Thomas J. Zega

Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Determination of interface atomic structure and its impact on spin transport using Z-contrast microscopy and density-functional theory.

We combine Z-contrast scanning transmission electron microscopy with density-functional-theory calculations to determine the atomic structure of the interface in spin-polarized light-emitting diodes. A 44% increase in spin-injection efficiency occurs after a low-temperature anneal, which produces an ordered, coherent interface consisting of a single atomic plane of alternating Fe and As atoms. First-principles transport calculations indicate that the increase in spin-injection efficiency is due to the abruptness and coherency of the annealed interface.

Fine-grained-rim mineralogy of the Cold Bokkeveld CM chondrite

Abstract A chrysotile-like phase, cronstedtite, polygonal serpentine, pentlandite, and finely intergrown tochilinite comprise the fine-grained rim (FGR) mineralogy of the Cold Bokkeveld CM chondrite. Transmission electron microscope images combined with compositional data indicate reaction among cronstedtite, the chrysotile-like phase, and polygonal serpentine. The Mg/(Mg+Fe) ratios of the cronstedtite are higher than those reported for the less altered Murchison CM chondrite. Cronstedtite grains exhibit layer separations, particularly at their boundaries. The FGRs surround different chondrule types but have similar bulk compositions and mineralogy. Ca is depleted in the FGRs relative to the bulk CM chondrite. The FGRs display non-uniform thicknesses, especially where they coat embayed chondrule areas, and they exhibit grain-size coarsening outward from the chondrules they enclose. FGR formation in Cold Bokkeveld is most plausibly explained by multiple accretionary episodes during which progressively coarser dust was deposited onto chondrules, presumably in the solar nebula. The compositional and mineralogic data are consistent with aqueous alteration on the parent body.

Opaque minerals in the matrix of the Bishunpur (LL3.1) chondrite: constraints on the chondrule formation environment

Abstract The chemistry and mineralogy of a group of opaque mineral assemblages in the matrix of the Bishunpur LL3.1 ordinary chondrite provide insight into the nebular environment in which they formed. The assemblages consist of a kamacite (Fe,Ni) core that is rimmed by troilite (FeS) and fayalite (Fe2SiO4). Accessory phases in the rims include silica (SiO2), chromite (FeCr2O4), whitlockite (Ca3(PO4)2), maricite (FeNaPO4), magnetite (Fe3O4), and tetrataenite (FeNi). We suggest that the metal melted in and equilibrated with an igneous chondrule under high-temperature, reducing conditions. In this environment the molten alloys incorporated varied amounts of Si, Ni, P, Cr, and Co, depending on the oxygen fugacity and temperature of the melt. Some of the metal was subsequently expelled from the chondrule interiors into the surrounding nebular gas. As the temperature dropped, the alloy solidified and volatile elements corroded the metal. The main reaction products were troilite and fayalite. Thermodynamic equilibrium calculations are used to constrain the conditions under which these two phases can form simultaneously in the solar nebula. Kinetic factors are used to place a lower limit on the formation temperature. We determine that the metal corroded between 1173 and 1261 K at a total pressure in the range of 10−5.0 to 10−4.1 bars and a dust/gas ratio of 302 to 355 x relative to solar composition. These conditions are consistent with our model that the metal corroded in a dust-rich region of the solar nebula that was cooling after a chondrule formation event.

Oxygen Reduction Reaction on Platinum/Tantalum Oxide Electrocatalysts for PEM Fuel Cells

We investigate platinum supported on tantalum oxide as a possible catalyst for oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells. Three synthetic routes are evaluated to compare activities of tantalum-oxide-supported platinum fuel cell electrocatalysts: (i) deposition of platinum colloids on tantalum oxide followed by mechanical grinding with Vulcan carbon (VC); (ii) deposition of tantalum oxide on VC, followed by the deposition of platinum colloids; and (iii) deposition of Pt colloids on VC, followed by deposition of tantalum oxide. These are compared to a Pt/VC standard made with the same Pt colloids. The area-specific activities for the ORR at 0.9 V are a factor of 1.5 higher for catalysts synthesized via preparation route (ii) compared to a Pt/VC standard. The area-specific activities of the catalysts synthesized via routes (i) and (iii) are close to that of Pt/VC. The higher area-specific activity of the catalyst synthesized by route (ii) may be due to the preferential adsorption of OH groups to the oxide vs platinum surface.

Nanometer-scale measurements of Fe3+/ΣFe by electron energy-loss spectroscopy: A cautionary note

Abstract The effects of electron-beam damage on the Fe3+/ΣFe (total iron) ratio were measured by electron energy-loss spectroscopy (EELS) with a transmission electron microscope (TEM). Spectra were acquired from crushed and ion-beam-thinned cronstedtite. For fluences below 1 × 104 e/Å2, the Fe3+/ΣFe values from crushed grains range between 0.43 and 0.49, consistent with undamaged material. These measurements were acquired from flakes 180 to 1000 Å thick. With increase influence, samples <400 Å thick become damaged and exhibit Fe3+/ΣFe values >0.5. The critical fluence for radiation damage by 100 kV electrons as defined by Fe3+/ΣFe <0.5 for cronstedtite at 300 K, is 1 × 104 e/Å2. The absorbed dose to the speciman during acquisition of a typical EELS spectrum is large, with values around 2.2 × 1010 Gy (J/kg), equivalent to the deposition of 620 eV/Å3. Cooling to liquid N2 temperature did not significantly slow the damage process. Ion-beam thinning produces an amorphous layer on crystal surfaces. Spectra from the thinnest regions, which are amorphous, exhibit Fe3+/ΣFe >0.7. With increase in sample thickness, the Fe3+/ΣFe values decrease to a minimum, consistent with data from the undamaged material. The increase of Fe3+/ΣFe with respect to electron-beam irradiation is likely caused by loss of H. At low fluences, the loss of H is negligible, thus allowing consistent Fe3+/ΣFe values to be measured. The cronstedtite study illustrates the care required when using EELS to measure Fe3+/ΣFe values. Similar damage effects occur for a range of high-valence and mixed-oxidation state metals in minerals. EELS is the only spectroscopic method that can be used routinely to determine mixed-valence ratios at the nanometer scale, but care is required when measuring these data. Consideration needs to be given to the incident beam current, fluence, fluence rate, and sample thickness.

Nanometer-scale measurements of iron oxidation states of cronstedtite from primitive meteorites

Abstract We report the first nanometer-scale measurements of the iron (III) to total iron (Fe3+/ΣFe) ratios from primitive meteorites. These ratios from the matrices and fine-grained rims (FGRs) of the Murchison, Murray, and Cold Bokkeveld CM chondrites fall within a tight range, from 0.45 to 0.54 (±0.02). The measurements were made using electron energy-loss spectroscopy (EELS) on cronstedtite, which is a product of aqueous alteration early in the history of the solar system. The results indicate that the alteration of these meteorites, which display a broad range of alteration intensity, occurred under similar redox conditions and, further, that alteration likely occurred in situ on asteroidal bodies rather than in the solar nebula.

Microchemical and structural evidence for space weathering in soils from asteroid Itokawa

Here we report microchemical and microstructural features indicative of space weathering in a particle returned from the surface of asteroid Itokawa by the Hayabusa mission. Space weathering features include partially and completely amorphous rims, chemically and structurally heterogeneous multilayer rims, amorphous surface islands, vesiculated rim textures, and nanophase iron particles. Solar-wind irradiation is likely responsible for the amorphization as well as the associated vesiculation of grain rims. The multilayer rims contain a nanocrystalline outer layer that is underlain by an amorphous inner layer, and both have compositions that are distinct from the underlying, crystalline orthopyroxene grain. The multilayer rim features could be derived from either radiation-induced sputter deposition or vapor deposition from micrometeorite impact events. The amorphous islands on grain surfaces have a distinctive morphology and composition suggesting that they represent surface deposits of melt derived from micrometeorite impact events. These observations indicate that both irradiation damage and micrometeorite impacts play a role in surface modification and space weathering on asteroid Itokawa.

Brearleyite, Ca12Al14O32Cl2, a new alteration mineral from the NWA 1934 meteorite

Abstract Brearleyite (IMA 2010-062, Ca12Al14O32Cl2) is a Cl-bearing mayenite, occurring as fine-grained aggregates coexisting with hercynite, gehlenite, and perovskite in a rare krotite (CaAl2O4) dominant refractory inclusion from the Northwest Africa 1934 CV3 carbonaceous chondrite. The phase was characterized by SEM, TEM-SAED, micro-Raman, and EPMA. The mean chemical composition of the brearleyite is (wt%) Al2O3 48.48, CaO 45.73, Cl 5.12, FeO 0.80, Na2O 0.12, TiO2 0.03, -O 1.16, sum 99.12. The corresponding empirical formula calculated on the basis of 34 O+Cl atoms is (Ca11.91 Na0.06)Σ11.97(Al13.89Fe0.16Ti0.01)Σ14.06O31.89Cl2.11. The Raman spectrum of brealryeite indicates very close structural similarity to synthetic Ca12Al14O32Cl2. Rietveld refinement of an integrated TEM-SAED ring pattern from a FIB section quantifies this structural relationship and indicates that brearleyite is cubic, I4̅3d; a = 11.98(8) Å, V = 1719.1(2) Å3, and Z = 2. It has a framework structure in which AlO4 tetrahedra share corners to form eight-membered rings. Within this framework, the Cl atom is located at a special position (3/8,0,1/4) with 0.4(2) occupancy and Ca appears to be disordered on two partially occupied sites similar to synthetic Cl-mayenite. Brearleyite has a light olive color under diffuse reflected light and a calculated density of 2.797 g/cm3. Brearleyite is not only a new meteoritic Ca-,Al-phase, but also a new meteoritic Cl-rich phase. It likely formed by the reaction of krotite with Cl-bearing hot gases or fluids.

CIRCUMSTELLAR MAGNETITE FROM THE LAP 031117 CO3.0 CHONDRITE

We report the first microstructural confirmation of circumstellar magnetite, identified in a petrographic thin section of the LaPaz Icefield 031117 CO3.0 chondrite. The O-isotopic composition of the grain indicates an origin in a low-mass (∼2.2 Me), approximately solar metallicity red/asymptotic giant branch (RGB/AGB) star undergoing first dredge-up. The magnetite is a single crystal measuring 750 × 670 nm, is free of defects, and is stoichiometric Fe3O4. We hypothesize that the magnetite formed via oxidation of previously condensed Fe dust within the circumstellar envelope of its progenitor star. Using an empirically derived rate constant for this reaction, we calculate that such oxidation could have occurred over timescales ranging from approximately ∼9000–500,000 years. This timescale is within the lifetime of estimates for dust condensation within RGB/AGB stars.