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Institut de minéralogie, de physique des matériaux et de cosmochimie

Crystal chemistry of mineral tracers

Low-temperature minerals constitute archives of the geochemical functioning and history of our planet’s surface. In many cases, the information is sequestered in the form of trace elements whose abundance, speciation and isotopic composition are determined by the physicochemical and biological parameters prevailing during the crystal growth. Information on geochemical processes can also be gained from the imprint of ambient radioactivity as point defects in the structure of minerals. It is therefore the role of the mineralogist to specify the complex links between information at the molecular level and their geochemical significance and to ensure the quality of the preservation of this information. In this context, our work concerns the understanding of isotopic fractionation processes, the incorporation of molecular impurities in biogenic minerals, the identification of radiation-induced defects in crystals and the understanding of fossilization mechanisms.


Ab initio modeling of isotopic fractionations (EB, MB).Our pioneering studies carried out for about 10 years in the field of ab initio modeling of isotopic fractionation coefficients has enabled us to efficiently approach the processes of isotopic fractionation between solids and solutions [Pinilla et al. 2014; 2015]. We have especially applied this approach to new isotopic systems such as iron and zinc [Blanchard et al. 2015; Ducher et al. 2016], particularly studied by geochemists (Fig. 1).


Local environment of impurities and geochemical tracers (EB, MB, GM).The combination of experimental approaches (vibrational and NMR spectroscopies) and theoretical (DFT calculations) approaches, allowed us for the first time to propose and validate molecular models for the incorporation of borate and sulfate impurities in calcium carbonates, considered respectively as "proxies" for pH and redox conditions of ancient oceans [Balan et al. 2014; 2016 a, b]. These approaches also make it possible to trace efficiently the mechanisms of transformation of biological materials during fossilization. Thus, analyzing the changes in the molecular environment of carbonate groups in apatites reveals the modifications at the smallest spatial scales of the materials constituting the vertebrate skeleton [Yi et al. 2013, 2014 a, b; Balan et al. 2016], "INSU News" 2013 (http://www.insu.cnrs.fr/en/node/4370).

Figure 1.

Left: Density of vibrational states of iron in goethite, measured by NRIXS or calculated by DFT + U, from which it is possible to determine the equilibrium constant of isotopic iron exchange [Blanchard et al. 2015].

Right: Theoretical models for the borate ion in the structure of the main natural calcium carbonates [Balan et al. 2016]. The hydrogen bonds are indicated as dotted lines.

At the same time, we have contributed to a better understanding of the spectroscopic signatures of micro-divided minerals, such as those of soils and sediments. The main results concerned the crystal-chemistry of iron-aluminum oxyhydroxides [Delattre et al. 2012; Blanchard et al. 2014; Ducher et al. 2016] and the modeling of Attenuated Total Reflectance spectra, a technique increasingly used to record the infrared spectrum of powder minerals such as biogenic phosphates and carbonates [Aufort et al. 2016]. In collaboration with the IMPMC MIMABADI team, we have finally identified original pathways leading to the transformation of nanogoethite to nanomagnetite, which could potentially be used as markers of sediment diagenesis conditions [Till et al. 2015]. Thanks to the expertise developed on ab initio modeling of the vibrational properties of hydroxylated minerals, we have been able to develop strong collaborations on the incorporation of hydrogen and volatile elements in nominally anhydrous minerals, whose major contribution to the functioning of the planet is established [Balan et al. 2013 a, b; 2014 a, b; 2016; Blanchard et al. 2013, 2016; Crepisson et al. 2014 a, b; Ingrin et al. 2013, 2014]. Transmission Electron Microscopy (TEM) is a powerful tool to solve nanoscale issues in almost all the MINENV projects. The study of nanoscale-minerals (Ferraris & Lorand, 2015, Vitale Bovarone et al., 2017), new mineral species (Ferraris et al. 2014) and synthetic compounds (Bakie et al. 2014), are good example of this interdisciplinary activity.

Study and characterization of minerals and synthetic products by MET. Since 2014, the Electron Microscopy activity in Transmission around different families of minerals and synthetic products has been integrated into the work of the team mainly in the form of assistance and supervision of PhD student activities. In the stricter sense of research, the publication of two new mineralogical species (Ferraris et al., 2014), crystallographic studies of lead apatites (Bakie et al., 2014) or nano-inclusions in mantle rocks (Ferraris et al. Lorand, 2015), studies on the origin of abiotic methane (Vitale Bovarone et al., 2017) and results on the porosity of synthetic and inorganic films (Brigo et al., 2016) Support that will surely intensify and will be renewed over time and in the scientific orientations of the different groups of the equipment.

Experimental modeling of radiation-induced defects in clay minerals (TA, EB, MG).In the thesis of Thomas Riegler and within the framework of a GUTEC / CNRS program, the measurement of thermal stability of radiation-induced defects, detected by Electron Paramagnetic Resonance, was applied to illites, giving access to the tracing of past radioactivity by new clayey parageneses, which were previously limited to kaolinites and smectites. We have shown that defects of type Ai in illites can be used on a geological time scale [Riegler et al., 2016a], and we have revealed several types of radiation-induced defects in talcs that are considered as simple models of 2/1 clays. A strong production of hydrogen by radiolysis, a critical parameter for the safety assessment of a nuclear waste storage site, was also observed for synthetic clay with a magnitude comparable to that of water [Lainé and Al., 2016]. Natural talc showed a much lower hydrogen production suggesting a moderating role for structural impurities. In addition, we have demonstrated a metamic state of graphite produced by alpha recoil nuclei from uranium disintegration chains in a paleoproterozoic uranium deposit in Canada. This is supported by experimental calibration of amorphization doses [Riegler et al., 2016b].

Cécile Duflot - 21/11/17

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