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  • 11
    Electronic Resource
    Electronic Resource
    Compression and coordination changes in pyroxenoids: an EXAFS study of MgGeO3 enstatite and CaGeO3 wollastonite (1992)
    Springer
    Physics and chemistry of minerals 18 (1992), S. 506-513 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract The evolution of the distortion of MgGeO3 enstatite and CaGeO3 wollastonite with increasing pressure, has been investigated using X-ray absorption spectroscopy (XAS) in a diamond anvil cell. At room temperature and low pressure (P〈7 GPa), the compressibility of the GeO4 tetrahedron is higher in MgGeO3 enstatite (K [GeO4]∼135 GPa) than in CaGeO3 wollastonite (K [GeO4]≥ 280 GPa). The compression mechanisms of the two compounds are different: the whole mineral compressibility of Ge-enstatite appears to be very homogeneous, in contrast to that of Ge-wollastonite which exhibits an inhomogeneous tretrahedral compressibility. This result is consistent with the variation of the Debye-Waller factors of the two compounds with increasing pressure. At higher pressures, the coordination of germanium atoms in the two compounds gradually changes from fourfold to sixfold. For CaGeO3 the coordination change starts at 7 GPa and is complete a 12 GPa, whereas it starts at about 8.5 GPa for MgGeO3 and is not complete at 31 GPa. The progressive evolution of the measured Ge-O distances as well as the modification in the X-ray absorption near-edge structure indicate two coexisting different sites rather than a progressive site modification. The transformation is found to be partially reversible in CaGeO3 wollastonite, whereas it is totally reversible in MgGeO3 enstatite.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00205265
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  • 12
    Electronic Resource
    Electronic Resource
    Electron microscopy of high-pressure phases synthesized from natural olivine in diamond anvil cell (1989)
    Springer
    Physics and chemistry of minerals 16 (1989), S. 320-330 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00199551
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  • 13
    Electronic Resource
    Electronic Resource
    A vibrational study of phase transitions among the GeO2 polymorphs (1991)
    Springer
    Physics and chemistry of minerals 18 (1991), S. 7-18 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Infrared and Raman spectra of the quartz, rutile and amorphous forms of GeO2 have been recorded under pressure and/or temperature, in order to study the crystalline to crystalline — or amorphous — transformations of this compound in the solid state. X-ray diffraction data shown that crystalline quartz-GeO2 subjected to high pressure amorphizes. Infrared data are consistent with a gradual amorphisation of this compound at static pressures between 6 to 12 GPa at 300 K. With increasing pressure, the Ge-O distance appears to remain constant and amorphization is associated with a progressive change in the coordination of germanium atoms from fourfold to sixfold. This apparent change in coordination is not quenchable at room pressure. On decompression, the Ge in the amorphous form returns to tetrahedral coordination. The anharmonic parameters for the Raman modes of the quartz and rutile forms of GeO2, have also been estimated from pressure and temperature shifts. These data have been used to calculate heat capacities and entropies of the two polymorphs at different pressures, with the Kieffer vibrational model. The calculated heat capacities at room pressure are within 1% of the experimental values between 20 and 1500 K. The calculated entropies are used to estimate the phase boundary in the (P, T) plane. The slope of the curve at room pressure (17 bar/K) is in good agreement with experimental values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00199038
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