Damien Jacob

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.

Structure refinement from precession electron diffraction data

Electron diffraction is a unique tool for analysing the crystal structures of very small crystals. In particular, precession electron diffraction has been shown to be a useful method for ab initio structure solution. In this work it is demonstrated that precession electron diffraction data can also be successfully used for structure refinement, if the dynamical theory of diffraction is used for the calculation of diffracted intensities. The method is demonstrated on data from three materials - silicon, orthopyroxene (Mg,Fe)(2)Si(2)O(6) and gallium-indium tin oxide (Ga,In)(4)Sn(2)O(10). In particular, it is shown that atomic occupancies of mixed crystallographic sites can be refined to an accuracy approaching X-ray or neutron diffraction methods. In comparison with conventional electron diffraction data, the refinement against precession diffraction data yields significantly lower figures of merit, higher accuracy of refined parameters, much broader radii of convergence, especially for the thickness and orientation of the sample, and significantly reduced correlations between the structure parameters. The full dynamical refinement is compared with refinement using kinematical and two-beam approximations, and is shown to be superior to the latter two.

Igneous Ca-rich pyroxene in comet 81P/Wild 2

Abstract The Stardust spacecraft successfully returned dust from comet 81P/Wild 2 to Earth in January 2006. Preliminary examination of the samples showed abundant crystalline silicates comparable to those found in chondritic meteorites presumably formed in the asteroid belt. Here, we report results of a transmission electron microscopy (TEM) study of a pyroxene-bearing terminal particle, which contains lamellar intergrowths of pigeonite and diopside on the (001) plane. This microstructure is typical for an igneous process and formation by exsolution during cooling. Width and wavelength of the lamellae indicate a cooling rate within the range 10-100 °C/h, in close agreement with those of chondrules or lava from an asteroidal igneous rock. This observation shows that some Stardust material experienced periods of igneous processing similar to material found in the inner early solar system. This implies that igneous materials were common materials in a large region of the protoplanetary disk and were not restricted to the asteroid belt. Their presence in comet Wild 2 also supports the favored view of large radial mixing from the inner to the outer regions before the comet’s accretion.

Structure refinement using precession electron diffraction tomography and dynamical diffraction: tests on experimental data.

The recently published method for the structure refinement from three-dimensional precession electron diffraction data using dynamical diffraction theory [Palatinus et al. (2015). Acta Cryst. A71, 235-244] has been applied to a set of experimental data sets from five different samples - Ni2Si, PrVO3, kaolinite, orthopyroxene and mayenite. The data were measured on different instruments and with variable precession angles. For each sample a reliable reference structure was available. A large series of tests revealed that the method provides structure models with an average error in atomic positions typically between 0.01 and 0.02 Å. The obtained structure models are significantly more accurate than models obtained by refinement using kinematical approximation for the calculation of model intensities. The method also allows a reliable determination of site occupancies and determination of absolute structure. Based on the extensive tests, an optimal set of the parameters for the method is proposed.

Application of precession electron diffraction to the characterization of (021) twinning in pseudo-hexagonal coesite

Abstract Precession electron diffraction is used to characterize a (021) twin observed in coesite. Due to the quasi-hexagonal dimensions of coesite (monoclinic space group C12/c1 with β = 120.34°), indexing of conventional spot patterns is ambiguous and the twin law determination is impossible. With precession, spot intensities enable the absolute indexing of the patterns. The method we used is based on the analysis of the departure from hexagonal symmetry. This ensures that all possible pseudo-equivalent monoclinic orientations are taken into account for the indexing. The orientation relationships between adjacent parts of the twin are then characterized. The twin is described as a mirror along (021), which is consistent with original descriptions of twinning in synthetic coesite and a previous characterization performed using large-angle convergent-beam electron diffraction (LACBED).

A precession electron diffraction study of α, β phases and Dauphiné twin in quartz

Precession electron diffraction is used to distinguish between the hexagonal beta high-temperature and the trigonal alpha low-temperature phases of SiO2 quartz. The structures just differ by a kink of the SiO4 tetrahedra arranged along spiraling chains, which induces a loss of the two-fold axis and subsequent twinning in the low-temperature phase. Conventional selected-area electron diffraction (SAED) does not enable the phases distinction since only the intensity of reflections is different. It becomes possible with precession that reduces the dynamical interactions between reflections and makes their intensity very sensitive to small variations of the electron structure factors. Distinction between the twinned individuals in the low-temperature phase is then easily made and the twin law is characterized using stereographic projections. The actual symmetry of precessed zone axis patterns is also examined in detail. Using dynamical intensity simulations, it is shown that under certain thickness conditions, the diffraction class symmetry can be observed on selected area patterns that are to be used in the case of beam sensitive materials such as quartz.

Surface relaxation of strained semiconductor heterostructures revealed by finite-element calculations and transmission electron microscopy

Abstract Surface relaxation of a cross-section strained In0.13Ga0.87As layer buried in GaAs is studied by finite-element calculations and transmission electron microscopy (TEM). Strain fields in the In0.13Ga0.87As layer and in the cladding layers are calculated by the finite-element method and they are then used to compute the corresponding TEM image contrast, in the framework of the column approximation. A good qualitative match between experimental and computed TEM images is obtained, which demonstrates that, using the column approximation, the finite-element approach can be adapted to this type of study. The sensitivity of TEM contrasts to parameters such as foil thickness, deviation parameter, anomalous absorption and chemical composition is also analysed. In particular, the results of our exhaustive study, of TEM contrasts as a function of foil thickness show that surface strain relaxation is significant whatever the foil thickness and that this relaxation should not thus be mistaken for what is comm...

A systematic method to identify the space group from PED and CBED patterns part I--theory.

This systematic method allows the unambiguous identification of the extinction and diffraction symbols of a crystal by comparison of a few experimental Precession Electron Diffraction (PED) patterns with theoretical patterns drawn for all the extinction and diffraction symbols. The method requires the detection of the Laue class, of the kinematically forbidden reflections and of the shift and periodicity differences between the reflections located in the First-Order Laue Zone (FOLZ) with respect to the ones located in the Zero-Order Laue Zone (ZOLZ). The actual space group can be selected, among the possible space groups connected with each extinction symbol or diffraction symbol, from the identification of the point group. This point group is available from observation of the 2D symmetry of the ZOLZ on Convergent-Beam Electron Diffraction (CBED) patterns.

Ordering state in orthopyroxene as determined by precession electron diffraction

Abstract Fe2+ and Mg distribution on octahedral M1 and M2 sites of the orthopyroxene structure is an indicator of the cooling rate and closure temperature of the mineral. It is generally obtained by single-crystal X‑ray diffraction, which is limited in spatial resolution. In this work, we determine the cationic distribution at a submicrometer scale in a transmission electron microscope using precession electron diffraction. Two orthopyroxene samples coming from the same metamorphic rock are studied, a naturally ordered one and a disordered one. The latter was obtained from the ordered sample by annealing at high temperature and rapid quenching. Both samples have been first studied in X‑ray diffraction and then in precession electron diffraction. Intensities recorded in zone-axis precession electron diffraction experiments have been quantitatively analyzed and compared to simulations, taking into account dynamical interactions between diffracted beams. Our structure refinement results are in good agreement with those obtained by single-crystal X‑ray diffraction. They enable to distinguish between the ordered sample and the disordered one in terms of the observed molar fractions of Fe at M1 and M2 sites. We discuss the sensitivity of the method as a function of experimental parameters. The larger dispersion of the results obtained on the ordered specimen is attributed to structural heterogeneities inherent to the sample.

Large-angle convergent-beam electron diffraction (LACBED) characterization of (021) twinning in natural coesite

Large-angle convergent-beam electron diffraction (LACBED) is used to characterize a (021) twinning in metamorphic coesite from Parigi, Dora-Maira Massif, Western Alps. The orientation relationships between the adjacent parts of the twin are characterized in detail and the twin is described as a mirror along (021). This microscopic description is fully consistent with original descriptions of twinning in synthetic coesite. Due to the quasi-hexagonal dimensions of coesite (belonging to the monoclinic space group C 12/ c 1 with a ~ c and β ~ 120°) such a result could not have been obtained using conventional electron diffraction experiments.