Periodic Hartree-Fock study of minerals: Tetracoordinated silica polymorphs

Periodic Hartree-Fock STO-3G calculations have been performed on several tetracoordinated silica polymorphs: low and high quartz, low and idealized high cristobalite and prototype tridymite. The optimized structural parameters are in overall good agreement with experimental data. In the particular case of -quartz, the SiO₄ tetrahedra are found to be irregular. The optimized values of the two different SiO bond lengths are respectively 1.608 Å and 1.613 Å. The potential energy versus tilt angle curves suggest a picture of the high temperature phases in terms of delocalized oxygen atoms which is consistent with a disordered structure. Finally, the bonding in silica polymorphs is discussed from electron density maps and Mulliken population analysis.     

Halite oolites and ripples in the Dead Sea, Israel

Clastic features in recent halite deposits are observed along the beaches of an artificially dammed part of the Dead Sea. These features include halite oolites (termed halolites in this paper) and ripples. Halite precipitates initially either at the brine surface or on the floor. It is suggested that moderate increase of wave agitation shifts the balance towards brine-surface crystallization, and keeps the growing halite grains in constant motion. In this way rippled structures are formed. A further increase of wave energy leads to the growth of coated halite grains. The accumulation of the various halite grains along the beach, to form soft rippled floor and oolitic beach ridge is brought about during shoreward winds. During calm periods the bulk of the halite crystallizes directly on the floor. It develops into a hard crust which assumes the morphology of the substrate, including the ripple forms.     

Climatic and environmental characteristics of Moscow megalopolis according to the data of the Moscow State University Meteorological Observatory over 60 years

Analyzed are the results of meteorological and environmental measurements performed over the 60-year period (1954–2013) at the Meteorological Observatory of Lomonosov Moscow State University. The significant positive temperature trend (0.04°C/year for 1954–2013) was obtained; it increased up to 0.07°C/year in 1976–2012. Considered are the features of seasonal variations of different atmospheric characteristics. Discussed are the type and causes of low-frequency changes in meteorological parameters, radiation balance components, radiation in different spectral ranges, and chemical composition of precipitation. Demonstrated are possible mechanisms of the more significant increase observed in air temperature in Moscow as compared with that in Central Federal District and their connection with the greenhouse effect in the urban atmosphere.     

Changes in the Period of Negative Superhumps of Type SU UMa Dwarf Novae. II. NY Her (2017 and 2020)

Astrophysics - Photometric studies of the type SU UMa dwarf nova NY Her were made during 2017 and 2020 at the Crimean Astrophysical Observatory and at the Sanglokh Observatory over a total of 77...     

Performance of sand and shredded rubber tire mixture as a natural base isolator for earthquake protection

The performance of a well-designed layer of sand, and composites like layer of sand mixed with shredded rubber tire(RSM) as low cost base isolators, is studied in shake table tests in the laboratory. The building foundation is modeled by a 200 mm by 200 mm and 40 mm thick rigid plexi-glass block. The block is placed in the middle of a 1m by 1m tank filled with sand. The selected base isolator is placed between the block and the sand foundation. Accelerometers are placed on top of the footing and foundation sand layer. The displacement of the footing is also measured by LVDT. The whole setup is mounted on a shake table and subjected to sinusoidal motions with varying amplitude and frequency. Sand is found to be effective only at very high amplitude( 0.65 g) of motions. The performance of a composite consisting of sand and 50% shredded rubber tire placed under the footing is found to be most promising as a low-cost effective base isolator.     

CO2 feedbacks and the 100 K year cycle in climate

Summary In an earlier paper (Lindzen, 1986), it was shown that allowing CO₂ to vary with snow/sea ice position could lead to a greatly enhanced response in glaciation to 100 K year orbital forcing—even when 20 K year forcing was much stronger. In that model, snow/sea ice position (SSIP) and glaciation were different: the former was the forcing for the latter. However, SSIP and glaciation were not decorrelated. Observations (Berner et al., 1979; Lorius et al., 1985; Neftel et al., 1982) suggest that CO₂ may be independently related to both SSIP and glaciation. In the present paper, we allow (in a highly simplified manner) such independent dependence, and show how it alters the earlier results. Briefly, the dependence of CO₂ on glaciation can contribute to and even cause a highly enhanced response to the 100 K year component of the forcing. However, the CO₂ dependence on SSIP is, on the whole, more effective in this regard. Thus, we expect time series of CO₂ to show variation on the faster time scales than does glaciation.With 5 Figures