The high-pressure C2/c–P21/c phase transition along the LiAlSi2O6–LiGaSi2O6 solid solution

Two synthetic single-crystals with composition Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆ and space group C2/c at room conditions have been studied under pressure by means of X-ray diffraction using a diamond anvil cell. The unit-cell parameters were determined at 12 and 10 different pressures up to P = 8.849 and P = 7.320 GPa for Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆, respectively. The sample with mixed composition shows a C2/c to P2₁/c phase transformation between 1.814 and 2.156 GPa, first-order in character. The transition is characterised by a large and discontinuous decrease in the unit-cell volume and by the appearance of the b-type reflections (h + k = odd) typical of the primitive symmetry. The Ga end-member shows the same C2/c to P2₁/c transformation at a pressure between 0.0001 and 0.39 GPa. The low-pressure value at which the transition occurred did not allow collecting any data in the C2/c pressure stability field except that on room pressure. Our results compared with those relative to spodumene (LiAlSi₂O₆, Arlt and Angel 2000a) indicate that the substitution of Al for Ga at the M1 site of Li-clinopyroxenes strongly affects the transition pressure causing a decrease from 3.17 GPa (spodumene) to less than 0.39 GPa (LiGaSi₂O₆) and decreases the volume discontinuity at the transition. As already found for other compounds, the C2/c low-pressure phases are more rigid than the P2₁ /c high-pressure ones. Moreover, the increase of the M1 cation radius causes a decrease in the bulk modulus K _T0. The axial compressibility among the Li-bearing clinopyroxenes indicates that the c axis is the most rigid for the C2/c phases while it becomes the most compressible for the P2₁ /c phases.     

Optical spectroscopic study of synthetic NaScSi<Subscript>2</Subscript>O<Subscript>6</Subscript>–CaNiSi<Subscript>2</Subscript>O<Subscript>6</Subscript> pyroxenes at normal and high pressures

Six synthetic NaScSi₂O₆–CaNiSi₂O₆ pyroxenes were studied by optical absorption spectroscopy. Five of them of intermediate (Na_1−x , Ca _x )(Sc_1−x , Ni _x )Si₂O₆ compositions show spectra typical of Ni²⁺ in octahedral coordination, more precise Ni²⁺ at the M1 site of the pyroxene structure. The common feature of all spectra is three broad absorption bands with maxima around 8,000, 13,000 and 24,000 cm⁻¹ assigned to ³ A _2g → ³ T _2g, ³ A _2g → ³ T _1g and →³ T _1g (³ P) electronic spin-allowed transitions of ^VINi²⁺. A weak narrow peak at ∼14,400 cm⁻¹ is assigned to the spin-forbidden ³ A _2g → ¹ T _2g (¹ D) transition of Ni²⁺. Under pressure the spin-allowed bands shift to higher energies and change in intensity. The octahedral compression modulus, calculated from the shift of the ³ A _2g → ³ T _2g band in the (Na_0.7Ca_0.3)(Sc_0.7Ni_0.3)Si₂O₆ pyroxene is evaluated as 85±20 GPa. The Racah parameter B of Ni²⁺(M1) is found gradually changing from ∼919 cm⁻¹ at ambient pressure to ∼890 cm⁻¹ at 6.18 GPa. The Ni end-member pyroxene [(Ca_0.93 Ni_0.07)NiSi₂O₆] has a spectrum different from all others. In addition to the above mentioned bands of Ni²⁺(M1) it displays several new relatively intense and broad extra bands, which were attributed to electronic transitions of Ni²⁺ at the M2 site. In difference to CaO₈ polyhedron geometry of an eightfold coordination, Ni²⁺(M2)O₈ polyhedra are assumed to be relatively large distorted octahedra. Due to different distortions and different compressibilities of the M1 and M2 sites the Ni²⁺(M1)- and Ni²⁺(M2)-bands display rather different pressure-induced behaviors, becoming more resolved in the high-pressure spectra than in that measured at atmospheric pressure. The octahedral compression modulus of Ni²⁺(M1) in this end-member pyroxene is evaluated as 150 ± 25 GPa, which is noticeably larger than in Ni_0.3 pyroxene. This is due to a smaller size and, thus, a stiffer character of Ni²⁺(M1)O₆ octahedron in the (Ca_0.93Ni_0.07)NiSi₂O₆ pyroxene compared to (Na_0.7Ca_0.3)(Sc_0.7Ni_0.3)Si₂O₆.

Field measurement and modeling of near-bed sediment transport processes with fluid mud layer in Tokyo Bay

Tokyo Bay is one of the estuaries in Japan with a high population of almost 26 million people in the basin area. One of the major concerns for the environment in this water area is the decreasing ecosystem functions including the deterioration of water and sediment qualities caused by various anthropogenic activities. Since the bottom sediments around almost the entire area of the inner bay consist of fine materials with a high organic content, which cause the deterioration of water quality through processes such as hypoxia, an understanding of the fine sediment dynamics in the Bay is crucial for an environmental assessment of the water area. This paper proposes a model for the key processes of fine sediment dynamics, which reflects field data about muddy bed structures and their dynamics obtained during the monitoring campaign in 2007. One of the specific features of the sediment in the Bay at present is the persistent existence of fluid mud layers (water content over 300?%) with a thickness of around a few decimeters, which might be caused by deposition of abundant organic particles due to eutrophication. The present study shows that diffusion flux model delivers quite reliable results for estimating erosion flux from the top of fluid mud layers after calibrating the model parameter against the time series data of vertical flux measured by an acoustic Doppler velocimeter system. This study also derives analytical solutions, based on the Bingham fluid concept, of advection flux in the fluid mud layer on which external shear stress force is applied.     

Symplectic integrators and their application to dynamical astronomy

Symplectic integrators have many merits compared with traditional integrators:     

Changes of microbial populations in a ship's ballast water and sediments on a voyage from Japan to Qatar

Colony-forming eutrophic marine microorganisms in ballast water were counted in samples taken on board in 2002 and 2003. In the ballast water in Japan, viable cell numbers were highly variable but not by more than 10(5.1) colony-forming units (CFU)ml(-1) regardless of season. Even when ballast water was discharged offshore, values varied but not by more than 10(5.0) CFUml(-1). The effectiveness of the ballast water exchange was unconfirmed, except for the February 2003 voyage. No microbial colonies were counted in the reloaded ballast water in the high seas on that voyage, which contributed to the reduction of the total number of viable cells sampled in the discharged ballast water at the Ras Laffan port in Qatar. In sediment samples, the values of 10(5.2) - 10(6.0) CFUml(-1) were estimated for all seasons in which voyages took place. The maximum of the marine Vibrio species, 110 CFUml(-1), was observed in the ballast water sample taken in July 2003. The estimated total viable cell numbers in sediments were higher than those counted in the ballast water throughout the experiments, indicating the importance of sediment management as well as ballast water management on vessels traveling from Japan.     

Long-term change in the status of water pollution in Tokyo Bay: recent trend of increasing bottom-water dissolved oxygen concentrations

Journal of Oceanography - Long-term trends of bottom-water concentrations of dissolved oxygen (DO), total nitrogen, and total phosphorus at 42 sites in Tokyo Bay were analyzed using monthly data...