# Quantitative nanoscale mapping of the Mn redox states in silicates by scanning transmission X-ray microscopy at the Mn L2,3-edges

Franck Bourdelle, Cyril Durand, Erwan Paineau, Emily Lloret, Laura Airaghi

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Franck Bourdelle, Cyril Durand, Erwan Paineau, Emily Lloret, Laura Airaghi. Quantitative nanoscale mapping of the Mn redox states in silicates by scanning transmission X-ray microscopy at the Mn L2,3-edges. Chemical Geology, 2025, pp.122739. 10.1016/j.chemgeo.2025.122739. insu-04991843v2

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# Quantitative nanoscale mapping of the Mn redox states in silicates by scanning transmission X-ray microscopy at the Mn L<sub>2,3</sub>-edges

Franck Bourdelle<sup>a,∗</sup>, Cyril Durand<sup>b</sup>, Erwan Paineau<sup>c</sup>, Emily Lloret<sup>b</sup>, Laura Airaghi<sup>d</sup>

## ARTICLE INFO

Editor: Oleg Pokrovsky

Keywords:

Manganese oxidation states
STXM
XANES
L<sub>2,3</sub>-edges
Redox mapping

## ABSTRACT

We present a novel analytical protocol for quantifying and mapping Mn oxidation states in silicates at the nanoscale. Based on X-ray Absorption Near Edge Structure spectra of the Mn L<sub>2,3</sub>-edges (630–660 eV) acquired thanks to a Scanning transmission X-ray microscope available at various Synchrotron facilities, we calibrate two equations using reference Mn-bearing samples to precisely determine the Mn mean valence. We detail the spectra processing and the methodology for deriving quantitative Mn redox maps from the proposed calibrations, with a high spatial resolution (20–30 nm). Furthermore, we explore the potential for coupling Mn and Fe L<sub>2,3</sub>-edge redox calibrations over the 630–730 eV energy range. The effectiveness of these calibrations, the coupling of Mn–Fe results, and the subsequent redox mapping are evaluated through two case studies, showcasing the benefits of this non-destructive approach.

## 1. Introduction

Assessing manganese (Mn) oxidation state at a submicrometric scale within silicates holds great significance in Earth Sciences or Chemistry. This is essential, for instance, in constraining the chemical conditions during Mn-rock formation or investigating the biogeochemical cycling of Mn in ecosystems. Various methods were proposed to probe Mn oxidation state, each offering some advantages, but also coming with limitations. In this framework, we can mention the X-ray photoelectron spectroscopy (XPS; Ilton et al., 2016) which allows to determine the Mn redox at the surface of the studied sample with a high spectra resolution. However, XPS often implies a large measured area and can induce damages due to heat production. Electron microprobe (EMP; Albee and Chodos, 1970) is an easy-to-use possibility to analyse all types of Mn-samples with limited damages, but the analysed volume (interaction volume) is larger than a micron. On its side, Electron Energy Loss Spectroscopy (EELS; e.g., Garvie and Craven, 1994; Tan et al., 2012) performed on Transmission Electron Microscope (TEM) is capable of analyzing very thin samples (< 100 nm), with very high spatial resolutions (< 10 nm). Nevertheless, EELS needs the latest generation microscope to reach a high spectral resolution (0.1 eV), and can induce

beam damages (e.g., Livi et al., 2012). X-ray absorption near edge structure spectroscopy (XANES) at the Mn K-edge (Manceau et al., 2012) is a good compromise for mapping Mn oxidation states on a large area with a high spectral resolution, but the spatial resolution is often insufficient to detect compositional variations at the crystallite scale. Spatial resolution is particularly critical because Mn-containing minerals are frequently present as very small crystallites that can form mixtures of crystallites with different Mn, or can exhibit Mn redox heterogeneities inside them. XANES at the Mn L<sub>2,3</sub>-edges appears as a viable alternative. The energy window, typically ranging from 630 to 660 eV, allows high spectral (< 0.1 eV) and spatial resolutions (~30 nm) which are easily attained at existing synchrotron facilities. In addition, the X-ray beam damage is negligible relative to electron beam used for EELS.

The absorption spectroscopy depends mainly on the metal valence (and, to a lesser extent, to other parameters like the atom coordination) paving the way to a useful method for determining Mn redox state at nanoscale. The XANES Mn L<sub>2,3</sub>-edge spectrum results from the 2p → 3d electronic transition – the absorption process leading to a core-excited 2p<sup>5</sup>d<sup>n+1</sup> final state from a 2p<sup>6</sup>3d<sup>n</sup> ground state. Practically, the transmitted soft X-ray intensity can be recorded using a scanning

<footnote><sup>a</sup> CY Cergy Paris Université, Sorbonne Université, CNRS INSU, Institut des Sciences de la Terre de Paris, ISTE>P, F-95000 Cergy, France</footnote>

<footnote><sup>b</sup> Univ. Lille, IMT Lille Douai, Univ. Artois, Junia, ULR 4515 – LGCgE, Laboratoire de Génie Civil et géo-Environnement, F-59000 Lille, France</footnote>

<footnote><sup>c</sup> Université Paris-Saclay, CNRS, UMR 8502 – LPS, Laboratoire de Physique des Solides, F-91405 Orsay, France</footnote>

<footnote><sup>d</sup> Université d'Orléans, CNRS, BRGM, UMR 7327 – ISTO, Institut des Sciences de la Terre d'Orléans, F-45100 Orléans, France</footnote>

<footnote>* Corresponding author at: Institut des Sciences de la Terre de Paris (ISTeP) – CY Cergy Paris Université, Maison Internationale de la Recherche (MIR), 1 rue Descartes, 95000 Neuville-sur-Oise, France.

E-mail address: franck.bourdelle@cyu.fr (F. Bourdelle).</footnote>
<table><caption>**Table 1**
Samples used for STXM-XANES Mn L<sub>2,3</sub>-edge investigations to Mn mean valence quantification.</caption><thead><tr><th>Sample</th><th>Location</th><th>Mn oxides wt%</th><th>Mn oxidation states</th><th>Mn<sup>x+</sup> mean valence<sup>a</sup></th></tr></thead><tbody><tr><td><strong>Silicates</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Rhodonite</td><td>Gambaseta (Liguria, Italia)</td><td>13</td><td>2+</td><td>2 ± 0.1</td></tr><tr><td>Ardennite-(As)</td><td>Salm-Château (Ardennes, Belgium)</td><td>19</td><td>2+</td><td>2 ± 0.2</td></tr><tr><td>Spessartite</td><td>Longchang (Guangdong, China)</td><td>43</td><td>2+</td><td>2 ± 0.1</td></tr><tr><td>Inesite</td><td>Hale Creek (California, USA)</td><td>37</td><td>2+</td><td>2 ± 0.2</td></tr><tr><td>Friedelite</td><td>Adervielle (Pyrénées, France)</td><td>53</td><td>2+</td><td>2 ± 0.2</td></tr><tr><td>Hubeite</td><td>Funsen (Hubei province, China)</td><td>11</td><td>2+</td><td>2 ± 0.2</td></tr><tr><td>Carpholite</td><td>Hornf Slavkov (Karlovy Vary, Czech Rep.)</td><td>22</td><td>2+</td><td>2 ± 0.2</td></tr><tr><td>Mn-Cummingtonite</td><td>Gambaseta (Liguria, Italia)</td><td>4</td><td>2+</td><td>2 ± 0.1</td></tr><tr><td>Franklinfurnaceite</td><td>Franklin mine (New Jersey, USA)</td><td>26/9</td><td>2+/3+</td><td>2.25 ± 0.2</td></tr><tr><td>Långbanite</td><td>Långban mine (Filipstad, Sweden)</td><td>22/44</td><td>2+/3+</td><td>2.70 ± 0.3</td></tr><tr><td>Braunite</td><td>Prabornaz mine (Aosta, Italia)</td><td>12/78</td><td>2+/3+</td><td>2.85 ± 0.2</td></tr><tr><td>Piemontite</td><td>Prabornaz mine (Aosta, Italia)</td><td>9.5</td><td>3+</td><td>3 ± 0.1</td></tr><tr><td>Stavelotite-(La)-like</td><td>Eveslogchorr (Russia)</td><td>71</td><td>4+</td><td>4 ± 0.3</td></tr><tr><td><strong>Oxides</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Jacobsite</td><td>Langban (Filipstad, Sweden)</td><td>31</td><td>2+</td><td>2 ± 0.1</td></tr><tr><td>Bustamite</td><td>Broken Hill (Australia)</td><td>42</td><td>2+</td><td>2 ± 0.1</td></tr><tr><td>Groutite</td><td>Zavalye (Kirovohrad, Ukraine)</td><td>100</td><td>3+</td><td>3 ± 0.1</td></tr><tr><td>Manganite</td><td>Lucy mine (Michigan, USA)</td><td>100</td><td>3+</td><td>3 ± 0.1</td></tr><tr><td>Cryptomelane</td><td>Castelo mine (Folgosinho, Portugal)</td><td>19/71</td><td>2+/4+</td><td>3.75 ± 0.1</td></tr><tr><td>Pyrolusite</td><td>Imini mine (Ouarzazate, Morocco)</td><td>100</td><td>4+</td><td>4 ± 0.1</td></tr></tbody></table>

transmission X-ray microscope (STXM), allowing to obtain 2D hyper-spectral images. This STXM-XANES association is of great interest for mapping variations in the metal oxidation state within crystallites of sub-micrometer size (e.g., Airaghi et al., 2020). Several approaches have been proposed to determine the Mn oxidation state from Mn L<sub>2,3</sub>-edge spectra (Pecher et al., 2003; Tan et al., 2012), but mostly within oxides rather than silicates, and rarely going as far as a mapping of oxidation state heterogeneities. For instance, Pecher et al. (2003) proposed STXM-XANES mapping but only semi-quantitative, due to the absence of a suitable linear spectral parameter-energy calibration. To fill this gap, Bourdelle et al. (2021) evaluated the potential of the Mn L<sub>2,3</sub>-edge XANES as a probe for Mn redox in natural silicates. The authors defined two empirical spectral calibrations, enabling the construction of quantitative Mn redox maps with high spatial resolution (20 nm per pixel). This promising approach remains exploratory as the spectral calibration was based on a limited set of 5 samples, and 2D maps were obtained on powders, not allowing to constrain the relationship between Mn redox variation and textural signature. Here, we propose a complete validation of these spectral calibrations based on a broad range of 19 Mn-bearing minerals, including natural silicates and oxides with various Mn contents and Mn oxidation states (2+, 3+, 4+). We then test our approach on two specific case studies, one to challenge our calibrations, and one to apply mapping. Considering that Mn and Fe are the most common variable-valence metals in silicates, we also demonstrate the effectiveness of our approach by integrating the current Mn redox quantification protocol with the established Fe protocol (Bourdelle et al., 2013), thus providing a robust characterization method.

## 2. Material and methods

### 2.1. Reference samples

The reference natural silicates and oxides samples with a wide range of Mn<sup>2+</sup>/ΣMn, Mn<sup>3+</sup>/ΣMn, Mn<sup>4+</sup>/ΣMn ratios are listed in Table 1. The structural formulae of each reference sample were checked by comparing chemical analyses – done from EMP, Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) coupled with an Energy Dispersive X-ray Spectrometer (EDX), and STXM-XANES for Fe<sup>3+</sup>, Fe<sup>2+</sup> contents – and the theoretical formulae to ensure the Mn valence.

Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) analysis were carried out on thin section cut across

the samples or on carbon coated small, isolated crystals, using a QUANTA 200 SEM instrument (FEI, Hillsboro, Oregon, USA; LOG laboratory, Université de Lille, Lille, France) operating at 15 kV with a 1.5 nA current intensity.

Chemical analyses were also performed on thin sections with a CAMECA SX 100 electron microprobe (EMP, Cameca, Gennevilliers, France) equipped with wavelength dispersive spectrometers (WDS). The equipment belongs to the Chevreul Institute (Michel-Eugène Chevreul Institute IMET; CNRS Research Federation-FR2638) in Lille (France). The analytical conditions were the following: 15 nA, 15 kV, spot size of 1 µm, counting time of 20 s/element. The analysed elements were Si, Al, Fe, Mg, Mn, Ti, Ca, Na and K. The system was calibrated with albite for Na, orthoclase for Al and K, wollastonite for Ca and Si, MgO for Mg, MnTiO<sub>3</sub> for Ti and Mn, and Fe<sub>2</sub>O<sub>3</sub> for Fe.

For STXM-XANES investigations, samples need to be transparent to soft X-rays, implying a crystallite thickness lower than 100 nm. Consequently, samples were prepared as disoriented powders dispersed in ethanol, then dropped onto a carbon holey support film placed on a 200-mesh copper grid, or as ultra-thin sections cut using a Focused Ion Beam (FIB, Bourdelle et al., 2012), preserving the crystal texture and structure. For the present STXM-XANES study, ultra-thin foils were cut by focused ion beam (FIB) milling into petrological thin sections. Ultra-thin foils were cut using the FEI strata Dualbeam 235 instrument of the IEMN (Institut d'Electronique, de Microélectronique et de Nanotechnologie, Lille, France) with a size of ~10–15 µm by 5 µm.

### 2.2. Application to two geological case studies

Arfvedsonite is an alkali amphibole that theoretically contains a mix of iron oxidation states in a 4Fe<sup>2+</sup>/1Fe<sup>3+</sup> proportion, leading to the general formula Na<sub>3</sub>Fe<sub>5</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>. The arfvedsonite-like minerals from Tanohata mine (Japan) also incorporate Mn. This variant, named kôzulite, is identified by Barkley et al. (2010) as having Mn<sub>tot</sub> = Mn<sup>2+</sup> alongside Fe<sub>tot</sub> = Fe<sup>3+</sup>, with potentially minor amounts of Mn<sup>3+</sup>. This silicate is therefore an ideal candidate for testing the proposed calibrations. For that purpose, a natural sample of kôzulite was selected, EMP analysis giving the following structural formula:

$$ (K_{0.18}Na_{0.82})(Na_{1.90}Ca_{0.10})(Mn^{2+}_{3.42}Mg_{1.08}Fe^{3+}_{0.54}Al_{0.06})(Si_{7.92}Al_{0.01}Ti_{0.08})O_{22}(OH)_2 $$

The Praborna mine site (Aosta valley, Italy), located on the north-eastern side of the Gran Paradisio massif, is recognized for deposits of

<footnote><sup>a</sup> Assumed error deviation (chemical heterogeneity of specimens).</footnote>
Mn-minerals (e.g., piemontite; Kenngott, 1853). Micrometric garnets and piemontite exhibit large chemical Mn zonations (Cenky-Tok and Chopin, 2006). Two selected samples (one Mn-garnet and one piemontite) were analysed by EMP and prepared as an ultra-thin section for STXM-XANES investigations.

## 2.3. STXM-XANES experiments

Data were acquired at the Swiss Light Source (Switzerland; PolLux beamline), and SOLEIL synchrotron (France; Hermes beamline). XANES experiments were performed on a STXM, allowing to record the transmitted soft X-ray intensity on each point of the pluri-micrometric-sized area of interest and for each defined energy. With this setup, stacks and linescans can be achieved, i.e. spectral maps of an area and sums of spectra for each pixel of a line, respectively. Spectra and stacks were obtained over two energy ranges – 630-660 eV for Mn L$_{2,3}$-edge and 700–730 eV for Fe L$_{2,3}$-edge – using an energy increment of 0.1 eV around the maximum absorption peaks. Spectra were adjusted in energy on the Fe L$_{2}$-b peak, at 723.5 eV. The dwell time per pixel and energy point was adjusted between 10 and 20 ms. The focus was achieved systematically for each sample, and precision in the determination of the focus position was better than the focus depth. Image stacks were aligned, and XANES spectra were derived from areas of interest using the aXis2000 software (Hitchcock, 2014).

## 2.4. XANES spectra processing

Optical density (OD) spectra were extracted from stacks and linescans such that $OD = -\ln(I/I_0)$, I being the X-ray intensity transmitted through the sample, and $I_0$ the one recorded within the blank. The processing consists of two steps (see Fig. S-1 in supplementary material):

(i) applying a linear background correction to remove the contribution of lower energy absorption edges so that the pre-edge region is set to 0 optical density.

(ii) subtracting the two edge steps corresponding to $2p \rightarrow continuum$ transitions with a double arctan function (e.g., Brotton et al., 2007) as:

$$f(\Delta E) = \frac{h_1}{\pi} \left( \arctan \left[ \frac{\pi}{w_1} (\Delta E - E_1) \right] + \frac{\pi}{2} \right) + \frac{h_2}{\pi} \left( \arctan \left[ \frac{\pi}{w_2} (\Delta E - E_2) \right] + \frac{\pi}{2} \right) \quad (1)$$

where $h_1$ and $h_2$ are the step heights of the two arctan functions, $w_1$ and $w_2$ are fixed peak widths and $E_1$ and $E_2$ are the positions of the inflection points resulting in an energy near the edge onset. Brotton et al. (2007) recommended setting the function slope w to 5 eV, to account for the slow onset of the continuum, arguing that a value smaller than 5 eV could induce spurious structures in the background-corrected spectrum. Bourdelle et al. (2021) adopt this recommendation for Mn, Bourdelle et al. (2013) prefer a value of 1 eV for Fe. Following these two contributions, we apply here the recommendation made for each, namely a w of 5 eV for Mn and 1 eV for Fe.

# 3. Toward a spectral calibration for quantifying Mn mean valence

## 3.1. Evolution of Mn L$_{2,3}$-edge spectra with changes in Mn redox state

A selection of Mn L$_{2,3}$-edge spectra from reference samples is shown in Fig. 1. Each L$_{3}$ and L$_{2}$ peak consists of one major modulation accompanied on either side by several minor peaks identified by letters. The energy position of the major peaks mainly depends on the core-excited final state, i.e. Mn redox state, corresponding to 641.6 and 654.1 eV for Mn$^{2+}$ (L$_{3}$-b and L$_{2}$-c), 643.2 and 654.4 eV for Mn$^{3+}$ (L$_{3}$-e and L$_{2}$-d), 644.6 and 655.2 eV for Mn$^{4+}$ (L$_{3}$-f and L$_{2}$-e), respectively. Here, the

<figure><img src="image_3.png" /><figcaption>Fig. 1. Representative Mn L$_{2,3}$-edge spectra for silicate and oxide reference samples with various Mn$^{x+}$ mean valence (mv). Spectra are normalised to the major L$_{3}$ peak intensity, and shifted vertically for clarity (normalised intensity with arbitrary units). The vertical lines indicate major (solid lines) and minor (dashed lines) peaks identified by letters. Redox states and core-excited final state configurations for each peak are also mentioned. The star symbol corresponds to silicate samples.</figcaption></figure>

position and the relative intensities of these different major peaks vary depending on the Mn redox state: the greater the electron deficit, the greater the energy of the major peak position. Obviously, spectra are qualitatively similar to those reported previously by EELS or XANES spectroscopies (Garvie and Craven, 1994; Morales et al., 2004; Zhang et al., 2010; Kubin et al., 2018).

The number, intensity and shape of minor peaks vary from one sample to another, arising from factors other than redox such as
coordination environment, crystal field strength, nature and length of metal–ligand bonds, or intra-atomic 3d-3d and 2p-3d Coulomb and spin–orbit interactions (e.g., van der Laan and Kirkman, 1992; de Groot et al., 2010). Upon disregarding these minor peaks, the overall shape of the spectra demonstrates broad similarities between silicates and oxides. Therefore, the spectral parameter-energy calibrations proposed below are based only on the major peaks, and it will be shown later that they are similarly influenced by redox for silicates and oxides, the complex study of minor peaks being not covered in the following.

## 3.2. Determination of Mn redox state from XANES Mn L<sub>2,3</sub>-edge intensity ratio

Building a quantitative map of the Mn redox requires the extraction and processing of energy images (subtraction of one energy image from another, energy image ratio, ...). Only an empirical calibration that correlates a straightforward spectral parameter – typically the intensity ratio – with the Mn mean valence can make this possible. Consequently, methods relying on spectral areas (Risch et al., 2017) should be discarded. The spectral parameter chosen for the empirical calibration does not necessarily require to have any specific physical significance, but should be as sensitive as possible to the Mn oxidation state. In this way, we can propose two types of calibration (Fig. 2). The first one is based on

<figure><img src="image_4.png" /><figcaption>Fig. 2. L<sub>2,3</sub>-edge intensity ratios from XANES spectra versus Mn mean valence for the reference samples. (a) R<sub>L2,3</sub> ratio, using selected intensities at one energy point (641.6 eV) of the L<sub>3</sub>-edge and one energy point (655.2 eV) of the L<sub>2</sub>-edge. (b) R<sub>L3</sub> ratio, using selected intensities at two energy points (641.6 and 644.6 eV) of the L<sub>3</sub>-edge. Vertical error bars represent the standard deviation calculated on the base of 4 or 5 spectra for each sample (82 spectra in total). Horizontal error bars represent the assumed error deviation due to chemical heterogeneity of samples. In dashed lines: confidence interval (95 %).</figcaption></figure>

the ratio between intensities at two energy points along the L<sub>3</sub>-edge, i.e. at the L<sub>3-b</sub> (641.6 eV, major peak of Mn<sup>2+</sup> L<sub>3</sub> edge spectra) and at the L<sub>3-f</sub> (644.6 eV, major peak of Mn<sup>4+</sup> L<sub>3</sub> edge spectra), such as:

$$ Mn mean valence = \frac{R_{L3} + 1.317}{0.730} \quad \text{with} \quad R_{L3} = \frac{I(at 644.6 eV)}{I(at 641.6 eV)} \quad (2) $$

considering that the Mn mean valence is a xMn<sup>2+</sup> + yMn<sup>3+</sup> + zMn<sup>4+</sup> combination, i.e.

Mn mean valence = $\frac{2x+3y+4z}{x+y+z}$ with x, y and z in atom(s) per formula unit.

Here, as the three oxidation states can be present in Mn silicates and oxides, it is not possible to determine the relative proportion of each of them directly and empirically; the Mn mean valence represents a good proxy.

The second one involves the intensity ratio between the L<sub>2-e</sub> (655.2 eV, major peak of Mn<sup>4+</sup> L<sub>2</sub>-edge spectra) and the L<sub>3-b</sub> (641.6 eV, major peak of Mn<sup>2+</sup> L<sub>3</sub> spectra) energy points, such as:

$$ Mn mean valence = \frac{R_{L2,3} + 1.001}{0.540} \quad \text{with} \quad R_{L2,3} = \frac{I(at 655.2 eV)}{I(at 641.6 eV)} \quad (3) $$

The coefficients of determination (R<sup>2</sup>) are very high in both cases, 0.97 and 0.96 for R<sub>L3</sub> (Eq. 2) and R<sub>L2,3</sub> (Eq. 3), respectively (Fig. 2).

## 3.3. Mn redox state mapping

The Mn redox map construction follows the approach proposed previously by Bourdelle et al. (2021), inspired from Fe mapping (Bourdelle et al., 2013). The scanning properties of the STXM allow a stack of ~240 energy images to be recorded over the 630–660 eV. Since eqs. (2) and (3) estimate the Mn mean valence from the ratio of absorption intensities at two energy points, only two energy images extracted from the stack are necessary to obtain a map. Three additional energy images are used to subtract the background (linear and arctan) from the two energy images of interest, as detailed in Fig. 3.

## 3.4. Crystal lattice orientation dependency of XANES L<sub>2,3</sub>-edge spectra

Although the XANES spectra at the L<sub>2,3</sub>-edges express the X-ray absorption involved in the 2p → 3d electronic transition – leading to a spectral sensitivity to the Mn and Fe oxidation states – the relative height of peaks may also vary according to the orientation of the crystallographic axes with respect to the light polarization (Brouder, 1990). In this way, Elmaleh et al. (2015) tested the angular dependence of X-ray absorption measuring XANES spectra on a FIB section of cronstedtite at the Fe L<sub>2,3</sub>-edges. The authors highlighted that, under a linearly polarized light with a normal incidence (i.e. perpendicular to the c axis of cronstedtite crystal), the spectra present a noticeable dichroism between vertical and horizontal directions of the polarization vector (noted ε), especially when ε // c. However, authors underlined that (1) this dichroism is not observed when the light incidence is parallel to the c axis, (2) the dichroism is particularly present for crystals with a trigonal symmetry (like the studied cronstedtites), (3) the low probability to encounter the specific orientation ε // c reduces the possibility of a polarization effect on the redox quantification. STXM beamlines often offer the possibility to use a circularly polarized light, similar to averaging out ε orientations. In the same way, the analysis of powders limits the probability of an unfavourable crystallite orientation, due to the relative disorientation of dropped particles.

Despite these considerations, we tested the angular dependence of X-ray absorption measuring XANES spectra on a FIB section of piemontite at the Mn L<sub>2,3</sub>-edges. This piemontite comes from Praborna (Italy), and displays a chemical zonation, from a Mn-rich core to a Mn-poor rims. The FIB section was cut across the zonation following the direction of the c axis (c axis in the plane of the section), and was analysed under a linearly polarized light with different ε vector orientations (tested angles
<figure><img src="image_5.png" /></figure>

(caption on next page)
**Fig. 3.** Mapping of the Mn mean valence from 5 selected energy images: two images in the pre-edge region (to apply the “linear background correction” at each pixel of the image), two at energies recommended by the Eq. 2 (641.6 eV and 644.6 eV) and one at 650 eV to remove the first edge step of the arctan function. Finally, the ratio of the resulting 641.6 and 644.6 eV images (noted A and B) can be used to determine the R<sub>1,3</sub> parameter at each pixel of the image and obtain Mn redox mapping. The map building from Eq. 3 (i.e. using the R<sub>1,2,3</sub> parameter) was similarly explained, thanks to additional images extracted at 655.2 eV and 660 eV (to remove the second edge step). All images are OD images, where the Mn-rich and Mn-poor zones of a chemically zoned garnet are the white- and light-grey phases, respectively. As an illustration, spectrum was retrieved from 240 images (i.e. 240 energy points) on the Mn-rich zone (brown rectangle on image A). Case of FIB section of garnet from Praborna. Scale bar: 200 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

between $\epsilon_{\text{horizontal}}$ and FIB section: 0 ($\epsilon$ // c), 45 and 90° ($\epsilon$ ⊥ c)). Results are shown by the Fig. 4. For each orientation, three spectra were extracted: one Mn spectrum into the Mn-rich zone, and one Fe spectrum and one Mn spectrum into the Mn-poor zone. By comparing the different spectra obtained according to the polarization vector orientation, no azimuthal angular dependence is observed. The few differences that could be observed with a fine eye concern the Fe spectra, not the Mn ones, and still, with no consequences on Fe redox quantification. Admittedly, the piemontite has not a trigonal symmetry, but these observations combined with the above ones, tend to show that the light polarization orientation has a negligible impact on the Mn mean value estimates.

Considering the minor effect of the light polarization orientation (especially using circularly polarized light), the undetectable effects of incident photon beam damages, the cautions to obtain spectrum free of saturation effects (Bourdelle et al., 2021), the low standard deviation calculated from 4 or 5 spectra from several particles for each reference samples – showing the great homogeneity and repeatability of the measurements, and the uncertainty on the data acquisition and quantification at the L<sub>2,3</sub>-edges, the total uncertainty of the redox quantification method below is of the order of 10 %.

# 4. Application to geological cases

## 4.1. Crystallochemistry of kôzulite

Considering that kôzulite may contain Fe<sup>2+</sup>, Fe<sup>3+</sup>, Mn<sup>2+</sup> and Mn<sup>3+</sup> in various proportion, obtaining a structural formula requires to couple chemical analyses with a full quantification of both Mn and Fe redox states. In this way, the XANES protocol outlined here was applied on a dozen of kôzulite crystallites. Fig. 5 illustrates a representative XANES spectrum of kôzulite over the full 630–730 eV range. The Mn L<sub>3</sub>-edge of kôzulite strongly resembles to pure Mn<sup>2+</sup> spectra, with a major peak at 641.6 eV (L<sub>3,b</sub>) and 2 additional modulations at around 643.0 eV (L<sub>3-d</sub>) and 644.6 eV (L<sub>3-f</sub>). However, the latter displays a shoulder with a higher starting point on L<sub>3-d</sub> peak than observed in other pure Mn<sup>2+</sup>-phases, leading to a higher L<sub>3-d</sub>/L<sub>3-b</sub> intensity ratio. Similarly, the Mn L<sub>2</sub>-edge has a general shape in accordance with a Mn<sup>2+</sup>-phase, except for a small energy shift of the maximum peak toward higher energies (i.e. from the L<sub>2-c</sub> to the L<sub>2-d</sub> position) and for the presence of the L<sub>2</sub>-edge tail around the 655.2 eV point. These subtle variations force to consider a small amount of Mn<sup>3+</sup>, like in franklinfurnaceite. Conversely, the Fe L<sub>2,3</sub>-edges distinctly mirror the spectral features of a pure Fe<sup>3+</sup> phase (Fig. 5), as

<figure><img src="image_6.png" /></figure>

**Fig. 4.** X-ray absorption spectra of zoned piemontite at the Fe L<sub>2,3</sub>-edges measured in transmission mode (STXM) (background correction). a we used a linear polarization with different polarization vector ($\epsilon$) orientations. c axis into the FIB section plane. Images of the FIB section, showing the chemical zonation of the targeted piemontite crystal. Spectra extraction areas are located by coloured squares. b Spectra obtained on each area, for Mn and Fe.
<figure><img src="image_7.png" /><figcaption>Fig. 5. Spectrum of kôzulite over the 630–730 eV energy range. Background is subtracted. Energy points selected for redox quantifications are highlighted (in blue for Mn, in red for Fe). Quantification results are indicated. In grey: representative spectrum of spessartite (Mn<sub>total</sub> = Mn<sup>2+</sup>) and nontronite (Fe<sub>total</sub> = Fe<sup>3+</sup>) for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

described by Bourdelle et al. (2013).

To confirm these initial findings, the proposed Mn<sup>n+</sup> calibrations were applied, in combination with the Fe<sup>n+</sup> (L<sub>3</sub>) calibration of Bourdelle et al. (2013). Indeed, Fe and Mn are the most common variable-valence metals, and consequently, calculating a structural formula knowing the redox of only one of the two can only give an approximate or even unsatisfactory result. Results are also summarized in Fig. 5. The purely ferric nature of the kôzulite sample is confirmed by a R<sub>L3-(Fe3+)</sub> around 2.20 (i.e. Fe<sup>3+</sup>/ΣFe = 1.00), while the Mn mean valence calculated from Eq. 2 and 3 is of 2.21 and 2.18, respectively, demonstrating the excellent convergence of these two calibrations. Coupled with EMP data, these results support a Mn<sup>2+</sup>-Mn<sup>3+</sup> mixture with a possible ratio of 4Mn<sup>2+</sup> per 1Mn<sup>3+</sup>, allowing a refinement of kôzulite formula:

$$ (K_{0.18}Na_{0.82})(Na_{1.90}Ca_{0.10})(Mn^{2+}_{2.70}Mn^{3+}_{0.67}Mg_{1.06}Fe^{3+}_{0.54}) (Si_{7.84}Al_{0.07}Ti_{0.08})O_{22}(OH)_2 $$

This structural formula of natural kôzulite specimen is in line with the description proposed by Barkley et al. (2010). In this way, the new Mn<sup>n+</sup> quantification calibrations demonstrate their validity. Moreover, the coupling of the Mn<sup>n+</sup> – Fe<sup>n+</sup> protocols allows to calculate a rigorous structural formula, since these two variable-valence metals are well characterized and each oxidation state is quantified. This is an undeniable advantage, particularly in phases where Fe is in a mixed oxidation state. In practical terms, this coupling does not involve analyzing the same area twice (once for Fe and once for Mn), but rather carrying out a single acquisition over the 600–730 eV range and then applying both calibrations to the same dataset.

## 4.2. Chemically-zoned Mn-garnet from Praborna mine (Aosta valley, Italy)

The first case study allowed us to validate new Mn<sup>n+</sup> quantification calibrations and to test the coupling of the Mn<sup>n+</sup> and Fe<sup>n+</sup> protocols to calculate structural formulae. Here, calibrations are used to map potential small-scale redox variations of these metals in chemically-zoned crystals.

For this, the proposed protocol was implemented on a chemically-zoned 70 µm-sized garnet from the Praborna region. Analyses were carried out on a FIB section in order to preserve the texture of minerals and to map the chemical variations on a sample with a constant thickness. The targeted garnet exhibits a circular shape with a Mn-enriched/

Fe-depleted core and a Mn-depleted/Fe-enriched rim, as shown by SEM-EDX map (Fig. 6a), in line with previous descriptions of spessartite-core/calderite-rim garnets in Praborna (Cenky-Tok and Chopin, 2006). A FIB section was cut across the boundary between Mn-rich and Fe-rich zones (Fig. 6b).

Two images extracted at the energies of Fe and Mn maximum peaks clearly evidence the Mn-rich zone on the right, Fe-rich zone on the left of the FIB section (Fig. 6c). These images display a clear boundary between them, or at least that the diffusion – if exists – occurs over a distance of less than 100 nm. Further images were extracted, and subsequent maps were built following the mapping protocol described above. As taking an intensity point on the L<sub>3</sub>-edge and another one on the L<sub>2</sub>-edge improves the signal-to-noise ratio, the Mn redox map was built from Eq. 3, while the Fe redox map was built from the R<sub>L3</sub> defined by Bourdelle et al. (2013). Mn redox map reveals that the Mn valence is 2+ and homogeneous in both zones (Fig. 6c), while Fe redox map indicates a pure Fe<sup>2+</sup> phase in the Fe rich zone, and a mix of Fe<sup>2+</sup>-Fe<sup>3+</sup> in the Fe poor zone, with a Fe<sup>3+</sup>/ΣFe ratio of 15–20 % (Fig. 6d). Here, the Fe<sup>3+</sup> rate varies inversely with the Fe concentration. To illustrate these redox distributions, several spectra were extracted from each chemical zone corroborating the maps: spectra from the Mn-rich and Mn-poor zones are similar, corresponding to a pure Mn<sup>2+</sup> phase (Fig. 6c), spectra from Fe-rich and Fe-poor zones show clear differences in their L<sub>3-b</sub>/L<sub>3-a</sub> ratios, resulting in an increase of the estimated Fe<sup>3+</sup> proportion from <5 to ~15–20 % across the chemical boundary (Fig. 6d). These redox distributions deviate from the description offered by Cenky-Tok and Chopin (2006) based on stoichiometric considerations. In this way, the chemically zoned garnet studied here likely differs from a spessartite-calderite specimen since Fe is mainly Fe<sup>2+</sup>. Although a purely divalent Mn implies a spessartite-calderite contribution from a solid-solution perspective, a low Fe<sup>3+</sup>/ΣFe ratio suggests a significant almandine contribution. In light of these STXM-XANES results, a reappraisal of the previous studies on Praborna garnets could be envisaged to provide new insights on their formation conditions, particularly considering this low Fe<sup>3+</sup>/ΣFe ratio.

## 5. Conclusion

We demonstrated the reliability of two calibrations based on 19 reference samples that allow quantitative determination of Mn redox state in silicate phases using STXM coupled with XANES spectroscopy at the Mn L<sub>2,3</sub>-edges. Coupled with the similar Fe redox state calibration, these protocols become non-destructive tools for quantifying and
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Mapping of Mn and Fe redox states on a FIB section cut across a chemically-zoned garnet from Praborna. (a) Back scattered electron image (left) and SEM-EDX map (right), highlighting the garnet chemical zoning: Mn-rich/Fe-poor core and Mn-poor/Fe-rich rims. FIB section is located. (b) OD image of the FIB section. The boundary between Mn-rich and Fe-rich zones is located. The yellow rectangle corresponds to the position of maps presented in c and d. (c, d) X-ray absorption images, in orange-yellow gradient, extracted at the Mn and Fe L₃-edge maximum intensity. Redox maps for Mn (Mn mean valence) and Fe (Fe³⁺/ΣFe; %) around the Mn-rich and Fe-rich zones boundary (see the mapping protocol). Mn and Fe spectra extracted for the two zones (extraction locations are indicated by red and green squares). Redox results are noted on the colour scales and beside spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

mapping Mn and Fe oxidation states on the same targeted area, offering valuable insights into the nanoscale crystallochemistry of silicates. Applied to two case studies, we proved the feasibility of mapping the Mn redox with a spatial resolution of 30 nm per pixel, and illustrated the benefits of coupling Mnⁿ⁺-Feⁿ⁺ calibrations (rigorous structural formula calculation, insights on redox conditions, comparison of oxidation state variations of these two metals through a chemically-zoned crystallite, ...). This unique approach opens up new perspectives to refine crystallochemistry of silicates by understanding how redox changes with concentration as well as determining chemical distributions along zonation with an unprecedented resolution.

**CCRediT authorship contribution statement**

**Franck Bourdelle:** Writing – original draft, Visualization, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Cyril Durand:** Writing – review & editing, Visualization, Methodology, Investigation, Conceptualization. **Erwan Paineau:** Writing – review & editing, Visualization, Methodology, Investigation, Conceptualization. **Emily Lloret:** Writing – review & editing, Visualization, Methodology, Investigation, Conceptualization. **Laura Airaghi:** Writing – review & editing, Investigation, Conceptualization.

**Declaration of competing interest**

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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**

We would like to thank Oleg S. Pokrovsky, Editor, and the two anonymous reviewers for their constructive comments and suggestions. We are most grateful to the Paul Scherrer Institute, Swiss Light Source, and especially Benjamin Watts for technical advice during experiments at PolLux beamline. Our acknowledgements are also extended to SUN synchrotron, Sufal Swaraj and Rachid Belkhou for assistance on Hermes beamline. We would like to thank the IEMN – Lille and David Troadec for the FIB section preparations, the Chevreul Institut – Lille and Séverine Bellayer for EMP analysis, and the Carmin platform and Sandra Ventalon for SEM-EDX analysis. Thanks also go to the Musée National d’Histoire Naturelle and Olivier Beyssac (IMPMC), the Franklin Mineral Museum and especially Earl Verbeek, and François Guillot (LOG) for providing some Mn-silicates. This study was financially supported by lab funds - LGCgE and GEC (ISTeP).

**Appendix A. Supplementary data**

Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemgeo.2025.122739.

**Data availability**

Data will be made available on request.

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