Mechanisms controlling Li incorporation and isotope fractionation in CaCO3 minerals – MeLiCa

-Li sorption onto Ca and Mg carbonate minerals (calcite, aragonite, hydromagnesite) and portlandite (Ca(OH)2) has been studied at 25 °C, high solid: liquid ratio (20 to 50 g/L), variable pH (8 to 13), ionic strength (0.01 to 1 M) and dissolved inorganic carbon (DIC) concentration. The exposure time varied from several days to several months. We found measurable adsorption of Li (5 to 15%, or 1-3 µg Li/g solid) which, however, poorly dependent on pH, ionic strength, and DIC concentration, regardless of the solid phase.

7Li NMR
The 7Li and 6Li NMR spectra were acquired using Bruker AVANCE NEO 850 (20 T) and 300 (7 T) NMR spectrometers. Prior to each NMR analysis, the synthesized products were verified by XRD. Pure polymorphs of CaCO3, such as calcite, aragonite, and vaterite, were analyzed.

Adsorption experiments did not evidence the interaction of Li+(aq) with >CaCO3- and >CO3- surface species, as hypothesized initially. Overall low adsorption does not allow using NMR spectroscopy for deciphering Li status on the carbonate mineral surfaces. Samples were prepared for future Li isotope analyses.
-Li behavior during the transformation of amorphous to crystalline CaCO3
Amorphous to crystalline CaCO3 experiments performed at 5, 25 and 40 °C with duration up to 90 days. Solids and fluid examined for their chemical composition and mineralogy. In selected samples 7Li NMR spectra were collected. Isotopic analyses are in progress.
-Li rich calcite, aragonite and vaterite were formed for 7Li NMR analyses
The selected ACC (amorphous to crystalline CaCO3 experiments) samples were analyzed by 7Li NMR. It was observed that these products contain a mixture of CaCO3 polymorphs. Currently, we are attempting to characterize the obtained spectra based on previous results for pure CaCO3 polymorphs. However, to confirm our hypothesis, more complex NMR experiments are needed, as homonuclear 7Li-7Li or heteronuclear 7Li-1H correlation NMR experiments. These experiments, in the case of low abundance lithium would be long and challenging. In the same way, the first 6Li NMR experiments were not conclusive due to the very low concentration of this isotope.

Biominerals
Some preliminary results have been obtained by ion microprobe on brachiopod shells. Ion probe measurements show no difference between primary and secondary calcite layers, as suggested by the results of Gasper et al (2021). However, the errors associated with the measurements are quite large due to the low lithium concentration in the shells (1 ppm or less). For species with a tertiary layer, it has not been possible to obtain reliable results for this structure because lithium abundance is very low (Rollion-Bard et al, 2019).

We have collected for the first time 7Li NMR spectra of Li-doped pure calcite, aragonite and vaterite. These spectra exhibit several signals that we propose to assign based on GIPAW DFT calculations.

Experiments with SO4 in solution are in progress

Samples from the Salda Lake (Turkey), the Sevan Lake (Armenia), Issyk-Kul (Kirgizhstan) and Balkhash (Kazakhstan) were collected and characterisation of these solid phases (TEM, SEM, XRD, chemical analyses) together with coupled water samples is in progress. This should allow comparing laboratory and field-based Li interaction with carbonate minerals.

Several scientific papers are in preparation for submission in the end of 2024

The reconstruction of Earth’s environmental conditions over geological history is commonly based on the chemical and isotopic composition of carbonate minerals. The robust interpretation of chemical and isotopic signals in these archives requires understanding of the mechanisms controlling elemental incorporation in the solid. Lithium has been proposed as an important proxy for a variety of environmental conditions including pH and temperature. The MeLiCa project aims to explore the mechanisms controlling the incorporation and isotope fractionation of Li in CaCO3 minerals, calcite and aragonite. Our hypothesis, which is supported by preliminary results, is that the [Li] and ?7Li composition of the solid is determined at the step of Li adsorption on the growing CaCO3 mineral phase. Lithium adsorption on mineral surfaces is controlled by the pH and the chemical composition of the fluid. Thus variations in [Li] and ?7Li are expected to occur due to different chemical compositions of the fluid. The focus of the MeLiCa project is to quantify the physicochemical parameters controlling [Li] and ?7Li composition in calcite and aragonite, which will allow the use of this element as a proxy tool. MeLiCa project will combine four components: i) experimental studies on Li sorption and co-precipitation with calcium carbonates that quantify [Li] and ?7Li of the solid under different physicochemical conditions, ii) characterization of Li bonding environment in the solid phase using 7Li NMR spectroscopy, iii) atomic-scale modelling based on the framework of density functional theory to provide insights on ?7Li composition of the carbonate minerals and iv) measurements of [Li] and ?7Li in natural samples formed under well characterized environmental conditions will be used to assess the findings of experimental and theoretical work. Together, this research will support [Li] and ?7Li proxy developments and biogeochemical modeling of climate evolution in the geological past.