Frictional properties of natural kaolinite‐bearing gouge samples from the Median Tectonic Line (SW Japan) have been studied using a high‐velocity rotary shear apparatus, and deformed samples have ...been observed with optical and electron (scanning and transmission) microscopy. For a slip velocity of 1 m s−1 and normal stresses from 0.3 to 1.3 MPa, a dramatic slip‐weakening behavior was observed. X‐ray diffraction analysis of deformed samples and additional high‐velocity friction experiments on pure kaolinite indicate kaolinite dehydration during slip. The critical slip‐weakening distance Dc is of the order of 1 to 10 m. These values are extrapolated to higher normal stresses, assuming that Dc is rather a thermal parameter than a parameter related to a true characteristic length. The calculation shows that dimensionally, Dc ∝ 1/σn2, where σn is the normal stress applied on the fault. The inferred Dc values range from a few centimeters at 10 MPa normal stress to a few hundreds of microns at 100 MPa normal stress. Microscopic observations show partial amorphization and dramatic grain size reduction (down to the nanometer scale) localized in a narrow zone of about 1 to 10 μm thickness. Fracture energy Gc is calculated from the mechanical curves and compared to surface energy due to grain size reduction, and energies of mineralogic transformations. We show that most of the fracture energy is either converted into heat or radiated energy. The geophysical consequences of thermal dehydration of bonded water during seismic slip are then commented in the light of mineralogical and poromechanical data of several fault zones, which tend to show that this phenomenon has to be taken into account in most of subsurface faults and in hydrous rocks of subducted oceanic crust.
Primitive interplanetary dust is expected to contain the earliest solar system components, including minerals and organic matter. We have recovered, from central Antarctic snow, ultracarbonaceous ...micrometeorites whose organic matter contains extreme deuterium (D) excesses (10 to 30 times terrestrial values), extending over hundreds of square micrometers. We identified crystalline minerals embedded in the micrometeorite organic matter, which suggests that this organic matter reservoir could have formed within the solar system itself rather than having direct interstellar heritage. The high D/H ratios, the high organic matter content, and the associated minerals favor an origin from the cold regions of the protoplanetary disk. The masses of the particles range from a few tenths of a microgram to a few micrograms, exceeding by more than an order of magnitude those of the dust fragments from comet 81P/Wild 2 returned by the Stardust mission.
A series of polished and unpolished sp super(2)-nanostructured carbons "nanographites" obtained from the pyrolysis of various precursor types have been systematically studied by both Raman ...spectroscopy and x-ray diffraction. The ratio between the intensities of the disorder-induced D band and the first-order graphite G band (ID/IG) commonly used up to now to estimate the "crystallite" diameter La displays, in the case of polished graphitized sp super(2) carbons, clear spatial heterogeneities and can lead to the overestimation of the intrinsic structural disorder. The full width at half maximum of the G band, which is shown to be insensitive to the polishing process, exhibits a linear dependence on the mean "crystallite" diameter FWHM(G) = 14 + 430/La and therefore can be used for an accurate structural characterization of these nanographites.
We performed mineralogical and petrographic studies of three UltraCarbonaceous Antarctic Micrometeorites (UCAMMs) by analytical transmission electron microscopy (TEM). The UCAMMs were identified in ...the CONCORDIA micrometeorite collection (2002 and 2006) recovered from central Antarctic snow, and are of probable cometary origin. UCAMMs are dominated by disordered carbonaceous matter that extends over surfaces of up to ∼90% of the particle. Embedded in this carbonaceous matter, we observed small and complex assemblages of fine-grained mineral phases, isolated minerals, glassy phases that resemble Glass with Embedded Metal and Sulfides (GEMS) that were first found in Interplanetary Dust Particles (IDPs), and rounded objects containing both glass and crystalline materials. The mineral assemblages are chondritic in composition, within a factor of 2. Crystalline materials represent at least 25% of mineral phases. This value is much larger than the upper limit of crystallinity measured in the diffuse interstellar medium (<2.2
wt%). Crystalline phases are dominated by low-Ca, Mg-rich pyroxenes, Mg-rich olivine and low-Ni Fe-sulfides. Exotic phases such as Mn-, Zn-rich sulfide and perryite have also been found as accessory minerals. The variety of high temperature mineral phases observed in UCAMMs is similar to that reported in chondritic porous IDPs and 81P/Wild 2 samples. The close association of high temperature crystalline phases with the low temperature carbonaceous matter in UCAMMs supports the hypothesis of a large-scale radial mixing in the early solar nebula. This new type of carbon-rich micrometeorites containing crystalline material provides the opportunity to analyze
in situ, without any chemical processing, the association of materials formed at both low and high temperatures in the protoplanetary disk. A better knowledge of these samples is also important to prepare for future cometary missions, like ROSETTA in 2014.
The effect of flame temperature on soot properties was studied in premixed methane/oxygen flames burning at a constant mixture composition (C/O
=
0.60, Φ
=
2.4) and different cold-gas flow velocities ...(4 and 5
cm
s
−1). Temperature and concentration profiles of stable gases and condensed phases combustion products were measured along the flame axis. It was found that the high flame temperature conditions cause a larger decomposition of methane into hydrogen and C
2–C
4 hydrocarbons, thereby reducing the formation of benzene and condensed phases including condensed species and soot. Soot properties were studied by UV–Visible absorption spectroscopy, thermogravimetry and H/C elemental analysis. A description of soot nanostructural organization was also performed by means of high-resolution transmission electron microscopy. Different properties and nanostructures were found to develop in the soot, depending on the temperature and on soot aging associated. Soot dehydrogenation occurred to a larger extent in the high flame temperature conditions. As soot dehydrogenates the mass absorption coefficients of soot exhibited an increasing trend along the flame axis. However, mature soot retained a relatively high H/C ratio and low absorption coefficients with respect to other less hydrogenated fuels even in high temperature conditions. This indicates that the aromatization/dehydrogenation of soot in premixed flames is more dependent on the fuel characteristics rather than on the flame temperature.
Generally, it was assessed that mature soot produced from diverse hydrocarbon fuels with similar flame temperatures and flame types possess a different chemical composition and structure. To this regard the H/C atomic ratio and mass absorption coefficients appeared to be signatures of soot properties and structural evolution.
Our objective is to understand how graphite can be formed at “low” temperatures (<1200°C) in contrast to the high temperature of the industrial processes (∼3000°C), and from precursors which are ...non-graphitizable by a thermal treatment alone. Blends of iron and saccharose char were heated between 650 and 1600°C. The carbons obtained were characterized by SEM, TEM and Raman microspectrometry. Our work confirms that graphite can be formed from non-graphitizable carbons during a heat-treatment in the presence of iron. Carbon and iron migrations, below the eutectic temperature (1150°C), appear to be a key factor for carbon transformation. Iron migration and graphitization could be favored by nucleation of Fe nanoparticles and surface melting, detected as soon as 900°C. This allows formation of turbostratic macroporous carbons. Above the eutectic, all iron is liquid and graphitization occurs; it is complete at 1600°C. Heat-treatment duration, observed over 4 orders of magnitude, favors the structural improvement. Concerning applications in planetology these experimental samples are pertinent experimental analogues of natural carbons from differentiated parent-bodies (with an iron core), and explain how graphite can be formed at temperatures below 1200°C in these environments.
— We discuss the relationship between large cosmic dust that represents the main source of extraterrestrial matter presently accreted by the Earth and samples from comet 81P/Wild 2 returned by the ...Stardust mission in January 2006. Prior examinations of the Stardust samples have shown that Wild 2 cometary dust particles contain a large diversity of components, formed at various heliocentric distances. These analyses suggest large‐scale radial mixing mechanism(s) in the early solar nebula and the existence of a continuum between primitive asteroidal and cometary matter. The recent collection of CONCORDIA Antarctic micrometeorites recovered from ultra‐clean snow close to Dome C provides the most unbiased collection of large cosmic dust available for analyses in the laboratory. Many similarities can be found between Antarctic micrometeorites and Wild 2 samples, in terms of chemical, mineralogical, and isotopic compositions, and in the structure and composition of their carbonaceous matter. Cosmic dust in the form of CONCORDIA Antarctic micrometeorites and primitive IDPs are preferred samples to study the asteroid‐comet continuum.
A structural description of young and mature soot sampled from methane, ethylene, cyclohexane and benzene premixed flames, burning in fuel-rich conditions, was performed by means of high-resolution ...transmission electron microscopy (HRTEM). HRTEM image analysis was carried out to achieve semi-quantitative information about the soot organization. Different nanostructures were found to develop in the soot, depending on the fuel and on soot aging.
The soot characteristics were also investigated by elemental analysis, oxidative thermogravimetry and UV–Vis spectroscopy.
Soot nanostructure in terms of the length of the graphitic layers, the size of the coherent domains made of stacked layers was related in some way with UV–Vis spectroscopic characteristics, H/C molar ratio, and oxidation reactivity. This demonstrated that there is a structure–property relationship between soot nanostructure and bulk properties.
The different spectral features and chemical properties of soot in dependence on the fuel nature and on aging resulted to be related to a different internal structure of primary particles which in turn derives from a different soot inception mechanism occurring in flames more or less rich in aromatic precursors.
Geothermometers based on Raman spectrometry of carbonaceous matter and covering a wide range of temperatures (100–650 °C) have been developed over recent years. While Raman data have been largely ...interpreted in terms of temperature, they are also the fingerprint of certain metamorphic conditions, especially in the low temperature range relevant to poorly ordered carbonaceous matter. This study investigates the Raman spectra of two series of chondritic carbonaceous matter and coal samples formed from different precursors and under different metamorphic conditions.
The Raman spectra of Polyaromatic Carbonaceous Matter (PCM) from 42 chondrites and 27 coal samples, measured with visible (514 nm) and ultra-violet (244 nm) excitation wavelengths, are analyzed. The Raman spectra of low rank coals and chondrites of petrologic types 1 and 2, which contain the more disordered PCM, reflect the distinct carbon structures of their precursors. The 514 nm Raman spectra of high rank coals and chondrites of petrologic type 3 exhibit continuous and systematic spectral differences reflecting different carbon structures present during the metamorphism event. They result from differences in the chemical structures of the precursors concerning for instance the reticulation of polyaromatic units or an abundance of ether functional groups, or possibly from a lack of carbonization processes to efficiently expel oxygen heteroatoms, due to weak lithostatic pressure and confinement. These results suggest that the use of low temperature carbon thermometers should be restricted to a given geological context. At the same time, the sensitivity of Raman spectra to precursors and certain metamorphic conditions could be used to obtain information other than temperature.
The analysis also provides evidence of the accretion of
relatively homogeneous PCM precursors among ordinary CO and CV carbonaceous chondrite parent bodies, given that the 514 nm Raman spectra of PCM efficiently trace the metamorphism grades. Looking closer, however, the 514 nm Raman data are more scattered in chondrites than in coals and the maturity tracers are less sensitive and miscorrelate with the atomic H/C ratio, suggesting slight compositional and structural differences among the PCM precursors accreted.
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