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  • 1
    Electronic Resource
    Electronic Resource
    Atomistic modeling of silica based sol-gel processes (1997)
    Springer
    Journal of sol gel science and technology 8 (1997), S. 55-58 
    Details
    ISSN: 1573-4846
    Keywords: Molecular Dynamics ; Density Functional Theory ; sol-gel solutions ; silica clusters
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Density Functional Theory is used to study water, methanol, ethanol, TMOS, and TEOS molecules and the most important silica clusters participating in sol-gel processes. Calculated bond lengths, bond angles and electric dipole moments compare well with experimental data. The energy of these molecules is reported and used to discuss the energetics of the hydrolysis and condensation reactions. Molecular Dynamics is employed to simulate liquid water, methanol, ethanol, TMOS, TEOS and experimental sol-gel solutions. Calculated densities and enthalpies of vaporisation compare well with experimental data. Preliminary results are presented for MD simulations of sol-gel solutions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02436817
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  • 2
    Electronic Resource
    Electronic Resource
    Electron petrography of shock-produced veins in the Tenham chondrite (1979)
    Springer
    Contributions to mineralogy and petrology 71 (1979), S. 211-218 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Electron microscopy of the minerals in, and adjacent to the black veins of the Tenham meteorite reveals that the minerals have undergone varying degrees of shock. The orthopyroxenes of the bulk of the meteorite have been transformed to clinopyroxenes in areas adjacent to the veins, and to majorite in the black vein itself. The majorite, which occurs in both equant and dendritic habit, is associated with a glassy phase from which it crystallized, and with a microcrystalline clinopyroxene aggregate produced from the majorite upon the release of pressure. The olivines of the meteorite are also deformed, and some have been transformed to the high pressure spinel polymorph. The spinel has subsequently partially inverted to the β-phase polymorph.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00375437
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  • 3
    Electronic Resource
    Electronic Resource
    Oxidation phenomena in pleonaste bearing titanomagnetites (1979)
    Springer
    Contributions to mineralogy and petrology 69 (1979), S. 355-359 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Transmission electron microscopy revealed that optically homogenous pleonastes occurring in titanomagnetites had been oxidised. The pleonastes had decomposed by a discontinuous mechanism to give an intergrowth of a spinel and an intermediate phase, whose structure was based on a spinel superlattice. The intermediate phase was similar to one previously found in oxidised chromites from Rhum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00372261
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  • 4
    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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  • 5
    Electronic Resource
    Electronic Resource
    A mechanism for pyroxene-pyroxenoid and pyroxenoid-pyroxenoid transformations (1984)
    Springer
    Physics and chemistry of minerals 10 (1984), S. 236-243 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract A mechanism for the solid state replacement of clinopyroxene by pyroxenoids has been derived from consideration of the crystal structures. The mechanism involves the propagation of partial dislocations, with Burgers vectors of 1/4 $$[2\bar 3\bar 1]$$ , through the clinopyroxene matrix to generate a resultant shear; the passage of one dislocation gives rise to a single chain-repeat unit of wollastonite in each silicate chain. Pyroxenoids may thus be formed from clinopyroxene by the periodic introduction of these dislocations on parallel slip-planes, and pyroxenoid to pyroxenoid inversions may proceed by the introduction and removal of such shears by the propagation of the correct combination of partial dislocations. The Burgers vectors for these dislocations in each of the pyroxenoid mineral structures has been calculated. A two step mechanism is proposed for clinopyroxene to bustamite, and pyroxenoid to bustamite inversions, which utilises the same shear as above which generates a wollastonite structure. The second step is a shear of 1/2[001] on the plane $$(1\bar 10)$$ of this wollastonite intermediate, to give a bustamite structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00309316
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  • 6
    Electronic Resource
    Electronic Resource
    Atomic ordering around the oxygen vacancies in sillimanite (1992)
    Springer
    Physics and chemistry of minerals 18 (1992), S. 373-378 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Computer simulation is used to investigate the short range ordering around an isolated oxygen vacancy in sillimanite. The static lattice energy with the use of empirical potentials is calculated, for different Al/Si distributions around a vacancy in a supercell of sillimanite. A parametrisation of the total energy is built up and used to deduce the best Al/Si ordering around the oxygen vacancy. It is found that a strong ordering about the vacancy occurs. In the ab-plane two sets of aluminium cluster are found besides the vacancy, surrounded above and below by silicon atoms, a configuration that promotes local charge balance. By placing two vacancies on sites directly adjacent to the same oxygen, the central oxygen site is bonded to four cations: this situation is found to be energetically unfavourable.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00199418
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  • 7
    Electronic Resource
    Electronic Resource
    Thermodynamic properties of CaCO3 calcite and aragonite: A quasi-harmonic calculation (1993)
    Springer
    Physics and chemistry of minerals 19 (1993), S. 472-479 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract A quasi-harmonic model has been used to simulate the thermodynamic behaviour of the CaCO3 polymorphs, by equilibrating their crystal structures as a function of temperature so as to balance the sum of inner static and thermal pressures against the applied external pressure. The vibrational frequencies and elastic properties needed have been computed using interatomic potentials based on two-body Born-type functions, with O-C-O angular terms to account for covalency inside the CO3 molecular ion. A good agreement with experimental data is generally shown by simulated heat capacity and entropy, while the thermal expansion coefficient seems to be more difficult to reproduce. The results obtained for aragonite are less satisfactory than those of calcite, but they are improved by using a potential specifically optimized on properties of that phase itself.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00203187
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  • 8
    Electronic Resource
    Electronic Resource
    Interatomic potentials for CaCO3 polymorphs (calcite and aragonite), fitted to elastic and vibrational data (1992)
    Springer
    Physics and chemistry of minerals 19 (1992), S. 80-87 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Calcite and aragonite have been modeled using rigid-ion, two-body Born-type potentials, supplemented by O-C-O angular terms inside the CO3 groups. A shell model has also been developed for calcite. Atomic charges, repulsive parameters and force constants have been optimized to reproduce the equilibrium crystal structures, the elastic constants and the Raman and infrared vibrational frequencies. The rigid-ion potential RIM (atomic charges:z O= -0.995e,z C = 0.985e,z Ca = 2.0e) fitted to calcite properties is able to account for those of aragonite as well. Experimental unit-cell edges, elastic constants, internal and lattice frequencies are reproduced with average relative errors of 2.1, 5.5, 2.4, 15.1% for calcite and of 0.2, 19.4, 2.5, 11.8% for aragonite, respectively. The RIM potential is suitable for thermodynamic and phase diagram simulations in the CaCO3 system, and is discussed and compared to other potentials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00198605
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  • 9
    Electronic Resource
    Electronic Resource
    Dislocations in CaTiO3 perovskite deformed at high-temperature: a transmission electron microscopy study (1996)
    Springer
    Physics and chemistry of minerals 23 (1996), S. 337-344 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Dislocation configurations in natural single crystals of CaTiO3 perovskite deformed in high-temperature creep were examined and characterized by transmission electron microscopy. Screw dislocations with Burgers vector [100]pc and [011]pc, dissociated on the $$(01\bar 1)_{{\text{pc}}} $$ plane, form rectangular networks with extended four-fold nodes in the shape of octagons, a configuration never observed in any of the previously investigated perovskites, except CaGeO3. Screw dislocations with Burgers vector [101]pc and $$(\bar 101)_{{\text{pc}}} $$ , on the (010)pc plane, react to form a twist wall; the dislocations with Burgers vector [002] produced by the reaction decompose into two perfect dislocations [001]pc. This results in a new configuration, never observed before, with three-fold nodes at the corners of rectangles. Both the octagonal extended nodes and the junctions decomposed into perfect dislocations are seen in samples deformed indifferently by slip on {100}pc or {110}pc planes, but they seem to appear only above 1520 K, in the cubic phase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00199499
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  • 10
    Electronic Resource
    Electronic Resource
    Ab initio study of MgSiO3 C2/c enstatite (1995)
    Springer
    Physics and chemistry of minerals 22 (1995), S. 453-460 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract We have used a newly developed ab initio constant-pressure molecular dynamics with variable cell shape technique to investigate the zero temperature behaviour of high pressure clinoenstatite (MgSiO3-C2/c) from 0 up to 30 GPa. The optimum structure at 8 GPa, as well as structural trends under pressure, compare very well with experimental data. At this pressure, we find noticeable “fluctuations” in the chain configuration which suggests the structure is on the verge of a mechanical instability. Two distinct compressive behaviours then appear: one below and another above 8 GPa. This phenomenon may be related to the observed transition to a lower symmetry P21/c phase which involves a reconfiguration of the silicate chains, and suggests that the C2/c structure at low pressures found here, may be an artifact of the dynamical algorithm which preserves space group in the absence of symmetry breaking fluctuations. Comparison with calculations in other magnesium silicate phases, indicates that the size and shape of the silicate units (isolated and/or linked tetrahedra and octahedra) are generally well described by the local density approximation; however, the weaker linkages provided by the O-Mg-O bonds, are not as well described. This trend suggests that, as in the recently studied case of H2O-ice, the structural properties of more inhomogeneous systems, like enstatite, may be improved by using gradientcorrected density functionals.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00200323
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