Science –

Blocage de la polyspermie chez les Mammifères

Tristan FERROIR — Sat, 17 May 2014 10:02:44 +0000

Le blocage de la polyspermie chez les Mammifères fait débat depuis longtemps. Si la réaction corticale est clairement un des moyens, le fait qu’elle se produise « tardivement » après l’entrée du spermatozoïde compte pour blocage mais pas comme blocage précoce. Certains mettent en avant une dépolarisation de la membrane de l’ovocyte mais cette conclusion est loin d’être partagé par l’ensemble de la communauté. Le modèle Souris semble notamment ne pas montrer une telle dépolarisation.

Un article publié par BIanchi et al dans Nature permet de proposer un mécanisme alternatif particulièrement fin :

L’intéraction entre la protéine Izumo située sur la membrane du spermatozoïde après la réaction acrosomique ne se fait pas avec CD9 comme penser précédemment. On pensait que cette intéraction permettait la fusion entre les membranes du spermatozoïde et la membrane de l’ovocyte (oolemme). En fait, Izumo n’intéragit pas avec CD9 mais avec une autre protéine que les chercheurs ont nommé Juno (la déesse romaine du mariage). Cette intéraction est absolument nécessaire : un ovocyte sans protéine Juno est infécondable. C’est la partie a du schéma

Là où c’est encore plus fort c’est qu’après la fécondation de l’ovocyte par un spermatozoïde, la protéine Juno est expulsée de la membrane dans des vésicules (schéma b). En conséquence, l’absence de la protéine Juno sur la membrane rend donc l’ovocyte infécondable. Voilà le blocage rapide de la polyspermie chez les Mammifères tant recherché sans qu’il y ait intervention d’une dépolarisation de l’ovocyte!

La comète Ison

Tristan FERROIR — Sat, 30 Nov 2013 10:48:09 +0000

La comète Ison découverte en 2012 par deux astronomes amateurs est passée toute proche du Soleil mais les images de la NASA laisse penser qu’elle pourrait avoir survécu à son passage alors qu’encore hier, on ne le pensait pas. En fait, seul une partie du noyau de la comète serait encore intact. Espérons qu’il sera possible de prendre des clichés par la suite qui permettrait de connaître un peu mieux la chimie des noyaux cométaires.

Cette vidéo prise par SoHo permet de voir la trajectoire de la comète.

Un bon moment pour revoir un peu ce que sont les comètes grâce au dossier que j’avais publié ici.

Le polymorphisme du carbone dans les météorites choquées : découverte de forme ultra-dures, plus dures que le diamant

Tristan FERROIR — Fri, 05 Feb 2010 10:51:51 +0000

Nous avons recemment publié un papier dans EPSL (Earth and Planetary Science Letters) intitulé « Carbon polymorphism in shocked meteorites: Evidence for new natural ultrahard phases ». J’en fais un long résumé ci-dessous. (English version available here). Les commentaires sont les bienvenus

Nous avons étudié les différentes occurences du carbone présents dans les ureilites et plus particulièrement dans l’ureilite Havero. Pour ceci, nous avons réalisé des lames minces polies par polissage à la pâte diamantée. Nous avons ensuite repéré deux zones carbonées distinctes par microscopie réfléchie. Des observations plus précises ont ensuite été réalisées sur un MEB environnemental, pour éviter le dépôt d’une couche de carbone sur l’échantillon à observer pour éviter toute contamination de l’échantillon par du carbone amorphe. Les observations microscopiques montrent qu’on peut délimiter différentes régions au sein des zones carbonées que nous avons étudiées. La Figure 2.41 montre une de ces zones observées ainsi qu’un schéma d’interprétation de la structure.

FiG. 2.41 - Une des zones carbonées d'Havero. En haut à droite, l'aspect en microscopie optique et l'image MEB correspondante au centre. On remarque que cette zone est fortement mal polie et est en relief par rapport à la matrice silicatée. L'encart en bas à gauche montre l'arrangement spatial de la zone le long de la ligne blanche sur l'image MEB.

On constate que les zones carbonées sont faiblement polies, granuleuses et qu’elles peuvent présenter des proéminences au dessus de la matrice silicatée jusqu’à des hauteurs de 13 um. L’arrangement spatial est concentrique avec des zones à faible relief (Zone A) vers l’extérieur de la zone carbonée, des zones à forts reliefs vers le centre (Zone B) et un mamelon à très fort relief (Zone C) tout au centre. Afin de vérifier la pureté chimique de ces zones, nous les avons extraites au moyen d’une microforeuse et les avons montées dans des disques d’acier troués. Ceci permet une étude par fluorescence X et diffraction des rayons X, le trou permettant au faisceau de passer à travers l’échantillon sans interaction avec le disque d’acier. Grâce à ce montage, nous avons pu réaliser des cartographies en fluorescence et en transmission des rayons X (Figure 2.42). Ces résultats montrent d’une part que la zone a une densité faible puisque la transmission est maximum dans la zone carbonée. Ceci permet d’imager la zone carbonée entière et donne un indice sur sa pureté. D’autre part, on constate que cette même zone est très pauvre en Ca, Mn, et en éléments plus lourds tels que le Fe par exemple.

FlG. 2.42 - a) Cartographie en transmission et cartographie chimique du Mn (b) et du Ca (c)

Nous avons alors mené des études en spectroscopie Raman au sein des différentes zones identifiées précédemment. La zone A est essentiellement constituée de graphite désordonné (bandes G, D et D’ présentes) et de petits diamants (un seul pic à 1331 cm^-1). La zone B est quant à elle constituée d’un mélange de graphite ordonné et de diamants, nanodiamants ou lonsdaleite, le pic Raman pouvant aller de 1331 cm^-1 à 1322 cm^-1. Enfin, la zone C est composée d’une phase présentant les pics caratéristiques du diamant du graphite mais aussi de nombreuses bandes additionelles. Certaines de ces bandes ont été attribuées à des défauts dans le graphite (1080, 1200, 1350 et 1500 cm^-1), de la lonsdaleite (1280 cm^-1) ou à des effets de taille (580 cm^-1), la plupart de ces bandes n’ont jamais été observées ni prédites pour aucun composé carboné.

FlG. 2.43 - Les différents spectres Raman obtenus au sein des phases carbonées des ureilites. Les zones sont les mêmes que celles identifiées dans la figure 2.41. La présence de deux phases répertoriées dans la zone C vient du fait que deux zones carbonées distinctes ont été étudiées et qu'elles ont montré deux phases différentes.

Nous avons poursuivi ces études de spectroscopie Raman par des investigations par diffraction des rayons X. Les données obtenues sur la première zone carbonée confirment les données Raman à savoir la présence de diamant, de graphite et d’une nouvelle phase. L’analyse de l’image de diffraction montre une forte orientation préférentielle entre les directions [001] du graphite et [111] du diamant comme en témoigne l’élargissement simultané pour des angles identiques des cercles de diffraction. Le spectre intégré a pu être indexé avec du fer bec provenant de la partie inférieure de la lame, du diamant 3C, du graphite non compressé et un nouveau polytype du diamant le 21R. C’est la première fois que ce polytype est observé que ce soit dans des matériaux synthétiques ou naturels. Il n’était jusqu’à présent que prédit théoriquement.

Dans la deuxième zone carbonée, la plupart des spectres acquis peuvent être indexés avec du graphite et du diamant ou de la lonsdaleite. Dans la zone la plus élevée, du graphite compressé, du diamant et de la lonsdaleite ou du diamant ainsi qu’une nouvelle phase ont été observés. Cette nouvelle phase a un spectre de diffraction X différent de tous les polymorphes du carbone connu y compris le 21R décrit précédemment. Cette phase a été indexée comme appartenant au groupe d’espace cubique Pm3m bien que l’asymétrie de la ligne (111) puisse la faire appartenir au groupe rhomboèdrique R3m. C’est dans un tel cas que le meilleur raffinement structural a été obtenu avec a = 3.5610(9) et a = 90.2(2)°. Ce nouveau polymorphe n’a jamais été observé ni même prédit par les calculs ab initio.

N dobs., A h k 1 dcalc, A I, % Phase

1	3,276	002		2,9	Gr
2	2,182	1 00		3,1	L
3	2,055	1 1 1	2,055	100	N*
4	1,928	1 0 1		2,3	L
5	1,780	200	1,780	7,2	N
6	1,451	2 1 1	1,543	5,1	N
7	1,257	2 2 0	1,258	3,7	N
8	1,185	3 0 0	1,186	14,2	N
9	1,072	3 1 1	1,073	10,8	N
10	1,024	2 2 2	1,027	2,7	N

Tab. 1: Indexation des plans cristallographiques de la nouvelle phase de carbone ultra-dure

L’existence de graphite compressé, non compressé et de diamant suggère un scénario complexe de formation de ces différents polymorphes du carbone. L’existence de graphite compressé suggère fortement un processus de choc. De plus, l’étude de la position de la bande D du diamant ainsi que de sa largeur à mi hauteur (FWHM) est en bon accord avec cette hypothèse. En effet, en comparant, la dispersion de ces deux valeurs aux données compilées par Miyamoto et al. [144], on constate que la quasi totalité de nos mesures tombent dans le champ de diamant formé par impact. Les quatre points hors de cette zone correspondent aux bandes des nouvelles phases.

Une étude similaire de la position
des bandes G du graphite ainsi que de leur FWHM permet dec alculer une compression correspondant à une pression résiduelle de 2.5GPa. Par ailleurs, si on utilise l’équation d’état du graphite pour les paramètres de mailles mesurés en diffraction des rayons X, on trouve une pression résiduelle comprise entre 2.5 et 4GPa ce qui est en très bon accord avec les données Raman. On peut donc penser que le graphite a été compressé à partir de graphite automorphe par le choc et que cette compression est un stade initial vers la formation de diamants et de nouvelles phases.

FlG. 2.44 - a) Plaque image de la zone contenant le polytype 21R du diamant, b) Son spectre intégré c) Plaque image contenant le nouveau polymorphe du carbone d) Spectre intégré du nouveau polymorphe

Cependant, nous avons aussi noté la présence de graphite non compressé. Nous pensons que ce graphite provient d’une rétromorphose du diamant durant la phase post-choc comme suggéré par Nakamura et al. Un modèle de conversion du diamant en graphite à haute température (2000K) a été proposé comme pouvant expliquer la formation d’îlots de graphite dans les diamants CVD. Ce modèle prédit aussi la formation d’une relation de parallélisme entre le plan (111) du diamant et (002) du graphite. De plus, Guillou et al., ont montré qu’en compressant du carbone noir (black carbon) on pouvait aussi obtenir une relation entre ces deux plans cristallographiques et un spectre Raman relativement complexe. Par contre, la compression d’autres précurseurs de type graphite polycristallin ou graphite orienté ne permet pas les mêmes observations montrant que le matériel de départ est extrêmement important.

Ainsi, les phénomènes de choc sur le système carbone dans les ureilites sont des contre-points essentiels aux expériences statiques et aux expériences de choc puisqu’ils ouvrent de nouvelles aires sur le diagramme de phase du carbone notamment étant donné le temps de choc permis dans les cas naturels.

FlG. 2.45 - Dispersions des valeurs caractéristiques en Raman du diamant montrant qu'ils sont formés par un phénomène de choc départ est important.

Carbon polymorphism in shocked meteorites: Evidence for new natural ultrahard phases (our EPSL paper)

Tristan FERROIR — Fri, 05 Feb 2010 10:35:29 +0000

We published a paper entitled « Carbon polymorphism in shocked meteorites: Evidence for new natural ultrahard phases » in the recent issue of EPSL. This post is the uncorrected proof featuring all the references, tables and pictures we published in the paper. Comments are very welcome. If you want a reprint of the paper, please use thee Contact Form.

Carbon polymorphism in shocked meteorites : evidence for new natural ultrahard phases

Tristan Ferroir1, Leonid Dubrovinsky2, Ahmed El Goresy2, Alexandre Simionovici3, Tomoki Nakamura4 and Philippe Gillet1,
1 Laboratoire des Sciences de la Terre, Universite de Lyon, Ecole Normale Superieure de Lyon, Universite Claude Bernard Lyon 1, CNRS 46 Allee d’Italie, 69364 Lyon Cedex 07, France
2 Bayerisches Geoinstitut, Universitat Bayreuth, D-95440 Bayreuth, Germany
3 Laboratoire de Geodynamique des Chaînes Alpines, OSUG, BP 53X, 38041 Grenoble, France
4Department of Earth and Planetary Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan

Abstract

The ureilite class of meteorites is named after the type example Novo-Urei, Russia that fell in 1886 and is one of the unusual achondritic meteorites. Ureilites contain olivine and pyroxenes (pigeonite, augite, or orthopyroxene but seem to be barren of feldspars) with graphite-bearing veins (including tiny diamonds), Fe metal with very low Ni-content, troilite, Fe3C and other minor accessory phases. A slice of the Havero ureilite was cut and then polished as a thin section using a diamond paste.. We identified two carbonaceous areas which were standing out by more than 10 /jm in relief over the surface of the silicate matrix suggesting that the carbonaceous phases were not easily polishable by a diamond paste and would therefore imply larger polishing hardness. These areas were investigated by reflected light microscopy, high-resolution Field Emission SEM (FESEM), energy-dispersive X-ray (EDX) analysis, Raman spectroscopy, and were subsequently extracted for in situ synchrotron microbeam X-ray fluorescence (XRF), imaging and X-ray diffraction (XRD). We report here the natural occurrences of one new ultrahard rhombohedral carbon polymorph and the theoretically predicted 21R diamond polytype in the Havero meteorite thus demonstrating that the carbon system is even more complex than what is currently thought based on ab initio molecular dynamic simulations and high-pressure experiments..

1. Introduction

Since the discovery of fullerenes[14] and carbon nanotubes[12], four forms of pure carbon are now recognized including diamond and graphite. These discoveries enhanced the interest in exploring possible further occurrences of polymorphs and polytypes of carbon. Many applications are foreseen for these new materials such as the ultrahard carbon polymorphs, possibly harder than diamond[9, 5, 22, 20]. A new and theoretically unpredicted ultrahard carbon polymorph formed under a very intense and brief shock was found in gneisses from the Popigai impact crater [6] and theoretically investigated afterwards [19]. This lead us to document the nature of shocked carbon materials in the ureilite class meteorites, particulary the Havero ureilite [24], which contains about 3% wt of pure carbon.

2. Materials and methods

A piece of the Havero meteorite was polished as a thin section using a diamond paste powder and was observed in reflected light microscopy. We identified several carbonaceous areas which were standing out by more than 10 /jm in relief compared to the silicate matrix. These areas were investigated by SEM, EDX and Raman spectroscopy and were afterwards extracted to be investigated by means of Synchrotron Radiation X-ray fluorescence and X-ray diffraction. We conducted SEM imaging on two of the three areas on an environmental SEM, FEI XL 30 FEG ESEM at GEMPPM, INSA Lyon, France. Standard SEM could not be used for that kind of sample since we needed to avoid carbon or gold thin film deposition. Raman experiments were performed at the Laboratoire des Sciences de la Terre (ENS Lyon, France). The spectra were recorded on a XY ©DILOR Raman microspectrometer equipped with a CCD detector. The spectrometer was used in backscattering geometry. The laser beam (514.5 nm exciting lines of a Spectra Physics@ Ar+ laser) was focused through microscope objectives (x100) down to a 1 micrometer spot on the sample and the backscattered light was collected through the same objective. To avoid any amorphization of the carbon phase, density filters were used so that the power on the sample was no more than 50 pW. Spectra were collected in the spectral region of 1200 to 1650 cm-1. On specific regions, larger spectra were acquired in the range of 200 to 3200 cm-1. The areas were subsequently drilled out of the thin section and placed at the center of a drilled stainless steel holder to allow X-ray diffraction and X-ray fluorescence on two different synchrotron beamlines at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. On the /x-FID beamline (micro fluorescence imaging and diffraction beamline, ID22), the sample was placed at 45° from the incident beam, facing an optical microscope in order to precisely locate the area of interest. A Si(Li) X-ray fluorescence detector was disposed at 90° from the incident beam while a CCD camera was placed 10 cm behind the sample to collect X-ray diffraction images. The beam was collimated through slits and focused by K-B mirrors down to a 4×1,5 /лm2 beamspot and a high flux of 1011 ph/s. We used a 22 keV energy beam to collect X-ray diffraction. Both fluorescence and diffraction were collected simultaneously with a dwell time of 20 s. A movable PIN diode could be placed after the sample to collect transmission flux allowing identification and characterization of the areas according to their mean density. Preliminary analysis consisted of a fine scale mapping of the different areas both in X-ray fluorescence and diffraction. Fluorescence mapping results were then carefully studied to select specific points to be investigated more precisely by X-ray diffraction. A second run of investigation was done on the High Pressure beamline (ID09) mainly dedicated to high pressure experiments and equipped only for X-ray diffraction. At this beamline, the beam is only collimated using slits down to a size of 10 by 10 /jm. The sample holder is mounted on a rotating device which allows both visual observation when at 90° position and X-ray diffraction when facing the beam. The energy is higher than at the /x-FID beamline (30keV) and the set-up is optimized for X-ray diffraction.

3. Results

We identified two carbonaceous areas standing out by more than 10 /jm in relief over the surface of the silicate matrix. This height suggests that the carbonaceous phases were not easily polishable by a diamond paste and would therefore imply higher polishing hardness. These high relief zones are surrounded by flat polished culets and powdery darker areas. The reflected light image (Fig. 1) shows that the polished surface of the high relief zone is irregularly scalloped with several gouges, granular, microtextured with a concentric spatial arrangement. The height varies within individual multiphase grains between 5 and 12 /jm. This is strongly suggestive of the presence of different coexisting transparent materials with different polishing hardness. Synchrotron XRF analysis showed that the area is composed of pure carbon and depleted in high Z elements (including Fe, Mn, Ni and Zn) in comparison with the surrounding silicate matrix. Fe- and Ni-bearing metal particles are sprinkled in the C-enclave and also occur in the underlying silicates. Four different carbon phases were identified by Raman spectroscopy (Fig. 2) : graphite, diamond (3C-polytype) and two so far unknown carbon phases. A similar spatial (concentric relationship between the different zones) and mineralogical arrangement of the carbonaceous phases is identified in the two carbonaceous areas. The low-relief lithology in the carbon-rich enclave (Figure 1, zone A) mainly consists of disordered graphite depicting small graphite D and D’ bands, a sharp G band spectrum[3, 4] and diamond projecting 2 /jm over it with its characteristic one-phonon band [15]. The high relief area (Figure 1, zone B) standing 8 /jm above the carbonaceous matrix exhibits a small Raman band of diamond which is shifted to 1322 cm-1 (characteristic of lonsdaleite or nanodiamonds) and a sharp G band indicative of ordered graphite. Finally, the very high relief zone (Figure 1, zone C) shows diamond and main graphite bands together with several additional peaks not attributable to any known carbon phase. The new phases of zone C are also mixed with diamond and ordered graphite. The concentric occurrence and the relationship between the different carbon polymorphs have to be interpreted in terms of phase transitions during the pressure and temperature variations throughout the shock event undergone by the meteorite. Synchrotron X-ray diffraction data recorded on the first carbonaceous area is consistent with the Raman data confirming the presence of graphite, diamond and a new phase (Fig 3 a, b). The analysis of the 2D diffraction images shows that diamond and graphite have strong
preferred orientations and that the graphite [00l] reflection is parallel to the diamond’s [111]. The X-ray pattern can be successfully indexedindicating pure iron, mainly coming from a lower level in the thin section, diamond 3C, secondary uncompressed graphite and a new diamond polytype, the 21R[27, 11, 28, 16]. Best fit for the 21R polytype was achieved for a R-3m space group and cell parameters of a=2.553(1)A and c=44.490(6) A for a volume of 251.19(4) A3 close to the theoretical predictions[25, 10, 23]. This polytype is observed for the first time in natural materials. In the second studied area of the sample, most of the diffraction patterns can be indexed as graphite, diamond and/or lonsdaleite. In the highest part of the carbonaceous inclusion the diffraction pattern confirmed the following species : compressed graphite (with lattice parameters a=2.454(1)A and c=6,372(1)A versus lattice parameters of secondary (or re-crystallised) graphite a=2.462(1)A and c=6.704(1)A), lonsdaleite and/or diamond (3C polytype), and a new phase. The new phase has a distinctly new diffraction pattern also different from the 21R polytype, thus confirming the occurrence of a new carbon polymorph. The unit cell contains four structural positions C1 (0.276, 0.276, 0.276), C2 (0.273, -0.256, -0.256), C3 (0., 0., 0.), C4 (0.5, 0.5, 0.). Although the quality of the available diffraction data is not sufficient for a full-profile structural refinement, processing of the diffraction pattern with fixed structural positions and optimisation occupancy factors indicates that the C3 and C4 positions are just partially filled which makes the density of the new phase intermediate between the densities of graphite and diamond. Figs. 3c, 3d show the diffraction patterns collected from a small area of the grain (5×7 /лm2) with a very high relief. In addition, to the diffraction lines of graphite and diamond (or lonsdaleite) the pattern reveals additional lines. While the lines at 2.055 ˚ 1.257Д 1.450Д and 1.185A are close to the lines of lonsdaleite and/or diamond, the other lines cannot be assigned to diamond or its polytype. The presence of the common texturing features on all intense diffraction lines (Fig. 3c) indicates that all of them belong to the same phase. The X-ray pattern was successfully indexed in terms of the Pm3m space group. Indeed, all lines indexed in the framework of a cubic primitive lattice with the cell parameter 3.559(4)A for a cell volume V = 45.08A ˚ . However, some reflections are asymmetric (particularly, the (111) reflection shown in the inset in Fig. 3d), while others (for example, (200)) are not. This means the actual symmetry of the new phase is lower than cubic and the best fit was obtained for a rhombohedral lattice (R3m space group) with the parameters a=3.5610(9) ˚ a=90.2(2)°. The proposed crystal structure of the new carbon polymorph is closely related to the structure of diamond : all carbon atoms are tetrahedrally coordinated, but slightly shifted from their symmetric positions in the diamond structure and two of the four structural positions are partially occupied. The presence of weak satellite reflections at 2.182A and 1.928A (if not coming from lonsdaleite) may also indicate a significant degree of disorder of the crystal structure.

4. Discussion

The observation of both compressed and uncompressed graphites and diamond show that a complex history and relationship exists between the different carbon phases. The existence of the compressed graphite strongly advocates for a shock event, which would entail the compression of pre-existing graphite. The dispersion of the FWHM and positions of the diamond Raman band [15] in different spots of the carbonaceous inclusions also strongly suggests a shock event. Using the average positions of the main Raman band of graphite and the value of its shift with pressure[21], we can calculate a residual stress of about 2.5 GPa. Similar stresses ranging from 2.5 to 4 GPa are inferred from the X-ray diffraction data using the equation of state of graphite[8]. A theoretical model of the diamond to graphite conversion[26] has shown that the topotaxial relationship between diamond (111) and graphite (002) could explain the formation of graphite islands inside the CVD diamonds[29] at temperatures higher than 2000 K. However, observations of graphitic islands inside CVD diamonds can be made for lower temperatures. More recently, Guillou et al. [7] showed that by static compression of black carbon at high-pressure and high-temperature, it is possible to obtain such a topotaxial relationship between diamond and graphite. The obtained product also exhibits very complex Raman spectra which are not observed when polycrystalline graphite or highly oriented pyrolitic graphite (HOPG) is used as a starting material. Even though, their Raman results do not exactly match the same bands as our samples, they show that the nature and P-T history of the diamond precursor material has a strong influence on the formation of diamond and other related materials. Therefore, a shock event on a graphitic precursor material ( whose residues are now present as primary compressed graphite) would have allowed to partially transform graphite to diamond 21R polytype and a new high-pressure phase of carbon. A high post-shock temperature entailed a partial back-transformation of diamond to the secondary uncompressed graphite observed in the sample. Carbon phases with “diamond-like” diffraction patterns but in space group Fd3m and forbidden reflections have already been described as products of shock-wave, ion implantation experiments with graphite or chemical treatments of diamond[9, 18, 13, 2]. The possible existence of a diamond-like structure with only partially occupied atomic positions (within the Fm3m space group) was also already discussed[17]. However, we present for the first time a structural model based on powder diffraction data of a natural carbon polymorph most likely intermediate between graphite and diamond. Note that the existence of such a phase has not hitherto been theoretically predicted[20, 22, 17]. The high polishing hardness of the new polymorph and the 21R polytype could be explained by the preferred orientations confirmed from the x-ray diffraction patterns and the heterogeneous hardness along the different crystallographic planes : highest hardness is at the (111) plane in analogy with 3C polytype. These observations of diamond 21R polytype and the new rhombohedral carbon polymorph show that the carbon system is far more complex than previously thought. The main information arising from these discoveries is that shocked carbon opens a new vision of the carbon system. Notwithstanding, the conditions to reproduce natural shock conditions cannot be reached experimentally since only nano- to microseconds shock durations can be achieved, while for natural objects, due to the significant size of the impactors, durations of shock events could be several seconds long[1]. Therefore, theoretical calculations have to show whether shock simulations can predict these new phases. The new carbon phases found in the Havero ureilite are thought to be metastable at ambient conditions and indeed formed at high-pressure in a purely dynamic event. They could represent intermediate states in the transformation of graphite to diamond. We maintain that the study of naturally shocked samples can greatly improve our knowledge of the carbon system since it gives access to unreachable areas of the phase space.

Acknowledgements The authors thank Gilles Montagnac for technical assistance with Raman spec-troscopy. We also thank Remi Tucoulou (ID22, ESRF) and Michael Hanfland (ID09, ESRF) for help during X-ray diffraction experiments. MATEIS Lab and A. Bogner is also thanked for providing access to the SEM facility and operating the environmental SEM. This project was supported by the ANR project ECSS.

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N dobs., A h к 1 dcalc, A I, % Phase

1	3,276	002		2,9	Gr
2	2,182	1 00		3,1	L
3	2,055	1 1 1	2,055	100	N*
4	1,928	1 0 1		2,3	L
5	1,780	200	1,780	7,2	N
6	1,451	2 1 1	1,543	5,1	N
7	1,257	2 2 0	1,258	3,7	N
8	1,185	3 0 0	1,186	14,2	N
9	1,072	3 1 1	1,073	10,8	N
10	1,024	2 2 2	1,027	2,7	N

Tab. 1: The X-ray pattern was successfully indexed in terms of the Pm3m space group. Indeed, all lines easily indexed in the framework of rhombohedral symmetry and lattice parameters a = 3.5610(9)A, a = 90.2(2)°. It was then successfully refined using the LeBail algorithm found in the GSAS package and the derived structural model was obtained from the Endeavour program.

Fig. 1: One of the carbonaceous areas in the Havero ureilite. The optical microscopy image (top right) and SEM image (center) of a typical carbonaceous area in Havero (area containing the 21R polytype). The SEM image shows the different heights in the carbonaceous area. The lower left inset shows a scheme of the spatial concentric arrangement of the different carbonaceous areas through a section represented by the solid white line on the SEM picture.

Fig. 2: Raman spectra obtained from the different carbon phases in the Haver¨o ureilite. (The labeling of the spectra corresponds to the areas indicated in Figure 1). Diamond with its typical one phonon band at 1331 cm-1 together with either secondary graphite and its characteristic G band at 1582 cm-1 or disordered graphite with D and G band at 1375 and 1572 cm-1 are observed. The two new phases exhibit bands belonging to diamond and graphite but also additional bands which are located at 442, 538, 1010, 1177, 1214, 1412, 1495 cm-1 for the 21R-polytype and some Raman bands at 336, 380, 468, 567, 750, 863, 1027, 1122, 1211, 1419, 1508, 1604 and 1700 cm-1 for the new carbon phase. Although some bands of the precited phases are usually attributed to classical fundamental and defect modes of graphite (1080, 1200, 1350 and 1500 cm-1), to lonsdaleite (1280 cm-1) or to domain size effects (580 cm-1), most of these bands have never been observed in any known carbon species.

Fig. 3: 2D (left) and integrated (right) diffraction patterns recorded in the highest relief zones of the two studied carbonaceous area. (a) The image of area 1 (Fig 1) at a wavelength of 0.412A (ID09, ESRF) shows a simultaneous directional increase of the intensities of the graphite (002) and diamond (111) lines suggesting a topotaxial preferred orientation relationship between the two phases. (b) The integrated pattern was successfully indexed as a mixture of graphite (G), diamond (D), 21R polytype of diamond (21R) and bcc-structured iron-nickel alloy (I), and further refined using the LeBail algorithm as implemented in the GSAS package. * stands for peaks originating from the lower silicate matrix. Blow-up inset figure at the top right shows the area around the diamond (111) peak where most of the 21R diffracting planes can be seen and identified unambiguously.(c) The image of area 2 at a wavelength of 0.6199 A (ID22, ESRF) shows diffraction lines attributed to the new super hard carbon polymorph (marked by “N” and indexed in pseudo-cubical settings), graphite (G), and reflection rings from lonsdaleite (L). The integrated diffraction pattern is interpreted as a mixture of diamond (3C polytype)(D), small amounts of compressed graphite and a new carbon phase (N) with rhombohedral symmetry and lattice parameters a = 3.5610(9)A, a = 90.2(2)°. A close-up view shows the asymmetry of the N(111) peak suggesting a splitting into N(111) and N(11-1) advocating for the rhombohedral symmetry.

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

Tristan FERROIR — Thu, 21 Dec 2006 11:41:30 +0000

NEW PUBLICATION IN SCIENCE

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.

Comet 81P/Wild 2 Under a Microscope

Tristan FERROIR — Thu, 21 Dec 2006 11:40:06 +0000

NEW PUBLICATION IN SCIENCE

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

Equation of state and phase transition in KAlSi3O8 hollandite at high pressure

Tristan FERROIR — Tue, 21 Mar 2006 11:38:06 +0000

NEW PUBLICATION IN AMERICAN MINERALOGIST

The tetragonal hollandite structure (KAlSi3O8 hollandite) has been studied up to 32 GPa at room temperature using high-pressure in-situ X-ray diffraction techniques. A phase transformation from tetragonal I4/m phase to a new phase was found to occur at about 20 GPa. This transition is reversible on release of pressure without noticeable hysteresis and hence this new high-pressure phase is unquenchable to ambient conditions. The volume change associated with the transition is found to be small (not measurable), suggesting a second order transition. The diffraction pattern of the high-pressure phase can be indexed in a monoclinic unit cell (space group I2/m), which is isostructual with BaMn8O16 hollandite. The » angle of the monoclinic unit cell increases continuously above the transition. A Birch-Murnaghan equation of state Þ t to pressure-volume data obtained for KAlSi3O8 hollandite yields a bulk modulus K0 = 201.4 (7) GPa with K’0 = 4.0.

A new high-pressure form of KAlSi3O8 under lower mantle conditions

Tristan FERROIR — Tue, 14 Dec 2004 11:30:39 +0000

NEW PUBLICATION

In situ X-ray diffraction measurements have been made on KAlSi3O8 hollandite using diamond anvil cell and multianvil apparatus combined with synchrotron radiation.
Both of the measurements with different techniques demonstrated that K-hollandite transforms to a new highpressure phase (hollandite II) at 22 GPa upon increasing pressure at room temperature. The X-ray diffraction peaks of the new phase were reasonably indexed on the basis of a monoclinic cell with I2/m space group. Hollandite II was also confirmed to be formed at high temperatures to 1200°C and pressures to 35 GPa, which was quenched to room temperature under pressure but converted back to hollandite at about 20 GPa on release of pressure. The present result is contradictory to earlier studies based mainly on quench method, which concluded that hollandite is stable up to 95 GPa at both room temperature and high temperatures up to 2300°C.

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Motion of a solid object through a pasty (thixotropic) fluid

Tristan FERROIR — Mon, 26 Jan 2004 11:35:37 +0000

NEW PUBLICATION IN PHYSCIS OF FLUIDS

For materials assumed to be simple yield stress fluids the velocity of an object should continuously increase from zero as the applied force increases from the critical value for incipient motion. We carried out experiments of fall of a sphere in a typical, thixotropic, pasty material ~a laponite suspension!. We either left a sphere falling in the fluid in different initial states of structure or vibrated the fluid in a given state of structure at different frequencies. In each case three analogous regimes appear either for increasing restructuring states of the fluid or decreasing frequencies: A rapid fall at an almost constant rate; a slower fall at a progressively decreasing velocity; a slow fall at a rapidly decreasing rate finally leading to apparent stoppage. These results show that the motion of an object, due to gravity in a pasty material, is a more complex dynamical process than generally assumed for simple yield stress fluids. A simple model using the basic features of the ~thixotropic! rheological behavior of these pasty materials makes it possible to explain these experimental trends. The fall of an object in such a fluid thus appears to basically follow a bifurcation process: For a sufficiently large force applied onto the object its rapid motion tends to sufficiently liquify the fluid around it so that its subsequent motion is more rapid and so on until reaching a constant velocity; on the contrary if the force applied onto the object is not sufficiently large the fluid around has enough time to restructure, which slows down the motion and so on until the complete stoppage of the object.