecular dynamic simulations. Goldschmidt, Florence, Italie, 25-30 Aug. 2013
4. Blanchard M., Pinilla C., Poitrasson F., Méheut M., Lazzeri M., Mauri F. & Balan E.: First-principles investigation of equilibrium iron isotope fractionation in oxide and sulfide minerals. invited presentation, Goldschmidt, Florence, Italie, 25-30 Aug. 2013
5. Blanchard M., Balan E.: Theoretical investigation of vibrational and isotopic properties of iron (oxyhydr)oxides. Atelier Modélisation des Oxydes. GDR CNRS ModMat et Co-DFT, Paris, 16-17 sept. 2013.
6. Blanchard M.: Theoretical investigation of isotopic fractionations: application to iron-bearing minerals. AGU Fall meeting, San Francisco, USA, 9-13 Dec. 2013
7. Blanchard M., Balan E. Theoretical investigation of vibrational and isotopic properties of iron (oxyhydr)oxides. Atelier Modélisation des Oxydes. GDR CNRS ModMat et Co-DFT, 16-17 sept. 2013, Paris.
8. Blanchard M. Projet ANR CrIMin : Cristallochimie des minéraux ferrifères et implications dans le cycle géochimique des polluants métalliques. Séminaire ANR sur les changements environnementaux, 19-20-21 mars 2014, Lille.
   

Publications

1. Blanchard M., Morin G., Lazzeri M., Balan E., Dabo I. (2012) First-principles simulation of arsenate adsorption on the (1 -1 2) surface of hematite. Geochim. Cosmochim. Acta, 86, 182-195
2. Blanchard M., Balan E., Giura P., Béneut K., Yi H., Morin G., Pinilla C., Lazzeri M., Floris A. (2013) Infrared spectroscopic properties of goethite: anharmonic broadening, long-range electrostatic effects and Al substitution. Phys. Chem. Minerals, 41, 289-302
3. Balan E., Blanchard M., Pinilla C. & Lazzeri M. (2014) First-principles modeling of sulfate incorporation and ³⁴S/³²S isotopic fractionation in different calcium carbonates. Chemical Geology, 374-375, 84-91 
4. Pinilla C., Blanchard M., Balan E., Ferlat G., Vuilleumier R., Mauri F. (2014) Equilibrium fractionation of H and O isotopes in water from path integral molecular dynamics. Geochim. Cosmochim. Acta, 135, 203-216