François Guyot

Thermal equation of state of CaSiO3 perovskite

A comprehensive pressure-volume-temperature data set has been obtained for CaSiO3 perovskite up to 13 GPa and 1600 K, using synchrotron X ray diffraction with a cubic-anvil, DIA-6 type apparatus (SAM-85). For each volume measurement, nonhydrostatic stress is determined from the relative shift in the diffraction lines of NaCl, within which the sample was embedded. Heating to above 973 K greatly reduced the strength of NaCl (to below 0.05 GPa), making the measurements hydrostatic. At room temperature the cubic perovskite structure remains metastable at pressures as low as 1 GPa, below which the sample transforms into an amorphous phase as indicated by a large background, a marked decrease in diffraction signals, and an anomalous volume decrease of the remaining crystalline phase. Because our experimental uncertainties are significantly smaller than those in previous measurements, the new data provide a tighter constraint on the zero pressure bulk modulus for CaSiO3 perovskite. A new set of room temperature equation of state parameters are identified so that both our data and the diamond cell data of Mao et al. [1989] are compatible [KT0 = 232(8) GPa, K′T0 = 4.8(3), and V0 = 45.58(4) A3]. Volume measurements along several isotherms under both stable and metastable pressure conditions allow isochoric and isobaric interpolations within the range of experimental pressure and temperature conditions. Analyses using various approaches yielded consistent results for (∂KT/∂T)P of −0.033(8) GPa K−1, and (∂α/∂P)T of −6.3 × 10−7 GPa−1 K−1, with a zero-pressure thermal expansion α0 of 3.0 × 10−5 K−1. The thermal pressure is found to be virtually independent of volume, and thus the Anderson-Gruneisen parameter δT = K′T0 = 4.8. These results are used to predict the bulk modulus and density of CaSiO3 perovskite under lower mantle conditions. Along an adiabat with the foot temperature of 2000 K, the density of the perovskite agrees with that of the preliminary reference Earth model (PREM) within 1% throughout the lower mantle. The bulk modulus shows a smaller pressure dependence along the adiabat; it matches that of PREM at the top of the lower mantle but is about 10% too low near the core-mantle boundary.

Bacterial diversity and carbonate precipitation in the giant microbialites from the highly alkaline Lake Van, Turkey

Lake Van harbors the largest known microbialites on Earth. The surface of these huge carbonate pinnacles is covered by coccoid cyanobacteria whereas their central axis is occupied by a channel through which neutral, relatively Ca-enriched, groundwater flows into highly alkaline (pH ~9.7) Ca-poor lake water. Previous microscopy observations showed the presence of aragonite globules composed by rounded nanostructures of uncertain origin that resemble similar bodies found in some meteorites. Here, we have carried out fine-scale mineralogical and microbial diversity analyses from surface and internal microbialite samples. Electron transmission microscopy revealed that the nanostructures correspond to rounded aragonite nanoprecipitates. A progressive mineralization of cells by the deposition of nanoprecipitates on their surface was observed from external towards internal microbialite areas. Molecular diversity studies based on 16S rDNA amplification revealed the presence of bacterial lineages affiliated to the Alpha-, Beta- and Gammaproteobacteria, the Cyanobacteria, the Cytophaga-Flexibacter-Bacteroides (CFB) group, the Actinobacteria and the Firmicutes. Cyanobacteria and CFB members were only detected in surface layers. The most abundant and diverse lineages were the Firmicutes (low GC Gram positives). To the exclusion of cyanobacteria, the closest cultivated members to the Lake Van phylotypes were most frequently alkaliphilic and/or heterotrophic bacteria able to degrade complex organics. These heterotrophic bacteria may play a crucial role in the formation of Lake Van microbialites by locally promoting carbonate precipitation.

X-ray microanalysis of high-pressure/high-temperature phases synthesized from natural olivine in a diamond-anvil cell

The transformation products of natural (Mg0.83Fe0.17)2SiO4 olivine have been prepared at high pressures and high temperature in a laser-heated diamond-anvil cell (DAC). The study of the quenched high-pressure phases has been performed using analytical transmission electron microscopy. The iron/magnesium partition coefficient between perovskite (pv) and magnesiowustite (mw), defined as Kmw − pv = χFe/χMg)mw/χFe/χMgpv, was measured in samples synthesized at pressures from 25 to 75 GPa under various heating conditions. In all cases, iron goes preferentially into magnesiowustite. It was observed, however, that the partition coefficient decreases strongly between 25 and 40 Gpa. At pressures higher than 40 GPa, it is constant (3.5) within experimental error. Although there is a systematic increase in K as T decreases, the effect of temperature is weak at high pressures. From the values of the iron partition coefficient and previously existing thermodynamic values, we obtain a consistent thermodynamic data set for the major constituents of the lower mantle. The equilibrium compositions of (Mg,Fe)2SiO4 spinel, (Mg,Fe)SiO3 perovskite and (Mg,Fe)O magnesiowustite phases have been measured and a new high-pressure phase of composition (Mg,Fe)Si2O5 is shown to occur in very high-temperature regions, probably due to high-temperature breakdown or non-stoichiometry of (Mg,Fe)SiO3 perovskite.

Twinning in MgSiO3 Perovskite

Crystals of MgSiO3 perovskite synthesized at high pressures and temperatures have orthorhombic symmetry under ambient conditions. Examination by transmission electron microscopy shows that the microstructure of crystals synthesized at 26 gigapascals and 1600�C is dominated by a large number of twin domains that are related by reflection operations with respect to {112} and {110} planes. These twins may be associated with the transformations of MgSiO3 perovskite from the cubic to tetragonal and tetragonal to orthorhombic phases, respectively, upon decreasing pressure and temperature. These observations suggest that under the experimental synthesis conditions, and perhaps in the earths lower mantle, the stable phase of MgSiO3 might have the cubic perovskite structure.

Quasi-harmonic computations of thermodynamic parameters of olivines at high-pressure and high-temperature. A comparison with experiment data

Specific volumes of forsterite and San Carlos olivine have been measured by in-situ X-ray diffraction at simultaneously high pressure and high temperature up to 7 GPa and 1300 K in a cubic-anvil press coupled to synchrotron radiation. No difference could be evidenced between the thermal pressure of the two compounds. It has been shown that the parameter alpha K-T is volume-independent within the volume range investigated in this study for both forsterite and San Carlos olivine. A quasi-harmonic calculation of the high-pressure high-temperature specific volumes of forsterite has been performed and shown to be in good agreement with the experimental results. Then, a self consistent model of all the thermodynamic functions of forsterite was constructed, the input consisting of the experimental ultrasonic and vibrational spectroscopic data exclusively. The model accurately reproduces not only the experimental P-V-T data measured in this study, but also the high-temperature, 1 bar thermal expansion, the adiabatic incompressibility, the constant pressure specific heat and the entropy measurements, without using other a priori information.

Cr(VI) detoxification by Desulfovibrio vulgaris strain Hildenborough: microbe–metal interactions studies

Toxic heavy metals constitute a worldwide environmental pollution problem. Bioremediation technologies represent efficient alternatives to the classic cleaning-up of contaminated soil and ground water. Most toxic heavy metals such as chromium are less soluble and toxic when reduced than when oxidized. Sulfate-reducing bacteria (SRB) are able to reduce heavy metals by a chemical reduction via the production of H2S and by a direct enzymatic process involving hydrogenases and c3 cytochromes. We have previously reported the effects of chromate [Cr(VI)] on SRB bioenergetic metabolism and the molecular mechanism of the metal reduction by polyhemic cytochromes. In the current work, we pinpoint the bacteria–metal interactions using Desulfovibrio vulgaris strain Hildenborough as a model. The bacteria were grown in the presence of high Cr(VI) concentration, where they accumulated precipitates of a reduced form of chromium, trivalent chromium [Cr(III)], on their cell surfaces. Moreover, the inner and outer membranes exhibited precipitates that shared the spectroscopic signature of trivalent chromium. This subcellular localization is consistent with enzymatic metal reduction by cytochromes and hydrogenases. Regarding environmental significance, our findings point out the Cr(VI) immobilization mechanisms of SRB; suggesting that SRB are highly important in metal biogeochemistry.

Multidisciplinary evidences that Synechocystis PCC6803 exopolysaccharides operate in cell sedimentation and protection against salt and metal stresses.

Little is known about the production of exopolysaccharides (EPS) in cyanobacteria, and there are no genetic and physiological evidences that EPS are involved in cell protection against the frequently encountered environmental stresses caused by salt and metals. We studied four presumptive EPS production genes, sll0923, sll1581, slr1875 and sll5052, in the model cyanobacterium Synechocystis PCC6803, which produces copious amounts of EPS attached to cells (CPS) and released in the culture medium (RPS) as shown here. We show that sll0923, sll1581, slr1875 and sll5052 are all dispensable to the growth of all corresponding single and double deletion mutants in absence of stress. Furthermore, we report that sll0923, sll1581 and slr1875 unambiguously operate in the production of both CPS and RPS. Both sll1581 and slr1875 are more important than sll0923 for CPS production, whereas the contrary is true for RPS production. We show that the most EPS-depleted mutant, doubly deleted for sll1581 and slr1875, lacks the EPS mantle that surrounds WT cells and sorbs iron in their vicinity. Using this mutant, we demonstrate for the first time that cyanobacterial EPS directly operate in cell protection against NaCl, CoCl2, CdSO4 and Fe-starvation. We believe that our EPS-depleted mutants will be useful tools to investigate the role of EPS in cell-to-cell aggregation, biofilm formation, biomineralization and tolerance to environmental stresses. We also suggest using the fast sedimenting mutants as biotechnological cell factories to facilitate the otherwise expensive harvest of the producer cell biomass and/or its separation from products excreted in the growth media.

Electron microscopy of high-pressure phases synthesized from natural olivine in diamond anvil cell

The products of the transformation of natural (Mg0.83Fe0.17)2SiO4 olivine have been prepared at various high pressures (between 25 GPa and 90 GPa), and high temperature in a laser-heated diamond-anvil cell (DAC). Studies of the high-pressure phases have been made by transmission electron microscopy (TEM), and X-ray microanalysis.The olivine/spinel boundaries exhibit all the characteristics of a diffusionless shear transition, having a finely sheared structure and a constant orientation relationship between the close-packed planes of the two structures ((100)ol∥(111)sp).The TEM observations of zones where olivine (or spinel) transforms into post-spinel phases show that the transformation possesses the features of an eutectoïdal decomposition, leading to a lamellar intergrowth of magnesiowüstite (Mg,Fe)O and perovskite (Mg,Fe)SiO3. With increasing temperature and/or decreasing pressure, the grain size of the high-pressure phases increases and obeys an Arrhenius law with an activation volume equal to zero. (Mg,Fe)O grains exhibit a very high density of dislocations (higher than 1011cm−2), whereas (Mg,Fe)SiO3 grains exhibit no dislocations but systematic twinning. The composition plane of the twins is (112) of the GdFeO3-type perovskite, corresponding to the {110} plane of the cubic lattice of ideal perovskite.

Comparison of the raman microprobe spectra of (Mg, Fe)2SiO4 and Mg2GeO4 with olivine and spinel structures

Raman microprobe (RMP) spectra were produced for each of the olivine and spinel structured phases of Mg2GeO4 and (Mg, Fe)2SiO4.The assembled data show that bands due to the tetrahedra in silicate and germanate olivines shift in a way that indicates a dominant mass effect. This correspondence is difficult to make in spinels due to differences in structural type. Differences in Fe/Mg content of olivine shift the tetrahedral vibration bands only slightly, but their linear shifts could be used to indicate the composition of the phase.

A thermodynamic model for MgSiO3-perovskite derived from pressure, temperature and volume dependence of the Raman mode frequencies

Raman spectra of MgSiO3-perovskite (Mg-pv) were recorded at simultaneous high-pressure and low-temperature conditions. This allowed to estimate characteristic frequencies (nu(i)) and other mode parameters as a function of both pressure and temperature. The cross derivatives partial derivative(2)nu(i)/partial derivative T partial derivative P were measured for the first time, thus providing new insights into the lattice dynamics of Mg-pv. These parameters are negative for the two lowest frequency modes at 250 and 255 cm(-1) (approximate to-6 X 10(-4) cm(-1) GPa(-1) K-1) and positive for the other modes (+3 X 10(-4) to +5 X 10(-4) cm(-1) GPa(-1) K-1). These measurements were combined with previously published vibrational density of states (VDoS) for deriving entropy, specific heat, thermal pressure, equation of state (EoS), and various thermoelastic parameters of Mg-pv at mantle P-T conditions. The calculations were performed using a general anharmonic formulation including the spectroscopically measured parameters, It is shown that anharmonic effects are relatively small in this compound under geophysically relevant conditions especially for the EoS. The model is used to discuss the discrepancies in the pressure and temperature calibrations in diamond-anvil cells and multianvil presses. Finally, a complete thermodynamic data set for (Mg0.9Fe0.1)SiO3-perovskite is proposed along lower mantle geotherms, It is shown that a pure perovskite lower mantle is unlikely to exist