Zhongkuan Wu

Bruno Lanson
email: bruno.lanson@univ-grenoble-alpes.fr

Xionghan Feng

Hui Yin

Zhangjie Qin

Xiaoming Wang

Wenfeng Tan

Zhongjun Chen

Wen Wen

Fan Liu
email: liufan@mail.hzau.edu.cn

Transformation of Ni-containing birnessite to tectomanganate: Influence and fate of weakly

Keywords: nickel, buserite, asbolane, todorokite, Mn oxides

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INTRODUCTION

Phyllomanganates and tectomanganates are the two mineral families accounting for most of Mn oxides found in terrestrial and aquatic environments (Post, 1999;Peacock and Sherman, 2007a). In the former, edge-sharing MnO 6 octahedra form layers whose symmetry depends on the abundance and distribution of Mn(III) octahedra, whereas in the latter, chains of edge-sharing Mn(IV)O 6 octahedra form the walls or floor/ceiling of tunnels. Birnessite is, together with its disordered structural analogue vernadite, the most common phyllomanganate and consists of a MnO 2 layer, whose layer charge deficit is compensated for by the presence of cations sorbed at layer vacancy sites or present as interlayer hydrated species (7.0-7.2 Å layer-to-layer distance). Heterovalent Mn cations and/or foreign transition metal cations can be hosted in both families, either structurally substituted for Mn(IV) or as charge compensating cations located in phyllomanganate interlayers or in tectomanganate tunnels (Post, 1999). The occurrence of natural phyllomanganates as micro-to nano-sized cryptocrystalline minerals usually enhances their surface reactivity that originates from their high negative layer charge and from the coexistence of heterovalent Mn cations in these structures. As the result, natural phyllomanganates are highly reactive with respect to both organic [START_REF] Remucal | A critical review of the reactivity of manganese oxides with organic contaminants[END_REF] and inorganic compounds. Specifically, natural phyllomanganates are readily associated with transition metals such as Co, Cu, Zn, or Ni [START_REF] Chukhrov | Crystallochemical nature of Co-Ni asbolan[END_REF][START_REF] Manceau | Crystal chemistry of cobalt and nickel in lithiophorite and asbolane from New Caledonia[END_REF]Manceau et al., , 1992aManceau et al., ,b, 2002Manceau et al., , 2007aManceau et al., ,b, 2014;;[START_REF] Marcus | Mn, Fe, Zn and As speciation in a fast-growing ferromanganese marine nodule[END_REF][START_REF] Bodeï | Formation of todorokite from vernadite in Ni-rich hemipelagic sediments[END_REF]Peacock and Sherman, 2007a;[START_REF] Lanson | Formation of Zn-Ca phyllomanganate nanoparticles in grass roots[END_REF][START_REF] Spinks | Detection of zinc deposits using terrestrial ferromanganese crusts[END_REF][START_REF] Zawadzki | Geochemistry of cobalt-rich ferromanganese crusts from the Perth Abyssal Plain (E Indian Ocean)[END_REF], rare earth [START_REF] Kasten | Rare earth elements in manganese nodules from the South Atlantic Ocean as indicators of oceanic bottom water flow[END_REF][START_REF] Ohta | REE(III) adsorption onto Mn dioxide (delta-MnO 2 ) and Fe oxyhydroxide: Ce(III) oxidation by delta-MnO 2[END_REF][START_REF] Ohnuki | Sorption of trivalent cerium by a mixture of microbial cells and manganese oxides: Effect of microbial cells on the oxidation of trivalent cerium[END_REF][START_REF] Zawadzki | Geochemistry of cobalt-rich ferromanganese crusts from the Perth Abyssal Plain (E Indian Ocean)[END_REF], and radioactive elements [START_REF] Duff | Mineral associations and average oxidation states of sorbed Pu on tuff[END_REF]Post, 1999).

In an effort to disentangle the mobility, (bio)availability, and fate of these elements in different geological settings, structural interactions of phyllomanganates with such foreign elements, and more especially with transition metals, have thus attracted the attention of the scientific community over the last few decades [START_REF] Manceau | Structural mechanism of Co 2+ oxidation by the phyllomanganate buserite[END_REF][START_REF] Manceau | Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scales of resolution[END_REF][START_REF] Manceau | Mineralogy and crystal chemistry of Mn, Fe, Co, Ni, and Cu in a deep-sea Pacific polymetallic nodule[END_REF][START_REF] Ohta | REE(III) adsorption onto Mn dioxide (delta-MnO 2 ) and Fe oxyhydroxide: Ce(III) oxidation by delta-MnO 2[END_REF]Lanson et al., 2002a;[START_REF] Marcus | Mn, Fe, Zn and As speciation in a fast-growing ferromanganese marine nodule[END_REF][START_REF] Toner | Zinc sorption to biogenic hexagonal-birnessite particles within a hydrated bacterial biofilm[END_REF]Peacock and Sherman, 2007a,b;[START_REF] Peacock | Physiochemical controls on the crystal-chemistry of Ni in birnessite: Genetic implications for ferromanganese precipitates[END_REF][START_REF] Peña | Mechanisms of nickel sorption by a bacteriogenic birnessite[END_REF][START_REF] Roque-Rosell | Sorption of Ni by "lithiophorite-asbolane" intermediates in Moa Bay lateritic deposits, eastern Cuba[END_REF][START_REF] Sherman | Surface complexation of Cu on birnessite (δ-MnO2): Controls on Cu in the deep ocean[END_REF]Yin et al., 2011a,b;[START_REF] Kwon | Understanding the trends in transition metal sorption by vacancy sites in birnessite[END_REF][START_REF] Yu | Zinc Sorption During Bio-oxidation and Precipitation of Manganese Modifies the Layer Stacking of Biogenic Birnessite[END_REF][START_REF] Burlet | Study of the spectro-chemical signatures of cobalt-manganese layered oxides (asbolane-lithiophorite and their intermediates) by Raman spectroscopy[END_REF]Simanova et al., 2015). In most studies, foreign elements were structurally substituted for Mn(IV) in the MnO 6 octahedra or sorbed as inner-sphere complexes either at vacant layer sites and/or at particle edges [START_REF] Manceau | Structural mechanism of Co 2+ oxidation by the phyllomanganate buserite[END_REF]Yin et al., 2011a[START_REF] Yin | High Co-doping promotes the transition of birnessite layer symmetry from orthogonal to hexagonal[END_REF][START_REF] Yu | Zinc Sorption During Bio-oxidation and Precipitation of Manganese Modifies the Layer Stacking of Biogenic Birnessite[END_REF][START_REF] Ohnuki | Sorption of trivalent cerium by a mixture of microbial cells and manganese oxides: Effect of microbial cells on the oxidation of trivalent cerium[END_REF][START_REF] Simanova | Time-Resolved Investigation of Cobalt Oxidation by Mn(III)-Rich delta-MnO 2 Using Quick X-ray Absorption Spectroscopy[END_REF][START_REF] Qin | The preferential retention of VI Zn over IV Zn on birnessite during dissolution/desorption[END_REF]. Similar to other transition metals, presence of Ni(II) forming inner-sphere complexes species at vacancy or edge sites and/or structurally incorporated in phyllomanganate layers has been extensively documented [START_REF] Manceau | Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scales of resolution[END_REF](Manceau et al., , 2007a,b;,b;Peacock and Sherman, 2007a,b;[START_REF] Grangeon | Crystal structure of Ni-sorbed synthetic vernadite: a powder X-ray diffraction study[END_REF][START_REF] Peña | Mechanisms of nickel sorption by a bacteriogenic birnessite[END_REF][START_REF] Yin | Characterization of Ni-rich hexagonal birnessite and its geochemical effects on aqueous Pb 2+ /Zn 2+ and As(III)[END_REF][START_REF] Yin | Effects of Co and Ni co-doping on the structure and reactivity of hexagonal birnessite[END_REF][START_REF] Kwon | Understanding the trends in transition metal sorption by vacancy sites in birnessite[END_REF]Simanova et al., 2015). Under low pH conditions, Ni(II) occurs mainly as triple-or double-corner sharing (TCS and DCS, respectively) inner-sphere complex above/below vacancy or at edge sites of the phyllomanganate layers (Peacock and Sherman, 2007b;[START_REF] Grangeon | Crystal structure of Ni-sorbed synthetic vernadite: a powder X-ray diffraction study[END_REF][START_REF] Peacock | Physiochemical controls on the crystal-chemistry of Ni in birnessite: Genetic implications for ferromanganese precipitates[END_REF][START_REF] Peña | Mechanisms of nickel sorption by a bacteriogenic birnessite[END_REF]Simanova et al., 2015). Migration from TCS/DCS sites to octahedral layer sites is favored by increasing contact time and increasing pH [START_REF] Peacock | Physiochemical controls on the crystal-chemistry of Ni in birnessite: Genetic implications for ferromanganese precipitates[END_REF], this structural incorporation within the octahedral MnO 2 layer being reversible [START_REF] Peacock | Physiochemical controls on the crystal-chemistry of Ni in birnessite: Genetic implications for ferromanganese precipitates[END_REF][START_REF] Peña | Mechanisms of nickel sorption by a bacteriogenic birnessite[END_REF] and essentially restricted to low Ni contents [START_REF] Manceau | Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scales of resolution[END_REF][START_REF] Bodeï | Formation of todorokite from vernadite in Ni-rich hemipelagic sediments[END_REF]Peacock and Sherman, 2007a,b;[START_REF] Kwon | Understanding the trends in transition metal sorption by vacancy sites in birnessite[END_REF]Simanova et al., 2015;[START_REF] Atkins | Release of Ni from birnessite during transformation of birnessite to todorokite: Implications for Ni cycling in marine sediments[END_REF], consistent with natural occurrences [START_REF] Manceau | Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scales of resolution[END_REF](Manceau et al., , 2007a,b;,b;[START_REF] Bodeï | Formation of todorokite from vernadite in Ni-rich hemipelagic sediments[END_REF]Peacock and Sherman, 2007a).

In addition, hydrated Ni(II) or Ni forming outer-sphere complexes [START_REF] Turner | Todorokites: A new family of naturally occurring manganese oxides[END_REF]Peacock and Sherman, 2007a) and Ni(II) (hydr)oxides [START_REF] Chukhrov | Structural Transformations of Manganese Oxides in Oceanic Nodules[END_REF][START_REF] Manceau | Crystal chemistry of cobalt and nickel in lithiophorite and asbolane from New Caledonia[END_REF]Manceau et al., , 1992a,b) ,b) may also be associated with Mn oxides. Despite their widespread natural occurrences for high Ni contents (NiO ~10-20 wt.%, [START_REF] Llorca | Nouvelles données sur la composition et la structure des asbolanes (Nouvelle-Calédonie)[END_REF][START_REF] Roque-Rosell | Sorption of Ni by "lithiophorite-asbolane" intermediates in Moa Bay lateritic deposits, eastern Cuba[END_REF][START_REF] Ploquin | Phyllomanganate vein-infillings in faulted and Al-poor regoliths of the New Caledonian ophiolite: periodic and sequential crystallization of Ni-asbolane, Alk-birnessite and H-birnessite[END_REF], these weaker associations have received little attention compared to structurally incorporated Ni or to Ni inner-sphere complexes. The presence of hydrated Ni(II) species in phyllomanganate interlayers leads to the presence of two planes of interlayer water molecules, thus increasing the layer-to-layer distance from ~7.2 Å (birnessite or vernadite) to ~9.6-10.2 Å (buserite - [START_REF] Burns | A review of the todorokite-buserite problem; implications to the mineralogy of marine manganese nodules[END_REF][START_REF] Chukhrov | Structural Transformations of Manganese Oxides in Oceanic Nodules[END_REF][START_REF] Golden | Transformation of birnessite to buserite, todorokite, and manganite under mild hydrothermal treatment[END_REF][START_REF] Manceau | Structural mechanism of Co 2+ oxidation by the phyllomanganate buserite[END_REF][START_REF] Manceau | Mineralogy and crystal chemistry of Mn, Fe, Co, Ni, and Cu in a deep-sea Pacific polymetallic nodule[END_REF][START_REF] Bodeï | Formation of todorokite from vernadite in Ni-rich hemipelagic sediments[END_REF][START_REF] Wegorzewski | Mineralogical characterization of individual growth structures of Mn-nodules with different Ni+Cu content from the central Pacific Ocean[END_REF]. Such increased hydration of Ni-rich phyllomanganates has for example been reported by Manceau et al. (2007b).

The presence of hydrated Ni(II) in phyllomanganate interlayers has been described also as favorable to the transformation of phyllomanganate to tectomanganates upon hydrothermal treatment [START_REF] Krukowska | A new synthetic route to todorokite-type manganese oxides[END_REF][START_REF] Luo | Double-aging method for preparation of stabilized Na-buserite and transformations to todorokites incorporated with various metals[END_REF][START_REF] Onda | Synthesis of manganese oxide octahedral molecular sieves containing cobalt, nickel, or magnesium, and the catalytic properties for hydration of acrylonitrile[END_REF], similar to Mg(II) [START_REF] Golden | Synthesis of todorokite[END_REF][START_REF] Golden | Transformation of birnessite to buserite, todorokite, and manganite under mild hydrothermal treatment[END_REF]. In addition, Ni(II) (hydr)oxides may polymerize in phyllomanganate interlayers either as a complete or as an incomplete brucite-like octahedral layer, leading to litiophorite-or asbolane-like species, respectively, both having a layer-to-layer distance of 9.4-9.8 Å [START_REF] Chukhrov | Structural Transformations of Manganese Oxides in Oceanic Nodules[END_REF][START_REF] Manceau | Crystal chemistry of cobalt and nickel in lithiophorite and asbolane from New Caledonia[END_REF]Manceau et al., , 1992a,b;,b;[START_REF] Feng | Hydrothermal Soft Chemical Synthesis of Ni(OH) 2 -Birnessite Sandwich Layered Compound and Layered LiNi 1/3 Mn 2/3 O 2[END_REF].

The present article thus focuses on the association of these weakly bound Ni(II) species with phyllomanganates. Synthetic samples with contrasting Ni contents were prepared and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, extended X-ray absorption fine structure spectroscopy (Mn and Ni K-edges). In addition, phyllomanganate samples were hydrothermally treated and reaction products characterized to investigate the effect of Ni on the phyllomanganate-to-tectomanganate transformation [START_REF] Golden | Synthesis of todorokite[END_REF][START_REF] Shen | Manganese oxide octahedral molecular sieves: preparation, characterization, and applications[END_REF][START_REF] Feng | Synthesis of todorokite at atmospheric pressure[END_REF][START_REF] Feng | Formation of nano-crystalline todorokite from biogenic Mn oxides[END_REF][START_REF] Atkins | Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments[END_REF][START_REF] Grangeon | Solid-state transformation of nanocrystalline phyllomanganate into tectomanganate: influence of initial layer and interlayer structure[END_REF][START_REF] Grangeon | Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms[END_REF][START_REF] Zhao | Formation of todorokite from "c-disordered" H + -birnessites: the roles of average manganese oxidation state and interlayer cations[END_REF] and the fate of weakly associated Ni(II) species during this transformation. The negative impact of Ni, when present at high content, on this transformation is contrasted with that of Co. Buserite-, litiophorite-, and asbolane-like phyllomanganates all have interplanar periodicities similar to those of todorokite, a common tectomanganate with a uniform 3×3 tunnel structure [START_REF] Post | Rietveld refinement of the todorokite structure[END_REF], however, leading to their frequent confusion [START_REF] Burns | A review of the todorokite-buserite problem; implications to the mineralogy of marine manganese nodules[END_REF]. Special care was thus paid to allow differentiating these species using nitric acid treatment [START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF], Fourier transform infrared spectroscopy and anomalous X-ray diffraction. An optimum procedure allowing this differentiation is proposed.

MATERIALS AND METHODS

Synthesis of layered precursors and reflux products

Both initial layered precursors and reflux products described in the present study were prepared as in our previous work on their Co analogues [START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF].

Briefly, layered precursors were prepared after [START_REF] Feng | Synthesis of todorokite at atmospheric pressure[END_REF] and [START_REF] Song | Cobalt-doped todorokites prepared by refluxing at atmospheric pressure as cathode materials for Li batteries[END_REF] by adding a concentrated NaOH solution into a Ni(II) + Mn(II) solution, prepared from their chloride salts. The resulting solution was then mixed and bubbled with oxygen gas for 5 h. Resulting black precipitates were washed and one half was dried for subsequent analyses. Initial Ni / (Ni + Mn) ratios were 0.00, 0.01, 0.05, 0.10, 0.15, and 0.20. Accordingly, initial layered samples were named Bir, NiB1, NiB5, NiB10, NiB15 and NiB20, and those Ni-containing samples are collectively referred to as NiB. The other half of the precipitates was used to prepare todorokite through a reflux process following the protocol of [START_REF] Feng | Synthesis of todorokite at atmospheric pressure[END_REF]. Wet precipitates were thus exchanged with Mg 2+ , rinsed, re-suspended in deionized water, and finally refluxed at 100 °C for 24 h. Reflux products were finally filtered, washed, and dried.

Transformation products of Bir, NiB1, NiB5, NiB10, NiB15 and NiB20 are named Tod, NiT1, NiT5, NiT10, NiT15 and NiT20, respectively; they are collectively referred to as NiT except for Tod. Both layered precursors and reflux products were dried at 60 °C for three days and ground into fine powders.

Characterization of layered precursors and reflux products

The elemental composition of all samples was determined in triplicate using atomic absorption spectrometry (Varian AAS 240FS) from 0.1000 g of sample dissolved in 25 mL solution of 0.25 mol L -1 NH 2 OH•HCl and 5 mL of 1 mol L -1 H 2 SO 4 . In addition, the specific surface area (SSA) of the reflux products was determined with the Brunauer-Emmett-Teller (BET) method and a standard adsorption analyzer (Quantachrome Autosorb-1, JEDL-6390/LV). Finally, a non-reducing nitric acid treatment [START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF] was performed to differentiate tectomanganates (todorokite) from phyllomanganates (buserite-, lithiophorite-, or asbolane-like phases - [START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF]. Briefly, 0.1 g sample was put in 250 mL of a 1 M HNO 3 solution with soft stirring at ambient temperature (20 °C). At different time intervals, 5 mL of the suspension was collected and readily filtered through a 0.22 µm membrane to determine Mn, Ni, and Mg release. After 1 week of this HNO 3 treatment, residual samples were dried at 60 °C for 3 days and analyzed with X-ray diffraction (XRD) at a scan speed of l0 ° 2 .min -1 .

Mineralogy of all samples was determined from powder XRD analysis using a Bruker D8 Advance diffractometer equipped with Cu Kα source ( = 0.15418 nm)

and operated at 40 kV and 40 mA. XRD patterns were collected at 0.02 °2 intervals using a continuous scan mode. XRD data was collected on fresh samples (dried at 60 °C for three days) at a scan speed of l0 ° 2 .min -1 . To assess the stability of their hydration, XRD data was collected on fresh samples after heating to 140 °C. Finally, XRD patterns were collected at l ° 2 .min -1 for aged samples that were kept dry in the dark for three months. Synchrotron XRD patterns were also collected on selected reflux products on beamline BL14B1 at the Shanghai Synchrotron Radiation Facility (SSRF) and on beamline 4B9A at the Beijing Synchrotron Radiation Facility (BSRF).

At BSRF, XRD data was collected at the Mn K-edge (6.5 keV), Ni K-edge (8.3 keV), and 12 keV with a typical beam size of 2 × 1 mm 2 , whereas the 18 keV data was collected at SSRF with a typical beam size of ~0.2 × 0.3 mm 2 . In both cases, X-rays were monochromatized with a double-crystal Si (111) monochromator. Finally, high-energy X-ray scattering data was collected in transmission mode on beamline 11-ID-B at the Advanced Photon Source (Argonne National Laboratory, Argonne, IL), using a ∼90 keV X-ray energy. Pair distribution functions (PDF) and differential PDFs were calculated as described elsewhere [START_REF] Li | Differential pair distribution function study of the structure of arsenate adsorbed on nanocrystalline γ-alumina[END_REF][START_REF] Wang | Effect of ferrihydrite crystallite size on phosphate adsorption reactivity[END_REF].

Infrared spectra were collected for both layered precursors and reflux products using a Bruker VERTEX 70. For each sample, 64 spectra were collected and summed over the 4000-400 cm -1 range with a 4 cm -1 resolution [START_REF] Zhao | Formation of todorokite from "c-disordered" H + -birnessites: the roles of average manganese oxidation state and interlayer cations[END_REF].

Micromorphology of the reflux products and lattice periodicities were observed a Jeol JEM2100F transmission electron microscope (TEM) operated at 200 kV. TGA was performed on a NETZSCH TG 209 instrument with a heating rate of 10 °C min -1 and a N 2 flow of 20 mL min -1 . Finally, X-ray photoelectron O1s spectra were collected with a VG Multilab2000. X-ray photoelectron spectroscopy (XPS) measurements used an Al K X-ray source (1486.6 eV) at a chamber base pressure of 3 × 10 -9 Torr (Yin et al., 2011a). The O 1s spectra were collected with a 25 eV pass energy and 0.1 eV energy steps, and analyzed with the Avantage software. All samples were charge-corrected to C 1s with a binding energy of 284.80 eV collected from the surface adventitious carbon [START_REF] Wang | Study on the shape control and photocatalytic activity of high-energy anatase titania[END_REF]. A Shirley-type background was subtracted before deconvolution and fitting. A Lorentzian:Gaussian ratio of 30:70 was used for all the fittings.

Finally, X-ray absorption spectra were collected at the Mn and Ni K-edges on the 1W1B beamline of BSRF to determine the relative proportions of Mn(II), Mn(III),

and Mn(IV) species and the local environments of Mn and Ni in both layered precursors and reflux products. Data was recorded at room temperature in transmission (Mn) and fluorescence (Ni) modes, and the energy was calibrated using metallic Mn/Ni foils as references. Extended X-ray absorption fine structure (EXAFS) spectra were processed using Athena and Artemis [START_REF] Ravel | ATHENA and ARTEMIS: interactive graphical data analysis using IFEFFIT[END_REF]. X-ray absorption near-edge structure (XANES) spectra of both layered precursors and reflux products were used to determine Mn average oxidation state (AOS) with a specific linear combination fitting method (the so-called Combo method). Mn(II), Mn(III), and Mn (IV) reference compounds were those of the original study (Table 1 in [START_REF] Manceau | Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy[END_REF].

RESULTS

Elemental composition

With increasing Ni content, the Mn content decreases from ~55.4 to ~44.8 wt.% for layered precursors and from ~54.4 to ~49.2 wt. % for reflux products (Table 1).

The Ni/(Mn+Ni) molar ratios are essentially similar in layered precursors and their respective reflux products, with only a very limited fraction of Ni being lost either during the Mg-for-(Na,Ni) exchange or during the subsequent transformation process.

In addition, the Mg wt.% in the reflux products significantly decreases from ~3.95 wt.% in the Ni-free Tod sample to ~2.11 wt.% in NiT20, thus suggesting a competition between Mg and Ni for interlayer and/or tunnel sites of layered precursors and/or of reflux products. Specific surface area (SSA) of NiT samples, as measured with the BET method, is systematically higher than that of Ni-free Tod, but no systematic evolution of SSA is observed with the increase of Ni content.

Powder X-ray diffraction

X-ray diffraction (XRD) patterns collected on fresh Bir (not shown) are essentially similar to those recorded after a few months of aging and display reflections at 7.2, 3.6, ~2.45, and ~1.42 Å, typical for birnessite [START_REF] Drits | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: I. Results from X-ray diffraction and selected-area electron diffraction[END_REF]Lanson et al., 2002b). Similar reflections are observed for fresh NiB5 and NiB10 samples (Fig. S1), in addition to a broad maximum peaking at ~4.6 Å. The intensity of this maximum increases with increasing Ni concentration in the initial solution and is thus likely related to the presence of Ni (hydr)oxide (ICDD #13-0229), whose formation was reported under similar experimental conditions (oxidation of a Ni-rich alkaline solution [START_REF] Feitknecht | Zur Kenntnis der höhern Nickelhydroxyde; die Oxydation von Nickelhydroxyd[END_REF]. Consistently, the intensity of this maximum is increased further for NiB15 and NiB20. These two samples exhibit also reflections at ~2.45, and ~1.42 Å, typical for layered manganates, the basal reflections of which indicate a 9.9-10.0 Å layer-to-layer distance (Fig. S1). Upon moderate heating (140 °C), these reflections shift to indicate a 7.2 Å layer-to-layer distance typical for birnessite, as the result of interlayer cation partial dehydration [START_REF] Golden | Synthesis of todorokite[END_REF][START_REF] Manceau | Crystal chemistry of cobalt and nickel in lithiophorite and asbolane from New Caledonia[END_REF][START_REF] Drits | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: I. Results from X-ray diffraction and selected-area electron diffraction[END_REF]. The initial presence of two planes of interlayer H 2 O molecules in NiB15 and NiB20 (buserite) is most likely indicative of the presence of interlayer hydrated Ni(II) cations, whose hydration is reduced upon heating. Sample heating also results in a decrease of the intensity of the ~4.6 Å maximum.

After aging for about three months, all layered precursors but NiB20 display over the low-angle region (5-30 °2) strong basal reflections at ~7.2 and ~3.6 Å typical for birnessite and indicative of the spontaneous dehydration of NiB15 interlayers (Fig. 1).

Two planes of H 2 O molecules are still present in most NiB20 interlayers (buserite), however. Splitting of reflections at ~2.45 and ~1.42 Å, and more generally positions of the reflections over the high-angle region (30-85 °2 -Fig. 1), unambiguously indicate that initial Bir is a triclinic birnessite (ICDD #43-1456), consistent with the synthesis protocol (Lanson et al., 2002b). With increasing Ni content, high-angle reflections broaden significantly thus indicating a decrease in the coherent scattering domain size. In addition, the resolution of these reflections decreases also with increasing Ni content, suggesting a minor increase of layer symmetry from orthogonal (a > b√3, γ = 90°) to hexagonal (a = b√3, γ = 90° - [START_REF] Yin | High Co-doping promotes the transition of birnessite layer symmetry from orthogonal to hexagonal[END_REF] [START_REF] Post | Synchrotron X-ray diffraction study of the structure and dehydration behavior of todorokite[END_REF], respectively.

With increasing Ni content, the positions of both ~9.6 and ~4.8 Å reflections shift towards higher angles for NiT15 and NiT20, compared to Ni-free Tod (Fig. S2) and the ~4.8 Å reflection becomes predominant, consistent with previous reports of todorokite prepared in the presence of various divalent metals [START_REF] Shen | Manganese oxide octahedral molecular sieves: preparation, characterization, and applications[END_REF][START_REF] Shen | Effects of inorganic cation templates on octahedral molecular sieves of manganese oxide[END_REF][START_REF] Krukowska | A new synthetic route to todorokite-type manganese oxides[END_REF][START_REF] Nicolas-Tolentino | Effects of Cu 2+ ions on the structure and reactivity of todorokite-and cryptomelane-type manganese oxide octahedral molecular sieves[END_REF][START_REF] Fuertes | New Catalysts based on Ni-Birnessite and Ni-Todorokite for the Efficient Production of Hydrogen by Bioethanol Steam Reforming[END_REF] and asymmetric. Although not described as such in these references, this intensity ratio modification is consistent with the presence of asbolane, a phyllomanganate with an incompleteisland-likeoctahedral layer of metal (hydr)oxides in the interlayer [START_REF] Chukhrov | Structural Transformations of Manganese Oxides in Oceanic Nodules[END_REF][START_REF] Golden | Transformation of birnessite to buserite, todorokite, and manganite under mild hydrothermal treatment[END_REF][START_REF] Fan | Synchrotron micro-spectroscopic examination of Indonesian nickel laterites[END_REF]. The presence of these incomplete brucite-like octahedral layers between MnO 2 layers precludes the collapse of the layer-to-layer distance to ~7.2 Å upon heating (Fig. S3)

Anomalous XRD patterns

To investigate further the possible origin of the observed contrasting I 4.8 /I 9.6 intensity ratios, XRD patterns of Tod, NiT15, and NiT20 were recorded at different energies, including Mn and Ni K-edges to modify the contribution of these elements to the structure factor (F hkl ). These XRD patterns logically exhibit reflections at similar d-values (Fig. 2a-d). Except for the pattern recorded at the Mn K-edge all Tod patterns also display similar low I 4.8 /I 9.6 intensity ratios (Fig. S4), consistent with previous reports [START_REF] Shen | Characterization of the Fe-Doped Mixed-Valent Tunnel Structure Manganese Oxide KOMS-2[END_REF]. Contrastingly, the I 4.8 /I 9.6 intensity ratio is significantly higher for NiT15 and NiT20 compared to Tod, and varies significantly for these two samples as a function of the energy used for data collection. Specifically, NiT15 and NiT20 display highest I 4.8 /I 9.6 intensity ratios at the Mn K-edge and lowest ratios at the Ni K-edge, whereas similar ratios are obtained for XRD patterns recorded As reported in the early descriptions of asbolane, this compound exhibits high values of the I 4.8 /I 9.6 intensity ratio [START_REF] Chukhrov | Crystallochemical nature of Co-Ni asbolan[END_REF][START_REF] Llorca | Nouvelles données sur la composition et la structure des asbolanes (Nouvelle-Calédonie)[END_REF][START_REF] Fan | Synchrotron micro-spectroscopic examination of Indonesian nickel laterites[END_REF], consistent with XRD patterns calculated for such a structure hosting an incomplete Ni(OH) 2 layer sandwiched in between two MnO 2 layers (Fig. 2e). In addition, for a given composition, this ratio increases when the contribution of Mn atoms to the structure factor is reduced (XRD data recorded at the Mn K-edge -Fig. 2a) and decreases when the contribution of Ni atoms to the structure factor is reduced (XRD data recorded at the Ni K-edge -Fig. 2b), consistent with the data. In addition, this ratio increases with the completeness of interlayer plane (Fig. 2e). Together with the collapse of layer-to-layer distance upon acid-dissolution of the incomplete Ni(OH) 2 layer (see below), these variations of the I 4.8 /I 9.6 intensity ratio can be ascribed to the presence of asbolane, rather than todorokite, in the reflux products.

FTIR

FTIR spectra of todorokite and of other 3×n tectomanganates display a band at ~760 cm -1 , typical for the corner-sharing linkage in their structures (Fig. 3 - [START_REF] Golden | Synthesis of todorokite[END_REF][START_REF] Feng | Synthesis of todorokite at atmospheric pressure[END_REF][START_REF] Julien | Lattice vibrations of manganese oxides[END_REF][START_REF] Atkins | Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments[END_REF][START_REF] Zhao | Formation of todorokite from "c-disordered" H + -birnessites: the roles of average manganese oxidation state and interlayer cations[END_REF].

This band is clearly visible in Tod FTIR spectrum but steadily decreases in intensity with increasing Ni content, most likely as the result of the decreasing proportion of 3×n tectomanganates in Ni-rich reflux products. Consistent with their layered structure, this band is absent in FTIR spectra of layered precursors. Rather, two lines at ~820 and ~950 cm -1 are visible in the FTIR spectra of Ni-containing layered precursors. These two lines are visible also in the spectra of related reflux products, although their intensity is reduced after the reflux treatment. The peak at ~820 cm -1 visible in NiB and NiT was identified as a combination lattice mode band in Ni (hydr)oxides [START_REF] Hall | Nickel hydroxides and related materials: a review of their structures, synthesis and properties[END_REF], which is consistent with the XRD analysis.

HRTEM

Consistent with previous studies [START_REF] Turner | Todorokites: A new family of naturally occurring manganese oxides[END_REF][START_REF] Golden | Synthesis of todorokite[END_REF][START_REF] Shen | Manganese oxide octahedral molecular sieves: preparation, characterization, and applications[END_REF][START_REF] Feng | Synthesis of todorokite at atmospheric pressure[END_REF][START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF], crystals with fibrous and plate-like morphologies are systematically present in the reflux products (Fig. 4), the former dominating in Tod and NiT1 (Figs. 4a-c), whereas platy crystals dominate samples having higher Ni-contents (NiT5 and NiT15; Figs. 4f,i). Fiber-shaped crystals (Figs. 4 e, g, h) often display lattice fringes with a typical ~1 nm periodicity, corresponding to the 3×3 todorokite tunnel structure [START_REF] Golden | Synthesis of todorokite[END_REF][START_REF] Feng | Synthesis of todorokite at atmospheric pressure[END_REF]). As it is common in todorokite, fibers are often aligned at ±120° with respect to each other, thus forming trilling patterns [START_REF] Siegel | Crystalline todorokite associated with biogenic debris in manganese nodules[END_REF][START_REF] Golden | Synthesis of todorokite[END_REF][START_REF] Feng | Synthesis of todorokite at atmospheric pressure[END_REF][START_REF] Bodeï | Formation of todorokite from vernadite in Ni-rich hemipelagic sediments[END_REF][START_REF] Xu | HRTEM investigation of trilling todorokite and nano-phase Mn-oxides in manganese dendrites[END_REF][START_REF] Atkins | Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments[END_REF]. Visible lattice fringes in the plates are commonly more diverse than in the fibers (Figs. 4 b, d and j), typical for a-disordered todorokite [START_REF] Xu | HRTEM investigation of trilling todorokite and nano-phase Mn-oxides in manganese dendrites[END_REF][START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF]. Lattice fringes are also visible in short rods and small plates, indicating the initial development of tunnel structure therein (Figs. 4 g, h -Golden et al., 1986). Finally, smooth surfaces without visible fringes, that are most likely typical for phyllomanganate crystals [START_REF] Golden | Synthesis of todorokite[END_REF][START_REF] Golden | Transformation of birnessite to buserite, todorokite, and manganite under mild hydrothermal treatment[END_REF][START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF], are also visible (Figs. 4 gzone 4, 4i, jzone 1, 4kzone 5). Interestingly, some platy particles exhibit lattice fringes over specific zones with smooth surfaces (Figs. 4g,j). This dual character is possibly related to the initial development of tunnel structures in specific zones of the particles whereas other zones maintain their initial planar structure [START_REF] Golden | Transformation of birnessite to buserite, todorokite, and manganite under mild hydrothermal treatment[END_REF][START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF].

Nitric acid treatment

Upon equilibration in nitric acid, Mn release from layered precursors reaches equilibrium within 12 hrs, the proportion of Mn released from NiB is higher than that released from Bir, (Fig. 5a), possibly as an effect of reduced crystal size. For reflux products, the release of Mn is slower, reaching equilibrium after ~48 hrs, and is essentially the same for all NiT samples (Fig. 5b), except for NiT5. Ni release is much slower than that of Mn, reaching a plateau after ~168 hrs in both layered precursors (Fig. 5c) and reflux products (Fig. 5d). The proportion of Ni released differs however in the two series of samples as essentially all Ni is released from layered precursors whereas ~20% of Ni remains in the reflux products, possibly indicative of Ni structural incorporation. Finally, Mg is systematically totally released from reflux products, possibly at a slower rate with increasing Ni content (Fig. 5f).

After acid treatment, the XRD pattern of Tod is essentially alike that of the initial reflux product and typical for todorokite (Fig. 5e). On the other hand, XRD patterns recorded on NiT15 and NiT20 after acid treatment exhibit, over the low-angle region, peaks at ~7.2 and ~3.6 Å, typical for the phyllomanganate birnessite, rather than peaks at ~9.6, ~4.8, and ~3.2 Å, as expected for todorokite (Fig. 5e). These two XRD patterns are actually similar to the one recorded on Bir after acid treatment and match XRD pattern of hexagonal birnessite (ICDD #23-1239), which was obtained from the low-pH equilibration of an initial triclinic birnessite [START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF]. The evolution of XRD patterns recorded from reflux products upon acid treatment is consistent with that observed for an asbolane-like phyllomanganate [START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF].

The structure of these ~9.6 Å phyllomanganates consists of an octahedral MnO 2 layer similar to that of birnessite, with an incomplete octahedral Me(OH) 2 , MeOOH, or MeO 2 sheet, possibly incommensurate with the MnO 2 layer, in the interlayer [START_REF] Chukhrov | Crystallochemical nature of Co-Ni asbolan[END_REF][START_REF] Manceau | Crystal chemistry of cobalt and nickel in lithiophorite and asbolane from New Caledonia[END_REF][START_REF] Fan | Synchrotron micro-spectroscopic examination of Indonesian nickel laterites[END_REF]. This incomplete interlayer octahedral sheet is solubilized by the acid treatment leading to the observed 7.2 Å layer-to-layer distance [START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF]. Such a peak at ~7.2 Å is visible in the XRD pattern of acid-treated NiT5 and NiT10, and its intensity increases with increasing Ni content, becoming the sole visible diffraction signature in acid-treated NiT15 and NiT20 (Fig. 5e).

Thermogravimetric analysis

Thermogravimetric analyses of layered precursors systematically show a significant weight loss at 140-150 °C corresponding to the dehydration of interlayer Na + and Ni 2+ cations, this weight loss decreasing with increasing Ni content (Figs. 6a,c; Table 2). A minor weight loss (2.4-2.9% in Bir, NiB5, NiB10, and NiB15) is also observed for these samples below 100 °C, which is most likely related to the departure of adsorbed water [START_REF] Gaillot | Structure of synthetic K-rich birnessite obtained by high-temperature decomposition of KMnO 4 . I. Two-layer polytype from 800 C experiment[END_REF]. This low-temperature weight loss is significantly increased for NiB20 (9.25%), however, most likely as the result of a low-temperature buserite-to-birnessite transition associated with the loss of one plane of interlayer H 2 O molecules as observed during moderate heating of fresh NiB samples (Fig. S1). A significant weight loss (4.10 and 6.72% for NiB5 and NiB20, respectively) is observed also for NiB samples over the 170-400 °C range. This weight loss is essentially absent in the TGA curve of Bir (1.78%) and increases with Ni content. It is likely related to the presence of the Ni (hydr)oxide responsible for the broad XRD maximum peaking at ~4.6 Å (Fig. 1). Finally, a weight loss is observed at ~550 °C for all layered precursors, with no significant influence of the Ni content, probably indicating the reduction of MnO 2 to Mn 2 O 3 [START_REF] Bish | Thermal behavior of complex, tunnel-structure manganese oxides[END_REF].

TGA curves recorded for reflux products significantly differ from those of their layered precursors, with weight losses at ~150, ~285, ~440, and ~545 °C for Tod (Figs. 6b,d). The weight loss observed from ~110-240 °C is most likely related to the loss of the H 2 O molecules hydrating cations present in todorokite tunnels [START_REF] Bish | Thermal behavior of complex, tunnel-structure manganese oxides[END_REF]. With increasing Ni content, this weight loss decreases (from 8.05 to 6.12%

in Tod and NiT20, respectively) and shifts towards lower temperatures, most likely indicative of the mineralogical evolution evidenced by XRD data recorded on acid-treated reflux products (Fig. 5e). For NiT10, this low-temperature weight loss splits in a doublet owing to the coexistence of todorokite and asbolane (Fig. 5e). The weight loss occurring at ~285 °C is also ascribed to the departure of tunnel water [START_REF] Bish | Thermal behavior of complex, tunnel-structure manganese oxides[END_REF]. TGA curves obtained for Tod and NiT20 also differ significantly over this range, with a shift of the weight loss towards higher temperatures in NiT20 compared to Tod. Evolution with increasing Ni content, is not as systematic as that observed for the low-temperature weight loss, however (Fig. 6d).

The third weight loss occurring at ~440 °C appears as a distinct signature for the presence of todorokite, as its intensity decreases with increasing Ni content to vanish for NiT15 and NiT20 which essentially contain asbolane (Fig. 5e). A last weight loss occurs at 520-550 °C, a temperature similar to that reported for layered precursors at ~550 °C (Figs. 6b,d).

O1s X-ray photoelectron spectroscopy

Consistent with the systematic prevalence of [MnO 6 ] building octahedral in both phyllomanganates and tectomanganates, all O1s XPS spectra are dominated by a peak at ~529.6 eV (Fig. 7), typical for oxygen (O 2-) in metal oxides. This main peak is asymmetric and displays a tail on its high-energy side for both layered precursors and reflux products. This tail at ~531-535 eV can be decomposed into the contributions of oxygen atoms from hydroxyl groups (OH -; 530.9-531.6 eV) and from water molecules (532.2-533.4 eV). For layered precursors, the intensity of this tail, and more especially that of the OH -contribution, increases with increasing Ni content (Fig. 7; Table S1), whereas the H 2 O contribution is about constant for all NiB samples but NiB20 that shows a slightly higher proportion of O atoms from H 2 O molecules, consistent with TGA data. In reflux products the contribution of O atoms in hydroxyl groups is significantly higher in NiT samples compared to Tod, consistent with the presence of asbolane-like phyllomanganate in Ni-rich reaction products. On the other hand, the H 2 O contribution shows no systematic evolution with Ni content (Fig. 7;

Table S1). Finally, relative intensities of peaks corresponding to OH -and H 2 O contributions are significantly increased in NiT samples compared to their corresponding layered precursors, at least under the deep vacuum conditions prevailing during the XPS measurements.

X-ray absorption spectroscopy

Mn K-edge XANES

Relative proportions of Mn(II), Mn(III), and Mn(IV) species and Mn average oxidation state (AOS) have been determined using the Combo method (Table 3 - [START_REF] Manceau | Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy[END_REF]. Results consistently indicate high (~1/3) relative proportions of Mn(III) in both layered precursors and reflux products, which is considered a key condition for the phyllomanganate-to-tectomanganate conversion [START_REF] Cui | Influence of Mn(III) availability on the phase transformation from layered buserite to tunnel-structured todorokite[END_REF][START_REF] Atkins | Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments[END_REF][START_REF] Atkins | Release of Ni from birnessite during transformation of birnessite to todorokite: Implications for Ni cycling in marine sediments[END_REF][START_REF] Grangeon | Solid-state transformation of nanocrystalline phyllomanganate into tectomanganate: influence of initial layer and interlayer structure[END_REF][START_REF] Grangeon | Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms[END_REF][START_REF] Zhao | Formation of todorokite from "c-disordered" H + -birnessites: the roles of average manganese oxidation state and interlayer cations[END_REF][START_REF] Li | Reaction Network of Layer-to-Tunnel Transition of MnO 2[END_REF]. The measured proportion of Mn(III) is slightly lower in Ni-free samples compared to the Ni-bearing ones (Table 3) and no significant difference is observed between layered precursors and their reflux products. In all cases, the Mn(II) content is null within the experimental error.

Mn K-edge EXAFS

EXAFS spectra of both layered precursors and of reflux products show significant evolutions with increasing Ni content, thus indicating that the presence of Ni affects the local structure of manganates in the two series of samples (Fig. 8). The EXAFS spectrum collected for Ni-free layered precursor (Bir) is typical for triclinic birnessite over both the staircase and the indicator regions (3.5-6.0 Å -1 and 7.5-10.0 Å -1 , respectively - [START_REF] Marcus | Mn, Fe, Zn and As speciation in a fast-growing ferromanganese marine nodule[END_REF]. In particular, the splitting of the features at ~7.8 Å -1 and ~9.0 Å -1 (arrows in Fig. 8a) are indicative of a high Mn(III) content, of the systematic elongation of Mn(III) octahedra along the a axis, and of their ordered distribution in rows parallel to the b axis [START_REF] Marcus | Mn, Fe, Zn and As speciation in a fast-growing ferromanganese marine nodule[END_REF][START_REF] Gaillot | Structure of the synthetic K-rich phyllomanganate birnessite obtained by high-temperature decomposition of KMnO 4[END_REF], typical features for triclinic birnessite [START_REF] Webb | Structural characterization of biogenic Mn oxides produced in seawater by the marine bacillus sp. strain SG-1[END_REF][START_REF] Atkins | Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments[END_REF][START_REF] Yin | Effects of Co and Ni co-doping on the structure and reactivity of hexagonal birnessite[END_REF]. With increasing Ni content, the ~7.8 Å -1 feature becomes less pronounced even though its position remains unchanged, consistent with the similar Mn(III) contents of all layered precursors (Table 3) and with the similar overall frequency of the EXAFS spectra of these samples [START_REF] Gaillot | Structure of the synthetic K-rich phyllomanganate birnessite obtained by high-temperature decomposition of KMnO 4[END_REF]. The steady lowering of the ~7.8 Å -1 feature resolution is thus most likely related either to disorder in the distribution of elongated Mn(III) octahedral or to some randomness in the direction of the elongation, consistent with the evolution of XRD patterns with increasing Ni content. The split ~9.0 Å -1 feature observed for Bir and NiB1 also turns into a broad and more symmetrical feature with increasing Ni content and the induced vanishing of the sharp high-frequency contribution.

In k-space Mn K-edge EXAFS spectra of Tod and NiT1 exhibit sharp features at ~8.0 and ~9.2 Å -1 that are typical for todorokite [START_REF] Webb | Structural characterization of biogenic Mn oxides produced in seawater by the marine bacillus sp. strain SG-1[END_REF][START_REF] Bodeï | Formation of todorokite from vernadite in Ni-rich hemipelagic sediments[END_REF][START_REF] Feng | Formation of nano-crystalline todorokite from biogenic Mn oxides[END_REF][START_REF] Atkins | Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments[END_REF]. These two features essentially decrease in intensity at higher Ni content. In addition, the ~9.2 Å -1 feature shifts to lower wave numbers in NiT samples compared to Tod and NiB1 (Fig. 8b). EXAFS spectra of Ni-rich NiT samples and their Fourier transforms (FTs) are actually alike those of their corresponding layered precursors (Figs. 9a-b), consistent with the similar structure of the MnO 2 layer in both birnessite and asbolane. On the contrary, spectra obtained for Ni-free Bir and Tod differ significantly, as expected for layered and tunnel manganese oxides (Fig. 9c). In particular, the FTs of these spectra show a clear shortening of the average Mn-Me E distance in Tod compared to Bir, as shown by the shift of the imaginary part of the FT (Fig. 9d, arrow). In addition, Tod FT shows a clear maximum at R +R ~ 3.0 Å, typical of Mn-Me C pairs, which is not observed for its Bir precursor. For both Ni-free and Ni-rich samples, Mn-O distances are similar in both layered precursors and corresponding reflux products, consistent with their similar Mn AOS.

Ni K-edge EXAFS

EXAFS spectra collected at the Ni K-edge are similar for both NiB samples, on the one hand, and NiT, on the other hand (Fig. 10), thus implying similar Ni speciation in each of the two sample groups. Although it was not possible to obtain a decent fit to this data using a linear combination fitting and available Ni reference spectra, similarity of the hydrated Ni(II) spectrum with collected spectra suggests that this species possibly prevails in all samples. In addition, the evolution of Ni speciation during the reflux process was assessed by fitting EXAFS spectra of reflux products as a linear combination of their respective NiB precursors and of Ni reference spectra (Fig. S4). For all NiT samples, a good fit was obtained with ~15% of a Ni(OH) 2 contribution in addition to the layered precursor (Table S2), consistent with the polymerization of a partial Ni(OH) 2 layer in birnessite interlayers to form an asbolane-like structure.

Pair distribution functions (G(r))

Atomic PDFs computed from Tod, NiT10, and NiT20 high-energy X-ray scattering patterns show similar peak positions (Fig. 11), indicative of similar local structures [START_REF] Zhu | Structural study of biotic and abiotic poorly-crystalline manganese oxides using atomic pair distribution function analysis[END_REF] as expected from the major contribution of layers/ribbons of edge-sharing MnO 6 octahedra in both layered precursors and reflux products.

Consistently, the maxima at ~1.90 Å, ~2.90 Å, and ~3.50 Å essentially correspond to the first neighbor Mn-O, to edge-sharing Mn-Me, and to a combination of corner-sharing Mn-Me and of second neighbor Mn-O atomic pairs, respectively [START_REF] Zhu | Structural study of biotic and abiotic poorly-crystalline manganese oxides using atomic pair distribution function analysis[END_REF]. The increased intensity of the ~3.50 Å maximum and the decreased intensity of the one at ~5.00 Å in the PDF of Tod compared to NiT10 and NiT20 are typical of the prevalence of tunnel structures in the former sample and of layered structures in the latter [START_REF] Grangeon | Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms[END_REF]. The ~5.00 Å maximum is mainly related indeed to second neighbor Mn-Mn pairs (Fig. S4 - [START_REF] Zhu | Structural study of biotic and abiotic poorly-crystalline manganese oxides using atomic pair distribution function analysis[END_REF], that are favored by the lateral extension of MnO 2 layers. Finally, the peak at ~4.50 Å is most likely related to Mn2-Mn2 and Mn4-Mn4 pairs across "holes" in tunnel walls, floors, and ceilings (Fig. S6), consistent with the increased intensity of this ~4.50 Å peak with decreasing tunnel size [START_REF] Zhu | Structural study of biotic and abiotic poorly-crystalline manganese oxides using atomic pair distribution function analysis[END_REF].

Differential PDF (d-PDF) can be calculated to assess more precisely the specific modifications affecting the reflux products, and more especially the broadening of the ~1.90 and 2.90 Å peaks towards high d-values with increasing Ni content (Fig. 11).

The d-PDF curves obtained for NiT10 and NiT20, both show a first peak at ~2.05 Å typical for Ni-O pairs (Manceau et al., 2007a,b;Peacock and Sherman, 2007a,b;[START_REF] Yin | Characterization of Ni-rich hexagonal birnessite and its geochemical effects on aqueous Pb 2+ /Zn 2+ and As(III)[END_REF][START_REF] Kwon | Understanding the trends in transition metal sorption by vacancy sites in birnessite[END_REF]Simanova et al., 2015;[START_REF] Atkins | Release of Ni from birnessite during transformation of birnessite to todorokite: Implications for Ni cycling in marine sediments[END_REF]

DISCUSSION

Structural transformation of layered precursors

Addition of Ni during the synthesis of layered precursors does not impact significantly their crystal structure. In particular, the Mn(III) content of layered precursors remains about constant at ~1/3 (Table 3) whatever the initial Ni content.

Consistently, XRD results (Fig. 1) show that layer symmetry remains orthogonal (a > b√3) for all precursors, indicative of the systematic elongation of distorted Mn(III) octahedra along the a axis and of their ordered distribution. FTs of the EXAFS spectra consistently show similar Mn-Me E distances in all layered precursors (not shown).

Both XRD patterns and EXAFS spectra of layered precursors are significantly modified when increasing the Ni content, however. These changes reveal an increased disorder in the orientation of Mn(III) octahedra and/or in their distribution.

The increasing intensity of the ~4.6 Å diffraction maximum with increasing Ni content indicates that part of Ni initially introduced in solution actually precipitates as Ni (hydr)oxide. The presence of Ni in the interlayers of NiB15 and NiB20 is attested by the presence of two planes of interlayer water molecules, consistent with its more negative hydration enthalpy compared to Na + [START_REF] Smith | Ionic hydration enthalpies[END_REF][START_REF] Johnson | Water in the interlayer region of birnessite: Importance in cation exchange and structural stability[END_REF], in these fresh samples. All Ni species, including the two mentioned above, are readily mobilized by the acid treatment (Fig. 5c), and it is thus reasonable to hypothesize that no Ni(II) cations are incorporated in the octahedral layer. Similarly, the orthogonal layer symmetry is typical of octahedral layers containing few vacant layer sites [START_REF] Drits | Substructure and superstructure of four-layer Ca-exchanged birnessite[END_REF][START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF], if any, and the presence of Ni(II) cations sorbed above or below such vacant sites is thus unlikely.

Although weakly bound to the layered precursors, the presence of Ni(II) significantly impacts their transformation during the reflux process. Bir, a typical triclinic birnessite, converts readily and completely to an a-disorder todorokite (Wu et al., 2019). With increasing Ni content, disorder increases together with the proportion of non-ideal 3×n tunnel sizes forming trilling patterns. In addition, platy crystals are visible in all reflux products, their relative abundance increasing with increasing Ni content with only minimal fragments of tunnel oxides being detected in NiT15 and NiT20 (Fig. 4). Platy crystals are most likely asbolane, as shown by the prevalence of this phyllomanganate hosting an incomplete octahedral layer of metal (hydr)oxides in their interlayers in Ni-rich reflux products. These incomplete brucite-like octahedral layers are dissolved by the acid treatment, leading to birnessite-like structures (Fig. 5e). As reported in the early work of Chukhrov and coworkers on natural asbolane [START_REF] Chukhrov | Crystallochemical nature of Co-Ni asbolan[END_REF][START_REF] Chukhrov | Structural Transformations of Manganese Oxides in Oceanic Nodules[END_REF], XRD diffraction patterns of asbolane are characterized by high I 4.8 /I 9.6 intensity ratios. Such high I 4.8 /I 9.6 intensity ratios are commonly reported for "todorokite", especially when synthesized in the presence of metallic elements such as Co, Ni, Cu, Zn, Er, La, etc [START_REF] Shen | Effects of inorganic cation templates on octahedral molecular sieves of manganese oxide[END_REF][START_REF] Tian | Effect of Mg 2+ ions on the formation of todorokite type manganese oxide octahedral molecular sieves[END_REF][START_REF] Krukowska | A new synthetic route to todorokite-type manganese oxides[END_REF][START_REF] Luo | Double-aging method for preparation of stabilized Na-buserite and transformations to todorokites incorporated with various metals[END_REF]. In the present study, the combined use of XRD simulation, anomalous XRD, HRTEM, thermogravimetric analysis, and nitric acid treatment allowed uncovering the actual nature of this seldom described phyllomanganate, but its formation may be far more common than previously reported in these experimental studies.

As shown in Fig. 2e, this I 4.8 /I 9.6 intensity ratio strongly depends on the completeness of the interlayer brucite-like octahedral layer, and the ratio determined for NiT15 and NiT20 corresponds to a ~50% completeness of the Ni(OH) 2 octahedral layer. This interlayer occupancy is much higher than possibly achieved with the sole presence of Ni(II) in these octahedral layers as Ni / (Ni+Mn) ratios are 8-12% in these samples, with part of the Ni present as Ni (hydr)oxide. It is thus likely that Mg cations contribute to the electron density at the interlayer mid-plane, in addition to Ni. As Mg scattering factor is lower than that of Ni, the actual completeness of the interlayer brucite-like layer is likely higher than 50%.

Differentiation of asbolane and todorokite.

Although the I 4.8 /I 9.6 intensity ratio strongly depends on the completeness of the interlayer brucite-like octahedral layer, a 4.8 Å peak stronger than its 9.6 Å counterpart is a possible indication of the presence of the phyllomanganate asbolane, rather than the tectomanganate todorokite, in reaction products, or in natural samples.

The two compounds indeed share these two interplanar distances. With its two planes of interlayer H 2 O molecules, buserite also exhibits similar periodicity but a gentle heating (to ~100°C) should lead to the partial dehydration of interlayer cations and to the ~7.2 Å periodicity typical of birnessite (Fig. S1 - [START_REF] Johnson | Water in the interlayer region of birnessite: Importance in cation exchange and structural stability[END_REF].

Furthermore, buserite is unlikely to have a high enough content of interlayer cations to induce a I 4.8 /I 9.6 ratio higher than 1.0.

Although values of this ratio higher than 1.0 are typically observed for asbolane, the partial exchange of Mg present in todorokite tunnels by cations having a higher electronic content (such as Ni and other divalent metal cations) can also lead to such high I 4.8 /I 9.6 ratios (Fig. 11), thus not allowing an unambiguous differentiation between asbolane and todorokite. In this search for an unquestionable differentiation between asbolane and todorokite, the nitric acid treatment proposed by Wu et al.

(2019) thus appears as a simple and reliable method. In the case of asbolane, dissolution of the interlayer brucite-like layer induced by the treatment leads indeed to a collapse of the layer-to-layer distance to the ~7.2 Å periodicity. On the contrary, periodicities are maintained for todorokite, the tectomanganate framework being unaltered, and positions of XRD maxima are thus unchanged.

The role of Mn(III) and interlayer species during the transformation

It is now accepted that the presence of a high Mn(III) content (>25-30%) in layered precursors is required for their conversion to tectomanganates. This pivotal role is most likely related to the steric strains induced by the presence of Jahn-Teller distorted Mn(III) octahedra in the octahedral layers. Migration of these Mn(III) cations from layer to interlayer or layer kinking at these structurally weak points contribute to building tunnel walls [START_REF] Grangeon | Crystal structure of Ni-sorbed synthetic vernadite: a powder X-ray diffraction study[END_REF][START_REF] Grangeon | Solid-state transformation of nanocrystalline phyllomanganate into tectomanganate: influence of initial layer and interlayer structure[END_REF][START_REF] Atkins | Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments[END_REF][START_REF] Atkins | Release of Ni from birnessite during transformation of birnessite to todorokite: Implications for Ni cycling in marine sediments[END_REF][START_REF] Yang | Trivalent manganese on vacancies triggers rapid transformation of layered to tunneled manganese oxides (TMOs): Implications for occurrence of TMOs in low-temperature environment[END_REF]. In the present case, the ordered distribution of Mn(III) cations in rows parallel to the b axis and separated from each other along the a axis by two rows of Mn(IV) creates a structurally weak point favorable to the formation of tunnels walls [START_REF] Atkins | Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments[END_REF][START_REF] Grangeon | Solid-state transformation of nanocrystalline phyllomanganate into tectomanganate: influence of initial layer and interlayer structure[END_REF]. Consistently, the initial Ni-free precursor Bir, a triclinic birnessite, was fully converted to todorokite. Similar transformation of triclinic birnessite to tunnel structures was previously reported in the literature although 3×3 tunnels were not systematically obtained [START_REF] Yang | Trivalent manganese on vacancies triggers rapid transformation of layered to tunneled manganese oxides (TMOs): Implications for occurrence of TMOs in low-temperature environment[END_REF]. Furthermore, the actual distribution and orientation of Mn(III) octahedra present in the MnO 2 layer do not appear to be key factors for the phyllomanganate-to-tectomanganate conversion as tectomanganates were obtained from hexagonal birnessite precursors in which Mn(III) octahedra are distributed and/or oriented at random [START_REF] Grangeon | Crystal structure of Ni-sorbed synthetic vernadite: a powder X-ray diffraction study[END_REF][START_REF] Grangeon | Solid-state transformation of nanocrystalline phyllomanganate into tectomanganate: influence of initial layer and interlayer structure[END_REF][START_REF] Atkins | Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments[END_REF][START_REF] Atkins | Release of Ni from birnessite during transformation of birnessite to todorokite: Implications for Ni cycling in marine sediments[END_REF][START_REF] Zhao | Formation of todorokite from "c-disordered" H + -birnessites: the roles of average manganese oxidation state and interlayer cations[END_REF]. The absence of transformation of NiB precursors to tunnel structures thus does not appear to be related to their Mn(III) content, which is similar to or higher than that of the Ni-free Bir.

As layers of all NiB precursors are essentially devoid of Ni, both in the octahedral layer and sorbed above/below vacant layer sites, the absence of transformation of these precursors to tunnel structures is thus likely due to kinetic effects favoring the formation of asbolane. Specifically, polymerization of Ni as Ni(OH) 2 fragments in the interlayers of initial birnessite/buserite (Fig. S5, Table S2) is favored over the conversion to tectomanganates. In turn, the presence of interlayer Ni(OH) 2 fragments likely impedes the migration of Mn(III) from the layer to the interlayer thus preventing the conversion to tectomanganates. Such competing effects are supported also by the increase of (Mg + Ni)/Mn ratios with increasing Ni content (Table 1) and are likely to occur with a variety of other hydrolysable metal cations. Consistently, "todorokite" containing divalent metal cations and displaying high I 4.8 /I 9.6 ratios possibly indicate the presence of asbolane-like structure. Such competing effects might contribute also to the prevalence of phyllomanganates over tectomanganates in natural environments [START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF].

Negative influence of Ni and Co on the phyllo-to-tectomanganate transformation: Contrasting mechanisms

Both in the present work investigating the possible transformation of Ni-containing layered precursors to tectomanganates and in the related study investigating a similar process for Co-containing layered precursors, tectomanganate formation was inhibited, totally (Ni) or partially (Co) for Me-rich precursors. The origin of these similar negative impacts differs however for the two metals. In the present case, layered precursors contain sufficient amounts of Mn(III) (~1/3) to trigger the transformation, as Ni(II) is not structurally incorporated in the precursors.

As discussed above, polymerization of Ni(OH) 2 fragments in birnessite interlayers is kinetically favored over migration of Mn(III) from the layer to the interlayer, or layer kinking, however, leading to the formation of asbolane rather than tectomanganates.

In contrast and consistent with previous reports [START_REF] Manceau | Structural mechanism of Co 2+ oxidation by the phyllomanganate buserite[END_REF][START_REF] Yu | Structural factors of biogenic birnessite produced by fungus Paraconiothyrium sp. WL-2 strain affecting sorption of Co2+[END_REF][START_REF] Kwon | Understanding the trends in transition metal sorption by vacancy sites in birnessite[END_REF][START_REF] Simanova | Time-Resolved Investigation of Cobalt Oxidation by Mn(III)-Rich delta-MnO 2 Using Quick X-ray Absorption Spectroscopy[END_REF][START_REF] Yin | High Co-doping promotes the transition of birnessite layer symmetry from orthogonal to hexagonal[END_REF], Co(II) is readily oxidized by birnessite during the formation of initial layered precursors with Co(III) being structurally incorporated in these precursors owing to the similar ionic radii of Co(III) and Mn(IV) [START_REF] Wu | Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility[END_REF]. The symmetry of layered precursors is increased from orthogonal to hexagonal. As a consequence, the content of Mn(III) in Co-containing layered precursors is lowered and does not allow triggering the transformation to tunnel structures [START_REF] Grangeon | Crystal structure of Ni-sorbed synthetic vernadite: a powder X-ray diffraction study[END_REF][START_REF] Grangeon | Solid-state transformation of nanocrystalline phyllomanganate into tectomanganate: influence of initial layer and interlayer structure[END_REF][START_REF] Atkins | Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments[END_REF][START_REF] Atkins | Release of Ni from birnessite during transformation of birnessite to todorokite: Implications for Ni cycling in marine sediments[END_REF]. In both cases, little Me is released to solution during the reflux process and Me cations are structurally incorporated in the reflux products (~20% and ~80% for Ni and Co, from the nitric acid experiments and EXAFS simulation, respectively) thus enhancing their sequestration in both tecto-and phyllomanganates.

The fate of Nickel during the transformation of layered precursors

Previous studies have consistently shown that Ni can be incorporated in the birnessite structure, either within the octahedral layer or sorbed at vacant layers sites (Peacock and Sherman, 2007b;[START_REF] Grangeon | Crystal structure of Ni-sorbed synthetic vernadite: a powder X-ray diffraction study[END_REF][START_REF] Grangeon | In-situ determination of the kinetics and mechanisms of nickel adsorption by nanocrystalline vernadite[END_REF][START_REF] Peacock | Physiochemical controls on the crystal-chemistry of Ni in birnessite: Genetic implications for ferromanganese precipitates[END_REF][START_REF] Peña | Mechanisms of nickel sorption by a bacteriogenic birnessite[END_REF][START_REF] Yin | Characterization of Ni-rich hexagonal birnessite and its geochemical effects on aqueous Pb 2+ /Zn 2+ and As(III)[END_REF][START_REF] Kwon | Understanding the trends in transition metal sorption by vacancy sites in birnessite[END_REF][START_REF] Yin | Effects of Co and Ni co-doping on the structure and reactivity of hexagonal birnessite[END_REF]Simanova et al., 2015;[START_REF] Atkins | Release of Ni from birnessite during transformation of birnessite to todorokite: Implications for Ni cycling in marine sediments[END_REF][START_REF] Lefkowitz | Structural alteration of hexagonal birnessite by aqueous Mn(II): Impacts on Ni(II) sorption[END_REF], consistent with natural occurrences [START_REF] Bodeï | Formation of todorokite from vernadite in Ni-rich hemipelagic sediments[END_REF]Manceau et al., 2007a,b;Peacock and Sherman, 2007a). In their literature review, [START_REF] Grangeon | In-situ determination of the kinetics and mechanisms of nickel adsorption by nanocrystalline vernadite[END_REF] showed that the fraction of Ni structurally incorporated in birnessite octahedral layers may be high (up to ~50%) only for low Ni content (~1%), as reported for natural samples [START_REF] Bodeï | Formation of todorokite from vernadite in Ni-rich hemipelagic sediments[END_REF]Manceau et al., 2007a,b;Peacock and Sherman, 2007a). Structural incorporation of Ni(II) in the octahedral layer appears to be enhanced by circumneutral pH conditions and time (Manceau et al., 2007b;Peacock and Sherman, 2007b;[START_REF] Peacock | Physiochemical controls on the crystal-chemistry of Ni in birnessite: Genetic implications for ferromanganese precipitates[END_REF][START_REF] Peña | Mechanisms of nickel sorption by a bacteriogenic birnessite[END_REF]. Competitive sorption at TCS/DCS sites has been proposed also as a mechanism favoring structural incorporation in the octahedral layer [START_REF] Lefkowitz | Structural alteration of hexagonal birnessite by aqueous Mn(II): Impacts on Ni(II) sorption[END_REF]. Some studies report the prevalence of Ni sorbed at TCS/DCS sites even for Ni/Mn ratios higher than 10%, however (Manceau et al., 2007a,b;[START_REF] Peña | Mechanisms of nickel sorption by a bacteriogenic birnessite[END_REF][START_REF] Yin | Characterization of Ni-rich hexagonal birnessite and its geochemical effects on aqueous Pb 2+ /Zn 2+ and As(III)[END_REF]Simanova et al., 2015;[START_REF] Grangeon | In-situ determination of the kinetics and mechanisms of nickel adsorption by nanocrystalline vernadite[END_REF]. In such cases, sorption of Ni(II) as inner sphere complexes is favored by the presence of vacant octahedral sites and is thus negatively correlated with the Mn(III) content.

Consistently, in our study structural incorporation of Ni in vacancy-free NiB precursors appears very limited. In NiB layered precursors, Ni is rather present both as Ni (hydr)oxide and as hydrated cations in birnessite/buserite interlayers, as shown by the increased hydration of S1). The influence of Ni on phyllomanganate hydration has been reported also for natural samples in which buserite (or 10 Å vernadite) abundance increases with increasing Ni content [START_REF] Bodeï | Formation of todorokite from vernadite in Ni-rich hemipelagic sediments[END_REF]Manceau et al., 2007a).

As discussed above, hydrated Ni(II) initially present in birnessite interlayers are readily stabilized during the reflux products by forming Ni(OH) 2 fragments (Fig. S5, Table S2), consistent with previous experiments with vacancy-free phyllomanganate precursors [START_REF] Feng | Hydrothermal Soft Chemical Synthesis of Ni(OH) 2 -Birnessite Sandwich Layered Compound and Layered LiNi 1/3 Mn 2/3 O 2[END_REF]. Polymerization of Ni(OH) 2 brucite-like octahedral layers in phyllomanganate interlayers is consistent also with the natural predominance of asbolane in Ni-rich environments (NiO ~10-20 wt.%, [START_REF] Chukhrov | Crystallochemical nature of Co-Ni asbolan[END_REF][START_REF] Chukhrov | Structural Transformations of Manganese Oxides in Oceanic Nodules[END_REF][START_REF] Roque-Rosell | Sorption of Ni by "lithiophorite-asbolane" intermediates in Moa Bay lateritic deposits, eastern Cuba[END_REF][START_REF] Ploquin | Phyllomanganate vein-infillings in faulted and Al-poor regoliths of the New Caledonian ophiolite: periodic and sequential crystallization of Ni-asbolane, Alk-birnessite and H-birnessite[END_REF] . Although the Mn AOS of his synthetic samples (~4.0) was favorable for Ni sorption at layer vacancy sites, Manceau et al. (2007b) (Table 1) indicates that all Ni initially present in layered precursors is retained during the reflux process.

CONCLUSION

The present study reports on the association of weakly bound Ni species Formation of asbolane, rather than todorokite, may actually be more frequent than reported in the literature owing to the similar periodicities in both compounds. High values (> 1) of the I 4.8 /I 9.6 intensity ratio may suggest the presence of this seldom described phyllomanganate. Acid nitric treatment, aiming at the dissolution of the island-like interlayer Ni(OH) 2 layer, represents an easy and unambiguous way to differentiate asbolane from todorokite which is unaffected by the treatment.

Both the presence of Co and Ni in synthesized phyllomanganates impede their transformation to tectomanganates when refluxed, the origin of this similar impact being different for the two metals, however. Oxidation of Co(II) sorbed on the MnO 2 layer and the subsequent structural incorporation of Co(III) in the octahedral layer releases the steric strains related to the presence of Jahn-Teller distorted Mn(III) octahedra, which is required for the phyllomanganate-to-tectomanganate conversion.

In contrast, the presence of hydrated Ni(II) favors the polymerization of fragments of Ni(OH) 2 octahedral layers, that in turn prevents the formation of tectomanganates despite the favorable presence of Mn(III) in the octahedral MnO 2 layer. In both cases, "foreign" metal cations are retained in the refluxed products, with contrasting ratios of structural incorporation.

  at energies different from the two absorption edges (Figs. 2a-d; S4). Both features indicate contrasting structures of Tod and Ni-rich NiT products depending on the initial Ni content.

  also reported the overwhelming presence of Ni as Ni(OH) 2 at high Ni content (Ni/Mn ratio of ~0.10 and ~0.18 at pH 4 and 7, respectively), despite experimental conditions supposed to prevent Ni(OH) 2 precipitation. The reflux process appears to have little influence on the Ni (hydr)oxide formed together with the layered precursors, as evidenced by the stability of the corresponding maximum at ~4.6 Å in the reflux products (Fig.1b). Finally, part of Ni (~20 %) is stabilized during the reflux process, as shown by the incomplete Ni release during acid treatment (Fig.5d). Stabilized Ni is possibly structurally incorporated in the reflux products. On the contrary,[START_REF] Atkins | Release of Ni from birnessite during transformation of birnessite to todorokite: Implications for Ni cycling in marine sediments[END_REF] reported no structural incorporation of Ni(II) during the formation of todorokite from a Ni-bearing birnessite precursor (~1 wt% Ni in hexagonal birnessite). They reported also a decrease in the Ni content of their reflux products, whereas in our experimental conditions stability of Ni/(Mn+Ni) ratios

(

  hydrated Ni, Ni (hydr)oxides) with phyllomanganates, that possibly prevail in natural Ni-rich (>10% NiO) manganates. Whatever their Ni content, prepared phyllomanganates systematically exhibit an orthogonal layer symmetry, indicative of a high Mn(III) content and of a minimal number of octahedral layer vacancies. Ni is essentially present as charge-compensating hydrated Ni(II) and as Ni (hydr)oxide, with no detectable Ni(II) sorbed or structurally incorporated in the phyllomanganate structure. Despite their high content of Mn(III), a favorable condition for their conversion to tectomanganates, absolane rather than tectomanganates forms during the reflux processing of these Ni-rich phyllomanganates. Asbolane formation results from the kinetically favored polymerization of fragments of Ni(OH) 2 octahedral layers in birnessite interlayers. The structure of the initial manganate layers is essentially unaffected by this transformation.

Fig. 1 .

 1 Fig. 1. XRD patterns of layered precursors (top) and reflux products (bottom). Colors correspond to the Ni/(Ni+Mn) ratio of the samples. Green ticks at the bottom of upper and lower figures indicate the reflections of triclinic birnessite (ICDD #43-1456) and todorokite (ICDD #38-0475), respectively.

Fig. 3 .Fig. 5 .

 35 Fig. 3. FTIR spectra of layered precursors and reflux products (top and bottom, respectively). Colors as in Fig. 1.

Fig. 6 .

 6 Fig. 6. Thermogravimetric analysis traces obtained for (a) layered precursors and (b)reflux products, and their 1 st derivatives (c and d, respectively). 1 st derivative curves were smoothed using a Savitzky-Golay filter and a 15 point window. Colors as in Fig.1.

Fig. 8 .

 8 Fig. 8. Mn K-edge EXAFS spectra obtained for (a) layered precursors and (b) reflux products. Colors as in Fig. 1.

Fig. 12 .

 12 Fig. 12. Intensity distributions calculated for ideal todorokite (ICDD #38-0475) with (a) only Mg in todorokite tunnels and (b) with Mg and Ni present in a 1:1 ratio in todorokite tunnels. In both cases, the model of Post et al. (2003) was modified assuming the complete filling of the O8 site by H 2 O molecules (no cations) and the presence of 1.0 cation (Mg or Mg+Ni) in the Mg site.

Fig. 1 .

 1 Fig. 1. XRD patterns of layered precursors (top) and reflux products (bottom). Colors correspond to the Ni/(Ni+Mn) ratio of the samples. Green ticks at the bottom of upper and lower figures indicate the reflections of triclinic birnessite (ICDD #43-1456) and todorokite (ICDD #38-0475), respectively.

FigureFig. 2 .

 2 Figure

Fig. 3 .

 3 Fig. 3. FTIR spectra of layered precursors and reflux products (top and bottom, respectively). Colors as in Fig. 1.

Fig. 5 .

 5 Fig. 5. Proportion of metal cations released during the nitric acid treatment: (a-b): Mn, (c-d): Ni, and (f): Mg. The amount of metal released is normalized to the sample overall metal content (see text for details). (e): XRD patterns of samples treated with nitric acid. Colors as in Fig. 1 except for Bir in (e).

Fig. 6 .

 6 Fig. 6. Thermogravimetric analysis traces obtained for (a) layered precursors and (b) reflux products, and their 1 st derivatives (c and d, respectively). 1 st derivative curves were smoothed using a Savitzky-Golay filter and a 15 points window. Colors as in Fig. 1.

Fig. 8 .

 8 Fig. 8. Mn K-edge EXAFS spectra obtained for (a) layered precursors and (b) reflux

Fig. 10 .

 10 Fig. 10. Comparison of Ni K-edge EXAFS spectra obtained for layered precursors

Fig. 12 .

 12 Fig. 12. Intensity distributions calculated for ideal todorokite (ICDD #38-0475) with (a) only Mg in todorokite tunnels and (b) with Mg and Ni present in a 1:1 ratio in todorokite tunnels. In both cases, the model of Post et al.(2003) was modified assuming the complete filling of the O8 site by H 2 O molecules (no cations) and the presence of 1.0 cation (Mg or Mg+Ni) in the Mg site.

Fig. S1 .

 S1 Fig. S1. XRD patterns of fresh layered precursors after heating to 140 °C for different durations

Fig. S3 .

 S3 Fig. S3. XRD patterns of reflux products after heating to 140 °C for 12 hours. Colors

Fig. S4 .

 S4 Fig. S4. Intensity ratio of the intensity of 4.8 Å and 9.6 Å peaks (I4.8/I9.6) in reflux products as a function of Ni content and X-ray energy (see Fig. 2 for the data).

Fig. S5 .

 S5 Fig. S5. Linear combination fitting of NiT K-edge EXAFS spectra using Ni(OH)2 and corresponding NiB samples as standard references.

Fig. S6 .

 S6 Fig. S6. Interatomic distances calculated for a variety of Mn-Mn pairs and ideal todorokite structure (ICDD #38-0475). The Mn4-Mn4 distance (4.48 Å) which is not reported in the figure is similar to the Mn2-Mn2 distance. Figure modified from Fig. 1(b) in Wu et al. (2019).

Table 1

 1 Chemical composition and specific surface area of layered precursors and reflux

	products
	Samples	Mn wt. %	Ni wt. %	Ni/(Mn+Ni) mol%	Mg wt. %	(Mg+Ni)/Mn mol%	SSA (m 2 /g)
	Bir	55.4(1.0)	--	--	--	--	--
	NiB1	54.1(1.1) 0.39(1)	0.67	--	--	--
	NiB5	53.7(1.0) 1.86(4)	3.18	--	--	--
	NiB10	51.2(1.0) 3.89(7)	5.98	--	--	--
	NiB15	47.3(1.7) 5.32(3)	8.71	--	--	--
	NiB20	44.8(1.8) 7.14(7)	11.50	--	--	--
	Tod	54.4(2.1)	--	--	3.95(8)	16.4	90.7
	NiT1	54.3(8)	0.33(1)	0.57	3.51(12)	16.2	--
	NiT5	54.2(6)	1.57(2)	2.57	3.12(2)	15.6	93.7

Table 3 .

 3 Fractional and average valence states of Mn obtained for layered precursors and reflux products from the Combo fit of XANES spectra and 1 st derivatives. The estimated error for Combo method is ±4%[START_REF] Manceau | Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy[END_REF][START_REF] Yin | High Co-doping promotes the transition of birnessite layer symmetry from orthogonal to hexagonal[END_REF]

		Mn 2+	Mn 3+	Mn 4+
	Sample				Mn-AOS
		at. %	at. %	at. %
	Bir	3	26	72	3.69
	NiB1	2	25	74	3.71
	NiB5	2	30	67	3.64
	NiB10	2	31	68	3.66
	NiB15	0	31	69	3.69
	NiB20	0	35	65	3.65
	Tod	2	25	74	3.72
	NiT1	1	35	65	3.64
	NiT5	3	32	66	3.63
	NiT10	2	35	64	3.62
	NiT15	1	34	65	3.64
	NiT20	0	33	67	3.67

Transformation of Ni-containing birnessite to tectomanganate: Influence and fate of weakly bound Ni(II) species Supplementary material Geochimica Et Cosmochimica Acta Zhongkuan

  Wu a,b , Bruno Lanson b, *, Xionghan Feng a , Hui Yin a , Zhangjie Qin a , Xiaoming Wang a , Wenfeng Tan a , Zhongjun Chen c , Wen Wen d , Fan Liu a, * Relative proportions of O species in the near-surface of layered precursors and reflux products calculated by fitting XPS O1s data Relative proportions of NiB and Ni(OH)2 species obtained from linear combination fitting of NiT K-edge EXAFS spectra using Ni(OH)2 and corresponding NiB samples as standard references (Fig. S5).

	Table S1 Table S2		OH -	531.2		1.59	28.9
			H2O	532.5		2.12	23.4
			O 2-	529.6		1.59	50.3
	Sample NiT20	O1s (BE) 529.59	State OH -	BE(eV) 531.0	FWHM(eV) 1.59	At.% 29.8
			O 2-H2O	529.6 532.3		1.59 2.12	72.9 19.9
	Bir	529.35 Samples NiBn /% Ni(OH)2 /% Error /% OH -531.5	1.59	11.6
		NiT5	H2O 82.0	533.4 18.0	2.1	2.12	15.5
		NiT10	O 2-86.5	529.7 13.5	1.0	1.59	65.9
	NiB5	529.72 NiT15	OH -87.1	531.2 12.9	0.9	1.59	18.8
		NiT20	H2O 85.3	532.2 14.7	1.1	2.12	15.3
			O 2-	529.5		1.59	60.8
	NiB10	529.72	OH -	531.0		1.59	22.3
			H2O	532.2		2.12	16.9
			O 2-	529.8		1.59	66.6
	NiB15	530.06	OH -	531.2		1.59	18.5
			H2O	532.3		2.12	15.0
			O 2-	529.5		1.59	55.6
	NiB20	530.21	OH -	530.9		1.59	25.9
			H2O	532.4		2.12	18.5
			O 2-	529.9		1.59	58.1
	Tod	529.89	OH -	531.6		1.59	20.9
			H2O	533.0		2.12	21.0
	Institute Chinese Academy of Sciences, Shanghai 201204, China O 2-529.8 1.59	46.7
	NiT5	529.84	OH -	531.4		1.59	29.0
	* Corresponding author: bruno.lanson@univ-grenoble-alpes.fr; H2O 532.8 2.12	24.4
	liufan@mail.hzau.edu.cn	O 2-	529.8		1.59	47.6
	NiT10	530.08	OH -	531.3		1.59	31.0
			H2O	532.6		2.12	21.5
	NiT15	530.15	O 2-	529.8		1.59	46.8

a Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River) Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China b Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, F-38000 Grenoble, France c Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China d Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research

ACKNOWLEDGMENTS

Caroline Peacock (Leeds University, UK) is thanked for providing Ni K-edge EXAFS data of Bir-Ni S and Bir-Ni V references. The authors are grateful to Mengqiang Zhu (University of Wyoming, Laramie, Wy, USA) for helping collect high-energy X-ray scattering data and providing Ni(II)-H 2 O EXAFS reference. This work is supported by the National Key Research and Development Program of China (No. 2016YFD0800400) and the Natural Science Foundations of China (No. 41571448). Xionghan Feng, Bruno Lanson, and Zhongkuan Wu are supported by a Cai Yuanpei program between the Key Laboratory of Arable Land Conservation (Wuhan, China) and ISTerre (Grenoble, France)