Showing papers by "IPG Photonics published in 1995"

Low Z elements (Mg, Al, and Si) K-edge X-ray absorption spectroscopy in minerals and disordered systems

Abstract: Soft X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy have been performed at the Mg-, Al- and Si-K edges in order to establish the ability of this spectroscopy to derive structural information in disordered solids such as glasses and gels. Mg- and Al-K XANES are good structural probes to determine the coordination state of these elements in important minerals, glasses and gels. In a CaO MgO 2SiO2 glass Mg XANES spectra differ from that found in the crystalline equivalent, with a significant shift of the edge maxima to lower energy, consistent with a CN lower than 6. Mg-EXAFS on the same sample are in agreement and indicate the presence of 5-coordinated Mg with Mg O distances of 2.01A. In aluminosilicate gels, Al K XANES has been used to investigate the [4]Al/Altotal ratios. These ratios increase as the Al/Si ratios decrease. Aluminosilicate and ferric-silicate gels were studied by using Si K edge XANES. XANES spectra differ significantly among the samples studied. Aluminosilicate gels with Al/Si= 1 present a different Al and Si local environment from that known in clay minerals with the same Al/Si ratio. The gel-to-mineral transformation thus implies a dissolution-recrystallization mechanism. On the contrary, ferric-silicate gel presents a Si local environment close to that found in nontronite which may be formed by a long range ordering of the initial gels.

A Pliocene-Quaternary compressional basin in the Interandean Depression, Central Ecuador

Abstract: SUMMARY The segment of the Interandean Depression of Ecuador between Ambato and Quito is characterized by an uppermost Pliocene-Quaternary basin, which is located between two N-S trending reverse basement faults: the Victoria Fault to the west, and the Pisayambo Fault to the east. The clear evidence of E-W shortening for the early Pleistocene (between 1.85 and 1.21 Ma) favours a compressional basin interpretation. The morphology (river deviations, landslides, folded and flexure structures) demonstrates continuous shortening during the late Quaternary. The late Pliocene-Quaternary shortening reached 3400 f 600 m with a rate of 1.4 f 0.3 mm yr-'. The E-W shortening is kinematically consistent with the current right-lateral reverse motion along the NE-SW trending Pallatanga Fault. The Quito-Ambato zone appears to act as a N-S restraining bend in a system of large right-lateral strike-slip faults. The compressive deformation. which affects the Interandean Depression during the Pliocene is apparently coeval to the beginning subduction of very young oceanic lithosphere north of the Gulf of Guayaquil. The relatively buoyant new crust may have significantly increased the mechanical coupling in the subduction zone from Pliocene to Present.

Mineralogy and chemistry of solid aerosols emitted from Mount Etna

Abstract: Aerosols were sampled at Mount Etna over a five-month period in 1985, enclosing the March-July south upper flank eruption. Collection was performed both at effusive vents by manual sampling during the eruption, and in the summit plume by means of an automatic collector during and after the eruption. The automatic collector allows a regular sampling in the difficult-to-reach summit area. Aerosols were analysed for S, Cl, and some metallic elements by 3 complementary analytical techniques (XRF, PIXE and AAS). Aerosol mineralogy has also been investigated by means of SEM. The analysed elements are emitted with enrichments coherent with the known models dealing with their degree of volatility in magmatic conditions. Sulfuric acid-particle reactions in the summit plume and high-temperature fractionation phenomena cause large discrepancies in the mineralogy and chemistry of particles emitted from the summit craters and lateral vents.

Al-O - -Al paramagnetic defects in kaolinite

Abstract: Trapped holes located on Al-O-Al bonds in kaolinite were studied by electron paramagnetic resonance spectroscopy (EPR) at 9.3 and 35 GHz applied to well-crystallized, X-ray irradiated and oriented samples. The Q-band EPR spectrum is characterized by three clearly separated groups of 11 quasi-equidistant superhyperfine lines centered at gxx=2.040±0.0005, gyy=2.020±0.0005 and gzz=2.002±0.001. In each of these groups, the 11 superhyperfine lines exhibit intensities according to the ratios 1∶2∶3∶4∶5∶6∶5∶4∶3∶2∶1. An angular dependence of the Q-band EPR spectrum with respect to the magnetic field is demonstrated by measurements on oriented films of kaolinite. An appropriate numerical treatment of the EPR spectra is described, which allowed extraction of the SuperHyperfine Structures (SHFS). X-and Q-band spectra have also been simulated. It is concluded from these experiments that only one type of center is present. This center, labelled the B-center in the literature, is very probably a hole trapped on oxygen (O- center) atoms coupled to two octahedral aluminium.

Cations in glasses under ambient and non-ambient conditions

Abstract: Structural data on the short-and medium-range structure of multicomponent oxide glasses may be obtained by taking advantage of the chemical selectivity of X-ray Absorption Spectroscopy (XAS). Ni K edge XAS in silicate glasses shows that this element occurs in 4- and 5-coordination, the ratio between the two coordination states being composition dependent. [4] Ni is bound to the silicate network, with large alkalis completing charge-compensation. Coordination changes from [6] Ni to [5] Ni and [4] Ni are also observed in borate glasses with increasing potassium content. Low alkali glasses show a medium-range structure consistent with the grafting of nickel octahedra onto boroxol rings. Zirconium is another element which shows a coordination change from [6] Zr to [8] Zr as a function of glass composition. The structural environment of glass components may be modeled using the interatomic distances derived from EXAFS and a bond valence model derived from the Pauling rules. XAS experiments may also be performed in situ at high pressure or high temperature using synchrotron radiation. High pressure EXAFS spectra on germanate glasses show a coordination change of germanium from 4 to 6 with increasing pressure. These modifications are reversible in glasses, which illustrates the need for in situ measurements. High temperature EXAFS and XANES show also a coordination change of Ni during the melting of sodium silicate glass, with an increase of the [4] Ni content in the melt.

A new Late Triassic-Liasic Palaeomagnetic pole from superimposed and juxtaposed magnetizations in the Saharan craton

Abstract: SUMMARY Palaeomagnetic measurements, in the Late Triassic-Liasic Zarzaytine formations of the La Recul6e area, showed the existence of superimposed and juxtaposed magnetizations. The separation of the juxtaposed magnetizations yielded Late Triassic-Liasic magnetization directions of normal and reversed polarities. The new corresponding palaeomagnetic pole agrees well with a Pangea A configuration. It confirms, from Autunian to Lias, a latitudinal displacement of stable Africa towards the north, with important anti-clockwise rotation contrary to Laurussia which did not undergo significant rotation. This transition of Pangea from A2 to A ended before the Late Triassic. It corresponds to dextral movements in the HercynoAppalachian collision area, and perhaps to part of the northwards drift of microcontinents of Gondwanian origin located north of the Arabian promontory.

High pressure and high temperature XAFS study of germanate: Fourfold versus sixfold coordination changes

Abstract: The Ge coordination changes in oxides have been investigated by XAFS at high pressure and high temperature using the energy dispersive configuration of the DCI storage ring of LURE (Orsay, France). Two different experimental set-up were used to investigate both tetrahedra-to-octahedra (high pressure and temperature) and octahedra-to-tetrahedra (high temperature) changes. The network compression and dilatation are found to be precursor of phase transformations occuring under these extreme conditions.

In situ high-temperature (→ 2100 K) XAFS and anharmonicity

Abstract: High-temperature (up to 2100 K) XAFS data in the energy-dispersive mode has been collected at Ga and Ge K-edges on a new type of micro-furnace (heating wire) that can run up to 3500 K. Another advantage is the fast data collection time (few seconds per spectra). Data were collected in a variety of Ga- and Ge-oxide compounds. At Ge-K edge, [4]Ge4+ polyhedra shows a weak anharmonicity, related to the high GeO bond strength in those compounds. In contrast, [6]Ge4+ polyhedra has a slightly anharmonic behaviour, but much smaller compared to more weaker bonds, like in [4, 6]Ga2 3+O3 or olivine-[6]Ni22+ SiO4. The high-temperature, time-resolved analysis of a [6] Ge → [4] Ge coordination change can also be followed in situ, as well as the melting of these high temperature phases.

Coordination change around 2 wt% Ni in Na2Si2O5 glass/melt systems

Abstract: The coordination environment around Ni at minor concentration levels (2 wt% NiO) in Na 2 Si 2 O 5 glass and melt has been investigated by high temperature (293–1250 K) X-ray absorption fine structure (XAFS) spectroscopy. The derived average anharmonic NiO distances for the glass at 293 K and the melt above 1150 K are 1.99 (1) and 1.97 (1) A, respectively. The NiO distance in the glass is consistent with a mixture of IV Ni and V Ni, whereas that in the melt is consistent with dominantly IV Ni. The significant reorganization of the melt during quenching may help explain the enrichment of Ni in early-formed igneous minerals. These data also suggest the possibility of a pressure-induced coordination change around Ni, which may influence significantly the geochemical modeling of the Earth.