B. Winkler

Theoretical investigation of bonding in diaspore

The bulk modulus of diaspore, alpha -AlOOH, has been obtained from density functional theory based calculations. The value obtained, B = 148 GPa, is consistent with that previously obtained from elastic constant measurements, but in strong disagreement with values derived from high pressure x-ray diffraction experiments. A Mulliken bond population analysis of the electronic structure implies that the Al-O bonds are significantly covalent, in contrast to findings based on an earlier x-ray diffraction study. On compression, the main change is the increase in the hydrogen-bond strength.

Neutron imaging and neutron tomography as non-destructive tools to study bulk-rock samples

Neutron imaging and neutron tomography can be used as non-destructive tools for the analysis of bulk-rock samples. Here, we establish the spatial resolution currently available, and discuss applications such as phase analysis and the observation of dynamic processes.

Thermodynamics of mixing and ordering in pyrope–grossular solid solution

Abstract Static lattice energy calculations have been combined with cluster expansion formalism to predict pairwise ordering interactions in the pyrope-grossular solid solution. The ordering interactions, the Js, have been then used to calculate the activity-composition relations over a wide temperature range with the help of the Cluster Variation Method. It is shown that short-range ordering in the system is driven by size mismatch. The prediction of the right signs and magnitudes of the ordering interaction energies requires separation of the mixing enthalpy into the configuration-dependent (chemical) and the configuration-independent (elastic) components. The study predicts the existence of a miscibility gap below 500ºC.

Structure and properties of aluminosilicate garnets and katoite: an ab initio study

The structural properties of end-members of the aluminosilicate garnet family (pyrope, grossular, spessartine and almandine) and of one hydrogarnet (katoite) have been investigated as a function of applied pressure. The calculated static geometry, bulk modulus and its pressure derivative are in good agreement with available experimental data. The results of a systematic study of the effect of the size of the divalent cation on compressibility are presented. The bonding and dynamics of Mg in pyrope are investigated by using frozen phonon calculations.

Systematic ab initio study of the compressibility of silicate garnets

The structural properties of the silicate garnets andradite, Ca(3)Fe(2)Si(3)O(12), uvarovite, Ca(3)Cr(2)Si(3)O(12), knorringite, Mg(3)Cr(2)Si(3)O(12), goldmanite, Ca(3)V(2)Si(3)O(12), blythite, Mn(2+)(3)Mn(3+)(2)Si(3)O(12), skiagite, Fe(2+)(3)Fe(3+)(2)Si(3)O(12), calderite, Mn(2+)(3)Fe(3+)(2)Si(3)O(12), and khoharite, Mg(3)Fe(3+)(2)Si(3)O(12), have been investigated with a quantum-mechanical model as a function of applied pressure. The study has been performed with the density functional theory code CASTEP, which uses pseudopotentials and a plane-wave basis set. All structural parameters have been optimized. The calculated static geometries (cell parameters, internal coordinates of atoms and bond lengths), bulk moduli and their pressure derivatives are in good agreement with the experimental data available. Predictions are made for those cases where no experimental data have been reported. The data clearly indicate that the elastic properties of all silicate garnets are dominated by the compressibility of the dodecahedral site. The compression mechanism is found to be based on a bending of the angle between the centers of the SiO(4) tetrahedra and the adjacent octahedra, as in the aluminosilicate garnets. An analysis of the relationship between ionic radii of the cations and the compressibility of silicate garnets is presented.

Diamond formation from CaCO3 at high pressure and temperature

We studied the decomposition of CaCO3 by laser heated diamond anvil cell experiments at pressures between 9 and 21 GPa up to 4000 K. The quenched samples were characterized by micro-Raman spectroscopy. From the results we conclude that calcite decomposes into CaO + O2 + C across the whole pressure range investigated at temperatures around 3500 K, initially forming graphite nanoparticles with dimensions around 3–11 nm. The graphite particles may aggregate and transform into diamond with dimensions around 20 nm if the sample is annealed in the diamond stability field. We therefore conclude that diamond can be crystallized directly from carbonatitic melts by decomposition of CaCO3 at high pressures and temperatures, and that phase diagrams showing a decomposition into CaO + CO2 in this P , T -range need to be reevaluated.

An ab initio study of hydrogarnets

Abstract The structural properties of katoite, Ca3Al2(O4H4)3, are investigated as a function of applied pressure. The calculated structure at ambient pressure, the bulk modulus and its pressure derivative are in good agreement with the available experimental data. The strength of the hydrogen bond increases under pressure, accompanied by a lengthening of the O-H bonds. This conclusion agrees with experimental spectroscopic data and casts serious doubts on the reliability of neutron diffraction results that predict strong compression of the O-H bonds. Calculations for Mg3Al2(O4H4)3 have been used to predict the structure and relative stability of the hypothetical hydropyrope

Small-angle neutron scattering study of volcanic rocks

Small-angle neutron scattering (SANS) and scanning electron microscopy (SEM) have been used to study the microstructure of several volcanic rocks (basalt, rhyolite, phonolitic pumice, phonolite, rhyolitic obsidian). Magnitudes of the power-law scattering exponent were 3 < α < 4 or α ≈ 4 for the majority of the rocks. The influence of the thermal history on the rock microstructure has been investigated by SANS experiments on thermally treated samples. Specific surface areas were determined for pumice (3 m2/g) and a basalt (2.5 m2/g and 0.2 m2/g) depending on the thermal history. The small-angle scattering in obsidian is shown to be caused by Fe-rich partially crystalline aggregates. The obsidian has been further characterized by ultrasonic resonance and thermal expansion measurements.

Crystal structures of curium compounds: an ab initio study

The crystal structure and electronic properties of curium and its compounds are investigated using the density functional theory approach. The accuracy of the method is shown to be sufficient to describe structures with various types of chemical bonding: halides, pnictides, oxides, hydrides and intermetallic compounds. Selected examples are given of the studies of properties and phase stability of curium compounds under hydrostatic compression. Further applications of the theoretical approach to crystallography and solid state chemistry of actinides are suggested.

Order/disorder phase transition in cordierite and its possible relationship to the development of symplectite reaction textures in granulites

Based on a consistent set of empirical interatomic potentials, static structure energy calculations of various Al/Si configurations in the supercell of Mg-cordierite and Monte Carlo simulations the phase transition between the orthorhombic and hexagonal modifications of cordierite (Crd) is predicted at 1623 K. The temperature dependences of the enthalpy, entropy, and free energy of the Al/Si disorder were calculated using the method of thermodynamic integration. The simulations suggest that the commonly observed crystallization of cordierite in the disordered hexagonal form could be related to a tendency of Al to occupy T1 site, which is driven by local charge balance. The increase in the Al fraction in the T1 site over the ratio of 2/3(T1): 1/3(T2), that characterizes the ordered state, precludes formation of the domains of the orthorhombic phase. This intrinsic tendency to the crystallization of the metastable hexagonal phase could have significantly postponed the formation of the association of orthorhombic cordierite and orthopyroxene over the association of quartz and garnet in metapelites subjected to granulite facies metamorphism. The textures of local metasomatic replacement (the formation of Crd + Opx Or Spr + Crd symplectites between the grains of garnet and quartz) indicate the thermodynamic instability of the association of Qtz + Grt at the moment of the metasomatic reaction. This instability could have been caused by the difficulty of equilibrium nucleation of orthorhombic cordierite.