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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Iron Vacancies Accommodate Uranyl Incorporation into Hematite

Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (-Fe2O3) during mineral growth. The stability and properties of the resulting uranium-doped material depend on the local coordination environment of incorporated uranium. While measurements of uranium coordination have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional ab initio molecular dynamics (AIMD) simulations for various defect configurations to generate EXAFS spectra which were fitted to experimental U L3-edge EXAFS for U6+-doped hematite. We discovered that the hematite lattice accommodates a trans-dioxo uranyl-like configuration for U6+ that substitutes for structural Fe3+, which requires two partially protonated Fe vacancies situated at opposing corner-sharing lattice sites. Surprisingly, the best match to experiment included significant proportions of vacancy configurations other than the minimum-energy configuration, pointing to the importance of incorporation mechanisms and kinetics over thermodynamics in determining the state of an impurity incorporated in a host phase under hydrothermal conditions.

McBriarty, Martin E.↗
NASA NTRS2003

Characterization of the Effects of Precursor Mineralogy on Hematite Spectra: Application to Martian Hematite Mineralization

The Thermal Emission Spectrometer (TES) instrument aboard Mars Global Surveyor discovered several isolated deposits of gray, crystalline hematite in Sinus Meridiani, Aram Chaos, and Valles Marineris. A variety of formation mechanisms has been proposed for the martian hematite deposits, including aqueous and nonaqueous processes. Comparison of the average Sinus Meridiani hematite spectrum measured by TES to laboratory emissivity spectra for a variety of naturally occurring hematites shows small but potentially important differences. In particular, the emissivity minimum at 300 and 445/cm in the Sinus Meridiani (SM) spectrum is displaced 10-25/cm to lower frequencies compared to some natural hematite samples. In addition, these bands in the TES data are narrower than the broad bands seen in many natural hematite spectra. These differences may imply that the natural variability of hematite spectra has not been fully characterized, especially with respect to the reaction pathway (precursor mineralogy and temperature of hematite formation) and crystal morphology. Here, we describe the thermal infrared spectral characteristics of several series of synthetic hematite samples derived by direct precipitation, dehydroxylation of fine-grained goethite and the oxidation of magnetite. Several natural hematite sample spectra are also presented for comparison. Transmission electron microscopy (TEM) and Mossbauer spectral analyses of selected samples were performed in order to help determine the causes of the changes seen in the infrared spectra.

Glotch, Timothy D.↗
NASA NTRS2008

Synthetic (Hydrothermal) Hematite-Rich Mars-Analog Spherules from Acid-Sulfate Brines: Implications for Formation and Diagenesis of Hematite Spherules in Outcrops at Meridiani Planum, Mars

The Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) orbiter discovered a large area at Meridiani Planum (MP) covered with the Fe-oxide hematite (alpha-Fe2O3) [1,2]. This discovery and favorable landing site characteristics led to selection of MP as the landing site for the Opportunity Mars Exploration Rover (MER) [3]. The Athena science payload onboard the Opportunity rover identified hematite-rich spherules (mean spherule diameter approx.4.2+/-0.8 mm) embedded in S-rich outcrop rock and also as lag deposits of whole and broken spherules [4,5,6,7,8,9]. Although the chemical and mineralogical compositions of spherules are not fully constrained, Moessbauer spectrometer (MB) Miniature Thermal Emission Spectrometer (Mini-TES) and chemical analyses from the Alpha Particle X-Ray Spectrometer (APXS) are consistent with a hematite mineralogical composition and an oxide bulk chemical composition consisting of Fe2O3. MGS-TES, also provides an important constraint that emission from the hematite-rich spherules is dominated by emission along the crystallographic c-axis [1,2,10,11]. The formation of hematite-rich spherules with similar chemical, mineralogical, morphological, and crystallographic properties to the MP spherules is rare on Earth, to date, only two natural analogs have been proposed; one from Utah (Navaho Concretions) and the other from Mauna Kea, Hawaii [12,13]. In this study, we synthesized in the laboratory hematite-rich spherules using conditions that may have existed on Early Mars [14] and compared their properties to those for MP hematite spherules of Mars and the analog spherules from Utah and Mauna Kea in order to assess their relative merit as MP hematite spherule analogs. Such comparisons yield clues to the formation pathway for MP spherules.

Golden, D. C.↗
NASA NTRS2014

Chemical and Mineralogical Characterization of a Hematite-bearing Ridge on Mauna Kea, Hawaii: A Potential Mineralogical Process Analog for the Mount Sharp Hematite Ridge

The Mars Science Laboratory (MSL) rover Curiosity landed in Gale Crater in August 2012 and is currently roving towards the layered central mound known as Mount Sharp [1]. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) hyperspectral data indicate Mount Sharp contains an ~5 km stratigraphic sequence including Fe-Mg smectites, hematite, and hydrated sulfates in the lower layers separated by an unconformity from the overlying anhydrous strata [1,2,3]. Hematite was initially detected in CRISM data to occur in the lower sulfate layers on the north side of the mound [2]. [3] further mapped a distinct hematite detection occurring as part of a ~200 m wide ridge that extends ~6.5 km NE-SW, approximately parallel with the base of Mount Sharp. It is likely a target for in-situ analyses by Curiosity. We document here the occurrence of a stratum of hematite-bearing breccia that is exposed on the Puu Poliahu cinder cone near the summit of Mauna Kea volcano (Hawaii) (Fig.1). The stratum is more resistant to weathering than surrounding material, giving it the appearance of a ridge. The Mauna Kea hematite ridge is thus arguably a potential terrestrial mineralogical and process analog for the Gale Crater hematite ridge. We are acquiring a variety of chemical and mineralogical data on the Mauna Kea samples, with a focus on the chemical and mineralogical information already available or planned for the Gale hematite ridge.

Graff, T. G.↗
DOE OSTI2025

Facet-dependent growth and dissolution of hematite resulting from autocatalytic interactions with Fe(II) and oxalic acid

The ability to simultaneously monitor the flux of iron atoms within the solution and solid phases can provide considerable insight into mechanisms of iron oxide mineral transformations. The autocatalytic interaction between hematite and Fe(II)-oxalate has long been of interest for its environmental and industrial relevance. In this study we take advantage of iron isotopic labelling and mass-sensitive imaging at the single particle scale to determine how changes in solution composition correlate with the morphologic evolution of faceted, micrometer-sized hematite platelets. Net dissolution is confirmed through analyses of aqueous iron chemistry, as well as by quantitative atomic force microscopy. Isotopic mapping techniques show that Fe(II) readily adsorbs to (001) and (012) surfaces in the absence of oxalate, but when oxalate is present selective dissolution of the (001) surface prevails and 57Fe deposition via recrystallization is not observed. Comparison between particle microtopographies following reaction with Fe(II), oxalate, and Fe(II)-oxalate show substantially different behavior, consistent with distinct mechanisms of interaction with hematite surfaces. The extensive characterization conducted on the coupled solution/solid dynamics in this system provides new insight for distinguishing crystal growth, dissolution, and recrystallization processes.

Taylor, Sandra D. [BATTELLE (PACIFIC NW LAB)]↗
NASA NTRS1997

Lepidocrocite to Maghemite to Hematite: A way to have Magnetic and Hematitic Martian Soil

We examined decomposition products of lepidocrocite, which were produced by heating the phase in air at temperatures up to 525 C for 3 and 300 hr, by XRD, TEM, magnetic methods, and reflectance spectroscopy (visible and near-IR). Single-crystal lepidocrocite particles dehydroxilated to polycrystalline particles of disordered maghemite which subsequently transformed to polycrystalline particles of hematite. Essentially pure maghemite was obtained at 265 and 223 C for the 3 and 300 hr heating experiments, respectively. Its saturation magnetization (J(sub s)) and mass specific susceptibility are approximately 50 A(sq m)/kg and approximately 40 cubic micrometers/kg, respectively. Because hematite is spectrally dominant, spectrally-hematitic samples (i.e., characterized bv a minimum near 860 nm and a maximum near 750 nm) could also be strongly magnetic (J(sub s) up to approximately 30 A(sq m)/kg) from the masked maghemite component. TEM analyses showed that individual particles are polycrystalline with respect to both maghemite and hematite. The spectrally-hematitic and magnetic Mh+Hm particles can satisfy the spectral and magnetic constraints for Martian surface materials over a wide range of values of Mh/(Mh+Hm) and as either pure oxide powders or (within limits) as components of multiphase particles. These experiments are consistent with lepidocrocite as the precursor of Mh+Hm assemblages on Mars, but other phases (e.g., magnetite) that decompose to Mh and Hm are also possible precursors. Simulations done with a copy of the Mars Pathfinder Magnet Array showed that spectrally hematitic Mh+Hm powders having J(sub s) equal to 20.6 A(sq m)/kg adhered to all five magnets.

Morris, Richard V.↗
NASA NTRS2010

Spherulitic Growth of Hematite Under Hydrothermal Conditions: Insights into the Growth Mechanism of Hematite Spherules at Meridiani Planum Mars.

Hematite-rich spherules were discovered embedded in sulfate-rich outcrop rock and as lag deposits of whole and broken spherules by the Opportunity rover at Meridiani Planem [1-6]. The Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), which has a wider spectral range compared to the Mars Exploration Rover Mini-TES, provided an important constraint that hematite-rich spherules are dominated by emission along the crystallographic c-axis [7-10]. We have previously synthesized hematite spherules whose mineralogic, chemical, and crystallographic properties are strikingly similar to those for the hematite-rich spherules at Meridiani Planum [11]. The spherules were synthesized in the laboratory along with hydronium jarosite and minor hydronium alunite from Fe-Al-Mg-S-Cl acid sulfate solutions under hydrothermal conditions. The reaction sequence was (1) precipitation of hydronium jarosite, (2) jarosite dissolution and precipitation of hematite spherules, and (3) precipitation of hydronium alunite upon depletion of hydronium jarosite. The spherules exhibit a radial growth texture with the crystallographic c-axis aligned along the radial direction, so that thermal emission spectra have no hematite emissivity minimum at approx.390/cm similar to the emission spectra returned by MGS TES. The objective of this paper is to expand on our initial studies [11] to examine the morphological evolution during growth of spherules starting from sub-micrometer crystals to spherules many orders of magnitude in size.

Ming, Douglas W.↗
NASA NTRS2005

Hematite Spherules in Basaltic Tephra Altered Under Aqueous, Acid-Sulfate Conditions on Mauna Kea Volcano, Hawaii: Possible Clues for the Occurrence of Hematite-Rich Spherules in the Burns Formation at Meridiani Planum, Mars

Iron-rich spherules (>90% Fe2O3 from electron microprobe analyses) approx.10-100 microns in diameter are found within sulfate-rich rocks formed by aqueous, acid-sulfate alteration of basaltic tephra on Mauna Kea volcano, Hawaii. Although some spherules are nearly pure Fe, most have two concentric compositional zones, with the core having a higher Fe/Al ratio than the rim. Oxide totals less than 100% (93-99%) suggest structural H2O and/or /OH. The transmission Moessbauer spectrum of a spherule-rich separate is dominated by a hematite (alpha-Fe2O3) sextet whose peaks are skewed toward zero velocity. Skewing is consistent with Al(3+) for Fe(3+) substitution and structural H2O and/or /OH. The grey color of the spherules implies specular hematite. Whole-rock powder X-ray diffraction spectra are dominated by peaks from smectite and the hydroxy sulfate mineral natroalunite as alteration products and plagioclase feldspar that was present in the precursor basaltic tephra. Whether spherule formation proceeded directly from basaltic material in one event (dissolution of basaltic material and precipitation of hematite spherules) or whether spherule formation required more than one event (formation of Fe-bearing sulfate rock and subsequent hydrolysis to hematite) is not currently constrained. By analogy, a formation pathway for the hematite spherules in sulfate-rich outcrops at Meridiani Planum on Mars (the Burns formation) is aqueous alteration of basaltic precursor material under acid-sulfate conditions. Although hydrothermal conditions are present on Mauna Kea, such conditions may not be required for spherule formation on Mars if the time interval for hydrolysis at lower temperatures is sufficiently long.

Morris, R. V.↗
NASA NTRS1989

Evidence for pigmentary hematite on Mars based on optical, magnetic, and Moessbauer studies of superparamagnetic (nanocrystalline) hematite

Samples containing variable amounts of superparamagnetic hematite (sp-Hm) were prepared by a method in which the sp-Hm particles were dispersed throughout larger particles of silica gel, and the optical and magnetic properties of these samples were compared with those of larger-diameter hematite (bulk-Hm). It is shown that the optical properties of sp-Hm are different from those of bulk-Hm. Implications of the results for mineralogical interpretations of spectral data for the Martian surface and its terrestrial analogues are discussed. It is concluded that features resulting from ferric iron in the Martian spectral data and the results of the Viking magnetic properties experiment are both consistent with hematite present as both sp-Hm and bulk-Hm; the hematite particles most likely occur in pigmentary form, i.e., as particles dispersed throughout the volume of a spectrally neutral material.

Morris, Richard V.↗
NASA NTRS2007

Hydrothermal Synthesis of Hematite-Rich Spherules: Implications for Diagenesis and Hematite Spherule Formation in Outcrops at Meridiani Planum, Mars

The Athena science payload onboard the Opportunity rover identified hematite-rich spherules (mean diameter of 4.2 +/- 0.8 mm) embedded in outcrops and occurring as lag deposits at Meridiani Planum. They have formed as diagenetic concretions from the rapid breakdown of pre-existing jarosite and other iron sulfates when chemically distinct groundwater passed through the sediments. Diagenetic, Fe-cemented concretions found in the Jurassic Navajo Formation, Utah and hematite-rich spherules found within sulfate-rich volcanic breccia on Mauna Kea volcano, Hawaii are possible terrestrial analogues for Meridiani spherules. The Navajo Formation concretions form in porous quartz arenite from the dissolution of iron oxides by reducing fluids and subsequent Fe precipitation to form spherical Fe- and Si-rich concretions. The Mauna Kea spherules form by hydrothermal, acid-sulfate alteration of basaltic tephra. The formation of hematite-rich spherules with similar chemical, mineralogical, and morphological properties to the Meridiani spherules is rare on Earth, so little is known about their formation conditions. In this study, we have synthesized in the laboratory hematite-rich spherules that are analogous in nearly all respects to the Meridiani spherules.

Golden, D. C.↗
DOE OSTI2020

Enthalpy of Uranium Adsorption onto Hematite

The influence of temperature on the adsorption of metal-ions at the solid-water interface is often overlooked, despite the important role that adsorption plays in metal ion fate and transport in the natural environment where temperatures vary widely. Herein, we examine the temperature-dependent adsorption of uranium, a widespread radioactive contaminant, onto the ubiquitous iron oxide, hematite. The multi-temperature batch adsorption data and surface complexation models indicate that adsorption of uranium, as the hexavalent uranyl (UO 2 2+ ) ion, increases significantly with increasing temperature, with an adsorption enthalpy (ΔH ads ) of +71 kJ mol –1 . We suggest that this endothermic, entropically-driven adsorption behavior is linked to reorganization of the uranyl-ion-hydration and interfacial water structures upon U VI adsorption at the hematite surface. Altogether, this work provides fundamental insight into the thermodynamics driving metal-ion adsorption reactions and provides the specific enthalpy value necessary for improved predictive geochemical modeling of U VI adsorption in the environment.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2021

Radiation-Enhanced Anion Transport in Hematite

The influence of radiation-induced (1 MeV energy H+ to ~0.1 dpa at 450°C), non-equilibrium point defect populations on mass transport is studied with an integrated campaign of experimental and theoretical methods. Using epitaxial thin films of hematite with embedded 18O tracer layers and nanoscale atom probe tomography measurements, it is shown that anion self-diffusion is enhanced by at least 2 orders of magnitude under irradiation compared to thermal diffusion alone. Complementary scanning transmission electron microscopy of vacuum annealed specimens, reveals associated microstructural changes in the oxide films, including local phase transformation to Fe3O4 and the development of nanoscale voids from vacancy coalescence. Point defect formation and migration energies were computed from density functional theory and applied within the context of chemical rate theory to analyze contributions from both interstitial and vacancy mechanisms to self-diffusion in thermal and irradiation conditions. Comparisons are made between calculated, literature and newly measured self-diffusion values, revealing good agreement on the magnitude of radiation-enhanced anion diffusion. Further, the model suggests a transition from vacancy to interstitialcy mechanisms at low temperatures and high oxygen activity, consistent with the varied activation energies reported from prior studies.

iron oxides, isotopic tracers, atom probe tomograp↗
DOE OSTI2019

Reductive Dissolution Mechanisms at the Hematite-Electrolyte Interface Probed by In Situ X-ray Scattering

The electron-catalyzed dissolution and reprecipitation of iron (oxyhydr)oxides constitute critical steps in natural geochemical iron cycling. However, the atomic-scale mechanisms of reductive dissolution and oxidative precipitation of Fe2+ remain poorly understood because they are difficult to directly experimentally probe. Using in situ synchrotron X-ray scattering and a novel electrochemical cell, we interrogate the interfacial structure between the hematite (?-Fe2O3) (11 ¯02) surface and acidic aqueous solution (5 mM Na2SO4, pH 4.0) under controlled electrochemical potential from open circuit to cathodic bias as the reductive dissolution potential is approached and then exceeded. The crystalline order of the surface improves under mild reducing conditions, and the surface Fe stoichiometry changes with cathodic bias. After significant reductive dissolution occurs and cathodic bias is removed, dissolved Fe is re-deposited, forming a disordered interface. Unlike at circumneutral pH, water layers at the hematite interface with acidic solution are poorly ordered, likely due to the adsorption of sulfate from the electrolyte. These results provide a novel atomic-scale view into the behavior of reducible transition metal oxide surfaces under fluctuating (electro)chemical conditions.

McBriarty, Martin E.↗