Synthesis and crystal structure of a triple chain silicate,Na2Mg4Si6O16(OH)2

In the system Na₂CO₃-MgO-SiO₂-H₂O a new sodium magnesium silicate was synthesized under hydrothermal conditions; 450–600 ° C and 300–1000 Kg/cm₂. The structure of the specimen was determined by X-ray powder methods, and its properties were studied by chemical, infrared and TG analyses. The specimen has a triple chain structure (space group, C2/c) with the ideal chemical composition, 4 (Na₂Mg₄Si₆O₁₆(OH)₂) and lattice parameters, a= 10.152(2), b=27.137(4), c=5.276(1) Å, and = 106.97(3) °.The essential feature of the structure is shown by the presence of SiO₄ tetrahedra linked to form chains which have three times the width of those in pyroxene. These triple chains have a periodicity, 5.27 Å, along their lengths, and are bonded to each other laterally by the brucite layer made up by eight Mg cations and sandwiched between two inward pointing bands of tetrahedra. These units are linked back to back by cations (Mg or Na) in the Na(2) site and by a large cation (Na) at the Na(1) site.     

Pyroxenes from the Beaver Bay gabbro complex of Minnesota

Although the Beaver Bay ferrogabbro is a small-scale layered intrusion, Ca-rich pyroxenes show a strong iron enrichment during fractionation, ranging from augite (Mg₃₈Fe₂₄ Ca₃₈) to ferrohedengergite (Mg₁₀Fe₄₈Ca₄₂). Ca-poor pyroxenes from intermediate pigeonite (Mg₃₉Fe₅₀Ca₁₁) to ferriferous pigeonite (Mg₂₇Fe₆₅Ca₈) occur as intercumulus minerals. The pyroxenes from the non-layered Beaver River gabbro are included in the overall pyroxene fractionation trend of the Beaver Bay gabbro complex. The pyroxene trend of the Beaver Bay gabbro complex is similar to those of the Skaergaard and Bushveld; however, there is a slight difference in that the Ca-rich pyroxenes of Beaver Bay (having Mg content over 30%) are slightly richer in Ca than either the Skaergaard or Bushveld augites.     

Rupture process of the Miyagi-Oki,Japan, earthquake of June 12, 1978

The faulting mechanism and multiple rupture process of the M = 7.4 Miyagi-Oki earthquake are studied using surface and body wave data from local and worldwide stations. The main results are as follows. (1) P-wave first motion data and radiation patterns of long-period surface waves indicate a predominantly thrust mechanism with strike N10° E, dip 20°W, and slip angle 76°. The seismic moment is 3.1 × 10²⁷ dyne-cm. (2) Farfield SH waveforms and local seismograms suggest that the rupture occurred in two stages, being concordant with the two zones of aftershock activity revealed by the microearthquake network of Tohoku University. The upper and lower zones, located along the westward-dipping plate interface, are separated by a gap at a depth of 35 km and have dimensions of 37 × 34 and 24 × 34 km², respectively. Rupture initiated at the southern end of the upper aftershock zone and propagated at N20°W subparallel to the trench axis. About 11 s later, the second shock, which was located 30 km landward (westward) of the first, initiated at the upper corner of the lower aftershock zone and propagated down-dip N80°W. Using Haskell modelling for this rupture process, synthetic seismograms were computed for teleseismic SH waves and nearfield body waves. Other parameters determined are: seismic moment $\text{M}{}_0\text{= 1.7 × 10}{}^{27}\text{dyne-cm, slip dislocation}\text{u}\text{= 1.9}\text{m}\text{, Δσ = 95}\text{bar, rupture velocity}\text{ν = 3.2}\text{km s}{}^{\mathrm{?1}}\text{,}\text{rise time}\text{τ = 2}\text{s}$, for the first event; $\text{M}{}_0\text{= 1.4 × 10}{}^{27}\text{dyne-cm}\text{,}\text{u}\text{= 2.4}\text{m}\text{, Δσ = 145}\text{bar}$, for the second event; and time separation between the two shocks ΔT = 11 s. The above two-segment model does not explain well the sharp onsets of the body waves at near-source stations. An initial break of a small subsegment on the upper zone, which propagated down-dip, was hypothesized to explain the observed near-source seismograms. (3) The multiple rupture of the event and the absence of aftershocks between the two fault zones suggests that the frictional and/or sliding characteristics along the plate interface are not uniform. The rupture of the first event was arrested, presumably by a region of high fracture strength between the two zones. The fracture energy of the barrier was estimated to be 10¹⁰ erg cm^?2. (4) The possible occurrence of a large earthquake has been noted for the region adjacent to and seaward of the area that ruptured during the 1978 event. The 1978 event does not appear to reduce the likelihood of occurrence of this expected earthquake.     

Deccan basalts at Mahabaleshwar,India

Fortyone successive flows of the Deccan Traps have been investigated at Mahabaleshwar, India, and the rocks from the twenty two different flows have been newly analysed. All of these basalts are silica-saturated tholeiites; and the series shows minor gradual variation with the order of eruption. These seem to be a result of magmatic differentiation somewhat similar to that shown in the Skaergaard intrusion.     

Radiation Pressure and the Poynting-Robertson Effect for Fluffy Dust Particles

A correct understanding of the dynamical effect of solar radiation exerted on fluffy dust particles can be achieved with assistance of a light scattering theory as well as the equation of motion. We reformulate the equation of motion so that the radiation pressure and the Poynting-Robertson effect on fluffy grains are given in both radial and nonradial directions from the center of the Sun. This allows numerical estimates of these radiation forces on fluffy dust aggregates in the framework of the discrete dipole approximation, in which the first term of the scattering coefficients in Mie theory determines the polarizability of homogeneous spheres forming the aggregates.The nonsphericity in shape turns out to play a key role in the dynamical evolution of dust particles, while its consequence depends on the rotation rate and axis of the grains. Unless a fluffy dust particle rapidly revolves on its randomly oriented axis, the nonradial radiation forces may prevent, apart from the orbital eccentricity and semimajor axis, the orbital inclination of the particle from being preserved in orbit around the Sun. However, a change in the inclination is most probably controlled by the Lorentz force as a consequence of the interaction between electric charges on the grains and the solar magnetic field. Although rapidly and randomly rotating grains spiral into the Sun under the Poynting-Robertson effect in spite of their shapes and structures, fluffy grains drift inward on time scales longer at submicrometer sizes and shorter at much larger sizes than spherical grains of the same sizes. Numerical calculations reveal that the dynamical lifetimes of fluffy particles are determined by the material composition of the grains rather than by their morphological structures and sizes. The Poynting-Robertson effect alone is nevertheless insufficient for giving a satisfactory estimate of lifetimes for fluffy dust grains since their large ratios of cross section to mass would reduce the lifetimes by enhancing the collisional probabilities. We also show that the radiation pressure on a dust particle varies with the orbital velocity of the particle but that this effect is negligibly small for dust grains in the Solar System.     

中国西北帕米尔东北缘的活动断裂研究

在卫星遥感图像的详细地质解译分析基础上，结合野外地质与地貌观察，对帕米尔东北缘山前与印度－亚欧大陆碰撞相关的活动断层进行了分析。研究结果指出，NW－NWW走向的断层主要表现为南倾逆冲断层，并伴随有强烈的右旋走滑分量。流经活动断层带的水系显示出右旋累积位错：小水系的水平位错量为4.0-20.0m，大河流的水平位错量达8－12km。沿断层带的上新世至早更新世地层也显示出一致的水平位错，位错量为8－12km。这些证明表明，帕米尔东南山前的NW－NWW走向的断层很可能开始于上新世末期至早更新世早期（2.2-3Ma)。研究结果首次厘定了帕米尔东北缘山前与向北逆冲相伴随的右旋走滑速率在第四纪期间达4.0-6.8mm/a。根据与现代地震活动相关的活断层分析，推测帕米尔东北缘山前7级以上地震重复周期为1000a左右。     

Direct determinations of the rates of rhyolite dissolution and clay formation over 52,000 years and comparison with laboratory measurements

Four porous, glass-dominated rhyolites from Kozushima Island, different in age and extent of weathering, were studied. Because the four rhyolites are homogeneously weathered to considerable depth, and because their initial chemical compositions were equal, the different rock characteristics can provide information about rates of rhyolite dissolution and clay mineral formation over ∼52,000 yr. Because glass surfaces retreat without surface roughening, surface area (measured by Brunauer-Emmett-Teller method; BET) was assumed to be approximately constant over time. The field dissolution rate, as inferred from the rate of loss of Si, was ∼6 × 10⁻¹⁹ mol cm⁻² s⁻¹. The estimated clay mineral formation rate was ∼1 × 10⁻¹⁹ mol cm⁻² s⁻¹. About 20% of dissolved Si precipitated as clays. In order to investigate the factors affecting the field dissolution rate, dissolution experiments that used powdered and block rhyolite samples were conducted. Under relevant field conditions (20°C and pH 6∼7), the rates were ∼5 × 10⁻¹⁷ and ∼5 × 10⁻¹⁸ mol cm⁻² s⁻¹ for powdered rhyolite and blocks, respectively. The dissolution rates obtained in this study decrease in the order powder > block > field. Because all surface areas were directly measured by BET, the differences are not attributable to the errors in surface area. The most plausible explanations of the slower rates are the lower degree of flushing and resultant high-solution saturation states in the pores (both in the field and in the rhyolite blocks used in experiments) plus the formation of alteration/hydrated layers at the glass surface.     

Topography and landslides in weathered granitic rock areas—Hai Van landslide in central Vietnam

Many landslides occur every year during heavy rains at the Hai Van Pass and surrounding area in central Vietnam, where granitic rocks are distributed. As is common in granite areas, these landslides often occur as small-scale flow-type and slump-type landslides. However, several horseshoe-shaped loose slopes of widths and lengths of 500 to 800 m, which incorporate these landslides, are observed on slopes across the area. These topographies resemble those formed by past and present large-scale landslides. The presence of such a topography and the repeated occurrences of landslides within this topography are rare in granite areas, where shallow flow-type landslides are generally frequent. To understand the mechanism causing the landslides in the Hai Van region, and as a support for future risk assessment, the factors and processes leading to the formation of such a topography and their relationship with these landslides must be identified and assessed. This study investigated the history of past landslide movement in the Hai Van Pass and surrounding area through observations of drill cores and outcrops, and analysis of the direction of remanent magnetism in the granitic rocks. Mineral compositions, cracks, degrees of weathering, and topographic shapes of the granitic rocks and their relationship to the landslides occurring today were also investigated. The results of the study reveal no variation in the direction of remanent magnetism in the granitic rocks in the region that would indicate disturbance of the ground due to a past large-scale landslide. No evidence of such an event could also be found both in the drill cores and the rock outcrops. Further, results of the analysis of cracks and weathering pattern confirm that the topography of the region is affected by the weathering of the granitic rocks that progresses in concentric circles of various sizes. Thus, it can be concluded that these topographies were not formed by a singular large-scale landslide of the past, but rather by a composite of relatively shallow landslides occurring on the slope of dome structures unique to granite areas, which are formed by differential weathering and denudation regulated by cracks.