1. Technical aspect
We have developed at the ESRF during the first year of the project a liquid helium cryostat to minimize radiation damage (Figure 1).
2. Scientific aspect
The project is divided into four tasks. We report separately the work accomplished in each of them.
2.1. Enrichment of rare-earth elements in apatite : Experimental approach
The main objective of this task is to understand how strategic rare-earth elements and yttrium (REY) are enriched in sediment muds below the seafloor. The main host phase is fluorapatite (FAp, Ca10(PO4)6F2). Apatite grains are of two genetic types, they are the remains of fossilized bones and teeth from fish debris (bioapatite), or chemical precipitates from seawater (authigenic apatite, Figure 2). Marine FAp grains are constituted of nano crystals (Figure 3a), and therefore are nanoporous with extremely large surfaces exposed to circulating fluids, thus explaining their high REY-content.
Despite their minute size, marine FAp has the same crystal structure as FAp gemstones found in hydrothermal deposits (Figure 3b). The FAp structure consists of a ring of PO4 tetrahedra forming honeycomb channels (Figure 4). Ca occupies a Ca1 site on the wall of the phosphate rings and a Ca2 site on the rim of the rings and adjacent to the channels and at the mineral surface. The site distribution of REEs in marine FAp is unknown, and we hypothesized at the beginning of the project that it is a marker of the geochemical processes that FAp has undergone.
We started the project by interrogating how cerium (Ce) is incorporated in the hydrothermal FAp references from Durango (Mexico) and Imilchil (Marocco, Figure 3b). The two FAp references were stable under the beam at room temperature, and therefore we could start the project while the liquid helium cryostat was being developed. Indeed, preliminary measurement at room temperature on biogenic FAp showed that Ce(III) was rapidly photooxidized to Ce(IV) (Figure 5).
The ERC project is performed principally on the new ID24 beamline, which is equipped of a unique high-reflectivity five-crystal analyzer (spectrometer) for high energy-resolution fluorescence detected EXAFS spectroscopy (HERD-EXAFS). Figure 6 compares the standard (SDD) and HERFD Ce L3-edge EXAFS spectra of the Durango FAp. The benefits of HERFD-EXAFS is clearly visible. The ERC project cannot be performed anywhere else because of the uniqueness of the spectrometer. All our data are unprecedented.
To find out how Ce is incorporated in FAp, one needs to reproduce the experimental HERFD-EXAFS spectrum with different model structures. The two important parameters are (1) the site occupancy of Ce (Ca1 vs. Ca2, Figure 4), and (2) the charge balance of the Ce(III) for Ca(II) substitution. The model structures were built ab initio (Task 2).
2.2. Enrichment of rare-earth elements in apatite : Theoretical approach
Substitution of Ca(II) by a 3+ REY requires compensation of the excess of positive charge, which can be realized in two coupled substitutions: REY(III) + Na(I) ↔ 2Ca(II) and REY(III) + Si(IV) ↔ P(V) + Ca(II). The nature of the charge compensating cation was unknown, and this question was addressed by modeling the HERFD-EXAFS spectrum with density functional theory (DFT) models. There is another step in the process, however. DFT yields structure models and one needs next to calculate, also ab initio, and fit the theoretical EXAFS spectrum to the experimental HERFD-EXAFS spectrum. This analytical approach is also unprecedented and its implementation required to write a special code. The software is called DFT2FEFFIT and has been published.
https://doi.org/10.1107/S1600576724005454(opens in new window)In the end, we were able to show that Ce occupies the Ca2 site in hydrothermal FAp and that the +1 charge excess is compensated by a Si(IV) for P(V) substitution at short distance. The combined HERFD-EXAFS and DFT results are published (
https://doi.org/10.1021/acsearthspacechem.3c00274(opens in new window)). It remains to be seen whether this finding can be extended to marine FAp, which are prospective resources for REY.
2.3. Enrichment of trace metals in marine ferromanganese deposits : Experimental approach
The main objective of this task is to understand how redox-sensitive metals, such as thallium (Tl), platinum (Pt), and Ce, are enriched in ferromanganese (FeMn) crusts on marine seamounts and in ferromanganese nodules on the seafloor. In 2024, we started working on Pt. HERFD-EXAFS data were measured on FeMn crusts from the Pacific, Atlantic, and Indian Oceans. Data analysis is underway.
2.4. Enrichment of trace metals in marine ferromanganese deposits : Theoretical approach
We modeled by density functional theory (DFT) the oxidative uptake of Tl(I) to Tl(III) on d-MnO2 nanolayers.
First, we showed that Tl(I) is weekly oxidizable, thus explaining the co-occurrence of Tl(I) and Tl(III) in marine FeMn deposits.
Second, we showed that Tl(I) is oxidized by the stepwise reaction sequence Tl(I) → Tl(II) → Tl(III), and results in the formation of two Mn(III), and not Mn(II) as suggested previously in the literature.
Publication:
https://doi.org/10.1021/acsearthspacechem.3c00103(opens in new window)
