[show abstract][hide abstract] ABSTRACT: Defining chemical and mechanical alteration of wellbore cement by CO2-rich brines is important for the predicting long-term integrity of wellbores in geologic CO2 environments. We reacted CO2-rich brines along a cement-caprock boundary at 60°C and pCO2 = 3 MPa using flow-through experiments. The results show that distinct reaction zones form in response to reactions with the brine over the 8-day experiment. Detailed characterization of the crystalline and amorphous phases, and the solution chemistry show that the zones can be modeled as preferential portlandite dissolution in the depleted layer, concurrent calcium silicate hydrate (CSH) alteration to an amorphous zeolite and Ca-carbonate precipitation in the carbonate layer, and carbonate dissolution in the amorphous layer. Chemical reaction altered the mechanical properties of the core lowering the average Young's moduli in the depleted, carbonate, and amorphous layers to approximately 75, 64, and 34 % of the unaltered cement, respectively. The decreased elastic modulus of the altered cement reflects an increase in pore space through mineral dissolution and different moduli of the reaction products.
[show abstract][hide abstract] ABSTRACT: Sorption of 10(-18)-10(-5)M neptunium (Np) to goethite was examined using liquid scintillation counting and gamma spectroscopy. A combination approach using (239)Np and long lived (237)Np was employed to span this wide concentration range. (239)Np detection limits were determined to be 2×10(-18)M and 3×10(-17)M for liquid scintillation counting and gamma spectroscopy, respectively. Sorption was found to be linear below 10(-11)M, in contrast to the non-linear behavior observed at higher concentrations both here and in the literature. 2-site and 3-site Langmuir models were used to simulate sorption behavior over the entire 10(-18)-10(-5)M range. The 3-site model fit yielded Type I and II site densities of 3.56sites/nm(2) (99.6%) and 0.014±0.007sites/nm(2) (0.4±0.1%), consistent with typical "high affinity" and "low affinity" sites reported in the literature . Modeling results for both models suggest that sorption below ∼10(-11)M is controlled by a third (Type III) site with a density on the order of ∼7×10(-5)sites/nm(2) (∼0.002%). While the nature of this "site" cannot be determined from isotherm data alone, the sorption data at ultra-low Np concentrations indicate that Np(V) sorption to goethite at environmentally relevant concentrations will be (1) linear and (2) higher than previous (high concentration) laboratory experiments suggest.
Journal of Colloid and Interface Science 09/2012; · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Abstract– Samples returned from comet 81P/Wild 2 by the Stardust mission provided an unequaled opportunity to compare previously available extraterrestrial samples against those from a known comet. Iron sulfides are a major constituent of cometary grains commonly identified within cometary interplanetary dust particles (IDPs) and Wild 2 samples. Chemical analyses indicate Wild 2 sulfides are fundamentally different from those in IDPs. However, as Wild 2 dust was collected via impact into capture media at approximately 6.1 km s−1, it is unclear whether this is due to variation in preaccretional/parent body processes experienced by these materials or due to heating and alteration during collection. We investigated alteration in pyrrhotite and pentlandite impacted into Stardust flight spare Al foils under encounter conditions by comparing scanning and transmission electron microscope (SEM, TEM) analyses of preimpact and postimpact samples and calculating estimates of various impact parameters. SEM is the primary method of analysis during initial in situ examination of Stardust foils, and therefore, we also sought to evaluate the data obtained by SEM using insights provided by TEM. We find iron sulfides experience heating, melting, separation, and loss of S, and mixing with molten Al. These results are consistent with estimated peak pressures and temperatures experienced (approximately 85 GPa, approximately 2600 K) and relative melting temperatures. Unambiguous identification of preserved iron sulfides may be possible by TEM through the location of Al-free regions. In most cases, the Ni:Fe ratio is preserved in both SEM and TEM analyses and may therefore also be used to predict original chemistry and estimate mineralogy.
[show abstract][hide abstract] ABSTRACT: The subsurface migration of Pu may be enhanced by the presence of colloidal forms of Pu. Therefore, complete evaluation of the risk posed by subsurface Pu contamination needs to include a detailed physical/chemical understanding of Pu colloid formation and interactions of Pu colloids with environmentally relevant solid phases. Transmission electron microscopy (TEM) was used to characterize Pu nanocolloids and interactions of Pu nanocolloids with goethite and quartz. We report that intrinsic Pu nanocolloids generated in the absence of goethite or quartz were 2-5 nm in diameter, and both electron diffraction analysis and HRTEM confirm the expected Fm3m space group with the fcc, PuO2 structure. Plutonium nanocolloids formed on goethite have undergone a lattice distortion relative to the ideal fluorite-type structure, fcc, PuO2, resulting in the formation of a bcc, Pu4O7 structure. This structural distortion results from an epitaxial growth of the plutonium colloid on goethite, leading to stronger binding of plutonium to goethite compared with other minerals such as quartz, where the distortion was not observed. This finding provides new insight for understanding how molecular-scale behavior at the mineral-water interface may facilitate transport of plutonium at the field scale.
[show abstract][hide abstract] ABSTRACT: Synthetic CdZnTe or “CZT” crystals are highly suitable for γ-spectrometers operating at the room temperature. Secondary phases (SP) in CZT are known to inhibit detector performance, particularly when they are present in large numbers or dimensions. These SP may exist as voids or composites of non-cubic phase metallic Te layers with bodies of polycrystalline and amorphous CZT material and voids. Defects associated with crystal twining may also influence detector performance in CZT. Using transmission electron microscopy, we identify two types of defects that are on the nano scale. The first defect consists of 40 nm diameter metallic Pd/Te bodies on the grain boundaries of Te-rich composites. Although the nano-Pd/Te bodies around these composites may be unique to the growth source of this CZT material, noble metal impurities like these may contribute to SP formation in CZT. The second defect type consists of atom-scale grain boundary dislocations. Specifically, these involve inclined “finite-sized” planar defects or interfaces between layers of atoms that are associated with twins. Finite-sized twins may be responsible for the subtle but observable striations that can be seen with optical birefringence imaging and synchrotron X-ray topographic imaging.
[show abstract][hide abstract] ABSTRACT: Abstract— The Stardust sample return capsule returned to Earth in January 2006 with primitive debris collected from comet 81P/Wild-2 during the flyby encounter in 2004. In addition to the cometary particles embedded in low-density silica aerogel, there are microcraters preserved in the aluminum foils (1100 series; 100 μm thick) that are wrapped around the sample tray assembly. Soda lime spheres (˜49 μm in diameter) have been accelerated with a light gas gun into flight-grade aluminum foils at 6.35 km s−1 to simulate the capture of cometary debris. The experimental craters have been analyzed using scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDX) to locate and characterize remants of the projectile material remaining within the craters. In addition, ion beam-induced secondary electron imaging has proven particularly useful in identifying areas within the craters that contain residue material. Finally, high-precision focused ion beam (FIB) milling has been used to isolate and then extract an individual melt residue droplet from the interior wall of an impact. This has enabled further detailed elemental characterization that is free from the background contamination of the aluminum foil substrate. The ability to recover “pure” melt residues using FIB will significantly extend the interpretations of the residue chemistry preserved in the aluminum foils returned by Stardust.
[show abstract][hide abstract] ABSTRACT: This work focuses on the 3. Resultsanddiscussioncharacterization of 10×10×10mm3 THM-grown CdZnTe detector-grade crystals that have been post-growth annealed to remove the secondary phases (SPs). All three detectors showed an average energy resolution of ∼1.63% for a small guarded pixel with 3.5mm diameter, measured using 137Cs—662keV with an average peak-to-Compton ratio of 2.7. The characterization showed vestiges of SPs and micro-twins present in some of the crystals indicating that the SPs prior to annealing were large and had size in the range of 100–500μm. The various detectable structural features, such as micron twins, strains and sub-micron level of Te inclusions seemed to have little or no influence in the radiation spectrometer performance of the detectors; this is possibly because they are either having low density or electrically inactive.
Journal of Crystal Growth 01/2010; 312(4):507-513. · 1.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: Synthetic Cd1–x
Te or “CZT” crystals are highly suitable for γ-spectrometers operating at room temperature. Secondary phases (SP) within CZT, presumed to be Te metal, have detrimental impacts on the charge collection efficiency of fabricated device. Using analytical techniques rather than arbitrary theoretical definitions, we identify two SP morphologies: (i) many void, 20-μm “negative” crystals with 65-nm nanoparticle residues of Si, Cd, Zn, and Te and (ii) 20-μm hexagonal-shaped bodies, which are composites of metallic Te layers with cores of amorphous and polycrystalline CZT material that surround the voids.
Journal of Materials Research. 03/2009; 24(04):1361 - 1367.
[show abstract][hide abstract] ABSTRACT: Refractory Ti-bearing minerals in the calcium-, aluminum-rich inclusion (CAI) Inti, recovered from the comet 81P/Wild 2 sample, were examined using analytical (scanning) transmission electron microscopy (STEM) methods including imaging, nanodiffraction, energy-dispersive spectroscopy (EDX) and electron energy loss spectroscopy (EELS). Inti fassaite (Ca(Mg,Ti,Al)(Si,Al)2O6) was found to have a Ti3+/Ti4+ ratio of 2.0 ± 0.2, consistent with fassaite in other solar system CAIs. The oxygen fugacity (logfO2) of formation estimated from this ratio, assuming equilibration among phases at 1509 K, is −19.4 ± 1.3. This value is near the canonical solar nebula value (−18.1 ± 0.3) and in close agreement with that reported for fassaite-bearing Allende CAIs (−19.8 ± 0.9) by other researchers using the same assumptions. Nanocrystals of osbornite (Ti(V)N), 2–40 nm in diameter, are embedded as inclusions within gehlenite, spinel and diopside in Inti. Vanadium is heterogeneously distributed within some osbornite crystals. Compositions range from pure TiN to Ti0.36V0.64N. The possible presence of oxide and carbide in solid solution with the osbornite was evaluated. The osbornite may contain O, but C is not present at detectable levels. The presence of osbornite, likely a refractory early condensate, together with the other refractory minerals in Inti, indicates that the parent comet contains solids that condensed closer to the proto-sun than the distance at which the parent comet itself accreted. The estimated oxygen fugacity and the reported isotopic and chemical compositions are consistent with Inti originating in the inner solar system like other meteoritic CAIs. These results provide insight for evaluating the validity of models of radial mass transport dynamics in the early solar system. The oxidation environments inferred for the Inti mineral assemblage are inconsistent with an X-wind formation scenario. In contrast, radial mixing models that allow accretion of components from different heliocentric distances can satisfy the observations from the cometary CAI Inti.
Geochimica et Cosmochimica Acta 01/2009; · 3.88 Impact Factor
[show abstract][hide abstract] ABSTRACT: We used focused ion beam thin section preparation and scanning transmission electron microscopy (FIB/STEM) to examine the interfacial region between spinel and host melilite for spinel grains in type B1 inclusions from the Allende and Leoville carbonaceous chondrites. Boundary clinopyroxenes decorating spinel surfaces have compositions similar to those of coarser clinopyroxenes from the same region of the inclusion, suggesting little movement after formation. Host melilite displays no anomalous compositions near the interface and late-stage minerals are not observed, suggesting that boundary pyroxenes did not form by crystallization of residual liquid. Allende spinels display either direct spinel-melilite contact or an intervening boundary clinopyroxene between the two phases. Spinel-melilite interfacial regions in a Leoville B1 are more complex, with boundary clinopyroxene, as observed in Allende, but also variable amounts of glass, secondary calcite, perovskite, and an Mg-, Al-, OH-rich and Ca-, Si-poor crystalline phase that may be a layered double hydrate. One possible scenario of formation for the glass veins is that open system alteration of melilite produced a porous, hydrated aggregate of Mg-carpholite or sudoite + aluminous diopside that was shock melted and quenched to a glass. The hydrated crystalline phase we observed may have been a shocked remnant of the precursor phase assemblage, but is more likely to have formed later by alteration of the glass. In the mantle, boundary clinopyroxenes may have been crystallized from Ti-rich liquids formed by the direct dissolution of perovskite and an associated Sc-Zr-rich phase or as a reaction product between dissolving perovskite and liquid. In the core, any perovskite and associated Ti-enriched liquids that may have originally been present disappeared before the growth of boundary clinopyroxene, and the observed boundary clinopyroxene may have nucleated and grown from the liquid, along with the larger core clinopyroxene.
[show abstract][hide abstract] ABSTRACT: CdZnTe or "CZT" crystals are highly suitable for use as a room temperature based spectrometer for the detection and characterization of gamma radiation. Over the last decade, the methods for growing high quality CZT have improved the quality of the produced crystals however there are material features that can influence the performance of these materials as radiation detectors. For example, various structural heterogeneities within the CZT crystals, such as twinning, pipes, grain boundaries (polycrystallinity), and secondary phases (SP) can have a negative impact on the detector performance. In this study, a CZT material was grown by the modified vertical Bridgman growth (MVB) method with zone leveled growth without excess Te in the melt. Visual observations of material from the growth of this material revealed significant voids and SP. Samples from this material were analyzed using various analytical techniques to evaluate its electrical properties, purity and detector performance as radiation spectrometers and to determine the morphology, dimension and elemental /structural composition of one of the SP in this material. This material was found to have a high resistivity but poor radiation spectrometer performance. It had SP that were rich in polycrystalline aluminum oxide (Al2O3), metallic Te and polycrystalline CdZnTe and 15 to 50 mum in diameter. Bulk elemental analyses of sister material from elsewhere in the boule did not contain high levels of Al so there is considerable elemental impurity heterogeneity within the boule from this growth.
[show abstract][hide abstract] ABSTRACT: GEMS (glass with embedded metal and sulfides) grains in interplanetary dust particles (IDPs) were examined using 200 keV analytical transmission electron microscopy. The morphologies and crystallography of embedded relict grains reveal that GEMS are pseudomorphs formed by irradiation processing of crystals free-floating in space. Some GEMS retain a compositional and morphological "memory" of the crystal from which they formed. Pseudomorphism may rule out condensation, annealing, flash heating, or shock melting as alternative mechanisms of GEMS formation. A significant and often dominant fraction of the atoms in GEMS were sputter-deposited from other grains. Therefore, a normal (solar) isotopic composition is not a reliable indicator of whether GEMS formed in the solar system or in presolar interstellar or circumstellar environments.
The Astrophysical Journal 12/2008; 617(1):650. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: While a significant fraction of silicate dust in stellar winds has a crystalline structure, in the interstellar medium nearly all of it is amorphous. One possible explanation for this observation is the amorphization of crystalline silicates by relatively "low" energy, heavy-ion cosmic rays. Here we present the results of multiple laboratory experiments showing that single-crystal synthetic forsterite (Mg2SiO4) amorphizes when irradiated by 10 MeV Xe ions at large enough fluences. Using modeling, we extrapolate these results to show that 0.1-5.0 GeV heavy-ion cosmic rays can rapidly (~70 Myr) amorphize crystalline silicate grains ejected by stars into the interstellar medium.
The Astrophysical Journal 12/2008; 662(1):372. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: CdZnTe (or CZT) crystals can be used in a variety of detector-type applications. This large band gap material shows great promise for use as a gamma radiation spectrometer. Historically, the performance of CZT has typically been adversely affected by point defects, structural and compositional heterogeneities within the crystals, such as twinning, pipes, grain boundaries (polycrystallinity) and secondary phases (SP). The synthesis of CZT material has improved greatly with the primary performance limitation being attributed to mainly SP. In this presentation, we describe the extensive characterization of detector grade material that has been treated with post growth annealing to remove the SPs. Some of the analytical methods used in this study included polarized, cross polarized and transmission IR imaging, I-V curves measurements, synchrotron X-ray topography and electron microscopy.
[show abstract][hide abstract] ABSTRACT: (S)TEM study of Ti-pyroxene (fassaite) and osbornite inclusions in the refractory minerals within the CAI "Inti" from comet P81/Wild-2 suggests a common inner-solar origin for fassaite. Inti is unlikely to originate outside of the solar system.
[show abstract][hide abstract] ABSTRACT: A FEI high-base Titan 80-300 (scanning) transmission electron microscope (S/TEM)has been installed at LLNL. It is first 300 kV analytical superSTEM that combines double Cs correctors and monochromator in one. Some preliminary results will be shown.
[show abstract][hide abstract] ABSTRACT: We are investigating the effect of capture-heating on iron sulphides impacted into Al foils to aid interpretation of Stardust residues. We find melting, S-loss, and recrystallisation of phases produced by target-impactor mixing.
[show abstract][hide abstract] ABSTRACT: Abstract— Sample preparation is always a critical step in the study of micrometer-sized astromaterials available for study in the laboratory, whether their subsequent analysis is by electron microscopy or secondary ion mass spectrometry. A focused beam of gallium ions has been used to prepare electron transparent sections from an interplanetary dust particle (IDP), as part of an integrated analysis protocol to maximize the mineralogical, elemental, isotopic, and spectroscopic information extracted from one individual particle. In addition, focused ion beam (FIB) techniques have been employed to extract cometary residue preserved on the rims and walls of microcraters in 1100 series aluminum foils that were wrapped around the sample tray assembly on the Stardust cometary sample collector. Non-ideal surface geometries and inconveniently located regions of interest required creative solutions. These include support pillar construction and relocation of a significant portion of sample to access a region of interest. Serial sectioning, in a manner similar to ultramicrotomy, is a significant development and further demonstrates the unique capabilities of focused ion beam microscopy for sample preparation of astromaterials.