Anomalous birefringence in andradite–grossular solid solutions: a quantum-mechanical approach

The static linear optical properties (refractive indices, birefringence and axial angle) of andradite–grossular (Ca₃Fe₂Si₃O₁₂–Ca₃Al₂Si₃O₁₂) solid solutions have been computed at the ab initio quantum-mechanical level through the Coupled Perturbed Kohn–Sham scheme, using an all-electron Gaussian-type basis set. Geometry relaxation after substitution of 1–8 Al for Fe atoms in the primitive cell of andradite yields 23 non-equivalent configurations ranging from cubic to triclinic symmetry. Refractive indices vary quite regularly between the andradite (1.860) and grossular (1.671) end-members; the birefringence δ and the axial angle 2V at intermediate compositions can be as large as 0.02° and 89°, respectively. Comparison with experiments suffers from inhomogeneities and impurities of natural samples; however, semi-quantitative agreement is observed.     

A quantum mechanical study of the perovskite structure type of MgSiO3

The periodic ab-initio Hartree-Fock Self Consistent Field program CRYSTAL has been used to study the electronic structure and equation of state of MgSiO₃ perovskite. Three space groups were considered: Pm3m (cubic; ideal untilted SiO₆ octahedra), P4/mbm (tetragonal; the octahedra are allowed to deform along and rotate about the crystallographic c cell edge) and Pbnm (orthorhombic; octahedra are allowed to deform along and rotate about the three cell edges). The calculated orthorhombic structure is the most stable, in agreement with experiment. The relative stability of the three structures and the effect of pressure on the SiO₆ octahedra is interpreted in terms of bond population data and is mainly determined by the oxygen-oxygen repulsion.     

The chemical nature of Europa surface material and the relation to a subsurface ocean

The surface composition of Europa is of special interest due to the information it might provide regarding the presence of a subsurface ocean. One source of this information is the infrared reflectance spectrum. Certain surface regions of Europa exhibit distorted H₂O vibrational overtone bands in the 1.5 and 2.0 μm region, as measured by the Galileo mission Near Infrared Mapping Spectrometer (NIMS). These bands are clearly the result of highly concentrated solvated contaminants. However, two interpretations of their identity have been presented. One emphasizes hydrated salt minerals and the other sulfuric acid, although each does not specifically rule out some of the other. It has been pointed out that accurate chemical identification of the surface composition must depend on integrating spectral data with geochemical models, and information on the tenuous atmosphere sputtered from the surface. It is also extremely important to apply detailed chemistry when interpreting the spectral data, including knowledge of mineral dissolution chemistry and the subsequent optical signatures of ion solvation in low-temperature ice. We present studies of flash frozen acid and salt mixtures as Europa surface analogs and demonstrate that solvated protons, metal cations and inorganic anions all influence the spectra and must all, collectively, be considered when assigning Europa spectral features. These laboratory data show best correlation with NIMS Europa spectra for multi-component mixtures of sodium and magnesium bearing sulfate salts mixed with sulfuric acid. The data provide a concentration upper bound of 50-mol% for MgSO₄ and 40-mol% for Na₂SO₄. This newly reported higher sodium and proton content is consistent with low-temperature aqueous differentiation and hydrothermal processing of carbonaceous chondrite-forming materials during the formation and early evolution of Europa.     

Rare-gas dating,I. A demountable metal system with low blanks

The construction and method of operation of an all-metal system for KAr and U-He dating is described in detail. The system is capable of handling samples in batches of about 15 g total weight. Blanks are typically 6 × 10^?9 cm³ STP for⁴⁰Ar_atmos, <1.2 × 10^?9 cm³ STP for⁴He, and ～2 × 10^?9 STP for⁴⁰Ar_radiogenic. The last of these quantities is independent of the quantity of⁴⁰Ar_radiogenic processed in the preceding run because vapor transport of the sample out of the hottest region of the crucible is very substantial and release of the rare gases in the sample is therefore almost complete. The system is very stable manometrically; monitoring of spike peaks therefore permits detection of any gas loss which amounts to more than a few percent. The system is not very suitable for analysis of large samples.     

Paleomagnetism,magnetic fabric,and <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of Pliocene and Quaternary ignimbrites in the Arequipa area,southern Peru

⁴⁰Ar/³⁹Ar ages and paleomagnetic correlations using characteristic remanent magnetizations (ChRM) show that two main ignimbrite sheets were deposited at 4.86 ± 0.07 Ma (La Joya Ignimbrite: LJI) and at 1.63 ± 0.07 Ma (Arequipa Airport Ignimbrite: AAI) in the Arequipa area, southern Peru. The AAI is a 20–100 m-thick ignimbrite that fills in the Arequipa depression to the west of the city of Arequipa. The AAI is made up of two cooling units: an underlying white unit and an overlying weakly consolidated pink unit. Radiometric data provide the same age for the two units. As both units record exactly the same well-defined paleomagnetic direction (16 sites in the white unit of AAI: Dec = 173.7; Inc = 31.2; α95 = 0.7; k = 2749; and 10 sites in the pink unit of AAI; Dec = 173.6; Inc = 30.3; α95 = 1.2; k = 1634), showing no evidence of secular variation, the time gap between emplacement of the two units is unlikely to exceed a few years. The >50 m thick well-consolidated white underlying unit of the Arequipa airport ignimbrite provides a very specific magnetic zonation with low magnetic susceptibilities, high coercivities and unblocking temperatures of NRM above 580°C indicating a Ti-poor titanohematite signature. The Anisotropy of Magnetic Susceptibility (AMS) is strongly enhanced in this layer with anisotropy values up to 1.25. The fabric delineated by AMS was not recognized neither in the field nor in thin sections, because most of the AAI consists in a massive and isotrope deposit with no visible textural fabric. Pumices deformation due to welding is only observed at the base of the thickest sections. AMS within the AAI ignimbrite show a very well defined pattern of apparent imbrications correlated to the paleotopography, with planes of foliation and lineation dipping often at more than 20° toward the expected vent, buried beneath the Nevado Chachani volcanic complex. In contrast with the relatively small extent of the thick AAI, the La Joya ignimbrite covers large areas from the Altipano down the Piedmont. Ti-poor titanomagnetites are the dominant magnetic carriers and AMS values are generally lower than 1.05. Magnetic foliations are sub horizontal and lineations directions are scattered in the LJI. The AMS fabrics are probably controlled by post-depositional compaction and welding of the deposit rather than transport dynamics. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Seismic reliability analysis of buildings with torsional eccentricities

A study is presented of the influence of stiffness and strength eccentricities on the inelastic torsional response of buildings under the action of two simultaneous orthogonal horizontal ground motion components. Asymmetric buildings were obtained from their respective symmetric systems and were characterized by their stiffness and strength torsional eccentricities in both orthogonal directions. Based on the results of inelastic response of both building types (symmetric and asymmetric), the seismic reliability functions are determined for each system, and their forms of variation with different global system parameters are evaluated. Illustrative examples are presented about the use of this information for the formulation of seismic design criteria for in‐plan asymmetric multistory systems, in order to attain the same reliability levels implicit for symmetric systems designed in accordance with current seismic design codes. Copyright © 2014 John Wiley & Sons, Ltd.     

Seismic response of masonry-infilled steel frames via multi-scale finite-element analyses

Effects of masonry infills on the seismic vulnerability of steel frames is studied through multi-scale numerical modelling. First, a micro-modelling approach is utilized to define a homogenized masonry material, calibrated on experimental tests, which is used for modelling the nonlinear response of a one-story, single span, masonry-infilled portal under horizontal loads. Based on results of the micro-model, the constitutive behavior of a diagonal strut macro-element equivalent to the infill panel is calibrated. Then, the diagonal strut is used to model infill panels in the macro-scale analysis of a multi-span multi-story infilled moment-resisting (MR) steel frame. The seismic vulnerability of the MR frame is evaluated through a nonlinear static procedure. Numerical analyses highlight that infills may radically modify the seismic response and the failure mechanism of the frame, hence the importance of the infill correct modelling.     

Displacement capacity of masonry piers: parametric numerical analyses versus international building codes

The nonlinear behaviour of masonry piers loaded in their plane is investigated by parametric numerical simulations. Each pier has a cantilever scheme, is loaded by a constant axial load and is subjected to an increasing horizontal displacement at the top. The macro-modelling approach is used to perform numerical analyses, adopting two different constitutive laws: a total strain crack model and a plastic model. The numerical model is calibrated on a block-masonry type for which experimental tests are available in literature. Parametric numerical simulations are performed by varying the aspect-ratio and the compression level, in order to assess the influence of such parameters on both shear strength and displacement capacity. By comparing numerical results with formulas of international codes, a good agreement for the shear strength is obtained, while significant differences are observed for the displacement capacity, which is influenced by both parameters. The authors propose a simple empirical formula for the displacement capacity, obtained by fitting the numerical results. The expression can be useful in the practical design for considering the influence of aspect-ratio and compression level, currently neglected by building codes.