High pressure polymorphism of dicalcium silicate Ca2SiO4. A transmission electron microscopy study

Ca₂SiO₄ dicalcium silicate has been transformed at high pressure in a diamond-anvil cell (DAC) coupled with a YAG laser heater, in order to study the high-pressure modifications of this compound. Starting material was the olivine form of Ca₂SiO₄ (γ-polymorph). Several samples have been synthesized at loading pressures of 4.5, 10 and 15 GPa respectively, at room temperature. Other samples have been obtained at pressures ranging between 4.5 and 45 GPa and temperatures estimated to be about 2500 °C. The study of the quenched high pressure and/or high temperature phases has been performed using analytical transmission electron microscopy (ATEM) and X-ray diffraction (XRD). All the polymorphs of Ca₂SiO₄ usually produced with high temperatures, including α-Ca₂SiO₄, have been observed in the samples recovered from the high-pressure experiments. The α′-Ca₂SiO₄ and α-Ca₂SiO₄ polymorphs have been obtained at ambient conditions for the first time without stabilizing impurities. A new modification of α′-Ca₂SiO₄ has also been synthesized. Finally, the breakdown at high-pressure and temperature of Ca₂SiO₄ into CaSiO₃ and CaO is reported.     

Complex conductivity response to microbial growth and biofilm formation on phenanthrene spiked medium

Several laboratory studies have recently demonstrated the utility of geophysical methods for the investigation of microbial-induced changes over contaminated sites. However, it remains difficult to distinguish the effects due to the new physical properties imparted by microbial processes, to bacterial growth, or to the development of bacterial biofilm. We chose to study the influence of biofilm formation on geophysical response using complex conductivity measurements (0.1-1000 Hz) in phenanthrene-contaminated media. Biotic assays were conducted with two phenanthrene (PHE) degrading bacterial strains: Burkholderia sp (NAH1), which produced biofilm and Stenophomonas maltophilia (MATE10), which did not, and an abiotic control. Results showed that bacterial densities for NAH1 and MATE10 strains continuously increased at the same rate during the experiment. However, the complex conductivity signature showed noticeable differences between the two bacteria, with a phase shift of 50 mrad at 4 Hz for NAH1, which produced biofilm. Biofilm volume was quantified by Scanning Confocal Laser Microscopy (SCLM). Significant correlations were established between phase shift decrease and biofilm volume for NAH1 assays. Results suggest that complex conductivity measurements, specifically phase shift, can be a useful indicator of biofilm formation inside the overall signal of microbial activity on contaminated sites.     

Experimental and numerical study of the effect of variable bathymetry on the slow-drift wave response of floating bodies

An experimental campaign is reported on the slow-drift motion of a rectangular barge moored at different positions along an inclined beach, at waterdepths ranging from 54 cm to 21 cm, and submitted to irregular beam seas. The beach is achieved by inclining the 24 m long false bottom of the tank at a slope of 5%, from a depth of 1.05 m. The slow-drift component of the measured sway motion is first compared with state-of-the-art calculations based on Newman’s approximation. At 54 cm depth a good agreement is obtained between calculations and measurements. At 21 cm depth the Newman calculations exceed the measured values. When the flat bottom setdown contribution is added up, the calculated values become 2 to 3 times larger than the measured ones. A second-order model is proposed to predict the shoaling of a bichromatic sea-state propagating in varying water-depth. This model is validated through comparisons with an extension of Schäffer’s model for a straight beach [Schäffer HA. Infragravity waves induced by short-wave groups. J Fluid Mech 1993;247:551-88] and with a fully nonlinear Boussinesq model. It appears that the long wave amplitude is much less than predicted by the flat bottom model, and that its phase difference with the short wave envelope also deviates from the flat bottom model prediction. As a result of this phase shift the actual second-order wave loads can be lower than predicted by Newman’s approximation alone. Application of the shoaling model to the barge tests yields a notably better agreement between numerical and experimental values of its slow-drift sway motion.     

High-pressure Raman spectroscopic study of Mn2GeO4, Ca2GeO4, Ca2SiO4, and CaMgGeO4 olivines

We present a Raman spectroscopic study of the structural modifications of several olivines at high pressures and ambient temperature. At high pressures, the following modifications in the Raman spectra are observed: 1)?in Mn₂GeO₄, between 6.7 and 8.6?GPa the appearance of weak bands at 560 and 860?cm^?1; between 10.6 and 23?GPa, the progressive replacement of the olivine spectrum by the spectrum of a crystalline high pressure phase; upon decompression, the inverse sequence of transformations is observed with some hysteresis in the transformation pressures; this sequence may be interpreted as the progressive transformation of the olivine to a spinelloid where Ge tetrahedra are polymerized, and then to a partially inverse spinel; 2)?in Ca₂SiO₄, the olivine transforms to larnite between 1.9 and 2.1?GPa; larnite is observed up to the maximum pressure of 24?GPa and it can partially back-transform to olivine during decompression; 3)?in Ca₂GeO₄, the olivine transforms to a new structure between 6.8 and 8?GPa; the vibrational frequencies of the new phase suggest that the phase transition involves an increase of the Ca coordination number and that Ge tetrahedra are isolated; this high pressure phase is observed up to the maximum pressure of 11?GPa; during decompression, it transforms to a disordered phase below 5?GPa; 4)?in CaMgGeO₄, no significant modification of the olivine spectrum is observed up to 15?GPa; between 16 and 26?GPa, broadening of some peaks and the appearance of a weak broad feature at 700–900?cm^?1 suggests a progressive amorphization of the structure; near 27?GPa, amorphization is complete and an amorphous phase is quenched down to ambient pressure; this unique behaviour is interpreted as the result of the incompatibilities in the high pressure behaviour of the Ca and Mg sublattices in the olivine structure.     

Groundwater beneath the urban area of Khan Younis City,southern Gaza Strip (Palestine): hydrochemistry and water quality

Arabian Journal of Geosciences - This paper presents the results of a groundwater hydrochemical assessment in the urban area of Khan Younis City in the southern Gaza Strip by measuring its...     

Earthquake activity in the Southern Vanuatu arc recorded by the Texas digital OBS

We have reconfigured the Texas digital ocean bottom seismograph (OBS) to operate in a triggered mode and record regional earthquake signals. This paper reports the results of a deployment program designed to test these digital OBS, by moni toring earthquake activity in and near the trench in southern Vanuatu (formerly, the New Hebrides). We successfully recorded hundreds of earthquakes, including 133 located regional earth quakes recorded by three or more stations. We also report J-B residuals for 21 earthquakes reported and located by the ISC. Fourier analysis of seismograms from regional earthquakes suggest that the frequencies of spectral peaks at any station were nearly the same for large, small, nearby, and distant events. However, we obtained very disparate frequencies when we analyzed seismograms for the same earthquake recorded at different stations. The most plausible interpretation is that spectral peaks do not depend on the characteristics of the earthquake source, but instead on site characteristics, or, more specifically, on the coupling of the instrument to the seafloor. To record reliable spectral data, we need to overcome this problem.     

Geodynamic aspects of the basic intrusions of central and Western Nepal

A petrological study of the basic rocks of magmatic activity in the Pokhara and Beri Khola regions of Central and Western Nepal suggests that the volcanic activities were not related to collision or continental subduction.     

Groundwater beneath the urban area of Khan Younis City,southern Gaza Strip (Palestine): assessment for multi-domestic purposes

The present study research investigation is aimed to assess the groundwater quality for the urban area in Khan Younis City, southern Gaza Strip, for multi-domestic purposes. The physicochemical analysis of the groundwater wells shows the major ions in the order of Na⁺ > Mg²⁺ > Ca²⁺ > B³⁺ > K⁺ and Cl^? > HCO₃ ^? > SO₄ ^2? > NO₃ ^2? > F^? > PO₄ ^3?. Groundwater quality is classified as very hard-brackish water type. Ninety-five percent of the wells are classified as saline water type with high NO₃ ^2? concentrations. Based on water quality index (WQI), the groundwater falls into one of three categories: fair water (10%), poor water (15%), very poor (45%), and worst (30%). The high WQI values are because of high Na⁺, Cl^?, SO₄ ^2?, and NO₃ ^2? concentrations, while synthetic pollution index (SPI) values indicate that most about 80% of the wells are seriously polluted. Langelier Saturation Index (LSI) indicates that most of data are either slightly scale forming or corrosive water or slightly corrosive but non-scale forming, and 75% of the wells are suitable for construction purposes (have SO₄ ^2? concentrations <300 mg/L). The groundwater reaches alarming situation, where almost chemically unsuitable for drinking purposes and the water to be used after proper treatment such as desalination.