A MEG study of the olivine and spinel forms of Mg2SiO4

In this paper we present a theoretical investigation of the structures and relative stability of the olivine and spinel phases of Mg₂SiO₄. We use both a purely ionic model, based on the Modified Electron Gas (MEG) model of intermolecular forces, and a bond polarization model, developed for low pressure silica phases, to investigate the role of covalency in these compounds. The standard MEG ionic model gives adequate structural results for the two phases but incorrectly predicts the spinel phase to be more stable at zero pressure. This is mainly because the ionic modeling of Mg₂SiO₄ only accounts for 95 percent of the lattice energy. The remainder can be attributed to covalency and many-body effects. An extension of the MEG ionic model using “many-body” pair potentials corrects the phase stability error, but predicts structures which are in poorer agreement with experiment than the standard ionic approach. In addition, calculations using these many-body pair potentials can only account for 10 percent of the missing lattice energy. This model predicts an olivine-spinel phase transition of 8 GPa, below the experimental value of 20 GPa. Therefore, in order to understand more fully the stability of these structures we must consider polarization. A two-shell bond polarization model enhances the stability of both structures, with the olivine structure being stabilized more. This model predicts a phase transition at about 80 GPa, well above the observed value. Also, the olivine and spinel structures calculated with this approach are in poorer agreement with experiment than the ionic model. Therefore, based on our investigations, to properly model covalency in Mg₂SiO₄, a treatment more sophisticated than the two-shell model is needed.     

Response of base-isolated buildings to wind loading

The response of base-isolated buildings to gusting wind is analysed theoretically based on experimental data obtained in a boundary layer wind tunnel. A comparison is made with the response evaluated using existing codes. A special procedure, similar to that of current codes, for computing the alongwind and torsional responses of base-isolated buildings is presented.     

Tropical weathering control of Ni, Cu, Co, and platinum group element distributions at the O'Toole Ni-Cu sulphide deposit, Minas Gerais, Brazil

The O'Toole nickel sulphide deposit is located in the Southern Plateau morphological and structural province of Minas Gerais, which has a humid tropical climate characterized by a mean temperature of 19°C and rainfall of 1600 mm. The primary ore reserve is 5.44 million tonnes at 2.72% Ni, 0.45% Cu, 0.06% Co, and 1.3 ppm combined platinum-group elements. Tropical weathering of the vertically dipping orebody has created gossans at the surface, an oxidized zone to a depth of about 20 m, and a zone of transition above fresh ore to a depth of up to 70 m. The weathering profile is similar mineralogically to the profiles at the Kambalda, Mt. Windarra, and Agnew Ni sulphide deposits in Western Australia, but differs in the depth extents of its mineralogically distinctive zones. These zones exhibit geochemically distinctive enrichments and depletions of various elements in this environment based on mass balance calculations and on the relative immobility of iridium. The work presented characterizes the geochemical behavior of Ni, Cu, Co, and platinum-group elements in the weathering of the O'Toole deposit.     

Transition enthalpies and entropies of high pressure zinc metasilicates and zinc metagermanates

The enthalpies of transition at T= 298 K between zinc metasilicate assemblages, measured by molten oxide solution calorimetry, are:

Cation disorder in garnets along the Mg3Al2Si3O12-Mg4Si4O12 join: an infrared,Raman and NMR study

We have obtained infrared and Raman spectra for garnets synthesized at high (static) pressures and temperatures along the join Mg₃Al₂Si₃O₁₂ (pyrope) — Mg₄Si₄O₁₂ (magnesium majorite). The vibrational spectra of Mg-majorite show a large number of additional weak peaks compared with the spectra of cubic pyrope garnet, consistent with tetragonal symmetry for the MgSiO₃ garnet phase. The Raman bands for this phase show no evidence for line broadening, suggesting that Mg and Si are ordered on octahedral sites in the garnet. The bands for the intermediate garnet compositions are significantly broadened compared with the end-members pyrope and Mg-majorite, indicating cation disorder in the intermediate phases. Solid state ²⁷Al NMR spectroscopy for pyrope and two intermediate compositions show that Al is present only on octahedral sites, so the cation disorder is most likely confined to Mg-Al-Si mixing on the octahedral sites. We have also obtained a Raman spectrum for a natural, shock-produced (Fe,Mg) majorite garnet. The sharp Raman peaks suggest little or no cation disorder in this sample.