Publication Search Results

ID: 151349
Type: article
Authors: Yang, Peng; Post, Jeffrey E.; Wang, Qian; Xu, Wenqian; Geiss, Roy; McCurdy, Patrick R.; Zhu, Mengqiang
Keywords: NMNH; NH-Mineral Sciences; Peer-reviewed
Abstract: Hexagonal birnessite, a typical layered Mn oxide (LMO), can adsorb and oxidize Mn(II) and thereby transform to Mn(III)-rich hexagonal birnessite, triclinic birnessite, or tunneled Mn oxides (TMOs), remarkably changing the environmental behavior of Mn oxides. We have determined the effects of coexisting cations on the transformation by incubating Mn(II)-bearing δ-MnO2 at pH 8 under anoxic conditions for 25 d (dissolved Mn < 11 μM). In the Li+, Na+, and K+ chloride solutions, the Mn(II)-bearing δ-MnO2 first transforms to Mn(III)-rich δ-MnO2 or triclinic birnessite (T-bir) due to the Mn(II)-Mn(IV) comproportionation, most of which eventually transform to a 4 × 4 TMO. In contrast, Mn(III)-rich δ-MnO2 and T-bir form and persist in the Mg2+ and Ca2+ chloride solutions. However, in the presence of surface adsorbed Cu(II), Mn(II)-bearing δ-MnO2 turns into Mn(III)-rich δ-MnO2 without forming T-bir or TMOs. The stabilizing power of the cations on the δ-MnO2 structure positively correlates with their binding strength to δ-MnO2 (Li+, Na+, and K+ < Mg2+ and Ca2+ < Cu(II)). Since metal adsorption decreases the surface energy of minerals, our finding suggests that the surface energy largely controls the thermodynamic stability of LMOs. Our study indicates that the adsorption of divalent metal cations, particularly transition metals, can be an important cause of the high abundance of LMOs, rather than the more stable TMO phases, in the environment.

ID: 146005
Type: article
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.

ID: 145570
Type: article
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
Type: article
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: 148392
Type: article
Authors: Gardner, James E.; Nazworth, Caroline; Helper, Mark A.; Andrews, Benjamin J.
Keywords: Peer-reviewed; NMNH; NH-Mineral Sciences
Abstract: The laterally directed pyroclastic density current (blast surge) that erupted from Mount St. Helens (Washington State, USA) on 18 May 1980 devastated similar to 600 km(2) of thick conifer forest, with most large trees uprooted or broken off near the base. Throughout this area, at least 51 patches of trees were left standing, mostly where the blast surge had reached 95 /- 5% of its distance traveled. Analysis of post-eruption digital topography and topographic profiles aligned in the direction that the surge traveled (defined by orientations of locally toppled trees) show that the stands of trees are on lee sides of hills and, rarely, on downstream sides of plateaus. All hills are higher than neighboring (within 500 m) hills that lack tree stands. In all cases, toppled trees upstream are orientated toward the patches of standing trees, arguing that the surge was not blocked. The patches thus reflect where the base of the surge temporarily left the ground. Because the stands are only found near the end of the surge runout, yet where inferred speeds of the surge were as slow as 40 m s(-1) and as fast as 115 m s(-1), the most likely reason the surge briefly detached from the ground was that its density had locally decreased enough that it approached that of the atmosphere. Numerical modeling shows that the distance the surge traveled before reattaching to the ground is controlled by its density and trajectory (i.e., hill height and stoss-side rise). This study shows that the path of destruction left by pyroclastic density currents can provide clues about their internal mechanics.

ID: 146557
Type: article
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
Type: article
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
Type: article
Authors: Megaw, Peter K. M.; Huizing, Terry E.; Richards, R. P.; Post, Jeffrey E.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed

ID: 149355
Type: article
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
Type: article
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
Type: article
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: 141449
Type: article
Authors: Beck, Andrew W.; Lawrence, David J.; Peplowski, Patrick N.; Viviano-Beck, Christina; Prettyman, Thomas H.; McCoy, Timothy J.; McSween Jr, Harry Y.; Yamashita, Naoyuki
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: We use data collected by the Dawn Gamma-Ray and Neutron Detector (GRaND) at Vesta to map compositions corresponding to nearly pure igneous lithologies in the howardite, eucrite, diogenite (HED) meteorite clan (samples likely from Vesta). At the ~300-km spatial scale of GRaND measurements, basaltic eucrite occurs on only 3% of the surface, whereas cumulate eucrite and orthopyroxenitic diogenite are not detected. The basaltic eucrite region is generally coincident with an area of the surface with thick regolith, elevated H, and moderate crater density, and may represent the best compositional sample of primordial vestan crust. We observe an absence of pure orthopyroxenitic diogenite terrains in the Rheasilvia basin and its ejecta, an observation corroborated by VIR (0.1%), which suggests the south-polar crust was a polymict mixture of igneous lithologies (howardite) at the time of the Rheasilvia impact, or was a thick basaltic eucrite crust with heterogeneously distributed orthopyroxenitic diogenite plutons. The most dominant igneous composition detected (11% of the surface) corresponds to one of the least-abundant igneous lithologies in the HED meteorite collection, the Yamato Type B diogenites (plagioclase-bearing pyroxenites). The distribution of Type B diogenite is spatially correlated with post-Rheasilvia craters in the north-polar region that are in close proximity to the Rheasilvia basin antipode. This suggests that north-polar Type B plutonism may have been associated with the Rheasilvia impact event. We propose that this was either through 1) uplift of pre-existing plutons at the antipode through focusing of Rheasilvia impact stress, or 2) Rheasilvia impact antipodal crustal melting, creating magmas that underwent fractionation to produce Type B plutons.

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

ID: 140155
Type: article
Authors: Cutler, Nick A.; Arróniz-Crespo, María; Street, Lorna E.; Jones, David L.; Chaput, Dominique L.; DeLuca, Thomas H.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Our study used a ~360-year fire chronosequence in northern Sweden to investigate post-fire microbial community dynamics in the boreal bryosphere (the living and dead parts of the feather moss layer on the forest floor, along with the associated biota). We anticipated systematic changes in microbial community structure and growth strategy with increasing time since fire (TSF) and used amplicon pyrosequencing to establish microbial community structure. We also recorded edaphic factors (relating to pH, C and N accumulation) and the physical characteristics of the feather moss layer. The molecular analyses revealed an unexpectedly diverse microbial community. The structure of the community could be largely explained by just two factors, TSF and pH, although the importance of TSF diminished as the forest recovered from disturbance. The microbial communities on the youngest site (TSF = 14 years) were clearly different from older locations (>100 years), suggesting relatively rapid post-fire recovery. A shift towards Proteobacterial taxa on older sites, coupled with a decline in the relative abundance of Acidobacteria, suggested an increase in resource availability with TSF. Saprotrophs dominated the fungal community. Mycorrhizal fungi appeared to decline in abundance with TSF, possibly due to changing N status. Our study provided evidence for the decadal-scale legacy of burning, with implications for boreal forests that are expected to experience more frequent burns over the course of the next century.

ID: 145240
Type: article
Authors: Grocke, Stephanie B.; de Silva, Shanka L.; Wallace, Paul J.; Cottrell, Elizabeth; Schmitt, Axel K.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Quartz-hosted melt inclusions from climactic and post-climactic deposits erupted from the 3.49 ± 0.01 Ma Guacha II Caldera (G2C) in SW Bolivia provide insights into the volatile and melt evolution leading into and following a major caldera-forming eruption of monotonous silicic ignimbrite. The G2C, with its petrological and volcanological framework already in place, provides a unique opportunity to investigate the role of volatiles in controlling the plinian to ignimbrite transition as well as the evolution from explosive to post-climactic effusive behavior at a large silicic caldera-forming system. Melt inclusions from the fallout (800 km3 DRE) pumice have overlapping major and trace element compositions and pre-eruptive H2O and CO2 contents (2.1 to 6.0 wt %, 36 to 630 ppm, respectively), indicating that they crystallized from the same pre-climactic monotonous silicic magma reservoir. Based on trace element and Cl systematics, the variations in H2O content likely reflect post-entrapment diffusive H loss from melt inclusions. We estimate that quartz crystallization took place at pressures of ~200 MPa, in agreement with pressures derived from amphibole in the ignimbrite pumice (~200 MPa). These results indicate that the pre-climactic magma was vapor saturated at shallow crustal conditions (between 5 and 8 km). Melt inclusions from the post-climactic Chajnantor Dome lava are the most evolved with respect to trace elements and REE. They have low H2O and Cl contents, and contain no detectable CO2, indicative of relatively shallow entrapment pressures (<100 MPa). Melt inclusions from the post-climactic dome lava represent a pod of eruptible melt that was extracted from degassed, remnant climactic magma that shallowed (<4 km) and evolved following caldera collapse causing resurgent uplift. Melt inclusion trace element systematics can be explained by extensive degrees of crystal fractionation (>=70%) of plagioclase, quartz, sanidine, biotite, Fe-Ti oxides, apatite, and zircon in the pre-climactic melt that produced variable melt inclusion compositions in the fallout and ignimbrite pumice (between 100 and 500 ppm Rb). In addition, quartz-hosted melt inclusions from the fallout pumice are more evolved than their host pumice glass, supporting a model whereby some andesitic recharge magma fractionated within the pre-climactic reservoir and produced a relatively small volume of residual rhyolitic melt. Quartz from the monotonous silicic pre-climactic reservoir was inmixed into this less evolved melt prior to eruption and deposition as fallout pumice. The shift from fallout to ignimbrite is not accompanied by changes in pre-eruptive dissolved H2O content in the melt, suggesting that differences in other factors such as vent and conduit evolution were more important in controlling explosive eruptive style. In contrast, melt inclusions from the dome support the interpretation that lava domes represent degassed, remnant magma indicating a more primary role of volatile content on the explosive to effusive eruptive transition. Pre-eruptive volatile contents of the Tara magmas are broadly similar to those of other monotonous silicic magmas in the Central Andes and elsewhere, suggesting that similarities in the petrological and geochemical evolution of these supervolcanic systems extend to their volatile signatures.

ID: 142277
Type: article
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
Type: article
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: 139236
Type: article
Authors: Grocke, Stephanie B.; Cottrell, Elizabeth; de Silva, Shanaka; Kelley, Katherine A.
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: The composition of the continental crust is closely tied to subduction zone magmatism. Elevated oxygen fugacity ( f O 2 ) plays a central role in fostering crystallization of oxide minerals and thereby aids in generating the calc-alkaline trend of iron depletion that characterizes the continents. Along continental margins, arc magmas erupt through continental crust and often undergo extensive differentiation that may modify magmatic f O 2 . The importance of the subducting slab and mantle wedge relative to the effects of this differentiation on the f O 2 recorded by continental arc magmas remains relatively unconstrained. Here, we focus on the effect of differentiation on magmatic f O 2 using a suite of 14 samples from the Central Volcanic Zone (CVZ) of the Andes where the continental crust is atypically thick (60–80 km). The samples range in composition from ~55 to 74 wt% SiO2 and represent the Neogene history of the arc. Samples are basaltic andesite to rhyolite and span a range of radiogenic isotopic compositions (87Sr/86Sr = ~0.705–0.712) that represent 30 to 100% crustal assimilation. We use several proxies to estimate the f O 2 recorded by lavas, pumice, and scoria: (1) whole rock Fe 3 + / S Fe ratios, (2) Fe 3 + / S Fe ratios in quartz-hosted melt inclusions, and (3) Fe–Ti oxide oxygen-barometry. Comparison of the f O 2 calculated from bulk Fe 3 + / S Fe ratios (post-eruptive) with that derived from Fe–Ti oxides or melt inclusion Fe 3 + / S Fe ratios (pre-eruptive), enables us to quantify the effect of syn- or post-eruptive alteration, and to select rocks for bulk analysis appropriate for the determination of pre-eruptive magmatic f O 2 using a strict criterion developed here. Across our sample suite, and in context with samples from the literature, we do not find evidence for systematic oxidation due to crystal fractionation or crustal contamination. Less evolved samples, ranging from 55 to 61 wt% SiO2, record a range of >3 orders of magnitude in f O 2 , spanning the f O 2 range recorded by all samples in our suite. Among these less evolved magmas, we find that those erupted from volcanic centers located closer to the trench, closer to the Benioff Zone, and with more geochemical evidence of subducted components in the mantle source (elevated La/Nb) result in magmas that record systematically higher f O 2 . We conclude that the slab/mantle source can exert greater control on magmatic f O 2 than processes occurring in even the thickest continental crust. Thus, the f O 2 of arc magmas, and hence their calc-alkaline nature, may be inherited from the mantle.

ID: 138799
Type: article
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
Type: article
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.