# Hydrothermal smectite synthesis in presence of Glycine: Synergistic chemical evolution

Rihab Fkiri, Fabien Baron, Clément Comminges, Sabine Petit, Claude

Geffroy, Brian Grégoire

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Rihab Fkiri, Fabien Baron, Clément Comminges, Sabine Petit, Claude Geffroy, et al. Hydrothermal smectite synthesis in presence of Glycine: Synergistic chemical evolution. Applied Clay Science, 2025, 276, pp.107902. 10.1016/j.clay.2025.107902. hal-05370979

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Research Paper

# Hydrothermal smectite synthesis in presence of Glycine: Synergistic chemical evolution

Rihab Fkiri*, Fabien Baron, Clément Comminges, Sabine Petit, Claude Geffroy, Brian Gregoire

Université de Poitiers, CNRS, Institut de Chimie des Milieux et Matériaux de Poitiers-IC2MP, Poitiers, France

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

- Clay mineral hydrothermal synthesis

- Organo-mineral interaction

- Organic-induced structural modification

- Glycine polymerization

## ABSTRACT

The role of smectite surfaces in organic matter dynamics is well documented, with clear evidence of their capacity to concentrate, preserve, and chemically modify organic compounds. However, the reciprocal influence—how organic molecules might guide clay mineral formation—remains a relatively unexplored scientific frontier. The present study investigates the dual role of glycine in Fe-smectite crystallization under hydrothermal conditions (150 °C for 5 days in an inert atmosphere), exploring how this ubiquitous amino acid influences mineral formation processes. X-ray diffraction confirmed the crystallization of Fe-smectite across glycine concentrations ranging from 0.08 to 0.6 M. The increase in glycine concentration induced a decrease in tetrahedral Fe(III) content and promoted the reduction of octahedral Fe(III) to Fe(II), as revealed by X-ray diffraction, Fourier transform infrared spectroscopy, and electrochemical analyses, consequently modifying the measured cation exchange capacity. Glycine polymerization was markedly enhanced in experiments with Fe-smectite crystallization compared to experiments without Fe-smectite. The presence of Fe-smectite favored linear peptides over cyclic one, thus facilitating the production of longer peptide chains. The results reveal a mutual interaction between glycine and minerals, resulting in minerals with distinct structural properties and reactivities that enhance organic matter transformation.

## 1. Introduction

The intricate relationship between living organisms, organic matter, and minerals makes subsurface environments such as soils and sediments both highly effective natural reactors and fragile systems, susceptible to biogeochemical disturbances. Even in abiotic environments, the association between organic matter and minerals plays a fundamental role in controlling soil and sediment porosity, cation exchange capacity (CEC), nutrient and pollutant mobility, or biogeochemical cycle of elements (Meier et al., 2001; Carvalho et al., 2012; Xu and Tsang, 2024).

Among minerals, clay minerals, particularly smectites, are of special interest due to their high surface reactivity and widespread occurrence in terrestrial subsurface environments. Smectites are 2:1 swelling clay minerals with a layered structure composed of an octahedral sheet sandwiched between two tetrahedral sheets. Depending on its cation occupancy, smectite is classified as dioctahedral (Al³⁺, Fe³⁺) or tri-octahedral (Mg²⁺, Fe²⁺), with isomorphic substitutions generating a net negative charge balanced by exchangeable interlayer cations (Na⁺,

Ca²⁺, etc.) (Mermut and Lagaly, 2001; Brigatti et al., 2006). This layer charge governs key properties such as cation exchange capacity, swelling ability, and specific surface area. Their colloidal size enhances interactions with organic compounds, enabling adsorption, protection, and even chemical transformation (Odom, 1997; Theng, 2018). Notably, Fe-smectites have been shown to be particularly relevant in subsurface environmental studies, where fluctuating redox conditions and limited oxygen create favorable conditions for their formation (Benzine et al., 2013; Pothanamkandathil et al., 2024). Fe-smectites exhibit distinct properties compared to the well-studied Al- or Mg-smectites including the ability to incorporate structural iron in mixed valence states in octahedral sites, which confers redox activity that is central to their environmental reactivity (Manceau et al., 2000a, 2000b; Pothanamkandathil et al., 2024). Fe-smectites serve as redox buffers in subsurface environments, modulating biogeochemical cycles of elements, nutrients, and pollutants (Neumann et al., 2011; Stucki, 2013; Fan et al., 2023). Laboratory experiments on Fe-smectite formation under subsurface conditions focus on biotic processes, abiotic process driven by redox conditions, and temperature or ionic composition influence

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: rihab.fkiri@univ-poitiers.fr (R. Fkiri).</footnote>
(Rozenson and Heller-Kallai, 1976; Komadel et al., 2000, 2006; Lee
et al., 2006; Stucki and Kostka, 2006; Ribeiro et al., 2009; Latta et al.,
2012; Pentráková et al., 2013; Zuo et al., 2021; Fan et al., 2023).
However, research conducted on natural environments suggests that
authigenic precipitation of clay minerals appears unrelated to microbial
respiration but seems to be promoted only by organic matter (Steiner
et al., 2022). However, the nature or properties of the organic matter or
of the involved functional groups actively influencing the formation of
mineral phases, is technically difficult to identify in natural systems
(Zhao et al., 2023). While the role of organic molecules (oxalate, succi-
nate or urea) in modulating the crystallization of Mg-rich smectites has
gained some attention, the influence of simple biomolecules, such as
amino acids, on the genesis of Fe-bearing smectites, arguably more
redox-active and environmentally reactive, is still poorly understood
(Vogels et al., 2005; Schumann et al., 2012; Zhou et al., 2017; Ponce and
Kloprogge, 2020; Kloprogge and Hartman, 2022). Glycine is an
adequate model of organic ligands owing to its chemical simplicity,
environmental relevance and well characterized interaction with clay
mineral surfaces. Glycine possesses both amine and carboxyl functional
groups, enabling it to act as a chelator that can influence metal specia-
tion, solubility and precipitation pathway. Moreover, glycine is a com-
mon component of dissolved organic matter in soils and sediment,
where it can reach sufficient concentrations to affect mineral formation
processes. Thus, glycine provides a robust model system to investigate
mineral – organic interactions under environmentally relevant condi-
tions, without the complexity introduced by larger, multifunctional
biomolecules (Fischer et al., 1992; Bujdák et al., 1996; Benincasa et al.,
2000; Kalra et al., 2003; Ramos et al., 2011; Ramos and Huertas, 2013;
Zhang et al., 2019; Zhu et al., 2019; Kloprogge and Hartman, 2022).

The objective of this study is to investigate the influence of glycine, a
representative, reactive amino acid, on Fe-smectite formation. Unlike
previous studies using clay minerals, our approach incorporates glycine
during the mineral formation process itself. This strategy enables us to
probe how organic ligands influence the structural properties of Fe-
smectites from the earliest stages of mineral genesis. Because the natu-
ral formation of these minerals occurs under low-temperature, long-
duration conditions that are difficult to reproduce experimentally, we
employed accelerated synthesis protocols (Baron et al., 2016; Petit et al.,
2017). Glycine concentrations were also increased relative to typical
environmental levels, allowing us to systematically monitor both min-
eral and organic evolution throughout the synthesis. This dual focus
provides a more integrated understanding of mineral-organic co-
development in subsurface environments, offering novel insights into
the mechanisms of biomolecule-assisted clay mineral formation.

**2. Experimental procedures**

Unless otherwise stated, all sample preparations and sample han-
dlings were performed exclusively in an anaerobic glove box (< 0.5 ppm
O₂) with an N₂ atmosphere (mBRAUN LABSTAR). Fe-smectites were
synthesized following the procedure previously described (Baron et al.,
2016; Petit et al., 2017). This procedure consists in forming a gel by co-
precipitation between a 0.2 M sodium metasilicate solution
(Na₂SiO₃·5H₂O) and a 0.2 M ferric chloride solution (Fe(III)Cl₃·6H₂O)
with a ratio of 4:2. The resulting gel was filtered (0.45 µm) under vac-
uum and then brought into contact with 0, 2.5, 10, and 20 mL of glycine
solution (1 M) in Teflon®-lined metal autoclaves (Parr®, type: 4744).
The final volume was then adjusted with Milli-Q® ultrapure water (18
MΩ.cm) to reach a total volume of 30 mL. The final concentration of
glycine in the reactor corresponded to 0 M (sample Fe-smectite-Gly 0
M), 0.08 M (Fe-smectite-Gly 0.08 M), 0.3 M (Fe-smectite-Gly 0.3 M),
and 0.6 M (Fe-smectite-Gly 0.6 M). The autoclaves were then closed in the
glovebox and then transferred into an oven for hydrothermal treatment
for 5 days at 150 °C, with a saturated vapor pressure of 5 bars in the
reactor. After the hydrothermal treatment, the final pH (pHₐ) and redox
potential (Eh) were measured (see SI), and the samples were filtered

under vacuum through 0.45 µm and then 0.1 µm filters to collect the
solid phase and thoroughly washed to remove excess salt. A portion of
the recovered solid was left to dry, while the remaining portion was
dispersed in Milli-Q® ultrapure water for storage. The solid phase color
evolved from brown color for Fe-smectite-Gly 0 M to a green color for
Fe-smectite-Gly 0.6 M. As references, organic blanks were prepared
under similar hydrothermal conditions (5 days at 150 °C) by adding the
same amount of glycine (i.e., 2.5, 10, and 20 mL of glycine solution (1
M)) to the reactor and adjusting the total volume to 30 mL with Milli-Q®
ultrapure water. These blanks named Gly-0.08 M, Gly-0.3 M and Gly-0.6
M, were used to evaluate the impact of synthesis conditions (T, P) on
glycine's reactivity.

Structural characterization of the synthesized minerals was per-
formed using X-ray diffraction (XRD), Fourier-transform infrared (FTIR)
spectroscopy, chemical analysis, and CEC measurements. The redox
state of structural iron was assessed through Mediated Electrochemical
Oxidation and Reduction (MEOR). Glycine transformation products
were analyzed using Ultra-High Performance Liquid Chromatography
coupled with High-Resolution Mass Spectrometry (UHPLC-MS) and Gas
chromatography-Mass Spectrometry (GC-MS). Detailed descriptions of
experimental procedures, analytical parameters, and data processing are
provided in the supporting information.

**3. Results**

**3.1. Mineralogy**

The XRD patterns of the smectite synthesized with glycine concen-
trations ranging from 0 to 0.6 M exhibited a prominent dome-like
feature, attributed to the sample holder used to maintain the same at-
mosphere as that inside the glovebox (Fig. 1). The powder XRD patterns
of the Fe-smectite-Gly 0 M synthetic samples exhibited reflections at
around 12.80 Å (001), 4.59 Å (02.11), 3.15 Å (004), 2.61 Å (13.20),
1.74 Å (15.24.31) and 1.532 Å (06.33) corresponding to a Fe-rich dio-
ctahedral smectite (nontronite) with tetrahedral [⁴]Fe (III) (Fig. 1a)
(Decarreau et al., 2008; Baron et al., 2016). For samples synthesized in
the presence of glycine, all XRD reflections were attributed exclusively
to clay mineral phases, indicating the absence of additional crystalline
phases. Transmission electron microscopy (TEM) analyses confirmed the
formation of Fe-smectite through the presence of crystalline phases
(Figs. S1 and S2, SI).

The *d*<sub>06-33</sub>-value decreased from 1.532 Å in Fe-smectite-Gly 0 M to 1.519 and 1.517 Å for samples Fe-smectite-Gly 0.08 M and Fe-smectite-Gly 0.3 M, respectively. These values remain consistent with a predominantly dioctahedral character of Fe-smectite although they suggest a decrease in tetrahedral Fe(III) (Petit et al., 2017). However, in the sample Fe-smectite-Gly 0.3 M, a new reflection appeared at 1.559 Å, indicating the formation of additional segregated trioctahedral domains. A similar feature was observed in the Fe-smectite-Gly 0.6 M sample, which also exhibited a *d*<sub>06-33</sub>-value of 1.559 Å, confirming its trioctahedral character. Interestingly, the introduction of glycine during smectite synthesis led to noticeable broadening of all reflections, suggesting a decrease in the size of coherent scattering domains. The evolution of the other reflections was discussed in supplementary information (SI-2).

Insight into the interlayer domain composition was partly obtained
by analyzing the XRD pattern of oriented preparations. The low relative
humidity of the glovebox atmosphere (<5 ppm) ensured that the sam-
ples analyzed were strongly dehydrated. The XRD pattern of the Fe-
smectite-Gly 0 M sample exhibited reflection at 10.03 Å (001), 5.01 Å
(002), and 3.27 Å (003) (Fig. 2). 00 l reflections were rational to each
other (with ratios *d*<sub>001</sub>-value/*d*<sub>002</sub>-value = 2.00 and *s* *d*<sub>001</sub>-value/*d*<sub>003</sub>-value = 3.07). As the concentration of glycine increased, the (001)
reflection shifted progressively to higher *d*-values at 10.31, 11.26, and
13.86 Å, respectively for Fe-smectite-Gly 0.08, 0.3 and 0.6 M. These *d*-
values exceeded those typically observed for dehydrated Na-smectite (e.
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Powder XRD patterns of the synthesized samples (a) and zoomed-in view of the 06–33 reflection (b). Note that the uncommon background of the XRD patterns is due to the dome that sealed the sample holder.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Oriented XRD patterns in dehydrated conditions of smectites synthesized in the presence of glycine at 0, 0.08, 0.3, and 0.6 M.</figcaption></figure>

g. 9.6 Å), suggesting alterations of the interlayer space likely caused by the incorporation of glycine or its transformation products. The *d*002-values increased from 5.01 Å (Fe-smectite-Gly 0 M) to 5.14 Å, 5.37 Å, and 5.32 Å, respectively for Fe-smectite-Gly 0.08, 0.3 and 0.6 M. The *d*003-values shifted to higher *d*-values at 3.33, 3.31, and 3.51 Å, for Fe-smectite-Gly 0.08, 0.3 and 0.6 M, respectively (Fig. 2). The lack of rationality in the 00 l reflection series of Fe-smectite-Gly samples indicates heterogeneity in interlayer occupancy. Accordingly, the 00l reflections became progressively broadened as the glycine concentration increased, revealing a loss of periodic arrangement in layer stacking.

## 3.2. Crystal chemistry

The Mid-IR spectrum of Fe-smectite-Gly 0 M was similar to that of [4]Fe(III)-nontronite synthesized under similar conditions as reported by Baron et al. (2016, 2019). However, the FTIR spectra of the Fe-smectite-

Gly samples synthesized in the presence of glycine revealed variations in band intensities and positions highlighting the influence of glycine on the crystal chemistry of the synthesized smectites.

In the OH stretching (νOH) region (3700–3400 cm⁻¹), a band was observed at 3554 cm⁻¹ for Fe-smectite-Gly 0 M which was assigned to νFe(III)₂-OH in dioctahedral smectites (Fig. 3) (Baron et al., 2016, 2019). In the spectra of Fe-smectite-Gly 0.08 M and Fe-smectite-Gly 0.3 M samples, the νOH band shifted to 3540 cm⁻¹ and a new band appeared at 3624 cm⁻¹ for the Fe-smectite-Gly 0.3 M sample (Fig. 3). According to Neumann et al. (2011), this band at 3624 cm⁻¹ was attributed to νFe (II)₃-OH indicating trioctahedral environment. In contrast, the band at 3540 cm⁻¹ was associated with νFe(III)₂-OH, clearly characterizing a dioctahedral environment. The shift from 3550 cm⁻¹ to the unusually low 3540 cm⁻¹ wavenumber was interpreted as resulting from modifications in the crystal chemistry of Fe-smectite aligning well with the emergence of Fe(II). Indeed, a νFe(III)₂-OH band at a wavenumber lower than 3550 cm⁻¹ has been previously observed in dioctahedral iron-rich smectites containing Mg with greater octahedral occupancy (Grauby et al., 1994; Petit et al., 2002) or in griffithite, an iron-rich saponite exhibiting dioctahedral domains (Komadel et al., 2000). For the Fe-smectite-Gly 0.6 M sample, trioctahedral environments became prominent, as evidenced by the enhanced absorbance of the 3624 cm⁻¹ band and the concomitant decrease of the 3540 cm⁻¹ band (Fig. 3). An additional band appeared in the spectra of Fe-smectite-Gly at 3583 cm⁻¹ (Fig. 3). While its exact assignment remained uncertain, it was likely associated with mixed Fe(II)- and Fe(III)-OH groupings within either dioctahedral or trioctahedral local environments.

In the OH bending (δOH) region, natural and synthetic Fe(III)-rich nontronites exhibited two main absorption bands at around 815 and 850 cm⁻¹. The former band, observed in the Fe-smectite-Gly 0 M sample, was attributed to νFe(III)₂-OH vibration (Fig. 3) (Baron et al., 2016, 2019). The attribution of the 850 cm⁻¹ band was still a matter of debate (Baron et al., 2016). For the samples synthesized in the presence of glycine a band at 757 cm⁻¹ emerged (Fig. 3). This band was tentatively attributed here to δFe(II)-Fe(III)-OH by analogy with Fe(III)-Mg- rich dioctahedral smectites where δMg-Fe(III)-OH band was observed at ~760 cm⁻¹ (Grauby et al., 1994; Petit et al., 2002; Gaudin et al., 2004, 2005). As the concentration of glycine increased, the band at 675 cm⁻¹ corresponding to the Fe(III)-Oₐₚ out of plane vibration underwent a
<figure><img src="image_6.png" /><figcaption>Fig. 3. Mid-IR spectra of smectites synthesized in the presence of 0, 0.08, 0.3 and 0.6 M glycine in the range 3700 to 3400 cm⁻¹ and 1200 to 400 cm⁻¹.</figcaption></figure>

decrease in intensity, while intensity of the band at 656 cm⁻¹ increased (Fig. 3). The band at 656 cm⁻¹ was generally observed in Fe(II)-rich trioctahedral smectites (Komadel et al., 2000; Fialips et al., 2002; Sakuma et al., 2022) and was unequivocally attributed to δFe(II)₃-OH, which characterized trioctahedral environments, which is fully consistent with the appearance of the corresponding O—H stretching vibration at 3624 cm⁻¹. For the Fe-smectite-Gly 0.6 M sample, the broad band at 802 cm⁻¹ was mainly attributed to amorphous silica as suggested by the Si—O band at 1100 cm⁻¹ (Ocaña et al., 1989). This amorphous silicate material was identified through TEM analyses (Figs. S1 and S2, SI).

The main vSi-O stretching absorption band for the Fe-smectite-Gly 0 M at 999 cm⁻¹ corresponded to Fe(III)-rich smectite (nontronite) with tetrahedral Si(IV) for Fe(III) substitutions (Fig. 3) (Baron et al., 2016). Based on the position of this band, the rate of tetrahedral substitution was estimated using the equation −0.02279 x vSi-O + 23.30771 = 0.54 ± 0.10 per half unit formula (phuf), according to adapted equation from Baron et al. (2016) for samples prepared from hydrated gel, as described by Petit et al. (2017). The wavenumber of the main Si—O stretching bands for samples Fe-smectite-Gly 0.08 M and Fe-smectite-Gly 0.3 M was observed at 1009 cm⁻¹, which was higher than the Fe-smectite-Gly 0 M reference sample (Fig. 3). Interestingly, such a frequency was observed in synthesized series with lower Si for Fe(III) substitution in the tetrahedral sheet of nontronite (Baron et al., 2016), meaning that the amount of tetrahedral Fe(III) decreased for syntheses conducted in the presence of glycine. However, the presence of a significant amount of Fe (II) cations in the octahedral sheets and the possible influence of glycine prevented the application of the equation to estimate tetrahedral Fe(III) content.

The decrease in tetrahedral Fe(III) content in Fe-smectite-Gly 0.08 M and Fe-smectite-Gly 0.3 M samples was evidenced by the upshift of the Si—O stretching vibration to 1009 cm⁻¹, accompanied by a decrease in the intensity of the 707 cm⁻¹ band attributed to the vibrations of tetrahedral [⁴Fe(III)-O (Decarreau et al., 2008)]. The Fe-smectite-Gly 0.6 M sample displayed a Si—O stretching band at 999 cm⁻¹, comparable to that of Fe-smectite-Gly 0 M. Notably, trioctahedral Fe(II)-rich phases such as chlorite are known to exhibit Si—O stretching vibrations around 980 cm⁻¹ (Borggaard et al., 1982). The downshift of the Si—O vibration in the Fe-smectite-Gly 0.6 M sample, compared to the Fe-smectite-Gly 0.08 M and Fe-smectite-Gly 0.3 M samples, may result from (i) the development of significant Fe(II) trioctahedral domains within the structure, through increased octahedral occupancy, and (ii) significant Fe(III) substitution in the tetrahedral sheets.

In the lattice region of the spectrum (600–400 cm⁻¹), two bands

were observed for Fe-smectite-Gly 0 M at 487 cm⁻¹ attributed to Si-O·⁶M and a broader band at 437 cm⁻¹ attributed to bending Si-O-Si (Baron et al., 2016) (Fig. 3). Both bands were characteristic of dioctahedral Fe(III)-rich smectites (Madejová et al., 2017; Petit et al., 2017). For Fe-smectite-Gly 0.08 M, these bands became sharper while maintaining the same positions at 487 and 437 cm⁻¹. However, for Fe-smectite-Gly 0.3 M, the band at 437 cm⁻¹ split into two overlapping bands at 449 and 432 cm⁻¹. In the case of Fe-smectite-Gly 0.6 M, only a single band appeared in this region at 468 cm⁻¹, which was characteristic of trioctahedral smectite (Madejová et al., 2017).

### 3.3. Chemical analyses

Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) analyses revealed a significant decrease in sodium content with increasing glycine concentration. Sodium levels decreased from 5.08 % in Fe-smectite-Gly 0 M to 2.77 %, 1.34 %, and 0.70 % in the presence of 0.08 M, 0.3 M, and 0.6 M glycine, respectively (Table 1). This decrease in Na content might be attributed to the incorporation of other species into the interlayer, compensating for the smectite’s layer charge density. For the Fe-smectite-Gly 0 M, the Fe/Si ratio was 0.71, which corresponded to a tetrahedral Fe(III) content of x = 0.49 in the [Si₄-xFe(III)x]Fe(III)₂O₁₀(OH)₂Naₓ half-unit cell (Table 1). This value was consistent with the 0.54 value determined by FTIR using the position of the main Si—O band (cf. Section 3.2) confirming the presence of Fe(III) in both tetrahedral and dioctahedral environments. When 0.08 M glycine was introduced, the Fe/Si ratio dropped to 0.55, suggesting a dioctahedral environment with a significant decrease of tetrahedral Fe (III) compared to Fe-smectite-Gly 0 M (Table 1). The tetrahedral Fe(III) content could be estimated around x = 0.13 from Fe/Si ratio if the octahedral occupancy was defined at 2 per half unit cell and the sample

<table><caption>**Table 1**

Chemical analysis of samples (at. %) obtained from SEM-EDS analyses.</caption><thead><tr><th>Samples</th><th>O (at. %)</th><th>Na (at. %)</th><th>Si (at. %)</th><th>Fe (at. %)</th><th>Fe/ Si</th><th>Na/ Fe</th></tr></thead><tbody><tr><td>Fe-smectite-Gly 0 M</td><td>59.19</td><td>5.08</td><td>20.86</td><td>14.88</td><td>0.71</td><td>0.34</td></tr><tr><td>Fe-smectite-Gly 0.08 M</td><td>60.98</td><td>2.77</td><td>23.34</td><td>12.92</td><td>0.55</td><td>0.21</td></tr><tr><td>Fe-smectite-Gly 0.3 M</td><td>61.27</td><td>1.34</td><td>22.88</td><td>14.51</td><td>0.63</td><td>0.09</td></tr><tr><td>Fe-smectite-Gly 0.6 M</td><td>62.42</td><td>0.70</td><td>24.87</td><td>12.01</td><td>0.48</td><td>0.06</td></tr></tbody></table>
was considered as fully dioctahedral as observed by XRD and FTIR (Fig. 1b and 3). As the glycine concentration increased to 0.3 M, the Fe/Si ratio rose to 0.63 (Table 1). The presence of both trioctahedral and dioctahedral segregated domains, as revealed by XRD, precluded accurate estimation of the structural formula of the Fe-smectite-Gly 0.3 M sample. For the Fe-smectite-Gly 0.6 M sample, the Fe/Si ratio decreased to 0.48; however, the presence of amorphous Silica, as revealed by FTIR (Fig. 3) and TEM analyses (Figs. S1 and S2, SI), precluded any calculation based on bulk chemical analyses (Table 1).

### 3.4. Cation exchange capacity (CEC)

The measured CEC of Fe-smectite synthesized in presence of glycine showed significant variation compared to Fe-smectite-Gly 0 M. It increased from $121.3 \times 10^{-5}$ eq/g for Fe-smectite-Gly 0 M to $138.3 \times 10^{-5}$ eq/g for Fe-smectite-Gly 0.3 M, indicating a small increase in exchangeable cations for samples synthesized in the presence of glycine. However, when the glycine concentration was increased to 0.6 M, the CEC decreased to $112.7 \times 10^{-5}$ eq/g. The amount of interlayer Na decreased as glycine concentration rose, dropping from $137.7 \times 10^{-5}$ mol/g at 0 M of glycine to 68.5, 45.4, and $29.7 \times 10^{-5}$ mol/g for glycine concentrations of 0.08 M, 0.3 M, and 0.6 M, respectively. This decrease in Na content was fully consistent with the SEM-EDS chemical analysis (Table 1). Conversely, the interlayer Fe content increased with glycine concentration, rising from $2.3 \times 10^{-5}$ mol/g at 0 M glycine to 3.2, 7.5, and $9.5 \times 10^{-5}$ mol/g for glycine concentrations of 0.08 M, 0.3 M, and 0.6 M, respectively (Table 2).

For Fe-smectite-Gly 0 M, the measured CEC of $121.3 \times 10^{-5}$ eq/g was slightly lower than the expected CEC of $144.6 \times 10^{-5}$ eq/g, likely due to residual Na that was not fully removed during the washing process (Table 2). Surprisingly, the calculated CEC values for samples synthesized in the presence of glycine were significantly lower than those measured. This discrepancy could be attributed to the presence of positively charged organic molecules, which might have compensated for the net negative charge of the Fe-smectites.

### 3.5. Fe redox state by Mediated Electrochemical Oxidation and Reduction

In Fe-smectite-Gly 0 M, iron was present exclusively as Fe(III), resulting in a Fe(II)/Fe<sub>tot</sub> ratio of 0. In contrast, Fe-smectite samples

<caption>**Table 2**</caption>

Atomic absorption spectroscopy (AAS) analysis of Fe-Smectite-Gly samples at different glycine concentrations. Na and Fe corresponded to extracted cations after Ca-saturation. Expected CEC corresponded to the sum of charge calculated from extracted cations.

<table><thead><tr><th>Samples</th><th>Measured CEC (eq/g clay) [1]</th><th>Na (mol/g clay) [2]</th><th>Fe (mol/g clay) [2]</th><th>Expected CEC (eq/g clay) [3]</th></tr></thead><tbody><tr><td>Fe-smectite-Gly 0 M</td><td>121.3 × 10<sup>-5</sup></td><td>137.7 × 10<sup>-5</sup></td><td>2.3 × 10<sup>-5</sup></td><td>144.6 × 10<sup>-5</sup></td></tr><tr><td>Fe-smectite-Gly 0.08 M</td><td>133.9 × 10<sup>-5</sup> ± 3.1 × 10<sup>-5</sup></td><td>68.5 × 10<sup>-5</sup> ± 2.1 × 10<sup>-5</sup></td><td>3.2 × 10<sup>-5</sup> ± 0.7 × 10<sup>-5</sup></td><td>[75.2 × 10<sup>-5</sup> ± 0.7 × 10<sup>-5</sup> - 78.8 × 10<sup>-5</sup> ± 0.9 × 10<sup>-5</sup>]</td></tr><tr><td>Fe-smectite-Gly 0.3 M</td><td>138.3 × 10<sup>-5</sup> ± 1.4 × 10<sup>-5</sup></td><td>45.4 × 10<sup>-5</sup> ± 1.3 × 10<sup>-5</sup></td><td>7.5 × 10<sup>-5</sup> ± 1.3 × 10<sup>-5</sup></td><td>[59.8 × 10<sup>-5</sup> ± 1.3 × 10<sup>-5</sup> - 67.1 × 10<sup>-5</sup> ± 2.4 × 10<sup>-5</sup>]</td></tr><tr><td>Fe-smectite-Gly 0.6 M</td><td>112.7 × 10<sup>-5</sup> ± 0.8 × 10<sup>-5</sup></td><td>29.7 × 10<sup>-5</sup> ± 0.4 × 10<sup>-5</sup></td><td>9.5 × 10<sup>-5</sup> ± 2.4 × 10<sup>-5</sup></td><td>[53.4 × 10<sup>-5</sup> ± 6.8 × 10<sup>-5</sup> - 64.3 × 10<sup>-5</sup> ± 9.4 × 10<sup>-5</sup>]</td></tr></tbody></table>

[1] The measured CEC was obtained from the measurement of Ca concentration after NH₄-to-Ca exchange at pH 7.

[2] This corresponded to the release quantity of Na and Fe after Ca saturation.

[3] The expected CEC was determined by summing the molar quantities of Na and Fe. Since Fe was Fe²⁺ or Fe³⁺, the lowest value assumed that all extracted Fe was divalent, while the highest value considered only trivalent cations.

synthesized with 0.08 M and 0.3 M glycine exhibited Fe(II)/Fe<sub>tot</sub> ratios of $0.14 \pm 0.02$ and $0.56 \pm 0.03$, respectively. The highest degree of reduction was recorded in the Fe-smectite-Gly 0.6 M sample, with a Fe (II)/Fe<sub>tot</sub> ratio of $0.72 \pm 0.02$. The progressive increase in this ratio with rising glycine concentrations suggests that the extent of Fe(III) reduction to Fe(II) is proportional to the amount of glycine present. These results support the hypothesis that glycine participated in redox-mediated processes, contributing to iron reduction (Table 3).

### 3.6. Chromatographic analyses

Analysis of the organic fraction after smectite hydrothermal synthesis indicated that 82.1 % to 90.1 % of glycine remained untransformed, either as free glycine or polymerized forms (Table 4). This recovery is comparable to that observed in blank samples, where glycine solutions underwent the same hydrothermal treatment. Given that glycine does not significantly degrade under these conditions, its recovery mainly reflects the analytical and measurement error. Assuming a relative error of approximately 10 %, based on the lowest recovery observed for Gly 0.3 M, only a minor fraction (less than 10 %) of glycine may have been converted into undetected organic molecules (aqueous or gas phase) or longer peptides.

The glycine conversion rate was consistently higher in Fe-smectite-samples, reaching 15.4 %, 22.5 %, and 16 % for Fe-smectite-Gly 0.08 M, 0.3 M, and 0.6 M, respectively. While this trend indicated an active role of the mineral surface in organic transformations, the conversion rate did not correlate directly with either the initial glycine concentration or the expected reactivity associated with octahedral iron reduction. The high glycine recovery, along with the low conversion rate, indicated that the majority of the detected organic fraction consists of free glycine.

The peptidization ratio, representing the fraction of converted glycine incorporated into peptides, increased with the initial glycine concentration in both blank and Fe-smectite samples. Samples with Fe-smectite exhibited both a higher conversion rate and longer peptides than blank samples, suggesting that smectite enhanced peptide formation, particularly at higher initial glycine concentrations.

Regarding the nature of the peptides formed, glycine dimers were consistently more abundant than trimers, which, in turn, were more prevalent than tetramers. Interestingly, peptide formation in the presence of smectite resulted in longer peptide chains compared to the blank samples, highlighting the mineral's catalytic role in peptidization.

The glycine dimer (Gly₂) existed in two distinct configurations: linear and cyclic. The relative proportion of these forms was crucial as it fundamentally impacts the dimer's chemical behavior and potential reactions. In its linear form, Gly₂ maintained both a free amino group and a carboxyl group, allowing it to participate in further peptide bond formation and serve as a building block for longer peptide chains. In contrast, the cyclic form (diketopiperazine, DKP) had both reactive ends consumed in ring formation, significantly limiting its ability to participate in chain elongation reactions. The distribution between these forms could therefore determine whether the system favored peptide growth or ended at the dimer stage. In terms of DKP selectivity, using glycine during mineral synthesis under hydrothermal conditions favored the formation of linear peptides over DKP, in contrast to the blank sample containing only glycine. In this context, the presence of smectite

<caption>**Table 3**</caption>

Electrochemical analysis of Fe(III) and Fe(II) in Fe-Smectite-Gly samples at different glycine concentrations.

<table><thead><tr><th>Samples</th><th>Fe(III) (mol)</th><th>Fe(II) (mol)</th><th>Fe(II)/Fe<sub>tot</sub></th></tr></thead><tbody><tr><td>Fe-smectite-Gly 0 M</td><td>2.79 × 10<sup>-6</sup></td><td>0</td><td>0</td></tr><tr><td>Fe-smectite-Gly 0.08 M</td><td>5.01 × 10<sup>-7</sup></td><td>8.03 × 10<sup>-8</sup></td><td>0.14 ± 0.02</td></tr><tr><td>Fe-smectite-Gly 0.3 M</td><td>2.28 × 10<sup>-7</sup></td><td>2.85 × 10<sup>-7</sup></td><td>0.56 ± 0.03</td></tr><tr><td>Fe-smectite-Gly 0.6 M</td><td>3.03 × 10<sup>-7</sup></td><td>7.66 × 10<sup>-7</sup></td><td>0.72 ± 0.02</td></tr></tbody></table>
<table><caption>**Table 4**
Amount of Gly remaining (%) and peptide yield (%) in organic blanks and Fe-smectite-Gly.</caption><thead><tr><th rowspan="2">Samples</th><th rowspan="2">Gly recovery(%)<sup>[1]</sup></th><th rowspan="2">Conversion rate (%)<sup>[2]</sup></th><th colspan="5">Peptide production yield (%)</th><th rowspan="2">DKP selectivity (%)</th></tr><tr><th>Peptization ratio<sup>[3]</sup></th><th>DKP (%)</th><th>Gly<sub>2</sub> (%)</th><th>Gly<sub>3</sub> (%)</th><th>Gly<sub>4</sub> (%)</th></tr></thead><tbody><tr><td>Gly 0.08 M</td><td>96.7</td><td>4.3</td><td>23.5</td><td>11.7</td><td>11.8</td><td>0.0</td><td>0.0</td><td>49.8</td></tr><tr><td>Fe-smectite-Gly 0.08 M</td><td>86.0</td><td>15.4</td><td>8.3</td><td>0.9</td><td>7.4</td><td>0.0</td><td>0.0</td><td>10.8</td></tr><tr><td>Gly 0.3 M</td><td>90.8</td><td>12.9</td><td>28.7</td><td>22.7</td><td>5.9</td><td>0.0</td><td>0.0</td><td>79.3</td></tr><tr><td>Fe-smectite-Gly 0.3 M</td><td>82.1</td><td>22.5</td><td>20.8</td><td>6.7</td><td>6.7</td><td>7.2</td><td>0.1</td><td>49.8</td></tr><tr><td>Gly 0.6 M</td><td>97.0</td><td>8.7</td><td>65.3</td><td>29.2</td><td>16.6</td><td>19.4</td><td>0.0</td><td>63.8</td></tr><tr><td>Fe-smectite-Gly 0.6 M</td><td>90.1</td><td>16.0</td><td>37.9</td><td>13.4</td><td>14.0</td><td>10.5</td><td>0.2</td><td>49.0</td></tr></tbody></table>

[1] See formula S1 in SI; [2] See formula S2 in SI; [3] See formula S3 in SI; [4] See formula S4 in SI.

appeared to influence DKP selectivity, suggesting that the solid surface affected the equilibrium between the linear and cyclic dimers and may have played a role in promoting chain extension through polymerization.

## 4. Discussion

### 4.1. Evidence of smectite formation in the presence of glycine

Previous studies have reported that [4]Fe(III)-smectite has typically been synthesized successfully at pH values between 10 and 14 in the absence of organic molecules (Decarreau et al., 2008; Petit et al., 2015; Baron et al., 2016). Under our experimental conditions, the final pH of Fe-smectite synthesized without glycine (0 M Gly) was measured at 11.9, which is consistent with the findings of Baron et al. (2016) (Table S1). When glycine was added to the reactor, the final pH dropped below 10, which is generally unfavorable for [4]Fe(III)-smectite formation. However, mineral analyses confirmed the successful formation of Fe-smectite across all tested glycine concentrations, though amorphous silica was also detected at the highest glycine concentration (Figs. 1a, 2, 3). Interestingly, the increase in glycine concentration during the formation of Fe-smectite resulted in a concomitant decrease in potential Hydrogen (pH) and redox potential (Eh), thereby creating less alkaline and more reducing conditions when compared to Fe-smectite that was synthesized without glycine. In the case of organic blanks, the Eh values were almost identical regardless of the glycine concentration. This indicated that the Eh value was influenced by Fe redox couples and not by glycine or their by-products (Table S1). Fe-smectite formation in the presence of glycine at a lower-than-expected pH could be attributed to several mechanisms: (i) The buffering capacity of glycine, due to its ionizable amino and carboxylate groups, might have created micro environments with locally favorable pH conditions for smectite formation. (ii) The formation of Fe-glycine complexes could enhance the stability of iron in solution (Mandado and Cordeiro, 2010) and prevent the premature precipitation of iron (hydr)oxide or aegirine formation that could otherwise compete with smectite formation (Baron et al., 2019). (iii) The presence of organics could have significantly affected nucleation and growth rates, promoting the formation of more thermodynamically stable structures capable of incorporating mixed-valence Fe (II) and Fe(III) cations.

### 4.2. Influence of glycine on Fe-smectite crystal chemistry

To better understand the role of glycine in the Fe-smectite structure, we investigated its impact on crystal chemistry, with particular attention to modifications in the tetrahedral and octahedral sites. The shift of the vSi-O band from 999 cm⁻¹ to 1009 cm⁻¹, along with the decreased intensity of the 707 cm⁻¹ band between Fe-smectite-Gly 0 M and Fe-smectite-Gly 0.08 M, indicated a decrease in [4]Fe(III) content (Baron et al., 2016, 2019), as confirmed by chemical analyses (Fig. 3, Table 1). In the octahedral sheet, the downshift of the vFe(III)₂-OH band from 3554 to 3540 cm⁻¹ and the appearance of a new band at 757 cm⁻¹, assigned to δFe(II)-Fe(III)-OH in a dioctahedral environment, further

suggested partial reduction of octahedral Fe(III). Electrochemical analysis estimated the Fe(II)/Fe<sub>tot</sub> ratio at 0.14 for Fe-smectite-Gly 0.08 M (Table 3). For both samples, solid-state analyses revealed predominantly Fe(III)-rich dioctahedral structures, with layer charge densities decreasing from 0.49 to 0.43 per half unit cell (phuc). Interestingly, this decrease in layer charge was accompanied by an increase in CEC from 121.3 × 10⁻⁵ to 133.9 × 10⁻⁵ eq/g, suggesting that layer charge alone does not fully account for the observed CEC. For Fe-smectite-Gly 0.08 M, the CEC calculated from the structural formula (0.13 from tetrahedral charge and 0.30 from octahedral charge; see Section 3.3) was approximately 100 × 10⁻⁵ eq/g. Accounting for variable charge contributions (estimated at 20–30 % of total CEC for particle sizes below 200 nm; Dzene et al., 2016) brought the calculated value in line with the measured CEC, indicating that structural charges were fully captured by Ca-exchange measurements and that no irreversibly adsorbed species persisted. The discrepancies between the CEC values measured using Na and Fe compared to those obtained from NH₄-for-Ca exchange suggest the presence of organic compounds that contribute to charge compensation. This interpretation is further supported by basal spacing measurements, which indicate the presence of interlayer species larger than dehydrated cations alone.

For Fe-smectite-Gly 0.08 and 0.3 M samples, the *d*<sub>06-33</sub>-values (at 1.519 and 1.517 Å, respectively) indicate a predominantly dioctahedral environment for both samples. However, these *d*<sub>06-33</sub>-values commonly used to estimate the b dimension of the unit cell are lower than the *d*<sub>06-33</sub>-value of the Fe-smectite-Gly 0 M sample, meaning that the b dimension is influenced by both a decrease in [4]Fe(III) content and the presence of octahedral Fe(II). Segregated trioctahedral environments have been also observed in the Fe-smectite-Gly-0.3 M sample with a reflection at 1.559 Å, that evidence the crystal chemistry complexity of the sample.

Estimating the [4]Fe(III) content in Fe-smectite-Gly 0.3 M and 0.6 M was challenging due to significant structural modifications associated with increased Fe(II) incorporation and the coexistence of dioctahedral and trioctahedral domains, which influenced the position of the Si—O stretching vibration. This interpretation is further supported by FTIR data, which reveals characteristic bands at 757 cm⁻¹ (Fe(III)-Fe(II)-OH bending in dioctahedral environments) and at 3624 cm⁻¹ and 656 cm⁻¹, corresponding to Fe(II)₃-OH stretching and bending, respectively-both indicative of trioctahedral environments. FTIR and XRD analyses consistently demonstrate an increasing proportion of trioctahedral domains with rising glycine concentration, in agreement with electrochemical measurements showing an increase in Fe(II)/Fe<sub>tot</sub> ratios from 0.56 to 0.72 in Fe-smectite-Gly 0.3 M and 0.6 M, respectively. Moreover, the shift of the Si—O stretching vibration from 1010 to 990 cm⁻¹, accompanied by the disappearance of the 707 cm⁻¹ band, suggests substantial tetrahedral substitution within trioctahedral environments, resembling structural features observed in chlorite. This interpretation aligns with the findings of Petit et al. (2023), who demonstrated that significant tetrahedral substitutions are necessary to accommodate the distortion of the octahedral sheet induced by trioctahedral domain development. Collectively, these observations indicate a progressive decrease in octahedral charge contribution in favor of increased
tetrahedral charge for the highest glycine concentration. The measured CEC values for Fe-smectite-Gly 0.3 M and 0.6 M were 138.3 and 112.7 × 10⁻⁵ eq/g, respectively. In the latter sample, the lower CEC, despite the higher tetrahedral charge, may be attributed to the presence of amor- phous silica. For both samples, the CEC calculated from the release of Na⁺ and Fe²⁺/Fe³⁺ after Ca exchange did not align with the CEC measured from NH₄-for-Ca exchange, suggesting the involvement of additional charge-compensating species.

### 4.3. Influence of Fe-smectite formation on glycine polymerization

Chromatographic analyses demonstrated that between 82 % and 97 % of the organic compounds, including glycine and its peptides (Gly₂, Gly₃, Gly₄), were successfully quantified. The unquantified fraction might correspond to longer peptide chains at a concentration below the detection limit or other unidentified or adsorbed molecules. Earlier work (Marangoni et al., 1989) on the electro-oxidation of glycine identified cyanide, cyanate, formaldehyde, ethylamine, ammonia and CO₂ as possible oxidation products. In our experiments, none of these compounds were identified.

One of the most significant side reactions during Fe-smectite formation was the spontaneous polymerization of glycine. The presence of minerals enhanced this process, leading to a higher glycine conversion rate, an increased peptidization ratio, and the formation of longer peptide chains compared to blank samples prepared under the same conditions. The role of mineral surfaces in peptide formation has been extensively studied, with factors such as mineral composition, temperature, reaction duration, and concentration significantly influencing the results (Kitadai et al., 2017). In our experiments, challenges arose from the continuous nucleation and growth of Fe-smectite, which created diverse active sites and local physicochemical environments that could impact the thermodynamics of polymerization. Kitadai (2014) demonstrated that peptide formation from glycine was favored when the zwitterionic form was in equilibrium with its anionic or cationic counterparts. Increasing the temperature lowered the pKa of the amino group (from 9.8 at 25 °C to 8.3 at 140 °C, according to Sakata et al. (2010), indicating that at the synthesis pH (8.7–9.5, Table S1), a substantial fraction of glycine existed in both zwitterionic and anionic forms, contributing to peptide formation. Thermodynamic models further suggested that these conditions favored linear peptide formation over cyclic peptides in accordance with our results.

## 5. Conclusion

We investigated the influence of glycine concentration on Fe-smectite crystallization under hydrothermal conditions using a controlled synthesis approach. Our findings demonstrate that glycine significantly promotes Fe-smectite formation at concentrations ranging from 0.08 to 0.6 M, even under less alkaline conditions (pH < 10), which are typically unfavorable for Fe-rich smectite crystallization (Petit et al., 2017). The effect of glycine on the crystal chemistry of Fe-smectite is primarily attributed to its ability to reduce Fe(III) to Fe(II), with the extent of reduction being governed by glycine concentration. At low glycine concentrations (0.08 M), the majority of the total negative charge in Fe-smectite originates from Fe(II) within a dioctahedral environment, with only a minor contribution from tetrahedral substitution. As Fe(II) content increases (0.3 M), layers with a trioctahedral environment begin to emerge. At 0.6 M, only trioctahedral environments are detected, accompanied by stronger tetrahedral Fe(III) substitution (to accommodate the large size octahedra sheet dimension).

Although glycine oxidation products were not detected, smectite crystallization appears to facilitate the formation of cyclic polymers, such as diketopiperazines (DKPs), as well as linear peptides, including Gly₂, Gly₃, and Gly₄. The increasing initial glycine concentration, combined with structural modifications occurring during smectite formation, enhances glycine polymerization, leading to the formation of

longer peptides. Compared to reference samples, smectite crystallization preferentially promotes the formation of linear dipeptides over DKPs.

This study highlights the potential role of organic matter in the formation of mineral phases with unique structures and properties. Exploring the extent of these modifications with other organics offers a valuable perspective for future research.

### CRediT authorship contribution statement

**Rihab Fkiri:** Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Conceptualization.

**Fabien Baron:** Writing – original draft, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization. **Clément Comminges:** Writing – review & editing, Validation, Methodology, Investigation, Formal analysis, Conceptualization. **Sabine Petit:** Writing – review & editing, Validation, Methodology, Formal analysis. **Claude Geffroy:** Writing – original draft, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization. **Brian Gregoire:** Writing – original draft, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization.

### Declaration of competing interest

Claude Geffroy reports financial support was provided by French Space Agency. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors are grateful to the French government program "Investissements d'Avenir" (EUR INTREE, reference ANR-18-EURE-0010), the European Union (ERDF), the French National Space Agency (CNES) R&T R-S24/SU-0003-092-01 and APR SOMOM, PEPR ORIGINS (ANR-22-EXOR-0014 – Plan France 2023), the Université de Poitiers, and the Région Nouvelle-Aquitaine for providing financial support for this study.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.clay.2025.107902.

### Data availability

Data will be made available on request.

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