Falko Langenhorst

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.

The effect of valence state and site geometry on Ti L3,2 and O K electron energy-loss spectra of TixOy phases

Abstract Titanium L3,2 and O K electron energy loss near-edge structures (ELNES) of seven Ti oxides have been measured in a transmission electron microscope to obtain information on the valence state and site geometry of Ti. The coordination of Ti in all phases studied is octahedral, whereas the valence states occurring are Ti2+, Ti3+, and Ti4+. Effects of polyhedra distortions are particularly observed for two oxides with mixed Ti3+-Ti4+ valence state, i.e., the Magnéli phases Ti4O7 and Ti5O9. A prominent pre-peak in the Ti L3 edge is attributed to the orthorhombic polyhedra distortions in these compounds, leading to complex crystal field splitting. The effect of valence state manifests itself in a systematic chemical shift of Ti white lines by 2 eV per valence state. On the basis of collected Ti L3,2 ELNES spectra we propose a new quantification technique for the determination of Ti4+/Ti3+ ratios. Complementary O K ELNES spectra were well reproduced by Density Functional Theory calculation, revealing that the O K-edge is sensitive to the covalent bonding in all analyzed oxides.

Magnetization of exsolution intergrowths of hematite and ilmenite: Mineral chemistry, phase relations, and magnetic properties of hemo‐ilmenite ores with micron‐ to nanometer‐scale lamellae from Allard Lake, Quebec

Hemo-ilmenite ores from Allard Lake, Quebec, were first studied over 50 years ago. Interest was renewed in these coarsely exsolved oxides, based on the theory of lamellar magnetism as an explanation for the high and stable natural remanent magnetizations (NRMs), 32 to 120 A/m, reported here. To understand the magnetism and evolution of the exsolution lamellae, the microstructures and nanostructures were studied using scanning electron microscopy and transmission electron microscopy (TEM), phase chemistry, and relations between mineral chemistry and the hematite-ilmenite phase diagram. Cycles of exsolution during slow cooling resulted in lamellae down to 1-2 nm thick. Combined electron microprobe, TEM, and X-ray diffraction (XRD) results indicate that hematite hosts reached a composition approximately ilmenite (Ilm) 14.4, and ilmenite hosts ∼Ilm 98. The bulk of the very stable NRM, which shows thermal unblocking ∼595-620°C, was acquired during final exsolution in the two-phase region canted antiferromagnetic R3c hematite + R3 ilmenite. Hysteresis measurements show a very strong anisotropy, with a stronger coercivity normal to, than parallel to, the basal plane orientation of the lamellae. Magnetic saturation (M s ) values are up to 914 A/m, compared to 564 A/m predicted for a modally equivalent spin-canted hematite corrected for ∼15% R 2+ TiO 3 substitution. Low-temperature hysteresis, AC-susceptibility measurements, and Mossbauer results indicate a Neel temperature (T N ) of the geikielite-substituted ilmenite at ∼43 K. The low-temperature hysteresis and AC-susceptibility measurements also show a cluster-spin-glass-like transition near 20 K. Below T N of ilmenite an exchange bias occurs with a 40 mT shift at 10 K.

Mineralogy and defect microstructure of an olivine-dominated Itokawa dust particle: evidence for shock metamorphism, collisional fragmentation, and LL chondrite origin

We report here detailed analytical scanning and transmission electron microscopic investigations on an olivine-dominated dust particle (RB-QD04-0042) from the surface of asteroid 25143 Itokawa. The dust particle was returned to Earth by the Hayabusa spacecraft and was made available in the context of the first announcement of opportunity for Hayabusa sample investigation. Multiple thin slices were prepared from the precious particle by means of focused ion beam thinning, providing a unique three-dimensional access to its interior. The 40 × 50 μm sized olivine particle contains a spherical diopside inclusion and an intimate intergrowth of troilite and tetrataenite. The compositions of olivine (Fo69Fa31) and diopside (En48Wo42Fs10), as well as the high Ni content of the sulfide-metal alloy, indicate a LL ordinary chondrite origin in accord with previous classifications. Although no impact crater exists at the surface of RB-QD04-0042, transmission electron microscopy revealed the presence of various shock defects in constituent minerals. These defects are planar fractures and [001] screw dislocations in olivine, multiple {101} deformation twins in tetrataenite and basal (0001) stacking faults in troilite. These diagnostic shock indicators occur only in a small zone on one concave side of the dust particle characterized by a high fracture density. These observations can be explained by a collisional event that spalled off material from the particles surface. Alternatively, the dust particle itself could be a spallation fragment of an impact into a larger regolith target. This suggests that Itokawa dust particles lacking visible microcraters on their surfaces might have still experienced shock metamorphism and were involved in collisional fragmentation that resulted in the formation of regolith.

Aluminum solubility in TiO2 rutile at high pressure and experimental evidence for a CaCl2-structured polymorph

Abstract Aluminum incorporation into TiO2 has been studied in the TiO2-Al2O3 system as a function of pressure at temperatures of 900 and 1300 °C using commercial Al2TiO5 nanopowder as starting material. A new orthorhombic TiO2 polymorph with the CaCl2 structure has been observed in the recovered samples synthesized from 4.5 to 7 GPa and 900 °C and from 2.5 to 7 GPa at 1300 °C. The phase transition to the α-PbO2 type TiO2 phase takes place between 7 and 10 GPa at both temperatures. Two mechanisms of Al incorporation in TiO2 rutile have been observed in the recovered samples. The substitution of Ti4+ by Al3+ on normal octahedral sites is dominant at lower pressures. High pressure induces the incorporation of Al3+ into octahedral interstices of the rutile structure, which is responsible for an orthorhombic distortion of the TiO2 rutile structure and gives rise to a (110) twinned CaCl2 type structure. This phase is probably a result of temperature quench at high pressure. Aluminum solubility in TiO2 increases with increasing pressure. TiO2 is able to accommodate up to 9.8 wt% Al2O3 at 7 GPa and 1300 °C. Temperature has a large effect on the aluminum incorporation in TiO2, especially at higher pressures. High pressure has a strong effect on both the chemistry and the microstructure of Al-doped TiO2. Enhanced aluminum concentration in TiO2 rutile as well as TiO2 grains with a microstructure consisting of twins are a clear indication of high-pressure conditions.

Crystal chemistry of hydration in aluminous orthopyroxene

Abstract Hydrogen incorporation in aluminous orthopyroxene may control the generation of melt and dominate the seismic properties at the base of the Earth.s lithosphere. To clarify the substitution mechanism of H, we have synthesized, characterized, and refined the crystal structure of this potentially significant variant of orthopyroxene. The experimentally produced crystals are small needles up to approximately 20 × 20 × 100 μm in size. Electron microprobe chemical analysis indicates about 11.7 wt% Al2O3. FTIR spectra indicate 7500 ppmw H2O with absorbance features qualitatively similar to natural mantle orthopyroxenes. TEM imagery indicates that the phase is pure orthopyroxene with low concentrations of defects and inclusions. Cell-parameter refinement from single-crystal X-ray diffraction gives a = 18.1876(7) Å; b = 8.7352(7) Å; c = 5.1789 (5) Å, V = 822.79(11) Å3, which is 1.2% smaller than pure Mg anhydrous orthoenstatite. The crystal structure has been refined from single-crystal X-ray intensity data measured using a rotating anode X-ray generator, micro-focused X-ray beam, and CCD detector system. The refined structure indicates about 5% vacancy in M2 and significant Al occupancy in both M1 and T2, consistent with its composition, (Mg0.95,0.05)M2, (Mg0.79Al0.21)M1, (Al0.25Si0.75)T2 SiT1 O6. The existence of hydrous orthopyroxene in the mantle could absorb water released from olivine on decompression to delay the onset of melting in the spinel stability region in mantle peridotite compositions.

The mineralogy and space weathering of a regolith grain from 25143 Itokawa and the possibility of annealed solar wind damage

We report the results of detailed mineralogical investigations by analytical scanning and transmission electron microscopy of particle RA-QD02-0115 recovered from the surface of asteroid 25143 Itokawa. We divided the 65 μm × 50 μm small particle into eight individual subsample slices via the focused ion beam method. The particle dominantly consists of olivine and contains inclusions of merrillite, tetrataenite/taenite, troilite, chromite, kamacite, and Cl-bearing apatite (in approx. decreasing order of frequency). The composition of olivine (fayalite 29.8 ± 1.1 mol% and molar Fe/Mn ratio of 57 ± 2) as well as the Ni-rich metal assemblage indicates an LL-type affinity in accord with previous classifications. The particle shows effects of solar wind irradiation on one of its principal faces. Olivine developed an approximately 34 nm wide rim composed of low-angle misoriented, nanometer-sized crystallites accompanied by a small amount of amorphous material. Exposed troilite developed a 4 to 8 nm wide polycrystalline rim with large-angle misorientations of the iron sulfide nanocrystallites. Merrilite shows marginally discernable surface damage but was too unstable under the electron beam for a detailed study. Cl-bearing apatite was found fully crystalline with no discernable rim structure. We discuss the unusual polycrystalline nature of the olivine rim in terms of possible annealing and recrystallization effects, which may have occurred during periods of time when Itokawa’s surface temperature may have been warmer due to closer perihelion distances. Model calculations show that the dynamical orbital evolution of near-Earth asteroids could lead to complex space weathering processes, arising from the competing interplay between irradiation-induced damaging and thermally driven annealing.

Nanocrystalline Nickel and Cobalt Sulfides Formed by a Heavy Metal-Tolerant, Sulfate-Reducing Enrichment Culture

A soil enrichment culture of the sulfate-reducers Desulfosporosinus auripigmenti and Citrobacter freundii and of fermentative bacteria from a former uranium-mining site was studied for its metal retention potential by promoting metal sulfide precipitation. The culture could tolerate up to 30 mM Ni and 40 mM Co. XRD and TEM analyses revealed the formation of amorphous NiS together with nanocrystalline, metastable α-NiS, and nanocrystalline cobalt pentlandite. The α-NiS with a grain size of 5 nm shows probably an example of size-dependent phase stability and/or specific biomineralization precipitation paths. Detailed mineralogical characterizations are necessary to correctly assess the mineral inventory and thus metal bioavailability.

Crystal chemistry of synthetic Ti-Mg-bearing hibonites: A single-crystal X-ray study

Abstract Hibonite single crystals were synthesized using two different techniques: hot-pressing of polycrystalline hibonite by means of piston-cylinder apparatus and solid state reaction using citrate-based sol-gel precursors. Chemical analyses, UV/Vis spectroscopy, and single-crystal X-ray characterization were performed on four sets of Ti-Mg-bearing hibonites to investigate the substitution mechanism of Ti3+, Ti4+, and Mg2+, relevant for hibonites found in calcium-aluminum-rich inclusions in meteorites. The unit-cell volume of hibonite depends linearly on the concentration of Ti and Mg: V = 8.21(3)·(Titot + Mg)apfu + 586.06(1). Structural refinements, carried out in the space group P63/mmc, demonstrate that Ti occupies two sites: M2, a trigonal bipyramid, and M4, a distorted octahedron occurring in facesharing pairs. The Ti occupancy factor was refined at both sites. Due to the repulsion of neighboring Ti cations the bond distance M4-O3 increases with increasing Ti content and the cations are displaced from the central position of the polyhedron. The displacement factors (U33)M2 and the site occupancy factor for Ti in the M2 site positively correlate for the samples, which have more than 0.3-0.4 Tiapfu, while (U33)M2 remains that of pure hibonite for small Ti occupancies at the M2 site. This plateau of displacement factor reflects the local strain fields around the substituted Ti atoms and its magnitude indicates that these strain fields begin to overlap at a Ti-Ti separation of about 1-2 unit cells. For a sample synthesized at low oxygen fugacity we detected Ti3+ by means of UV/Vis absorption spectroscopy. The presence of Ti3+ has a clear effect on the M4-M4 distance, which deviates from the linear trend described by samples containing mainly Ti4+. The average bond length M3-O depends linearly on the Mg content of these synthetic hibonites clearly indicating that Mg occupies this site.

New insights into the toxicity of mineral fibres: A combined in situ synchrotron μ-XRD and HR-TEM study of chrysotile, crocidolite, and erionite fibres found in the tissues of Sprague-Dawley rats

Along the line of the recent research topic aimed at understanding the in vivo activity of mineral fibres and their mechanisms of toxicity, this work describes the morpho-chemical characteristics of the mineral fibres found in the tissues of Sprague-Dawley rats subjected to intraperitoneal/intrapleural injection of UICC chrysotile, UICC crocidolite and erionite-Na from Nevada (USA). The fibres are studied with in situ synchrotron powder diffraction and high resolution transmission electron microscopy to improve our understanding of the mechanisms of toxicity of these mineral fibres. In contact with the tissues of the rats, chrysotile fibres are prone to dissolve, with leaching of Mg and production of a silica rich relict. On the other hand, crocidolite and erionite-Na fibres are stable even for very long contact times within the tissues of the rats, showing just a thin dissolution amorphous halo. These findings support the model of a lower biopersistence of chrysotile with respect to crocidolite and erionite-Na but the formation of a silica-rich fibrous residue after the pseudo-amorphization of chrysotile may justify a higher cytotoxic potential and intense inflammatory activity of chrysotile in the short term in contact with the lung tissues.