Manganese (Mn) oxides are important hosts of trace metals via adsorption and/or redox processes. Previous adsorption studies on synthesized Mn oxides have provided valuable insights on metal adsorption mechanisms, and yet a comparison of synthesized and natural minerals is still rare. Here we focus on thallium (Tl), a paleoredox tracer with a major sink via Mn oxide burial, to investigate Tl adsorption and isotope fractionation on various natural minerals that have not been reported so far. We also compare our results on natural Mn oxide phases with previous adsorption experiments on synthetic phases. Adsorption experiments were performed on various natural Mn oxides with different structures and Mn oxidation states. Our results show that Mn2+ and Mn3+-rich oxides (hausmannite, feitknechtite, pyrolusite, and todorokite) have limited Tl affinity and isotopic fractionation upon adsorption. However, our data show unexpected Tl desorption and rapid isotope exchange on vernadite, leading to a much lower Tl affinity compared to synthesized structurally similar delta-MnO2. Using an isotope mass balance and X-ray absorption spectroscopy, we suggest that the observed Tl isotope exchange is likely to be non-oxidative with minor oxidative fractionation involved, which could be attributed to occupation of oxidative adsorption sites and competing adsorption by other trace metals (e.g., Co, Ni, Cu, etc). Our vernadite adsorption experiments thus suggest that Tl speciation changes may be prevalent in natural ferromanganese crusts. Should Tl desorption from vernadite occur periodically due to changes in ocean chemistry, the likely release of isotopically heavy 205Tl could complicate the use of Tl isotopes as an ocean oxygen proxy over geological history. As Mn oxides play an important role in metal biogeochemical cycles, our results highlight the necessity of additional research on desorption to better understand Tl cycling in nature and subsequent implications for interpreting paleo-redox conditions in the rock record.

Changes in the crystal structure of hematite (Fe2O3) during dissolution were analyzed using in situ, time-resolved X-ray diffraction (TRXRD) of synthetic hematite powders in 3 M HCl solutions. The refined Fe occupancy in hematite increased from 0.899(4) to 0.98(1) during dissolution, and lattice parameters (a, c) and unit-cell volume also increased. Because of the shared octahedral faces in the crystal structure of hematite, increasing Fe3+ occupancy enhanced repulsion between Fe3+ ions, promoting structural expansion. Previous TRXRD studies of nanohematite growth revealed that Fe occupancy also increases as crystals enlarge (Chen et al. 2023). The increase in Fe occupancy during both growth and dissolution suggests that structural changes induced by nanoscale sizing are not always reversible as a function of particle size. We attribute the increase in Fe occupancy as hematite dissolved to the sequential removal of defective surface layers. A higher concentration of Fe vacancies in the outer shells relative to the cores of the particles may account for the increase in Fe occupancy during both particle growth and dissolution.

Manganese oxides influence Ni partitioning in natural and metal-polluted systems through adsorption and incorporation. Understanding Ni behavior within environmental contexts requires knowledge of its binding to environmentally relevant minerals such as birnessite during and after precipitation. Here, we compare Ni binding at initial (48-h) and aged (14-day) time frames onto fungal (via Stagonospora sp. SRC1lsM3a) and abiotic birnessite, as fungi are important Mn oxidizers in contaminated sites, with two timing-of-addition scenarios: reacting preformed birnessite and coprecipitation. X-ray absorption spectra were collected to track the mineral structures and Ni binding modes. Aging abiotic and fungal birnessites reacted with Ni increase the hexagonal sheet symmetry and average Mn oxidation state (AMOS), while coprecipitation yields similar initial (and stable) structures. Aging also shifts Ni from biomass- and edge-bound sites in fungal and abiotic systems, respectively, to binding the above vacancies and incorporating into the mineral structure. Incorporated Ni is positively correlated with AMOS and Mn(IV) content, with both fungal- and abiotic-aged samples achieving 2.3 mol % (mol Ni:mol Mn) incorporation, albeit with different timing-of-addition scenarios. These results suggest an equilibrium structure for Ni-substituted phyllomanganates, with birnessite and fungal biomass acting as Ni sinks and sources in environmental systems.

Hematite (α-Fe2O3) is a low-cost, naturally abundant, and stable iron oxide with promising applications in energy applications such as photoelectrochemical water splitting, piezocatalysis, and microbial fuel cells. A superhydrous form, hydrohematite, incorporates significant structural water through cation vacancies, altering its physical and chemical properties. However, the positions and behavior of hydroxyl groups have not been fully characterized. In this study, we combined neutron powder diffraction, inelastic neutron scattering, pair distribution function analysis, and first-principles simulations to investigate the structure and dynamics of hydrogen in hydrohematite.Rietveld refinement of neutron data revealed that hydroxyl incorporation and Fe vacancies expands the unit cell, particularly along the c-axis. The refined c/a ratio of hydrohematite was 2.737(2), slightly higher than the range of 2.731-2.734 in anhydrous hematite. Difference Fourier maps can account for half of the hydrogen positions, with the remainder too disordered to be apparent. Magnetic structure analysis revealed enhanced moments in hydrohematite, with a refined magnetic moment of 4.710(12) μB, compared to 3.968(163) μB in stoichiometric hematite. This slight increase is attributed to Fe deficiencies in the structure. Additionally, in-situ neutron diffraction indicated that the Morin transition, observed around 260K in the stoichiometric hematite, was suppressed in hydrohematite.Combining inelastic neutron scattering and DFT calculations, we identified vibrational modes associated with hydroxyl motion at 800, 900, 1160, 1660, 2180, and 3450 cm−¹. Simulations indicated that the distinct 900 cm−¹ mode corresponds to out-of-plane OH deformation, while stretching modes between 2900–3500 cm−¹ confirm the presence of structural hydroxyls. DFT results also showed that hydrogen preferentially bonds with three of the six oxygens in an octahedral site, with calculated O-H bond lengths ranging from 0.97-1.02 Å and O–H⋅⋅⋅O hydrogen bond lengths between 1.63- 2.11 Å.

Coal mine drainage sites across Pennsylvania often contain high concentrations of Mn, along with contaminants such as Zn, Ni, Al, Co, and Fe. Passive treatment systems consisting of cobbled limestone/dolostone beds have been used to promote Mn oxidative precipitation, which can also remove other metals via incorporation into or adsorption onto the resulting Mn oxide minerals. Here, we examine Mn oxide precipitates and aqueous geochemistry of a dolostone-lined passive coal mine drainage treatment system in Glasgow near Bellwood, PA. We focus on characterizing the precipitated Mn oxide phases with exceptional detail to bridge our understanding of synthetic Mn oxides from the laboratory into the field. Mn oxides were collected and analyzed using X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, transmission electron microscopy, X-ray absorption fine structure analysis, and Raman spectroscopy. Water samples were also collected and analyzed for pH and for metal concentrations using inductively coupled plasma-optical emission spectroscopy (ICP-OES). Analyses of the coatings on the cobbles revealed mixtures of tectomanganates and phyllomanganates, including todorokite and both triclinic and hexagonal birnessite/buserite-like structures. Raman mapping showed that Mn oxide phases are intermixed at the micron-scale. Differences in the compositions of triclinic and hexagonal birnessite-like phases were observed in the samples, with triclinic birnessites containing higher Ca, Mg, Al, Co, and Ni concentrations than hexagonal birnessites. Water analyses indicated that the treatment system was effective at removing Mn, Zn, Ni, and Fe as water infiltrated the beds of cobbled dolostone.

To compare thermal expansion behaviors in isomorphic structures with different redox behaviors, we separately heated pyrolusite (Mn4+O2) and rutile (Ti4+O2) powders from 25 to-1000 degrees C at a rate of 0.05 degrees C/s at ambient pressure and analyzed them using synchrotron X-ray diffraction and Rietveld refinement. The pyrolusite exhibited two reduction-induced phase decompositions. Between 531 and 583 degrees C, pyrolusite decomposed to bixbyite (Mn3+2 O3), and between 972 and 998 degrees C, bixbyite decomposed to hausmannite (Mn2+Mn23+ O-4). The rutile experienced no phase decompositions or transitions over this same temperature interval. The refined bond lengths and angles for pyrolusite showed that the Mn4+ coordination octahedron became more distorted near the phase decomposition to bixbyite due to the approach of two coordinating oxygen atoms. The 3d electrons of each Mn4+ ion in pyrolusite make 7C bonds with the 2p electrons of the surrounding O-2-ions, whereas the Ti4+ of rutile has no 3d electrons. Thus, 7C bonding between octahedral Mn4+ and the surrounding O-2-anions increases the strength of the Mn-O bonds of pyrolusite relative to the Ti-O bonds of rutile. However, we observed a small decrease in O-O distances in pyrolusite before the decomposition, suggesting that at high temperature, increased 7C bonding between adjacent O anions precedes the release of O2 and the reduction of Mn4+ to Mn3+. Analogous behavior was observed for bixbyite before its reductive phase decomposition to hausmannite. In contrast, no anomalous changes in the O-O distances occurred for rutile. Our X-ray diffraction analyses provided accurate thermal expansion coefficients for these materials over a broader temperature range than reported in previous studies. The Mn-O bond lengths in pyrolusite were shorter and stronger than the Ti-O bonds of rutile; for example, at 60 degrees C, bond distances in pyrolusite refined to 1.8854(6) angstrom whereas the bond lengths of rutile were 1.9575(4) angstrom. Accordingly, the axial and volumetric thermal expansion coefficients refined for rutile were greater than those for pyrolusite. The axial coefficients of thermal expansion (CTE) for pyrolusite were 96% (a-axis) and 69% (c-axis) of the corresponding values for rutile, and the volumetric CTE for pyrolusite was 85% of that for rutile.

Abstract Studies suggest that feitknechtite (β-MnOOH) is a prevalent, and perhaps necessary, intermediate phase during the synthesis of birnessite-like phases, the abiotic oxidation of Mn2+, and the transformation of biogenic hexagonal phyllomanganates to more complex Mn oxides in laboratory and natural systems. Researchers have generally described feitknechtite as consisting of pyrochroite-like (or cadmium iodide-like) Mn-O octahedral layers, but a detailed crystal structure has not been reported. We used TEM/SAED and powder XRD and Rietveld refinements to derive the unit cell and, for the first time, report a complete structure description for feitknechtite (β-MnOOH). Rietveld refinements were also completed for three natural feitknechtite/hausmannite samples, and time-resolved synchrotron XRD experiments were used to follow the thermal transformation of feitknechtite to hausmannite. Additionally, we identified and report the structure for a second, and perhaps novel, MnOOH polymorph (proposed designation ε-MnOOH), mixed with the synthetic feitknechtite, that is similar to β-MnOOH but with a different layer stacking.

Microbial manganese (Mn) oxidation plays a critical role in Mn(III/IV) oxide formation in modern day environmental systems. These oxidation processes and resulting biominerals are sensitive to aqueous conditions, such as pH and dissolved constituent concentrations. With Mn and sulfur (S) biogeochemical cycling closely linked in many environmental systems, and dissolved organic sulfur comprising a substantial pool of total sulfur in several environments, the impact of dissolved organic sulfur compounds on Mn redox processes is important to consider. Sulfonic acids, environmentally ubiquitous organosulfur compounds, play substantial roles in S cycling in many natural and contaminated systems. Research to assess the effects of these abundant sulfonic acids on Mn biogeochemical cycling, microbial Mn oxidation processes, and Mn biominerals is needed for understanding and predicting the impact of coupled S and Mn biogeochemical cycles, particularly in environments with dynamic redox gradients or in anthropogenically contaminated systems. Further, with research on microbial and abiotic Mn oxidation processes often using aminosulfonic acids to control pH, understanding the impact of such sulfonic acids on microbial Mn oxidation processes is critical. Several recent studies found that commonly used zwitterionic N substituted aminosulfonic acids, known as Good's buffers, such as HEPES and MES, can alter abiotic birnessite sheet structures. Here we investigate the impact of two sulfonic acids with broad applications to natural and contaminated sites as well as laboratory settings (HEPES and MES) on fungal Mn oxidation relative to a carbonate buffer and a buffer-free control by three Ascomycete fungi known to oxidize Mn(II): Stagonospora sp. SRC1lsM3a, Paraphaeosphaeria sporulosa AP3s5–JAC2a, and Plectosphaerella cucumerina DS2psM2a2. Structural analyses of the products show that sulfonic acids promote Mn oxidation by P. cucumerina, producing hexagonally symmetric phyllomanganates analogous to hexagonal birnessite or c–disordered H+ birnessite [(Ca,Na,K)(Mn4+,Mn3+,□)O2 nH2O], with solid–associated Mn(II) bound to vacancy sites and biomass, while in their absence almost all Mn remains as either aqueous Mn(II) or solid–associated Mn(II) bound to biomass. In contrast, sulfonic acids exert the opposite effect on Mn oxidation by P. sporulosa, with their presence suppressing Mn(II) oxidation to Mn(IV), likely leading to the formation of mycogenic bixbyite (Mn3+2O3) while the buffer–free control forms a poorly crystalline phyllo- or tectomanganate. Meanwhile, all treatments exert a minimal effect on Mn(II) uptake from solution and Mn oxidation with Stagonospora sp., with all experimental systems and controls forming poorly crystalline, hexagonally symmetric phyllomanganates. The fact that the sulfonic acids here studied exert similar effects on Mn oxidation, but substantially different effects for each fungus, suggests they affect Mn oxidation via mechanistically similar pathways that are likely dependent on interactions with fungal exudates (which vary from species to species) or specific fungal Mn oxidation processes. Interestingly, for all fungi, MES increases Mn(III) in the resulting biominerals, while the carbonate buffer consistently decreases Mn(III). These results clearly demonstrate that sulfonic acids not only alter Mn oxide structures, as has been previously noted in abiotic studies, but can interfere with Mn oxidation reactions themselves, highlighting the incredible sensitivity of both Mn oxide structures and the Mn oxidation process to the aqueous environment.

Non-crystalline silica mineraloids are essential to life on Earth as they provide architectural structure to dominant primary producers, such as plants and phytoplankton, as well as to protists and sponges. Due to the difficulty in characterizing and quantifying the structure of highly disordered X-ray amorphous silica, relatively little has been done to understand the mineralogy of biogenic silica and how this may impact the material properties of biogenic silica, such as hardness and strength, or how biosilica might be identified and differentiated from its inorganic geological counterparts. Typically, geologically formed opal-A and hyalite opal-AN are regarded as analogs to biogenic silica, however, some spectroscopic and imaging studies suggest that this might not be a reasonable assumption. In this study, we use a variety of techniques (X-ray diffraction, Raman spectroscopy, and scanning electron microscopy) to compare differences in structural disorder and bonding environments of geologically formed hydrous silicas (Opal-A, hyalite, geyserite) and silica glass versus biogenic silicas from an array of organisms. Our results indicate differences in the levels of structural disorder and the Raman-observed bonding environments of the SiO2 network modes (D1 mode) and the Q-species modes (~1015 cm-1 ) between varieties of biogenic silicas and geologically formed silicas, which aligns with previous studies that suggest fundamental differences between biogenic and geologically formed silica. Biosilicas also differ structurally from one another by species of organism. Our mineralogical approach to characterizing biosilicas and differentiating them from other silicas may be expanded to future diagenesis studies, and potentially applied to astrobiology studies of Earth and other planets.

In this study, we investigated an unusual natural Mn oxide hollandite-group mineral from the Kohare Mine, Iwate Prefecture, Japan, that has predominantly water molecules in the tunnels, with K, Na, Ca, and Ba. The specimens are labeled as type manjiroite, but our analyses show that Na is not the dominant tunnel species, nor is it even the primary tunnel cation, suggesting either an error in the original analyses or significant compositional variation within samples from the type locality. Chemical analyses, X-ray photoelectron spectroscopy, and thermal gravimetric analysis measurements combined with Rietveld refinement results using synchrotron X-ray powder diffraction data suggest the chemical formula: (K0.19Na0.17Ca0.03Ba0.01H2O1.60)(Mn5.024+Mn2.823+Al0.14Fe0.02)O-13.47(OH)(2.53). Our analyses indicate that water is the primary tunnel species, and although water has been reported as a component in natural hollandites, this is the first detailed study of the crystal structure and dehydration behavior of a natural hydrous hollandite with water as the predominant tunnel species. This work underscores the rarity of natural Na-rich hollandite phases and focuses new attention on the role of hydrous components of hollandite-like phases in determining their capacities to exchange or accommodate various cations, such as Li+, Na+, Ba2+, Pb2+, and K+ in natural systems.

Phyllomanganates are ubiquitous in a variety of environments and commonly enriched in transition metal elements, such as Ni. The effect of such foreign metal cations on phyllomanganate transformation is widely documented under aqueous conditions together with the induced modification of Ni geochemical behavior. A similar knowledge is lacking however on phyllomanganate transformation and on the induced fate of associated metal elements that may occur under dry conditions, that prevail in deserts and arid areas increasingly exposed to severe droughts or wildfires. The present study shows that crystallinity, morphology, Mn oxidation state, and Ni binding mechanisms are essentially unaffected when aging hexagonal birnessite (Mn oxidation state similar to 3.90 and Ni/Mn molar ratios of 0.00 and 0.13) in the dry state at room temperature for up to 8 years. In contrast, heating aged Ni-doped birnessite to 25-200 ? results in an increased proportion of edge-sharing Ni-Ni(Mn) pairs with increasing temperature induced by the migration of interlayer Ni to birnessite octahedral layers and/or by an increased sharing of coordination oxygens by interlayer Ni/Mn from adjacent layers. Further heating to 400 ? does not change this proportion, with birnessite layer structure being retained. Transformation of Ni-doped birnessite to cryptomelane is complete at 500 ?, while that of Ni-free birnessite is achieved at 400 ?, suggesting that Ni doping increases birnessite thermal stability. Birnessite-to-cryptomelane transformation comes with a strong increase of Mn oxidation state, whereas this parameter remains unchanged in heated birnessite samples. Ni incorporation in the cryptomelane framework, reduces its release during reductive acid dissolution by a factor of 396 +/- 15 compared to initial birnessite. These results shed light on mineral transformation affecting layered manganates under dry conditions and on the fate of associated transition metal elements. (C) 2022 Elsevier Ltd. All rights reserved.

The crystallization of hematite from precursor ferrihydrite was studied using time-resolved, angle dispersive synchrotron X-ray diffraction in aqueous solutions at pH 10 and 11 and at temperatures ranging from 80 to 170 degrees C. Rietveld analyses revealed a non-classical crystallization pathway involving vacancy infilling by Fe as defective hematite nanocrystals evolved. At 90 degrees C and pH 11, incipient hematite particles exhibited an Fe site occupancy as low as 0.68(2), and after 30 min, Fe occupancy plateaued at 0.84(1), achieving a metastable steady state with a composition corresponding to "hydrohematite." During crystal growth, unit-cell volume increased with an increase in Fe occupancy. The increase in Fe occupancy in hydrohematite was accomplished by deprotonation, resulting in a shortening of the long Fe-O(H) bonds and decreased distortion of the octahedral sites. Once the occupancy stabilized, the unit-cell volume contracted following further nanoparticle growth. Our study documented various synthetic routes to the formation of "hydrohematite" with an Fe vacancy of 10-20 mol% in the final product.The structure refined for synthetic hydrohematite at 90 degrees C and pH 11 closely matched that of natural hydrohematite from Salisbury, Connecticut, with a refined Fe occupancy of 0.83(2). Dry heating this natural hydrohematite generated anhydrous, stoichiometric hematite, again by continuous infilling of vacancies. The transformation initiated at 150 degrees C and was complete at 700 degrees C, and it was accompanied by the formation of a minor amorphous phase that served as a reservoir for Fe during the inoculation of the defective crystalline phase.

Manganese oxides are ubiquitous marine minerals which are redox sensitive. As major components of manganese nodules found on the ocean floor, birnessite and buserite have been known to be two distinct water-containing minerals with manganese octahedral interlayer separations of ~7 Å and ~10 Å, respectively. We show here that buserite is a super-hydrated birnessite formed near 5 km depth conditions. As one of the most hydrous minerals containing ca. 34.5 wt. % water, super-hydrated birnessite, i.e., buserite, remains stable up to ca. 70 km depth conditions, where it transforms into manganite by releasing ca. 24.3 wt. % water. Subsequent transformations to hausmannite and pyrochroite occur near 100 km and 120 km depths, respectively, concomitant with a progressive reduction of Mn4+ to Mn2+. Our work forwards an abiotic geochemical cycle of manganese minerals in subduction and/or other aqueous terrestrial environments, with implications for water storage and cycling, and the redox capacity of the region.

Iron (oxyhydr)oxides are sensitive indicators of pH, Eh, temperature, microbial activity, and climate conditions in the Critical Zone. The most ubiquitous and environmentally significant iron oxides in most soils are two-line ferrihydrite, goethite, and hematite. Here we present a comprehensive study of the transformation of two-line ferrihydrite to hematite and goethite over a wide range of temperature (25-170 degrees C) and initial pH (2-13) conditions through ex situ batch and in situ synchrotron X-ray diffraction (XRD) experiments. Within the high time resolution of our experiments, goethite and hematite nucleated nearly simultaneously from ferrihydrite in nearly equal concentrations by mass. Hematite increased in abundance relative to goethite until a steady-state ratio was achieved, and both phases ceased growth on the depletion of ferrihydrite. Higher temperatures and lower water activities favored hematite formation at all pH values studied. In both our batch and our time-resolved, angle-dispersive synchrotron X-ray diffraction experiments, hematite was favored relative to goethite at an initial pH of 3 to 5. In contrast, goethite preferentially formed in neutral (initial pH 7-8) and highly alkaline conditions (initial pH >= 11). Surprisingly, mildly alkaline conditions (initial pH 9-11) induced the precipitation of a highly Fe-deficient (Fe-occ = similar to 0.80-0.90) variety of hematite known as "hydrohematite" in greater concentrations than goethite. Our results are useful for the application of hematite-goethite ratios as paleoclimate proxies for soil and sediment systems with low pH buffering capacities.

Hausmannite is a common low valence Mn oxide mineral, with a distorted spinel structure, in surficial sediments. Although natural Mn oxides often contain various impurities of transitional metals (TMs), few studies have addressed the effect and related mechanism of TM doping on the reactivity of hausmannite with metal pollutants. Here, the reactivity of cobalt (Co) doped hausmannite with aqueous As(III) and As(V) was studied. Co doping decreased the point of zero charge of hausmannite and its adsorption capacity for As(V). Despite a reduction of the initial As(III) oxidation rate, Co-doped hausmannite could effectively oxidize As(III) to As(V), followed by the adsorption and fixation of a large amount of As(V) on the mineral surface. Arsenic K-edge EXAFS analysis of the samples after As(V) adsorption and As(III) oxidation revealed that only As(V) was adsorbed on the mineral surface, with an average As-Mn distance of 3.25-3.30 Å, indicating the formation of bidentate binuclear complexes. These results provide new insights into the interaction mechanism between TMs and low valence Mn oxides and their effect on the geochemical behaviors of metal pollutants.