# High-resolution solid-state 7Li NMR study of lithium incorporation in calcite and aragonite

A. Zhadan, Valerie Montouillout, Julie Aufort, V. Mavromatis, E. Balan

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A. Zhadan, Valerie Montouillout, Julie Aufort, V. Mavromatis, E. Balan. High-resolution solid-state 7Li NMR study of lithium incorporation in calcite and aragonite. Chemical Geology, 2025, 690 (1), pp.122842. 10.1016/j.chemgeo.2025.122842. hal-05344715

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High-resolution solid-state <sup>7</sup>Li NMR study of lithium incorporation in calcite and aragonite

A. Zhadan<sup>1*</sup>, V. Montouillout<sup>2</sup>, J. Aufort<sup>1</sup>, V. Mavromatis<sup>3</sup> and E. Balan<sup>1</sup>

<sup>1</sup>Sorbonne Université, CNRS, MNHN, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), 4 place Jussieu, 75252 Paris cedex 05, France

<sup>2</sup>Conditions Extrêmes et Matériaux: Haute Température et Irradiation, CEMHTI, UPR 3079 - CNRS Univ Orleans 45071 Orléans, France.

<sup>3</sup>Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland

Received

Corresponding author Antonii Zhadan, e-mail: antonii.zhadan@sorbonne-universite.fr
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**Abstract:**

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The concentration of Li and its isotope compositions in CaCO₃ minerals have been proposed as emerging paleo-environmental tools. Yet the bonding environment of Li in these phases, a prerequisite for interpreting mechanisms controlling Li incorporation and isotope fractionation, is not known. In this study, for the first time ever the atomic-scale environment of lithium in two synthetic calcite and aragonite samples is determined using ⁷Li Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). The spectra display several overlapping signals with ⁷Li chemical shift ranging between -2 and +1 ppm in the calcite sample, and spanning a narrower range, from -1 to 0 ppm, in the aragonite sample. This attests to a diversity of Li atomic-scale environments, which are interpreted in the light of a theoretical study of Li-bearing calcite and aragonite models using the Density Functional Theory (DFT).

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**Keywords:**

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Calcite, Aragonite, lithium, NMR, DFT calculations, GIPAW

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## 1. Introduction

The trace element composition of CaCO₃ minerals calcite and aragonite are routinely used to inform about environmental changes of Earth's aquatic environments over the geological past. For example, the ratios of divalent cations such as Ba, Mg and Sr in biominerals such as foraminifera and corals have been used to assess variations in water temperature (e.g. Beck et al., 1992; Nürnberg et al., 1996) or terrestrial input (Lewis et al., 2018), among other parameters. Given the significant knowledge that can be extracted from the trace element composition of CaCO₃ minerals, a large number of studies has been devoted to understanding the parameters affecting trace element distribution in calcite and, to a lesser extent, aragonite, since the '80s (e.g. Lorens, 1981; Mucci, 1986, 1987; Zhong and Mucci, 1989; Burton and Walter, 1991; Tesoriero and Pankow, 1996; Dietzel et al., 2004; Lee and Reeder, 2006; Lakshtanov and Stipp, 2007; Balan et al., 2016; Mavromatis et al., 2022). In the past two decades, owing to the technological ability to precisely measure the isotopic compositions of trace elements in natural samples, experimental studies have also focused on unravelling the role of physicochemical parameters on the isotope distribution between solid and fluid (e.g. Tang et al., 2008, 2012; Immenhauser et al., 2010; Horner et al., 2011; Böhm et al., 2012; Balan et al., 2018; Brazier et al., 2024). The interpretation of elemental distribution between CaCO₃ mineral and aqueous fluid for divalent cations (Me²⁺) usually relies on an ideal replacement of one Ca²⁺ ion in the solid with a Me²⁺ trace, leading to the formation of a solid-solution. This approach is supported by synchrotron-based observations, which suggest that the presence of Me²⁺ in calcite or aragonite yields a coordination of Me²⁺ to 6 or 9 oxygen atoms, respectively, denoting ideal replacement for Ca²⁺ ion (e.g. Reeder et al., 1999; Allison et al., 2005; Finch and Allison, 2007; Finch et al., 2010).

Natural carbonates however, also contain monovalent ions such as sodium or lithium. Specifically, the concentration and isotopic composition of lithium in natural carbonate
samples have been seen as an emerging paleo-proxy for a number of environmental parameters. For example, changes in the isotopic composition of Li in carbonate rocks have been used to assess paleo-weathering and hydrothermal processes, and to provide a direct insight into the regulation of the global carbon cycle and Earth's climate over geological time (Vigier et al., 2009; Misra and Froelich, 2012; Vigier and Goddéris, 2015; Chen et al., 2023). Additionally, because Li does not directly participate in any biological processes, it has been used to infer physicochemical parameters such as dissolved inorganic carbon concentration, pH, degree of seawater saturation with respect to calcite, temperature, and growth rate (e.g. Hall and Chan, 2004; Marriott et al., 2004; Millot et al., 2004; Bryan and Marchitto, 2008; Lear et al., 2010; Lear et al., 2010; Vigier and Goddéris, 2015; Branson et al., 2024).

However, the mechanisms controlling the incorporation of the monovalent Li⁺ in calcite and aragonite lattice are still elusive, which is a serious obstacle to the use of Li as a proxy tool. Previous experimental studies involving the co-precipitation of Li with CaCO₃, have shown that lithium incorporation in these minerals is influenced by various factors including temperature, fluid composition, growth rate and crystal structure (Marriott et al., 2004; Füger et al., 2019; Seyedali et al., 2021; Brazier et al., 2024). Notably, Füger et al. (2019) argued that incorporation of Li in calcite cannot be associated to only one coordination environment in the solid, due to isotopic variations under different pH conditions. These results have been further confirmed by Branson et al. (2024) who reported a correlation between the Li partitioning coefficient and the solution pH. However, Branson et al. (2024) did not identify any parameter controlling Li isotope fractionation. In this context, an atomic-scale understanding of Li speciation in carbonates could bring important information about its incorporation mechanism.

In this study, the local environment of ⁷Li in two synthetic samples of Li-bearing calcite and aragonite was probed by high-resolution solid-state ⁷Li MAS NMR spectroscopy.
Solid-state NMR is a chemically selective technique that can provide detailed information on the local chemical environment of elements, even to trace amount in crystal (Mavromatis et al., 2015; Balan et al., 2016). NMR analyses was performed at a high magnetic field (17.6 T), significantly increasing the sensibility and spectral resolution. In parallel, the structure and 7Li NMR spectroscopic properties of models of Li-bearing calcite and aragonite were analysed using density functional theory. These models correspond to various schemes of Li incorporation at the Ca structural site or at interstitial sites of the crystal structures. The comparison of the theoretical results and NMR analytical data attests to the diversity of 7Li environments in the investigated calcite and aragonite samples.

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## 2. Materials and methods

### 2.1. Samples

Two synthetic samples of Li-bearing aragonite and calcite containing 18 and 550 ppm of Li, respectively, were used for 7Li NMR analyses. The aragonite sample LiAr-18 that used in this study derived from the earlier work by Mavromatis et al. (2022) and additionally contains ~100 ppm Mg. Briefly this sample was produced by co-precipitation of Li with aragonite on Li free aragonite seeds. For details, we refer the reader to Mavromatis et al., (2022). Note here that in the analysed sample Li is incorporated solely in aragonite as mentioned by Mavromatis et al. (2022) which is the only mineral precipitating in this experiment as denoted by the lower abundance of calcite in the overgrowth compared to the seed as quantified by XRD spectra and Rietveld refinement. The calcite sample was synthesized by mixing equal volumes (i.e. 50mL) of a Li₂CO₃ and a CaCl₂ solution. The concentrations of Ca²⁺ and CO₃²⁻ after mixing were 10 and 12 mM, respectively. The mixture, incubated at 40°C for 7 days under continuous stirring. The precipitated solid was separated
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from the fluid by suction filtration. The final solution had a pH of 6.5 and the concentration of Ca and alkalinity were 8.0 and 10.0 mM, respectively.

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## 2.2. Solid characterization

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### 2.2.1. Powder X-ray diffraction

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Powder X-ray diffraction (PXRD) patterns (Fig. S1) were recorded between 20° and 90° (2θ) with a 0.02° step size using a Bruker D8 Advance laboratory diffractometer (Cu Kα₁,₂ radiation) equipped with a LynxEye XE detector in Bragg–Brentano geometry.

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### 2.2.2 $^{7}$Li MAS NMR analyses

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Solid state $^{7}$Li NMR spectra were obtained using a Bruker AVANCE NEO 750 (17.6 T) NMR spectrometer operating at 291.5 MHz at spinning frequencies ranging from 5 to 14 kHz, using a 4 mm Bruker MAS probe. A $^{7}$Li radiofrequency field strength of 50 kHz was used and a small flip angle ($\pi/6$) was utilized with recycle delays of 30 s to ensure complete magnetisation for the $^{7}$Li MAS NMR analyses. The rotors were filled up in a glove-box under dried Ar atmosphere and approximately 2000 scans were accumulated during each acquisition. NMR MAS measurements were performed at -28, +22 and +65 °C. Temperature was controlled and measured using thermocouples inserted in the NMR probe and the temperature calibration was performed through the $^{79}$Br shift of KBr (Thurber and Tycko, 2009). The $^{7}$Li chemical shifts were referenced to a 1M LiCl solution.

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## 2.3 Theoretical modelling
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In the past thirty years, theoretical methods based on the density functional theory (DFT) have reached a level of accuracy high enough to predict the properties of geochemically relevant systems (e.g., Cygan and Kubicki, 2001). In the present study, the theoretical properties of models of Li-bearing calcium carbonates were investigated at the DFT level, following an approach similar to that previously used to investigate the boron and sulphate environment in calcite and aragonite (Balan et al. 2014; 2016). The generalized gradient approximation (GGA) to the exchange-correlation functional as proposed by Perdew, Burke, and Ernzerhof (PBE) (Perdew et al. 1996). was used and calculations were performed with the QUANTUM ESPRESSO package (Giannozzi et al., 2009). The ionic cores were described using norm-conserving pseudopotentials from the Optimized Norm-Conserving Vanderbilt (ONCV) library (Hamann, 2013). Electronic wave-functions and charge density were expanded with plane-wave basis sets with cutoffs 80 and 480 Ry, respectively, corresponding to a total energy convergence better than 1 mRy/atom.

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Periodic models of Li-bearing calcium carbonates were built for the two common CaCO₃ polymorphs: calcite (space group *R*3c, Z=2) and aragonite (Pmcm, Z=4). The models were built by inserting one or two Li atoms in 2×2×2 supercells of calcite (rhombohedral cell, 80 atoms) or aragonite (160 atoms). The sampling of the reciprocal space for the electronic integration was restricted to the centre of the Brillouin zone (Γ point). Cell parameters of the pure phases (a = 12.83 Å, α = 46.28° for the rhombohedral supercell of calcite; a = 10.03 Å, b = 16.02 Å, c = 11.63 Å for the orthorhombic supercell of aragonite) were optimized at zero pressure and were used without further relaxation to produce the Li-bearing supercells. The relaxation of atomic internal coordinates was performed until the residual forces were <10⁻⁴ Ry per atomic unit. The structure files (.cif) of the models are provided as Supplementary Information.

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The NMR properties of ⁷Li (nuclear spin I = 3/2) involve both the Zeeman effect and the interaction of the nuclear quadrupolar moment with the electric field gradient (EFG) at the nucleus. Accordingly, the ⁷Li NMR spectra are characterized by three parameters: the chemical shift δᵢₐ, the quadrupolar coupling constant C_Q and the asymmetry parameter η.

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The NMR chemical shift δ<sub>iso</sub> is defined as δ<sub>iso</sub> = - (σ - σ<sup>ref</sup>), where σ is the isotropic shielding (one-third of the trace of the NMR shielding tensor) and σ<sup>ref</sup> is the isotropic shielding of the same nucleus in a reference system. It can be obtained from the structural models by calculating the shielding of the nuclei relative to the electronic current induced by the external magnetic field, using the Gauge Including Projector Augmented Waves (GIPAW) approach, which allows reconstructing the all-electron magnetic response from the pseudo-wave-functions (Pickard and Mauri, 2001; Charpentier, 2011; Bonhomme et al., 2012). For this calculation, GIPAW pseudopotentials (Giannozzi et al., 2009) were used with cutoffs of 80 and 480 Ry for the wave-functions and charge density, respectively. The conversion of absolute shielding tensors in isotropic chemical shifts was done using a calibration curve relating the ⁷Li chemical shift δ<sub>iso</sub> and the calculated magnetic shielding σ<sub>iso</sub> for a series of Li-bearing compounds (Fig 1; Tables S1 and S2). The corresponding best linear fit is expressed as δ<sub>iso</sub> = -0.7532σ<sub>iso</sub> + 67.19 with a regression factor 0.9784. Crystallographic data and isotropic chemical shifts δ<sub>iso</sub> used in these benchmarking calculations were taken from Köcher et al. (2018). On average, the difference between measured and calculated ⁷Li chemical shifts is Δδ<sub>iso</sub> = ±0.3 ppm. The C_Q and η parameters are related to the eigenvalues (V<sub>zz</sub>, V<sub>xx</sub> and V<sub>yy</sub>) of the electric field gradient. V<sub>zz</sub> has the largest magnitude, while V<sub>xx</sub> and V<sub>yy</sub> correspond to the other two eigenvalues, ordered such that |V<sub>zz</sub>|≥|V<sub>yy</sub>|≥|V<sub>xx</sub>|. The asymmetry parameter η<sub>Q</sub>=(V<sub>xx</sub>-V<sub>yy</sub>)/V<sub>zz</sub> describes the deviation of the EFG from axial symmetry, tensor and reflect the symmetry of the Li site. These quantities were calculated for all the models.

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3. Results
## 3.1. <sup>7</sup>Li MAS NMR of calcite and aragonite

The <sup>7</sup>Li MAS NMR spectra of calcite and aragonite samples obtained at room temperature (Fig. 2) included the central transition as well as the complete set of spinning sidebands corresponding to the < ±1/2, ±3/2> satellites. The spectra of the two samples were recorded at various spinning speeds but only the most informative spectra, recorded at 5 kHz, are reported here and analysed. These spectra were chosen due to the full presence of spinning sidebands, which is a prerequisite for determining the quadrupolar coupling constant C<sub>Q</sub>. The central line in calcite is dominated by an asymmetric contribution with a maximum at 0.20 ppm and several shoulders at 0.87, -0.33 and -1.13 ppm. A weaker but well-defined peak is observed at -1.87 ppm. In aragonite, the central line corresponds to a single peak at -0.50 ppm with a slight asymmetry on the negative chemical shifts side. The asymmetric and structured shape of these spectra reflects the presence of several overlapping contributions, attesting to the occurrence of different lithium environments in the samples. At this stage, an unambiguous decomposition of the spectrum into individual contributions cannot be achieved solely based on the room temperature (22 °C) spectral data.

To better identify individual contributions present in the NMR spectra, the <sup>7</sup>Li NMR spectra of calcite and aragonite samples were recorded at different temperatures (65, 22, -28 °C) (Fig. 3). In both polymorphs, the position of the main line shifts by approximately +0.52 ppm from -28 °C to 65 °C. In calcite, the narrow signal at -1.87 ppm also displays a notable shift with temperature. In addition, a significant broadening of the main line is observed in the spectra recorded at -28 °C. While a detailed analysis of the evolution of the full width at half maximum (FWHM) of the individual peaks is not possible, the observed changes suggests that a broader static distribution of sites occurs at lower temperature while the lines are narrowed at higher temperature (Massiot, 1990; Wagemaker et al., 2001).

## 3.2. Analysis of NMR spectra
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A deconvolution of the collected spectra (Fig. 4), including all spinning sideband
patterns, was performed using the “disperse” model integrated in Dmfit software (Massiot et
al., 2002). This model enables the determination of the ⁷Li chemical shift δᵢₘ�_o and quadrupolar
coupling constant (Cq), allowing for a quantitative analysis of the proportion of different
contributions. It assumes a Gaussian distribution of the V<sub>zz</sub> component in the electric field
gradient (EFG) tensor, leading to a distribution of Cq. However, V<sub>xx</sub> and V<sub>yy</sub> are held
constant, which prevents the determination of the asymmetry parameter (ηq).

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The fitting of spectra at different temperature was carried out using the same Cq
values and the same proportion of lithium sites, while the chemical shift ⁷Li and FWHM of
each line were allowed to vary.

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The best fit accounting for both the central and the satellite transitions required seven
different ⁷Li structural environments for the calcite samples (Fig. 4). For the aragonite sample,
only two contributions were required (Fig. 4). The different ⁷Li contributions identified in
calcite are labelled as Lix (x=1,...,7) and cover a chemical shift range of ~2.74 ppm. A strong
overlap is however observed between the Li1, Li2 and Li3 contributions (~76% of the
spectrum area) and the Li4 and Li5 contributions (~17% of the spectrum area). The chemical
shifts of all these contributions exhibit a similar temperature dependence (Fig. 5).

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For the aragonite sample, only two contributions were required to fit the spectra at
22°C and 65 °C (Fig. 4). In the spectrum at 22 °C, the Li1 contribution is dominant and
accounts for 93% of the total area. It was necessary to add a third contribution (Li3) to
account for the broadening of the spectrum at -28 °C compared to the spectra recorded at
higher temperatures (Fig. 4). The NMR parameters determined at different temperatures are
reported in Tables 1 and S3. The ⁷Li chemical shifts in the aragonite sample cover a smaller
range (~0.45 ppm) than in the calcite sample. The Li1 contribution in aragonite is more
sensitive to the temperature change than the Li2 contribution (Fig. 5)

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In the calcite and aragonite samples, the average value of the ⁷Li quadrupolar coupling constants (CQ) is 0.06 MHz, while the CQ values determined for the various sites do not display significant variations with respect to the corresponding uncertainty range (±0.05 MHz) (Table 1).

**Table 1.** ⁷Li isotropic chemical shifts (δ<sub>iso</sub>), quadrupolar coupling constants (CQ), FWHM and Integral intensities (I) obtained from the simulation of the ⁷Li MAS NMR spectra calcite and aragonite obtained at 22 °C.

<table><thead><tr><th>Sample</th><th>Lithium<br>site</th><th>δ<sub>iso</sub>, ppm<br>(±0.02 ppm)</th><th>C<sub>Q</sub>, MHz<br>(±0.05 MHz)</th><th>FWHM (Hz)<br>(±10 Hz)</th><th>I, % (±1%)</th></tr></thead><tbody><tr><td rowspan="7">Calcite</td><td>Li1</td><td>0.87</td><td>0.08</td><td>120</td><td>8</td></tr><tr><td>Li2</td><td>0.33</td><td>0.06</td><td>230</td><td>53</td></tr><tr><td>Li3</td><td>0.17</td><td>0.01</td><td>130</td><td>15</td></tr><tr><td>Li4</td><td>-0.33</td><td>0.09</td><td>175</td><td>12</td></tr><tr><td>Li5</td><td>-0.68</td><td>0.06</td><td>170</td><td>5</td></tr><tr><td>Li6</td><td>-1.13</td><td>0.09</td><td>140</td><td>2</td></tr><tr><td>Li7</td><td>-1.87</td><td>0.04</td><td>100</td><td>5</td></tr><tr><td rowspan="2">Aragonite</td><td>Li1</td><td>-0.50</td><td>0.05</td><td>150</td><td>93</td></tr><tr><td>Li2</td><td>-0.95</td><td>0.10</td><td>200</td><td>7</td></tr></tbody></table>

### 3.3 Theoretical models of Li sites in calcite and aragonite

The NMR properties of selected models of Li bonding environments in calcite and aragonite have been considered to support the interpretation of experimental spectra (Fig. 6). The .cif files of these models are provided as supplementary information.
The model “Li<sub>sub</sub>” was obtained by substituting one Li<sup>+</sup> for one Ca<sup>2+</sup> ion in the supercells, using a homogeneous electrostatic background to ensure the electrostatic neutrality of the system. Such model is expected to mimic a relatively remote charge compensation, in which the macroscopic neutrality of the crystal is ensured by impurities or defects which are located sufficiently far apart from the substituted site to leave its local structure unaffected. In calcite, the coordination of Li is similar to that of Ca and corresponds to a weakly distorted octahedron with Li-O bond lengths ranging between 2.32 and 2.35 Å. In aragonite, the nine-fold coordination of Ca corresponds to a more complex and elongated geometry with varying Ca-O bond lengths and angles (Finch and Allison, 2007). The corresponding atomic-scale environment of substitutional Li in aragonite is significantly distorted with Li-O bond lengths ranging between 2.0 and 3.3 Å.

The model “Li<sub>int</sub>” was obtained by setting the Li atom at the most probable interstitial site position. In calcite, this interstitial site lies between carbonates groups CO<sub>3</sub><sup>2-</sup> groups and the CaO<sub>6</sub> coordination polyhedra along the c-axis, with a radius of approximately 0.86 Å (Ishikawa and Ichikuni, 1984). This site has been identified as a potential incorporation site for monovalent cations such as Na<sup>+</sup> and K<sup>+</sup> (Ishikawa and Ichikuni, 1984). The Li is in 6-fold coordination with Li-O bond lengths ranging between 1.92 and 2.42 Å. In the denser structure of aragonite, the interstitial environment of Li corresponds to a four-fold coordination with Li-O bond lengths ranging between 1.8 and 2.0 Å.

The “Li<sub>sub</sub>+Li<sub>int</sub>” model combines the two previous configurations, leading to an electrostatically neutral configuration. In this case, it was assumed that electrostatic interactions favour the configuration corresponding to the shortest distance between Li<sub>sub</sub> and Li<sub>int</sub>. In calcite, the two Li ions display an almost regular triangular coordination with a Li-O bond length of 1.877 Å, forming nearly parallel and off-centered LiO<sub>3</sub> triangles. In aragonite, the two Li atoms occupy the opposite faces of the vacant 9-fold Ca site. They display different
atomic-scale environments, with 4-fold (Li-O bond lengths between 1.9 and 2.15 Å) and 5-fold (Li-O bond length between 1.89 and 2.12 Å) coordinations.

Finally, two “Li<sub>sub</sub>+H” models have been considered. They correspond to a Li<sup>+</sup> for Ca<sup>2+</sup> substitution with an electrostatic charge compensation ensured by a proton bonded to a neighbouring CO<sub>3</sub> group. In this case, a position closer to the charge-deficient Li site was selected. In both polymorphs the OH group points out of the CO<sub>3</sub> triangle, sharing a moderate-strength H-bond with an oxygen atom belonging to an adjacent carbonate group. In calcite, the Li atom is in 3-fold coordination and located on the opposite face of the octahedral site, while it occurs in a distorted 4-fold coordination in aragonite (Fig. 6).

<table><caption>**Table 2.** Theoretical <sup>7</sup>Li isotropic chemical shifts (δ<sub>iso</sub>), quadrupolar coupling constants (C<sub>Q</sub>), and shortest Li-O bond length, predicted by the density functional theory calculations for the different Li-bearing calcite and aragonite models.</caption><thead><tr><th>Mineral</th><th>Model</th><th>Li site</th><th>shortest Li-O bond<br>length (Å)</th><th>δ<sub>iso</sub><br>(ppm)</th><th>C<sub>Q</sub><br>(MHz)</th></tr></thead><tbody><tr><td rowspan="5">Calcite</td><td>Li<sub>sub</sub></td><td>Li<sub>sub</sub></td><td>2.328</td><td>-2.28</td><td>0.06</td></tr><tr><td>Li<sub>int</sub></td><td>Li<sub>int</sub></td><td>1.923</td><td>0.00</td><td>0.32</td></tr><tr><td>Li<sub>sub</sub>+H</td><td>Li<sub>sub</sub>+H</td><td>1.919</td><td>-0.18</td><td>0.35</td></tr><tr><td>Li<sub>sub</sub>+Li<sub>int</sub></td><td>Li<sub>sub</sub></td><td>1.877</td><td>0.86</td><td>0.56</td></tr><tr><td>Li<sub>int</sub></td><td>Li<sub>int</sub></td><td>1.877</td><td>0.86</td><td>0.56</td></tr><tr><td rowspan="4">Aragonite</td><td>Li<sub>sub</sub></td><td>Li<sub>sub</sub></td><td>2.019</td><td>-1.38</td><td>0.22</td></tr><tr><td>Li<sub>int</sub></td><td>Li<sub>int</sub></td><td>1.797</td><td>0.14</td><td>0.18</td></tr><tr><td>Li<sub>sub</sub>+H</td><td>Li<sub>sub</sub>+H</td><td>1.974</td><td>-1.04</td><td>0.22</td></tr><tr><td>Li<sub>sub</sub>+Li<sub>int</sub></td><td>Li<sub>sub</sub></td><td>1.917</td><td>-0.61</td><td>0.22</td></tr></tbody></table>
<table><tr><td>Li<sub>int</sub></td><td>1.887</td><td>-0.40</td><td>0.46</td></tr></table>

Significant variations (~3.14 ppm) are observed in the theoretical ⁷Li chemical shift across the different Li-bearing calcite models (Table 2). These variations are enough to be observed. In particular, the Li<sub>sub</sub> substitutional model leads to a theoretical chemical shift at -2 ppm while the other models lead to theoretical chemical shifts ranging between -0.2 and 0.9 ppm. For aragonite, the range of theoretical ⁷Li chemical shifts is about half as large as that obtained for calcite (Table 2), which could make distinguishing signals in the acquired NMR spectrum of aragonite more challenging. However, it is noteworthy that the simple substitutional model leads to the more negative value, as in calcite. The range of calculated quadrupolar coupling constants (CQ) observed for the different types of lithium environments does not exceed 0.6 MHz (Table 2), indicating weakly distorted Li environments. The significant discrepancy between the fitted (Table 1) and calculated (Table 2) CQ parameters may stem from dynamic effects (Dunstan et al., 2015) that are not accounted for in the static GIPAW calculations.

Although it is difficult to precisely determine the leading structural parameter controlling the ⁷Li chemical shift, it can be noticed that the theoretical ⁷Li chemical shifts in calcite and aragonite (Fig. 7) define two almost parallel trends when plotted as a function of the shortest Li-O bond length of the Li site. The relation with the other Li-O bond lengths (Table S3), or with the average Li-O bond length of the corresponding sites, is poorer. In calcite, a difference in Li-O bond length of 0.451 Å results in a difference larger than 2 ppm in the ⁷Li chemical shift. The “Li<sub>sub</sub>+Li<sub>int</sub>” model displays similar values for the shortest Li-O bond length of the two site, leading to a similar ⁷Li chemical shift value for both lithium atoms. In the “Li<sub>sub</sub>+H” model, the presence of hydrogen modifies the local environment of lithium, leading to a decrease in the shortest Li-O bond length compared to the more
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symmetric substitutional “Li<sub>Ca</sub>” model. Compared with the Li<sub>sub</sub> (-2.28 ppm) this structural
change induces a significant change in <sup>7</sup>Li chemical shift which is close to 0 ppm.

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Regarding aragonite, the shortest Li-O bonds are found for the interstitial Li model
(Table 2). Protonation of CO<sub>3</sub><sup>2-</sup> group close to the substitutional lithium has a similar effect to
that observed in calcite, i.e. a shortening of Li-O bond lengths and an increase of the Li
chemical shift toward positive values. In the “Li<sub>sub</sub>+Li<sub>int</sub>” model, the shortest Li-O bond
length of the interstitial lithium is shorter than that observed on the substitutional Li in the
same model. This difference induces a slight difference in the <sup>7</sup>Li chemical shift of the two
atoms; while the equivalent model in calcite provided an identical value for the two atoms in
triangular coordination (Table 2).

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317

318

319

320

321

322

323

4. Discussion

324

4.1 Li environment in calcite

325

The ranges of theoretical <sup>7</sup>Li chemicals shifts are well consistent with those observed
on the calcite sample (Fig. 8; Table 2).

326

327

In calcite, the theoretical <sup>7</sup>Li NMR chemical shift of the “Li<sub>sub</sub>” model determined at -2.28 ppm occurs within 0.5 ppm of the acquired value determined for the well-defined narrow peak observed on the more negative side of the spectrum (Li7 contribution, -1.87; Table 1). Accordingly, the Li7 contribution most likely correspond to Li atoms substituted at the symmetric 6-fold Ca sites (“Li<sub>sub</sub>” model). In this case, the charge compensation should be ensured by a relatively more remote mechanism than in the other models. For instance, macroscopic electroneutrality of the crystal can be given by impurities or defects located sufficiently far apart from the substituted site. Based on this assignment and considering the relative variations in the theoretical chemical shifts of the different models, the Li4 (-0.33

328

329

330

331

332

333

334

335
336 ppm) and Li5 (-0.68 ppm) contributions could correspond to substitutional Li atoms locally
337 compensated by the presence of protons in their local environment. The "Li<sub>sub</sub>+H" model
338 leads to a slightly negative calculated shift of -0.18 ppm which is in an acceptable range
339 compared to the Li4 and Li5 values. In this case, the charge compensation is ensured by the
340 presence a nearby HCO<sub>3</sub><sup>-</sup> group. Such a local mechanism induces a stronger site distortion and
341 potentially multiple configurations depending on the location and orientation of the OH
group. The potential occurrence of several configurations is consistent with the observation of
342 several overlapping contributions appearing on the low chemical shift side of the main band.
343 The interstitial Li model "Li<sub>int</sub>" yields a theoretical shift of 0 ppm, consistent with the
344 acquired Li2 (0.33 ppm) and Li3 (0.17 ppm) signals which correspond to the main band,
345 which could suggest that the incorporation mechanism of Li in calcite predominantly involves
346 interstitial Li ions. However, it cannot be excluded that these signals also correspond to
347 Li<sub>sub</sub>+H type environments, considering the proximity of the theoretical chemical shifts
348 determined for the Li<sub>sub</sub>+H and Li<sub>int</sub> models. Finally, the "Li<sub>sub</sub>+Li<sub>int</sub>" model, displaying both
349 substitutional and interstitial lithium, corresponds to a theoretical <sup>7</sup>Li chemical shift of 0.86
350 ppm for both lithium sites. In this case, the interstitial Li<sup>+</sup> cation ensures the local charge
351 balance of the neighbouring Li<sup>+</sup> for Ca<sup>2+</sup> substitution. This specific Li environment could
352 correspond to the shoulder clearly observed on the high chemical shift side of the main
353 resonance (Li1 contribution, 0.87 ppm). This suggests that even at low Li concentrations,
354 electrostatic constraints can favour a coupled incorporation of interstitial and substitutional Li
355 in calcite.
356

357

358

## 4.2 Li environment in aragonite

359

360

In aragonite, the measured values range between -0.50 ppm for the dominant Li1
contribution and -0.95 ppm for the Li2 contribution at room temperature (Table 1).
Consistently, the range covered by the theoretical chemical shifts is smaller than in calcite, from -1.38 ppm to 0.14 ppm for substitutional and interstitial lithium, respectively. By comparison with the assignments proposed on calcite, the minor signal (Li2) could correspond to substitutional lithium, either with remote charge compensation ("Li<sub>sub</sub>" model, -1.38 ppm) or more local charge compensation by H atoms ("Li<sub>sub</sub>+H" model, -1.04 ppm). Due to the proximity of the theoretical values and strong overlap of the bands, it is not possible to assign the observed signals more precisely from the present data. The dominant contribution at -0.50 ppm actually corresponds to two components of similar intensity, which are resolved in the spectrum recorded at -28 °C and differ by 0.27 ppm (Table S3). These properties are consistent with those of the coupled incorporation "Li<sub>sub</sub>+Li<sub>int</sub>" model, which displays two Li sites with slightly different theoretical chemical shifts at -0.61 and -0.40 ppm (Table 2). A quantification based on the spectra at -28 °C (Table S3) suggests that, in the investigated aragonite sample, about half of the lithium occupies a substitutional position and half is located at the interstitial site, acting as charge compensator of the substitutional Li.

375

376

## 4.3 Implications for the Li incorporation in calcium carbonates

The obtained results offer insights into Li environment in carbonate group minerals. This is of significant importance because Li and its isotopes in carbonates have been proposed as emerging paleoenvironmental tools (e.g. Hall et al., 2004; Pogge von Strandmann et al., 2019; 2020). Understanding however how physicochemical parameters affect the concentration and isotope composition of Li in CaCO₃ minerals requires characterization of bonding environment. This is because coordination and metal-oxygen (Me-O) bond length of trace metal in the solid, can assist interpretations on trace metal concentration and isotope fractionation. For example, the incorporation of Ba in aragonite is hindered due to its large size compare to Ca in 9-fold coordination. However, the distribution coefficient of Ba in

378

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380

381

382

383

384

385
386 aragonite is significantly larger compared to that in calcite, because Ba²⁺ (aq) has to reduce its
387 coordination number to 6 (Dietzel et al., 2004; Mavromatis et al., 2018). Moreover,
388 information of Me-O difference between a mineral and a fluid can be used to assess
389 enrichment or depletion of an isotope under thermodynamics. For example, the shorter/stiffer
390 Ba-O bond length in aragonite compared to Ba²⁺ (aq) suggest enrichment of the heavier
391 isotope in the solid (e.g. Schauble, 2004; Blanchard et al., 2017). These predictions have been
392 recently confirmed in synthetic samples and modelled using first-principle calculations
393 (Mavromatis et al., 2020). However, the variations of Li environment probed by NMR at
394 different temperatures suggest a significantly enharmonic behaviour confirming that the
395 relation of the Li bonding to its isotopic fractionation properties could be less straightforward
396 than for other chemical elements (Dupuis et al. 2017).

397 The challenge of characterizing Li environments in CaCO₃ minerals is not easy to
398 resolve because the spectroscopic techniques available are limited. Here for the first time we
399 show that ⁷Li NMR spectroscopy can be used to assess environments of traces of Li in calcite
400 and aragonite. Note that earlier NMR spectroscopy has been successfully used to explore the
401 presence of B at low concentration in calcite and aragonite (e.g. Noireaux et al., 2015;
402 Mavromatis et al., 2021). Despite the limited number of investigated samples, the present
403 results offer a direct insight into Li environment in carbonate group minerals. The occurrence
404 of interstitial and substitutional Li in both calcite and aragonite samples confirms that
405 interstitial sites can play a key role in the incorporation of alkali ions in calcite (Ishikawa and
406 Ichikuni, 1984; Okumura and Kitano, 1986). The presence of several Li environments in a
407 single calcite sample is also consistent with the previous proposal that incorporation of Li in
408 calcite is likely associated to more than one coordination environments (Füger et al., 2022). In
409 contrast, the incorporation of Li in the investigated aragonite sample is dominated by a
410 coupled mechanism where Li occupies neighbouring substitutional and interstitial sites,
corresponding to incorporation as Li₂CO₃. We acknowledge here that analyses of only one sample for calcite and one for aragonite cannot provide concrete arguments for the differences observed in Li concentration and isotope fractionation among synthetic samples of different studies (e.g. Füger et al., 2022 vs. Branson et al., 2024). Yet additional NMR measurements together with the possible Li-O bond lengths presented in Table 2 can act as the basis for the interpretations of Li isotope compositions in these samples in the future.

Overall the differences between aragonite and calcite, as well as the relatively complex Li environment observed in each sample, underline the need of systematic analyses a series of samples, aiming at identifying specific relations between the proportion of Li environments and specific parameters such as the degree of fluid saturation and respective growth rate, solution pH and mineral growth mechanisms. However, it has to be emphasized that the incorporation of Li in natural carbonates could differ from those corresponding to inorganic precipitation experiments. In Earth's aquatic environments CaCO₃ minerals may also form via an amorphous precursor (Gilbert et al., 2022) a mineral formation pathway that can result in trace element composition different from that expected during classical growth (e.g. Loste et al., 2003; Littlewood et al., 2017; Purgstaller et al., 2017). As such, it should be considered that Li bonding environments may be different in crystalline solids formed via an amorphous precursor as for example it has been shown for Sr in calcite formed via ACC compared to typical overgrowth by Littlewood et al. (2017). Moreover, it has to be pointed out that recent works examining the role of mineral growth from solution enriched in SO₄²⁻ (aq) on uptake of traces such as Mg or B have shown significant variations in distribution coefficients (e.g. Goetschl et al., 2019; 2021; Uchikawa et al., 2023) associated with presence of sulphate in the solid, but also in Mg isotope fractionation (e.g. Mavromatis et al., 2025). Considering that Li occupies both substitutional positions and interstitial sites in both calcite and aragonite, as shown above, presence of sulphate may as well produce additional Li bonding environments.
These observations suggest that the investigation of natural samples, and potential identification of specific ⁷Li NMR signatures, could bring new information about the physicochemical conditions which prevailed during calcite or aragonite formation.

## 5. Conclusions

In this study we have for the first time shown that ⁷Li MAS NMR spectroscopy can differentiate Li bonding environments. Here we couple NMR analyses of two synthetic samples with DFT calculations and show that Li in CaCO₃ minerals can occupy both substitutional and interstitial sites. For aragonite NMR spectra show only two sites to occur, one substitutional and one interstitial. In contrast in calcite 7 different sites have been identified in which Li resides obtaining charge compensation with H⁺, OH⁻ and HCO₃⁻ groups. Overall, this study provides the first characterization of Li bonding environment in the two most abundant CaCO₃ polymorphs and sets the basis for additional work with NMR spectroscopy as a tool for the interpretation of Li chemical and isotopic composition in natural CaCO₃ minerals.

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463

464 Acknowledgements:

465 This work was performed using HPC resources from the SACADO MeSU platform at
466 Sorbonne Université. This work was funded by the French National Research Agency (ANR-
467 22-CE01-0001-04 MeLiCa). Financial support from the IR INFRANALYTICS FR2054 for
468 conducting the research is gratefully acknowledged. We thank the three anonymous reviewers
469 who provided insightful comments on this manuscript.

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