Publication Search Results

ID: 146005
Authors: Brooks, Alison S.; Yellen, John E.; Potts, Richard; Behrensmeyer, Anna K.; Deino, Alan L.; Leslie, David E.; Ambrose, Stanley H.; Ferguson, Jeffrey R.; d'Errico, Francesco; Zipkin, Andrew M.; Whittaker, Scott; Post, Jeffrey; Veatch, Elizabeth G.; Foecke, Kimberly; Clark, Jennifer B.
Keywords: NH-Anthropology; NH-Paleobiology; NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Previous research suggests that the complex symbolic, technological, and socio-economic behaviors that typifyHomo sapienshad roots in the middle Pleistocene <200 ka, but data bearing on human behavioral origins are limited. We present a series of excavated Middle Stone Age sites from the Olorgesailie Basin, southern Kenya, dated >=295 to ~320 ka by40Ar/39Ar and U-Series methods. Hominins at these sites made prepared cores and points, exploited iron-rich rocks to obtain red pigment, and procured stone tool materials from >=25-50 km distance. Associated fauna suggests a broad resource strategy that included large and small prey. These practices imply significant changes in how individuals and groups related to the landscape and one another, and provide documentation relevant to human social and cognitive evolution.
ORCID(s): 0000-0001-6008-0100,0000-0001-6857-368X,0000-0002-7978-8747

ID: 145570
Authors: Byrne, Keal S.; Butler, James E.; Wang, Wuyi; Post, Jeffrey E.
Keywords: NMNH; NH-Mineral Sciences; Peer-reviewed
Abstract: To date, the eponymous color-changing behavior of chameleon diamonds lacks an explanation in terms of an identified diamond defect structure or process. Well known, however, is that this color-change is driven by the influence of both light and heat. In this paper, we present observations of how luminescence emission in chameleon diamonds responds to temperature changes and optical pumping. Fluorescence, phosphorescence, and thermoluminescence experiments on a suite of natural chameleon diamonds reveal that a specific emission band, peaking near 550 nm, may be stimulated by several different mechanisms. We have observed thermal quenching of the 550 nm emission band with an activation energy of 0.135 eV. The 550 nm band is also observed in phosphorescence and thermoluminescence. Thermoluminescence spectra suggest the presence of low lying acceptor states at 0.7 eV above the valence band. When excited with 270 nm light, we observe emission of light in two broad spectral bands peaking at 500 and 550 nm. We suggest that the 550 nm emission band results from donor-acceptor pair recombination (DAPR) from low lying acceptor states at ca. 0.7 eV above the valence band and donor states approximately 2.5 to 2.7 eV above the valence band. We do not identify the structure of these defects. We propose a speculative model of the physics of the color change from 'yellow' to 'green' which results from increased broad-band optical absorption in the near-IR to visible due to transitions from the valence band into un-ionized acceptor states available in the 'green' state of the chameleon diamond. We report near-IR absorption spectra confirming the increased absorption of light in the near-IR to visible in the 'green' when compared to the 'yellow' state with a threshold at ca. 0.65 eV, supporting the proposed model.

ID: 144412
Authors: Fischer, Timothy B.; Heaney, Peter J.; Post, Jeffrey E.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: We used time-resolved synchrotron X-ray diffraction to follow the complete dissolution of synthetic triclinic Na-birnessite as promoted by the trihydroxamate siderophore desferrioxamine B (DFOB). Many microorganisms employ siderophores to increase the availability of Fe, Mn, and other trace metals for metabolic processes. Our primary goal was to quantify the DFOB-assisted dissolution rate by direct, continuous observation of the solid phase. Our kinetic model indicates that the rate of dissolution is dependent on DFOB] but not pH, and has a reaction order of 0.505 with a rate constant of 0.112 wt%birn min- 1. The unit-cell dimensions of birnessite remained virtually constant within error throughout the dissolution process, showing only a 0.3% contraction along the c-axis. Despite the small changes in unit-cell volume, Rietveld analysis revealed that the occupancy of Mn within the octahedral sheets decreased from 100% to ~ 80%, presumably as the result of complexation of structural Mn3 + with DFOB followed by extraction of Mn3 + from the crystal structure. These observations suggest a critical lacunarity of ~ 20 mol% Mn for triclinic Na-birnessite, below which the structure is destabilized. Moreover, this study reveals that DFOB-promoted dissolution must operate by a different mechanism from that engaged when bacterial membrane fractions directly transfer electrons to birnessite crystals. We propose that crystal structure analysis of minerals undergoing dissimilatory metal reduction can elucidate metabolic pathways employed by microorganisms.

ID: 146557
Authors: Lin, Xiayang; Heaney, Peter J.; Post, Jeffrey E.
Keywords: NMNH; NH-Mineral Sciences; Peer-reviewed; Mineralogy
Abstract: The authors investigated "rainbow" hematite from Minas Gerais, Brazil, using electron microscopy, atomic force microscopy, and synchrotron X-ray diffraction to determine the cause of its intense wide-angle iridescence. The study revealed that the interference is produced by a highly periodic microstructure consisting of spindle-shaped hematite nanocrystals containing minor Al and P impurities. The nanorods are 200-300 nm in length and 5060 nm in width. They are arranged in three orientations at 120 degrees angles with respect to each other and stacked layer by layer to form the bulk crystal. The distances between adjacent parallel spindle-shaped particles within the same layer fall in the range of 280-400 nm, generating a diffraction grating for visible light. The organized substructure is apparent on all freshly fractured surfaces, suggesting that it represents more than an exterior surface coating. The authors propose that this periodic substructure results from arrested crystal growth by the oriented aggregation of hematite nanorods.

ID: 145172
Authors: Ling, Florence T.; Post, Jeffrey E.; Heaney, Peter J.; Ilton, Eugene S.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Because of their nanocrystallinity and high cation exchange capacities, birnessite phases can control the cycling of heavy metals in soils and groundwaters, and they also are implicated in the oxidation of transition metals in natural environments. Birnessite reactivity is determined by crystal structure and composition. Because birnessites typically are poorly crystalline, synchrotron-based absorption spectroscopy (EXAFS, XANES) often is utilized for structural characterization. For example, linear combination fitting (LCF) of X-ray absorption spectra typically is applied to quantify mixed triclinic and hexagonal birnessite phases. This approach is challenged, however, because the structures of the standards are not always plainly apparent. Moreover, it is difficult to distinguish birnessites with nanoscale intergrowths of hexagonal and triclinic endmembers from homogeneous birnessite structures of "intermediate triclinicity". We explored these issues by synthesizing a host of cation-exchanged birnessite specimens whose long-range symmetrical character could be determined by X-ray diffraction without ambiguity. Through a combination of Fourier transform infrared spectroscopy (FTIR), extended X-ray absorption fine structure (EXAFS), and X-ray photoelectron spectroscopy (XPS), we have examined the relationships among structural symmetry, Mn oxidation state, and interlayer composition. Our results confirm prior models that as the concentration of Mn3+ increases, the departure from hexagonal symmetry also increases. Rietveld refinements indicate that the Jahn-Teller distortions associated with Mn3+ induce systematic variations in unit-cell parameters, particularly an increase in the a-axis and the ? angle of the unit cell. Interlayer cation composition also controls structural distortions, and Ca-rich birnessites showed less deviation from hexagonality than did Na-, K-, and Ba-birnessites. Our linear combination fits of X-ray absorption spectra sometimes yielded misleading results, reinforcing the difficulty and importance of selecting appropriate standards.

ID: 148689
Authors: Megaw, Peter K. M.; Huizing, Terry E.; Richards, R. P.; Post, Jeffrey E.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed

ID: 149355
Authors: Peterson, Kristina M.; Heaney, Peter J.; Post, Jeffrey E.
Keywords: NMNH; NH-Mineral Sciences; Peer-reviewed
Abstract: Synchrotron X-ray diffraction was used to monitor the hydrothermal precipitation of akaganeite (beta-FeOOH) and its transformation to hematite (Fe2O3) in situ. Akaganeite was the first phase to form and hematite was the final phase in our experiments with temperatures between 150 and 200 degrees C. Akaganeite was the only phase that formed at 100 degrees C. Rietveld analyses revealed that the akaganeite unit-cell volume contracted until the onset of dissolution, and subsequently expanded. This reversal at the onset of dissolution was associated with a substantial and rapid increase in occupancy of the Cl site, perhaps by OH- or Fe3 . Rietveld analyses supported the incipient formation of an OH-rich, Fe-deficient hematite phase in experiments between 150 and 200 degrees C. The inferred H concentrations of the first crystals were consistent with "hydrohematite." With continued crystal growth. the Fe occupancies increased. Contraction in both a- and c-axes signaled the loss of hydroxyl groups and formation of a nearly stoichiometric hematite. (C) 2018 International Centre for Diffraction Data.

ID: 146186
Authors: Rabenhorst, Martin; Post, Jeffrey E.
Keywords: NMNH; NH-Mineral Sciences; Peer-reviewed
Abstract: Following on earlier work that has shown the value of iron-based IRIS (Indicator of Reduction In Soils) technology, recent efforts have focused on developing a manganese-based IRIS coating. A synthesis procedure developed utilizing a high Na lactate to KMnO4 molar ratio followed by 3 d of dialysis, forms a crystalline birnessite that can be easily applied to PVC tubing and which forms a durable manganese oxide coating. In this study, a series of experiments were run to evaluate the impact of the following parameters on the synthesis of the birnessite: Na/K ratio; lactate/permanganate ratio; lactate source; reaction time; centrifuge washing; dialysis and ageing of the samples. Results confirm that to synthesize birnessite that forms a durable coating both high lactate and high Na/K ratios are required. The high lactate/permanganate ratio (6.7) facilitates a rapid nucleation of triclinic birnessite crystals within 10 min during which time the initial structural template is established, and where some of the manganese (approximately 38%) is reduced to Mn3+. The high Na/K facilitates the ordering of the triclinic birnessite along the c axis following synthesis, and based on analyses of x-ray diffraction (XRD) patterns, the crystallites grow from 2 to 12 nm in direction of c axis over the course of several days. Scanning electron microscopy (SEM) analyses show that oriented structured particles up to 1 mm form. X-ray photoelectron spectroscopy (XPS) analyses demonstrate that there is no difference in the charge structure of the Mn between birnessites that will, or will not, form a durable coating. We conclude that the differences in durability are related primarily to physical interaction and/or interlocking of crystals.

ID: 149195
Authors: Yang, Peng; Lee, Seungyeol; Post, Jeffrey E.; Xu, Huifang; Wang, Qian; Xu, Wenqian; Zhu, Mengqiang
Keywords: NMNH; NH-Mineral Sciences; Peer-reviewed
Abstract: Tunneled Mn oxides (TMOs) are common minerals in natural environment, particularly in ferromanganese nodules of oceanic and lake sediments. Their structures host a considerable amount of transition and rare earth metals, thus mediating metal cycling and bearing potential economic interest for exploiting these metals. TMOs form through topotactic transformation of layered Mn oxides (LMOs), such as vernadite, in natural environment. Trivalent Mn (Mn(III)) in the LMO structure is a critical player in the transformation, and the transformation is believed to be extremely slow at room temperature. However, the specific role of Mn(III) and its impacts on the transformation kinetics remain unknown. In the present study, we show that the formation of Mn(III) on vacancies of an LMO is the initial transformation step leading to TMOs, and that the transformation can be rapid at room temperature and circumneutral pH. Specifically, after pre-adsorbed with Mn(II) on vacancies at pH 4, delta-MnO2, a hexagonal birnessite analogous to vernadite, starts to transform to a 4 x 4 TMO at 1 h upon incubation at pH 7 and 21 degrees C under anoxic conditions. The rapid transformation is triggered by the comproportionation reaction between the vacancy-adsorbed Mn(II) and Mn(IV) in delta-MnO2 that produces Mn(III) on the vacancies. Such intermediate Mn(III)-rich product acts as a precursor for subsequent rapid structural rearrangement to form tunnels. An incubation at lower or higher pH retards the transformation due to an insufficient amount of Mn(III) (pH 6) or the formation of triclinic birnessite (pH 8) as an intermediate product. The presence of O-2 favors the formation of triclinic birnessite at pH 8 and thus retards the transformation whereas O-2 enhances production of Mn(III)-rich hexagonal birnessite at pH 6 and 7 and promotes the transformation. We propose a novel transformation mechanism of LMOs to TMOs, highlighting the role of vacancy-adsorbed Mn(III) in the transformation. This work changes our understanding of TMO formation kinetics and suggests TMOs can readily form in low-temperature redox-fluctuating environment, such as lake and oceanic sediments where Mn(II) often coexists with LMOs. (C) 2018 Elsevier Ltd. All rights reserved.

ID: 145571
Authors: Butler, James E.; Post, Jeffrey E.; Wang, Wuyi
Keywords: NMNH; NH-Mineral Sciences; Peer-reviewed

ID: 142277
Authors: Ling, Florence T.; Post, Jeffrey E.; Heaney, Peter J.; Kubicki, James D.; Santelli, Cara M.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: The characterization of birnessite structures is particularly challenging for poorly crystalline materials of biogenic origin, and a determination of the relative concentrations of triclinic and hexagonal birnessite in a mixed assemblage has typically required synchrotron-based spectroscopy and diffraction approaches. In this study, Fourier-transform infrared spectroscopy (FTIR) is demonstrated to be capable of differentiating synthetic triclinic Na-birnessite and synthetic hexagonal H-birnessite. Furthermore, IR spectral deconvolution of peaks resulting from MnO lattice vibrations between 400 and 750cm(-1) yield results comparable to those obtained by linear combination fitting of synchrotron X-ray absorption fine structure (EXAFS) data when applied to known mixtures of triclinic and hexagonal birnessites. Density functional theory (DFT) calculations suggest that an infrared absorbance peak at ~1628cm(-1) may be related to OH vibrations near vacancy sites. The integrated intensity of this peak may show sensitivity to vacancy concentrations in the Mn octahedral sheet for different birnessites.

ID: 141439
Authors: Zubkov, V. I.; Solomnikova, A. V.; Post, Jeffrey E.; Gaillou, Eloise; Butler, J. E.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Precision admittance spectroscopy measurements were carried out over wide temperature and frequency ranges for a set of natural single crystal type IIb diamond samples. Peaks of conductance spectra vs. temperature and frequency were used to compute the Arrhenius plots, and activation energies were derived from these plots. The capacitance-voltage profiling was used to estimate the majority charge carrier concentration and its distribution into depth of the samples. Apparent activation energies between 315 and 325 meV and the capture cross section of about 10- 13 cm2 were found for samples with uncompensated boron concentrations in the range of 1 to 5 × 1016 cm- 3 (0.06-0.3 ppm). The obtained boron concentrations are in good coincidence with FTIR results for the samples. Also, a reason for the difference between the observed admittance activation energy and the previously reported ionization energy for the acceptor boron in diamond (0.37 eV) is proposed.

ID: 138799
Authors: Ilton, Eugene S.; Post, Jeffrey E.; Heaney, Peter J.; Ling, Florence T.; Kerisit, Sebastien N.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Hydrous manganese oxides are an important class of minerals that help regulate the geochemical redox cycle in near-surface environments and are also considered to be promising catalysts for energy applications such as the oxidation of water. A complete characterization of these minerals is required to better understand their catalytic and redox activity. In this contribution an empirical methodology using X-ray photoelectron spectroscopy (XPS) is developed to quantify the oxidation state of hydrous multivalent manganese oxides with an emphasis on birnessite, a layered structure that occurs commonly in soils but is also the oxidized endmember in biomimetic water-oxidation catalysts. The Mn2p3/2, Mn3p, and Mn3s lines of near monovalent Mn(II), Mn(III), and Mn(IV) oxides were fit with component peaks; after the best fit was obtained the relative widths, heights and binding energies of the components were fixed. Unknown multivalent samples were fit such that binding energies, intensities, and peak-widths of each oxidation state, composed of a packet of correlated component peaks, were allowed to vary. Peak-widths were constrained to maintain the difference between the standards. Both average and individual mole fraction oxidation states for all three energy levels were strongly correlated, with close agreement between Mn3s and Mn3p analyses, whereas calculations based on the Mn2p3/2 spectra gave systematically more reduced results. Limited stoichiometric analyses were consistent with Mn3p and Mn3s. Further, evidence indicates the shape of the Mn3p line was less sensitive to the bonding environment than that for Mn2p. Consequently, fitting the Mn3p and Mn3s lines yielded robust quantification of oxidation states over a range of Mn (hydr)oxide phases. In contrast, a common method for determining oxidation states that utilizes the multiplet splitting of the Mn3s line was found to be not appropriate for birnessites.

ID: 141416
Authors: Peterson, Kristina M.; Heaney, Peter J.; Post, Jeffrey E.
Keywords: Peer-reviewed; NMNH; NH-Mineral Sciences
Abstract: The nucleation and growth of akaganeite and its transformation to hematite under hydrothermal conditions were monitored over a temperature range of 80 to 200 degrees C using time-resolved synchrotron X-ray diffraction. In each experiment, akaganeite was the first phase to form and hematite was the final phase. No intermediate phases were identified. The induction time to akaganeite nucleation was similar to 5525 s and 537 s at 80 degrees C and 100 degrees C, respectively, yielding an activation energy of 129 +/- 15 kJ/mol. However, akaganeite nucleated at a constant temperature of 123 +/- 5 degrees C when the heater set point was 150 degrees C or higher, suggesting an activation energy for akaganeite nucleation of 0 kJ/mol between 150 and 200 degrees C. Hematite nucleation induction times decreased with increasing temperature from 1723 s to 110 s between 150 and 200 degrees C. Based on a JMAK analysis, the activation energies for the crystal growth and dissolution of akaganeite were 74 +/- 8 kJ/mol and 125 +/- 7 kJ/mol, respectively. Our calculated activation energies for hematite nucleation and crystal growth were 80 +/- 13 kJ/mol and 110 +/- 21 kJ/mol, respectively. (C) 2016 Elsevier B.V. All rights reserved.

ID: 138658
Authors: Post, Jeffrey E.; Thomas, Ellen; Heaney, Peter J.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Synchrotron powder X-ray diffraction and scanning electron microscopy examinations of manganese oxide concretions/nodules (~0.3-1.0 mm diameter) from ODP Site 1262 on Walvis Ridge in the Southeastern Atlantic Ocean revealed that they consist primarily of the layered Mn oxide phase jianshuiite (Mg,Mn,Ca)Mn34+O7·3H2O]. The nodules are from an interval with severe carbonate dissolution that represents the Paleocene/Eocene (P/E) thermal maximum (~55.8 Ma). Most nodules from the middle of the carbonate dissolution interval contain internal open space, and consist almost entirely of euhedral plate-like jianshuiite crystals, 2-4 ?m in diameter and ~0.1-0.5 ?m thick. Backscattered electron images and energy-dispersive X-ray analyses revealed stacks of interleaved Al-rich and Al-poor jianshuiite crystals in some nodules. The crystals in other nodules contain predominantly Mg (with trace K and Al) in addition to Mn and O, making them near "end-member" jianshuiite. Rietveld refinements in space group R3 confirmed the isostructural relationship between jianshuiite and chalcophanite, with Mg occupying the interlayer position above and below the vacant sites in the Mn/O octahedral sheet, and coordinated to 3 octahedral layer O atoms (1.94 Å) and 3 interlayer water O atoms (2.13 Å). Final refined occupancy factors suggest that small quantities of Ni and possibly Mn2+ are located on the Mg site. The transient appearance of the Mg-rich birnessite-like phase jianshuiite, probably abiotically produced, must indicate an exceptional transient change in the chemistry of the pore fluids within deep ocean sediments directly following the P/E boundary, possibly as a result of decreasing oxygen levels and pH, followed by a return to pre-event conditions.

ID: 133993
Authors: Hwang, Gil Chan; Post, Jeffrey E.; Lee, Yongjae
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: In situ high-pressure synchrotron X-ray powder diffraction study of three tunnel manganese oxide minerals (hollandite with 2 × 2 MnO6 octahedra tunnels, romanechite with 2 × 3 tunnels, and todorokite with 3 × 3 tunnels) was performed using a diamond anvil cell and nominally penetrating alcohol and water mixture as a pressure-transmitting medium up to ~8 GPa. Bulk moduli (B 0) calculated using Murnaghan’s equation of state are inversely proportional to the size of the tunnel, i.e., 134(4) GPa for hollandite (I2/m), 108(2) GPa for romanechite (C2/m), and 67(5) GPa for todorokite (P2/m). On the other hand, axial compressibilities show different elastic anisotropies depending on the size of the tunnel, i.e., ßa0 \beta_{0}^{a} (a/a 0) = -0.00066(3) GPa-1, ßb0 \beta_{0}^{b} (b/b 0) = 0.00179(8) GPa-1, ßc0 \beta_{0}^{c} (c/c 0) = 0.00637(4) GPa-1 c > b > a] for hollandite; ßa0 \beta_{0}^{a} (a/a 0) = 0.00485(4) GPa-1, ßb0 \beta_{0}^{b} (b/b 0) = 0.0016(1) GPa-1, ßc0 \beta_{0}^{c} (c/c 0) = 0.00199(8) GPa-1 a > c > b] for romanechite; and ßa0 \beta_{0}^{a} (a/a 0) = 0.00826(9) GPa-1, ßb0 \beta_{0}^{b} (b/b 0) = 0.0054(1) GPa-1, ßc0 \beta_{0}^{c} (c/c 0) = 0.00081(8) GPa-1 a > b > c] for todorokite. Overall, the degree of tunnel distortion increases with increasing pressure and correlates with the size of the tunnel, which is evidenced by the gradual increases in the monoclinic ß angles up to 3 GPa of 0.62°, 0.8°, and 1.15° in hollandite, romanechite, and todorokite, respectively. The compression of tunnel manganese oxides is related to the tunnel distortion and the size of the tunnel.

ID: 137573
Authors: Ling, Florence T.; Heaney, Peter J.; Post, Jeffrey E.; Gao, Xiang
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Laboratory experiments that explore the bioprecipitation or redox transformations of layered Mn oxides commonly employ buffers, such as the HEPES and MES buffers, to maintain solution pH to near neutrality. The assumption is that holding solution pH constant does not serve as the primary control for the variety of Mn oxide produced. To test this assumption, synthetic triclinic Na-birnessite was reacted in batch experiments with a pH 7 HEPES buffer, a pH 7 MES buffer, and an unbuffered pH 7 solution for 14 days in total darkness. At the end of the experimental run, the Mn oxide solids were analyzed by conventional and synchrotron X-ray powder diffraction. These assays revealed that in the presence of the HEPES buffer, triclinic Na-birnessite completely transformed into highly crystalline hexagonal H-birnessite. In unbuffered solutions starting at pH 7 and in the presence of MES, which offers a lower buffering capacity than does HEPES, triclinic Na-birnessite partially transformed to poorly crystalline hexagonal H-birnessite. The unbuffered pH 7 solution exhibited an increase in pH to 8.03. We interpret the results to indicate that: 1) buffers can indirectly promote the transformation of triclinic Na-birnessite to hexagonal H-birnessite by serving as a source of H+, even at circumneutral pH; 2) triclinic Na-birnessite alone can stimulate hydrolysis, which in turn induces an exchange of H+ for interlayer Na+; and 3) H-birnessite and Na-birnessite operate as an acid-conjugate base pair.

ID: 134080
Authors: Peterson, Kristina M.; Heaney, Peter J.; Post, Jeffrey E.; Eng, Peter J.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: In ferruginous soils, nano- to microscale hematite (a-Fe2O3) plays a central role in redox processes and contaminant cycling. Hematite is known to incorporate structural OH- and water, and the requisite charge balance is achieved by iron vacancies. Prior researchers have suggested that the defective hematite structures form unique phases called “protohematite” and “hydrohematite.” Infrared and Raman spectroscopic studies have assigned a lower-symmetry space group to “hydrohematite” (R3c) relative to that of stoichiometric hematite (R3¯c). However, the existence and structure of these phases have been contentious, largely due to the lack of in situ X-ray diffraction data Here we present a new structure refinement for “hydrohematite” precipitated hydrothermally at 200 °C in a monoclinic space group (I2/a) using time-resolved synchrotron X-ray diffraction (TR-XRD) data collected during the in situ hydrothermal precipitation of akaganeite and its transformation to stoichiometric hematite. Distinct peak splitting was observed in the “hydrohematite” diffraction patterns, indicating a violation of the threefold rotational symmetry. A monoclinic unit cell with parameters of a = 7.3951(10), b = 5.0117(5), c = 5.4417(7) Å, ß = 95.666(5)° provided a good fit and significant reduction in ?2 and Rwp relative to space group R3¯c. Rietveld analyses revealed that water concentrations in the first-formed crystals of “hydrohematite” were comparable to water contents of akaganeite and goethite. Thus, the hydrothermal transformation of akaganeite to “hydrohematite” is promoted not by dehydration but by reconstruction of the oxygen framework.

ID: 118902
Authors: Post, Jeffrey E.; Farges, Francois
Keywords: Peer-reviewed; NMNH; NH-Mineral Sciences; si-federal

ID: 128097
Authors: Post, Jeffrey E.; Heaney, Peter J.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Time-resolved synchrotron X-ray powder diffraction data were used to investigate the dehydration behavior of the chalcophanite (ZnMn3O7·3H2O) structure from 300 to 1060 K. Rietveld refinements revealed two obvious phase changes, at ~450 and ~950 K, corresponding to the dehydration of chalcophanite followed by transformation to a spinel structure (Mn-hetaerolite). Only small changes were observed in the chalcophanite unit cell from RT to ~438 K; the volume increased by ~0.8%, mostly caused by thermal expansion of ~0.5% along c. Above ~427 K, the interlayer water molecules were lost, resulting in a collapse of the interlayer spacing from ~7 to ~4.8 Å. The newly formed anhydrous phase (ZnMn3O7) retained chalcophanite's R3̄ space group and a dimension, but the c dimension decreased from ~21 to 14.3 Å, and the Zn coordination changed from octahedral to tetrahedral. Above ~775 K the anhydrous chalcophanite began to transform to a spinel structure, corresponding to a Mn-rich hetaerolite (Zn0.75Mn2+0.25)Mn23+O4]. By ~973 K the transformation was complete. The diffraction patterns did not show a significant increase in background during the transformation, indicating that the reaction did not involve transient amorphization. The phase change was likely triggered by loss of 1.25 of seven O atoms in the original anhydrous chalcophanite structure with a corresponding reduction of Mn4+ to Mn3+ and Mn2+.