Bertrand Devouard

Icosahedral packing of B12 icosahedra in boron suboxide (B6O)

Objects with icosahedral symmetry (Ih) bear a special fascination; natural examples are rare, but include radiolaria and virus particles (virions). The discovery of C60, a molecule in the shape of a truncated icosahedron with Ih symmetry, has aroused widespread interest. In 1962, Mackay described a radiating packing of spheres in Ih symmetry, in which the centres of successive shells of spheres lie on the surfaces of icosahedra. There has been extensive investigation of the conditions under which such packing might be realized in assemblies of atoms or of molecules such as C60 (ref. 5). Here we report the preparation, at high temperatures and pressures, of boron suboxide (B6O) in which the preferred form of the material is as macroscopic, near-perfect, regular icosahedra, similar to the multiply-twinned particles observed in some cubic materials. A major difference is that B6O has a rhombohedral structure that nearly exactly fits the geometrical requirements needed to obtain icosahedral twins. These icosahedral particles have a structure that can be described as a Mackay packing of icosahedral B12 units, and thus has long-ranged order without translational symmetry.

Magnetite morphology and life on Mars

Nanocrystals of magnetite (Fe3O4) in a meteorite from Mars provide the strongest, albeit controversial, evidence for the former presence of extraterrestrial life. The morphological and size resemblance of the crystals from meteorite ALH84001 to crystals formed by certain terrestrial bacteria has been used in support of the biological origin of the extraterrestrial minerals. By using tomographic and holographic methods in a transmission electron microscope, we show that the three-dimensional shapes of such nanocrystals can be defined, that the detailed morphologies of individual crystals from three bacterial strains differ, and that none uniquely match those reported from the Martian meteorite. In contrast to previous accounts, we argue that the existing crystallographic and morphological evidence is inadequate to support the inference of former life on Mars.

Aqueous alteration in the Northwest Africa 817 NWA 817 Martian meteorite

Samples of a new Martian meteorite of the nakhlite family (NWA 817) contain traces of an iron-rich alteration product. Textural arguments indicate that this alteration product has been formed on the parent body of the meteorite (Mars). The chemical composition and structural data (X-ray diffraction, transmission electron microscopy and vibrational spectroscopy) show that the alteration mineral is a hydrous phase from the smectite family. Major elements and rare earth elements suggest that the formation of the alteration phase is related to the circulation of an aqueous fluid which composition is controlled by the dissolution of feldspars to account for a positive Eu anomaly, olivine and possibly apatite. Hydrogen isotope data display negative deltaD values ranging from -60 to -280parts per thousand in olivine and pyroxenes and from -140 to -181parts per thousand in the alteration phase. The values of deltaD for the alteration product show a small scatter with a mean value of -170 +/- 14parts per thousand. These values are lower than those previously obtained on other Martian meteorites, which give mainly positive deltaD values. These positive values have been interpreted as resulting from the interaction of the Martian meteorites with water from the Martian atmosphere. Ruling out the effect of terrestrial alteration, it is suggested that alteration in the NWA 817 meteorite was likely produced on Mars by the circulation of an aqueous fluid originating from a chemical reservoir, such as the Martian mantle, which has not equilibrated with a fractionated Martian atmosphere

Serpentinites from Central Cuba petrology and HRTEM study

We have sampled serpentinites and the closely associated metabasites in the paleosubduction zone context of Central Cuba in order to characterise the microstructures of serpentine minerals as a function of metamorphic grade. The samples were collected in the eclogitic unit of the Escambray Massif and in the Zaza Zone, where eclogitic conditions were locally attained. Serpentinites are associated to the metabasites in lenses embedded in a metasediment matrix (Escambray) or form the matrix that embeds the metabasites (Zaza Zone). Field and petrological evidence suggests that serpentinites and associated metabasites underwent the same metamorphic history. Serpentinites from the Escambray Massif have preserved high-grade structures. In the Zaza Zone, most of the sampled serpentinites only underwent low-grade metamorphism and the sample that underwent eclogitic conditions is strongly retrogressed. Low-grade serpentinites show pseudomorphic textures in thin section, a mixture of chrysotile, poorly crystallised serpentine and minor lizardite. High-grade samples, characterised by non-pseudomorphic textures, mainly consist of antigorite, associated to minor chrysotile. Transmission electron microscopy shows that antigorite from preserved samples (Escambray Massif) displays few microstructural defects such as stacking faults or modulation dislocations, in contrast to the higher density of defects encountered in highly retrogressed serpentinites (Zaza Zone). We thus propose that the record of high metamorphic grade in matrix antigorite is best characterised by the elimination of structural defects.

Crystal structure of kanemite, NaHSi2O5•3H2O, from the Aris phonolite, Namibia

Abstract Kanemite was studied by single-crystal X-ray diffraction. The mineral, ideally NaHSi2O5·3H2O, is orthorhombic (space group Pbcn); unit-cell parameters are a = 4.946(3), b = 20.502(15), c = 7.275(3) Å, with Z = 4. The structure is solved and refined to an R value of 0.058 for 825 independent reflections. The arrangement of atoms consists of alternating (010) sheets of corrugated [Si2O4OH]nn- and hydrated Na. The silicate sheets contain six-membered rings of HOSiO3-SiO4 units. Sodium atoms coordinate to six water molecules, forming layers of distorted octahedra. Residual electron densities were located that give reasonable positions for four H atoms. One H is part of a silanol group, and the other three H atoms are associated with water bonded to Na. Bonding between the silicate and Na sheets is through hydrogen bonding from H of the Na layer to O of the silicate sheet.

Serpentinites in an Alpine convergent setting: Effects of metamorphic grade and deformation on microstructures.

Alpine antigorite serpentinites associated with eclogites were investigated to determine if they can be used as indicators of the tectono-metamorphic conditions during subduction and exhumation processes. The detailed petrology of serpentinites sampled in the Monviso massif (Western Alps, Italy) was combined with a transmission electron microscopy study. Alpine serpentinites display a degree of serpentinization close to 100%. Antigorite is the main mineral present, forming non-pseudomorphic textures in the various studied samples and exhibiting a homogeneous chemical composition with limited cationic substitutions. Considering its oceanic origin, the Alpine serpentinite in the Monviso massif formed a lizardite + chrysotile assemblage that recrystallized under greenschist-facies conditions into poorly ordered antigorite, with a modulation wavelength showing significant variations at the crystal scale. Under blueschist-facies conditions, the modulation wavelength of antigorite becomes regular. Thus, periodic antigorites can be related to high-grade conditions, while poorly ordered antigorites characterize lower metamorphic grade. In the present study, we failed to observe any elimination of structural defects with increasing metamorphic grade. While around 50% of the antigorite crystals are highly ordered, it seems that this ordering is at least partly obliterated by retrogressive deformation. Antigorite displays strong evidence of deformation-sensitivity, and the observed microstructures can be directly related to the mechanical behaviour of serpentinites in subduction zones. We investigated the deformation-induced microstructures in serpentinites collected in the Erro-Tobbio eclogitic unit (Ligurian Alps, Italy), which appear to preserve prograde and retrograde structures formed during subduction. According to the microstructural evidence, shearing is accommodated by brittle and/or ductile deformation mechanisms. Collected samples were fractured at different scales (cm to nm) and have a well-developed schistosity characterized by a strong crystallographic fabric. With increasing metamorphic grade, the brittle behaviour gives way to pressure-solution, which persists up to eclogite-facies conditions. The common obliteration of high-grade microstructures in antigorite, as observed in the Monviso serpentinites, results from continuous recrystallization of this mineral during retrogressive deformation.

Pseudotachylyte (Frictionite) at the Base of the Arequipa Volcanic Landslide Deposit (Peru): Implications for Emplacement Mechanisms

The Arequipa volcanic landslide deposit (Peru) displays a pseudotachylyte (or “frictionite”) at its basal contact with an older ignimbrite. The pseudotachylyte consists of a dark 1‐cm‐thick vitreous layer that shows striations on its upper surface. It contains small mineral clasts from both the landslide rocks and the underlying ignimbrite embedded in an optically isotropic matrix. TEM observations revealed a clast‐melt suspension with a lower black obsidian‐like vein made of chemically heterogeneous glass. The pseudotachylyte is interpreted as the result of partial melting due to frictional heating during landslide emplacement. This is the third pseudotachylyte reported at the base of a landslide deposit and the first in a volcanic environment. Calculations show that basal melting only accounts for a small proportion of the total energy released during landslide emplacement, indicating that landslide transport was dominated by dissipative stresses affecting the whole landslide body and not concentrated only in a narrow basal layer. Melting most likely occurred during a short frictional episode that marked the transition from nondepositional to depositional conditions.

Topology and crystal growth of natural chrysotile and polygonal serpentine

Abstract Through detailed HRTEM examination of a number of serpentinite specimens, the microstructures of textures and individual chrysotile and polygonal fibers are reviewed and their probable growth processes anticipated. It is recalled that five-fold symmetry is ubiquitous among these fibers and that their axial diffraction pattern is insensitive to their deformation or unequal development. 15- and 30-sectored fibers of polygonal serpentine are described by a fan-like arrangement of flat lizardite sectors, with radiating sector boundaries occupied by arrays of partial dislocations. In each case, the layer stacking sequence varies from one sector to the next. Crystal growth of chrysotile probably occurs by thick, curved sectors closing up as tubules, which subsequently develop to a critical size controlled by local storage of elastic energy. For a strict elastic control we may envision the use of chrysotile geometry as an indicator of departure from equilibrium. A solid-state transformation, chrysotile-to-polygonal serpentine, is likely to be driven by the release of elastic energy stored in chrysotile. Diameter growth of polygonal serpentine is no longer limited by elasticity constraints, so allowing “giant” size and tight intergrowth of those fibers.

Sectors in polygonal serpentine. A model based on dislocations

Based on coexisting rolled chrysotile and polygonal serpentine fibers with 15 or 30 sectors each, a crystallographic model for polygonization of chrysotile is proposed. It is based on an assumed chrysotile-to-lizardite transition. Polygonization of chrysotile requires more likely 15 partial dislocations per turn, as required by polytype translational operators for serpentines. The observed number of sectors corresponds to the two most elastically stable arrays of dislocations. Homogeneous shear of the layer stacking arising from intersector kinking results in a cyclic distribution of twins and/or different polytypes. This makes the fiber axis a fivefold symmetry axis and consequently polygonal serpentine and chrysotile to be both forms of serpentine with local fivefold symmetry. This model is alternative to the recent crystallograpic model by Chisholm (1991, 1992).

Onion morphology and microstructure of polyhedral serpentine

Abstract We describe the shape and internal structure of polyhedral spheroids found in serpentinized peridotites. Serpentine spheroids resemble geodesic domes made of ~160 to 180 triangular facets. At facet edges, the nested layers bend by ~14° along their three <010> crystallographic directions, resulting in an onion-like structure with lateral continuity of the layers. The stacking of the serpentine layers within sectors is controlled by interlayer bonding. These polyhedral onions correspond to a novel type of spherical nanostructure for layered materials.