Vertical Profiles of Major Organic Geochemical Constituents and Extracellular Enzymatic Activities in Sandy Sediments of Aransas and Copano Bays, TX

Vertical distribution patterns of organic geochemical constituents and the enzymes aminopeptidase and β-glucosidase provide insights about the nature and reactivity of sediment organic matter in the sandy sediments of two shallow “South Texas” estuaries. Sediment total organic carbon (TOC) δ¹³C values indicated that the organic matter (OM) was derived more from a mixture of seagrass and phytoplankton than from terrigenous OM. Down-core amounts of TOC and total nitrogen (TN) were <0.2% of dry weight, respectively. Enzyme activities were highest near surface and ranged from 25 to 1 μM/h for aminopeptidase as compared to 5 to 0.2 μM/h for glucosidase. In Aransas Bay, aminopeptidase activity correlated with sediment TN content (r _s = 0.30) and β-glucosidase with TOC content (r _s = 0.27). In Copano Bay, aminopeptidase correlated with TOC, TN, and carbohydrate content (r _s = 0.89, 0.90, and 0.83, respectively). Variations of glucosidase activity also related positively to TOC, TN, and total carbohydrate content (r _s = 0.68, 0.77, and 0.48, respectively). Overall, enzyme activities in these low OM, sandy sediments resembled those for other benthic marine environments.     

Selection of bioindicators in coal-contaminated soils of Dhanbad,India

Coal handling, crushing, washing, and other processes of coal beneficiation liberate coal particulate matter, which would ultimately contaminate the nearby soils. In this study, an attempt was made to determine the status of soil bio-indicators in the surroundings of a coal beneficiation plant, (in relation to a control site). The coal beneficiation plant is located at Sudamudih, and the control site is 5 km away from the contaminated site, which is located in the colony of Central Institute of Mining and Fuel Research Institute, Digwadih, Dhanbad. In order to estimate the impact of coal deposition on soil biochemical characteristics and to identify the most sensitive indicator, soil samples were taken from the contaminated and the control sites, and analyzed for soil organic carbon (SOC), soil N, soil basal respiration (BSR), substrate-induced respiration (SIR), and soil enzymes like dehydrogenase (DHA), catalase (CAT), phenol oxidase (PHE), and peroxidase (PER). Coal deposition on soils improved the SOC from 10.65 to 50.17 g kg⁻¹, CAT from 418.1 to 804.11 μg H₂O₂ g⁻¹ h⁻¹, BSR from 8.5 to 36.15 mg CO₂–C kg⁻¹ day⁻¹, and SIR from 24.3 to 117.14 mg CO₂–C kg⁻¹ day⁻¹. Soils receiving coal particles exhibited significant decrease in DHA (36.6 to 4.22 μg TPF g⁻¹ h⁻¹), PHE (0.031 to 0.017 μM g⁻¹ h⁻¹), PER (0.153 to 0.006 μM g⁻¹ h⁻¹), and soil N (55.82 to 26.18 kg ha⁻¹). Coal depositions significantly (P < 0.01) decreased the DHA to 8.8 times, PHE to 1.8 times, and PER to 25.5 times, but increased the SOC to 4.71 times, CAT to 1.9 times, SIR to 4.82 times, and BSR to 4.22 times. Based on principal component analysis and sensitivity test, soil peroxidase (an enzyme that plays a vital role in the degradation of the aromatic organic compounds) is found to be the most important indicator that could be considered as biomarkers for coal-contaminated soils.     

Marine Macrophyte Wrack Inputs and Dissolved Nutrients in Beach Sands

We investigated the role of sandy beaches in nearshore nutrient cycling by quantifying macrophyte wrack inputs and examining relationships between wrack accumulation and pore water nutrients during the summer dry season. Macrophyte inputs, primarily giant kelp Macrocystis pyrifera, exceeded 2.3 kg m⁻¹ day⁻¹. Mean wrack biomass varied 100-fold among beaches (range = 0.41 to 46.43 kg m⁻¹). Mean concentrations of dissolved inorganic nitrogen (DIN), primarily NO_x⁻-N, and dissolved organic nitrogen (DON) in intertidal pore water varied significantly among beaches (ranges = 1 to 6,553 μM and 7 to 2,006 μM, respectively). Intertidal DIN and DON concentrations were significantly correlated with wrack biomass. Surf zone concentrations of DIN were also strongly correlated with wrack biomass and with intertidal DIN, suggesting export of nutrients from re-mineralized wrack. Our results suggest beach ecosystems can process and re-mineralize substantial organic inputs and accumulate dissolved nutrients, which are subsequently available to nearshore waters and primary producers.     

Review: evaporation of mercury from soils. An integration and synthesis of current knowledge

 Understanding the mechanisms of mercury evaporation from soil to the atmosphere is necessary for tracing the fate of mercury in the biological environment and for assessing potential health effects and the impact of anthropogenic mercury emissions on the environment. In this article an integrating overview of the current knowledge of the mechanisms of mercury evaporation is presented. Abiological and biological formation of Hg(0) and/or (CH₃)₂Hg in the uppermost soil layers are the rate limiting processes of mercury evaporation from soils in background areas; the evaporation rate in background areas is probably strongly influenced by deposited airborne mercury. The evaporation rate limiting factors in mercury enriched mineralized areas with large fractions of total mercury being volatile mercury species (relative to background soil in the non-mineralized vicinity) meteorological variations and the transport characteristics of soils for volatile mercury species. Mercury evaporation rates from background soils are usually <0.2 μg·m^–2·h^–1 and significantly smaller than from mercury-enriched mineralized areas. Received: 20 November 1995 / Accepted: 24 July 1996     

Determination of cation distribution in (Co,Ni,Zn)2SiO4 olivine by synchrotron X-ray diffraction

 The cation distribution of Co, Ni, and Zn between the M1 and M2 sites of a synthetic olivine was determined with a single-crystal diffraction method. The crystal data are (Co_0.377Ni_0.396Zn_0.227)₂SiO₄, M _r  = 212.692, orthorhombic, Pbnm, a = 475.64(3), b = 1022.83(8), and c = 596.96(6) pm, V = 0.2904(1) nm³, Z = 4, D _x  = 4.864 g cm⁻³, and F(0 0 0) = 408.62. Lattice, positional, and thermal parameters were determined with MoKα radiation; R = 0.025 for 1487 symmetry-independent reflections with F > 4σ(F). The site occupancies of Co, Ni, and Zn were determined with synchrotron radiation employing the anomalous dispersion effect of Co and Ni. The synchrotron radiation data include two sets of intensity data collected at 161.57 and 149.81 pm, which are about 1 pm longer than Co and Ni absorption edges, respectively. The R value was 0.022 for Co K edge data with 174 independent reflections, and 0.034 for Ni K edge data with 169 reflections. The occupancies are 0.334Co + 0.539Ni + 0.127Zn in the M1 sites, and 0.420Co + 0.253Ni + 0.327Zn in the M2 sites. The compilation of the cation distributions in olivines shows that the distributions depend on ionic radii and electronegativities of constituent cations, and that the partition coefficient can be estimated from the equation: ln [(A/B)_M1/(A/B)_M2] = −0.272 (IR _A -IR _B ) + 3.65 (EN _A −EN _B ), where IR (pm) and EN are ionic radius and electronegativity, respectively. Received: 8 April 1999 / Revised, accepted: 7 September 1999     

Effect of sheep manure and EDTA on the leaching of potassium from heavy metals contaminated calcareous soils

In this study, we examined the movement of potassium (K) in columns of contaminated calcareous soils by sheep manure and ethylene diamine tetraacetic acid (EDTA). Glass tubes, 4.9 cm in diameter and 40 cm in length, were packed with contaminated soils. The resulting 20-cm long column of soil had a bulk density of 1.3–1.4 g cm⁻³. Columns were leached with distilled water, 0.01 M EDTA, 0.01 M CaCl₂, and sheep manure extract solutions. The amounts of K leached varied considerably between different soils (sandy loam and loamy sand) and leaching solutions. The amount leached with EDTA solution, varied from 7.2 to 66.7% of the extractable K when 20 pore volumes had passed through the column. The breakthrough curves of K in the EDTA and CaCl₂ were approximately similar, indicating they have similar ability to displace K from these contaminated calcareous soils. Thus, among leaching solutions application of EDTA and CaCl₂ on contaminated soils might enhance the mobility of K and large amounts of K will be leached.     

Vertical transport of polycyclic aromatic hydrocarbons in different particle-size fractions of sandy soils

Vertical transport of selected polycyclic aromatic hydrocarbons (PAHs) in different particle-size fractions of sandy soils was investigated by simulation experiments in soil columns. Tested soil samples were fractionized into three particle-sizes including sand, coarse silt and fine silt (2,000–50, 50–20 and <20 μm). Rainfall simulations were conducted in artificially PAHs contaminated soil columns with 30 cm length and 5 cm diameter in 40 days. PAHs were extracted from soil samples and determined by high performance liquid chromatography (HPLC). Results showed that the residue level of PAHs in fine silt fraction reached 35.85 mg/kg, which was significantly higher than those in sand and coarse silt fraction (16.28 and 11.80 mg/kg, respectively), probably because PAHs in macroporous fractions were prone to volatilize or degrade compared with that in microporous fractions. Linear relationship between the residue levels of individual PAH (R _PAHs) and the value of partition coefficient (log K _oc) was regressed as R _PAHs = 1.55 × log K _oc − 5.86, R ² = 0.91, n = 9. These results indicated that vertical transport of the mixed PAHs in soils were controlled both by the nature of PAHs (i.e. log K _oc, molecular weight), soil particle size and soil organic contents, which could influence the transport of PAHs.     

Fracture flow and groundwater compartmentalization in the Rollins Sandstone, Lower Mesaverde Group, Colorado, USA

 This paper presents a site-specific conceptual model of groundwater flow in fractured damage zones associated with faulting in a package of sedimentary rocks. The model is based on the results of field and laboratory investigations. Groundwater and methane gas inflows from fault-fracture systems in the West Elk coal mine, Colorado, USA, have occurred with increasing severity. Inflows of 6, 160 and 500 L s⁻¹ discharged almost instantaneously from three separate faults encountered in mine workings about 460 m below ground level. The faults are about 600 m apart. The δ ²H and δ ¹⁸O compositions of the fault-related inflow waters and the hydrodynamic responses of each fault inflow indicate that the groundwaters discharge from hydraulically isolated systems. ¹⁴C data indicate that the groundwaters are as much as 10,500 years old. Discharge temperatures are geothermal (≈30°C), which could indicate upwelling from depth. However, calculations of geothermal gradients, analysis of solute compositions of groundwater in potential host reservoirs, geothermometer calculations, and results of packer testing indicate that the fractured groundwater reservoir is the Rollins Sandstone (120 m thick) directly beneath the coal seams. The packer test also demonstrates that the methane gas is contained in the coal seams. A geothermal gradient of 70–80°C km⁻¹, related to an underlying intrusion, is probably responsible for the slightly elevated discharge temperatures. Large discharge volumes, as great as 8.2×10⁵m³ from the 14 South East Headgate fault (14 SEHG), rapid declines in discharge rates, and vertical and horizontal permeability (matrix permeability generally <0.006 Darcy) indicate fracture flow. An in-mine pumping test demonstrates that the 14 SEHG fault has excellent hydraulic communication with fractures 50 m from the fault. Aeromagnetic data indicate that the faults are tectonically related to an igneous body that is several thousand meters below the coal seams. Exploratory drilling has confirmed a fourth fault, and two additional faults are projected, based on the aeromagnetic data. The conceptual model describes a series of parallel, hydraulically separate groundwater systems associated with fault-specific damage zones. The faults are about 600 m apart. Groundwater stored in fractured sandstone is confined above and below by clayey layers. Received March 1999 / Revised, November 1999 / Accepted, December 1999     

Effects of surfactants and electrolyte solutions on the properties of soil

Biosurfactants are frequently used in petroleum hydrocarbon and dense non-aqueous phase liquids (DNAPLs) remediation. The applicability of biosurfactant use in clayey soils requires an understanding and characterization of their interaction. Comprehensive effects of surfactants and electrolyte solutions on kaolinite clay soil were investigated for index properties, compaction, strength characteristics, hydraulic conductivities, and adsorption characteristics. Sodium dodecyl sulfate (SDS) and NaPO₃ decreased the liquid limit and plasticity index of the test soil. Maximum dry unit weights were increased and optimum moisture contents were decreased as SDS and biosurfactant were added for the compaction tests for mixtures of 30% kaolinite and 70% sand. The addition of non-ionic surfactant, biosurfactant, and CaCl₂ increased the initial elastic modulus and undrained shear strength of the kaolinite–sand mixture soils. Hydraulic conductivities were measured by fixed-wall double-ring permeameters. Results showed that the hydraulic conductivity was not significantly affected, but slightly decreased from 1×10⁻⁷ cm/s (water) to 0.3×10⁻⁷ cm/s for Triton X-100 and SDS. The adsorption characteristics of the chemicals onto kaolinite were also investigated by developing isotherm curves. SDS adsorbed onto soil particles with the strongest bonding strength of the fluids tested. Correlations among parameters were developed for surfactants, electrolyte solutions, and clayey soils.     

Adsorption of Lead (II) and Zinc (II) from Aqueous Solution by Bituminous coal

Activated carbons have been proven to be effective adsorbents for the removal of Pb (II) and Zn (II) dissolved in aqueous media. The study of adsorption of Pb (II) and Zn (II) on two different size fractions from a composite coal sample of Maghara coal mine, C₆₃ (63–125 μm) and C₂₅₀ (125–250 μm) is presented in this paper. C₆₃ and C₂₅₀ were treated in water solutions of 50 mM lead and zinc acetates. X-ray photoelectron spectroscopy (XPS) was used to characterize the starting and treated coal surfaces. The high surface area and surface functional groups (carboxy and phenolic) enable activated bituminous coal of Maghara to act as efficient adsorbents for removing dissolved Pb (II) and Zn (II) in alkaline medium.     

Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50)

 The partitioning of Fe and Mg between the M1 and M2 octahedral sites of olivine has been investigated by in situ time-of-flight neutron powder diffraction. The degree of M-cation order was determined from direct measurements of site occupancies in a synthetic sample of Fo50Fa50 heated to 1250 °C at the Fe-FeO oxygen buffer. Fe shows slight preference for M1 at temperatures below about 600 °C, progressively disordering on heating to this temperature. Above 630 °C, the temperature at which site preferences cross over (T _cr), Fe preferentially occupies M2, becoming progressively more ordered into M2 on increasing temperature. The cation-ordering behaviour is discussed in relation to the temperature dependence of the M1 and M2 site geometries, and it is suggested that vibrational entropy, crystal field effects and changes in bond characteristics play a part in the cross-over of partitioning behaviour. The temperature dependence of site ordering is modelled using a Landau expansion of the free energy of ordering of the type ΔG = −hQ + gTQ +  (T − T _c)Q ² +  Q ⁴, with a/h = 0.00406 K⁻¹, b/h = 2.3, T _c = 572 K and g/h = 0.00106 K⁻¹. These results suggest that the high-temperature ordering behaviour across the forsterite-fayalite join will have a bearing on the activity-composition relations of this important rock-forming mineral, and indicate that Fe-Mg olivine solid solutions become less ideal as temperature increases. Received: 12 August 1999 / Accepted: 25 April 2000     

Mössbauer study of synthetic Mg0.22Fe0.78SiO3 clinopyroxene

Summary  Transmission M?ssbauer spectra of synthetic Ca-free P2₁/c Mg_0.22Fe_0.78SiO₃ clinopyroxene were collected at temperatures in the range 4.2 to 745 K and in an external magnetic field of 60 kOe at 180 K. The magnetic order-disorder transition temperature was determined by M?ssbauer thermoscanning to be 21 ± 3 K. Above this temperature, all M?ssbauer spectra consist of a superposition of two doublets, respectively produced by Fe²⁺ ions at an almost regular octahedral M1 site and at a more distorted octahedral M2 site. The temperature variation of the Fe²⁺ center shifts were analyzed using the Debye model for the lattice vibrations. The characteristic M?ssbauer temperatures were found to be 356 K ± 35 K for M1 and 333 K ± 25 K for M2. From the external field (60 kOe) M?ssbauer spectrum recorded at 180 K, the principal component V _zz of the electric field gradient (EFG) was determined to be positive for both sites but precise values for the magnitudes of the asymmetry parameters η of the EFG could not be determined. The temperature variations of the M1 and M2 quadrupole splittings ΔE _Q(T ) are consistent with the higher distortion of the M2 octahedra. Using the crystal-field model to interpret ΔE _Q(T ), the energy gaps δ₁ and δ₂ of the first excited electronic states within the ⁵D orbital term were estimated to be 410 ± 50 cm⁻¹ and 730 ± 50 cm⁻¹ for M1, and δ₁ = 1050 ± 75 cm⁻¹ for M2. Received May 29, 2000;/revised version accepted July 13, 2001     

Clinopyroxene geobarometry of magmatic rocks. Part 2. Structural geobarometers for basic to acid, tholeiitic and mildly alkaline magmatic systems

The crystal structures of 212 experimentally synthesized, igneous clinopyroxenes were modeled from electronprobe chemical data. The coexisting melts span a wide range of petrologically relevant, dry and hydrous compositions, characterized by variable enrichment in silica and alkalis. Experimental conditions pertain to Earth's crust and uppermost mantle (P = 0–24 kbar; garnet absent) and a variety of f _O2 values (from CCO-buffered to air-buffered) and mineral assemblages (Cpx ± Opx ± Pig ± Ol ± Plag ± Spl ± Mt ± Amp ± Ilm). Unit-cell volume (V_cell) versus M1-polyhedron volume (V_M1) relations were investigated over a range of pressures and temperatures using data derived from structure modeling and corrected for thermal expansivity and compressibility. The relationships between pressure and clinopyroxene structural parameters were found to be dependent on the nature of the coexisting melt. To reduce compositional effects, only clinopyroxenes belonging to mildly alkaline (MA) and tholeiitic (TH) series were considered. Pressure was modeled as a linear function of V_cell, V_M1, and Mg/(Mg + Fe²⁺)^Cpx ratio. A calibration based on the whole data set (MA + TH) reproduced the experimental pressures within 1.4 kbar at the 1-σ level. The maximum residuals were 3.5 kbar and 3.9 kbar for MA- and TH-clinopyroxenes, respectively. Better statistics were obtained by considering MA- and TH-clinopyroxenes separately. A calibration based on the 69 MA-clinopyroxenes reproduced the experimental pressures within 1.1 kbar (1σ) and with a maximum residual of 2.7 kbar. A calibration based on the 143 TH-clinopyroxenes reproduced the experimental pressures within 1.0 kbar (1σ) and with a maximum residual of 3.4 kbar. When these geobarometers are applied to natural samples for which P is unknown, the correction for compressibility is necessarily made through a trial-and-error procedure. This expedient propagates an additional error that increases the above uncertainties and residuals by a factor of about 2. Applications to natural, igneous rocks for which the pressures of crystallization could be constrained based on experimental, petrological or geological evidence yielded pressure estimates that reproduced the expected values to within ca. 2 kbar. Compared to the MA-formulation, the TH-formulation appears to be less robust to variations in magma composition. When applied to high-pressure (>10 kbar) clinopyroxenes synthesized from very low Na (Na₂O < 1.5%) melts, the latter geobarometer can underestimate P by as much as 6 kbar. Calculation of P through the present geobarometers requires clinopyroxene major-element composition and an independent, accurate estimate of crystallization T. Underestimating T by 20 °C propagates into a 1-kbar increase in calculated P. The proposed geobarometers are incorporated in the CpxBar software program, which is designed to retrieve the pressure of crystallization from a clinopyroxene chemical analysis. Received: 11 June 1998 / Accepted: 12 November 1998     

The effect of Cr on the solubility of Al in orthopyroxene: experiments and thermodynamic modelling

The partitioning of chromium and aluminium between coexisting orthopyroxene and spinel in equilibrium with forsterite in the system MgO–Al₂O₃–SiO₂–Cr₂O₃ (MAS–Cr) has been experimentally determined as a function of temperature, pressure and Cr/(Cr + Al) ratio. Experiments were conducted at temperatures between 1300 and 1500 °C and at pressures from 5 to 54 kbar. Previous experimental results on the (Al, Cr)₂O₃ and Mg(Al, Cr)₂O₄ solid solutions have been combined with the present results plus relevant data from the CMAS system to derive a thermodynamic model for Al–Cr-bearing orthopyroxenes, spinels and corundum–eskolaite solid solutions. The orthopyroxene solid solution can be modelled within the accuracy of all experimental constraints as a ternary solid solution involving the components Mg₂Si₂O₆ ( E), MgAl₂SiO₆ (M) and MgCr₂SiO₆ (C), in which the activities are related to composition through the equations: The mole fractions are defined as where n _Al and n _Cr are the number of Al and Cr cations per orthopyroxene formula unit of six oxygens. These expressions reduce to one-site mixing for Mg₂Si₂O₆–MgAl₂SiO₆ orthopyroxenes in the Cr-free system, but are equivalent to two-site mixing for the exchange of Al and Cr between orthopyroxene and spinel, as required by the experimental data. We find W ^opx _EM =W ^opx _EC  = 20 kJ mol⁻¹ and W ^opx _MC =0. Received: 9 August 1999 / Accepted: 18 February 2000     

Suspended Matter,Chl-a,CDOM, Grain Sizes,and Optical Properties in the Arctic Fjord-Type Estuary,Kangerlussuaq, West Greenland During Summer

Optical constituents as suspended particulate matter (SPM), chlorophyll (Chl-a), colored dissolved organic matter (CDOM), and grain sizes were obtained on a transect in the arctic fjord-type estuary Kangerlussuaq (66°) in August 2007 along with optical properties. These comprised diffuse attenuation coefficient of downwelling PAR (K _d(PAR)), upwelling PAR (K _u(PAR)), particle beam attenuation coefficient (c _p), and irradiance reflectance R(−0, PAR). PAR is white light between 400 and 700 nm. The estuary receives melt water from the Greenland Inland Ice and stations covered a transect from the very high turbid melt water outlet to clear marine waters. Results showed a strong spatial variation with high values as for suspended matter concentrations, CDOM, diffuse attenuation coefficient K _d(PAR), particle beam attenuation coefficients (c _p), and reflectance R(−0, PAR) at the melt water outlet. Values of optical constituents and properties decreased with distance from the melt water outlet to a more or less constant level in central and outer part of the estuary. There was a strong correlation between inorganic suspended matter (SPMI) and diffuse attenuation coefficient K _d(PAR) (r ² = 0.92) and also for particle beam attenuation coefficient (c _p; r ² = 0.93). The obtained SPMI specific attenuation—K _d^*(PAR) = 0.13 m² g⁻¹ SPMI—and the SPMI specific particle beam attenuation—c _p^* = 0.72 m² g⁻¹—coefficients were about two times higher than average literature values. Irradiance reflectance R(−0, PAR) was comparatively high (0.09−0.20) and showed a high (r ² = 0.80) correlation with K _u(PAR). Scattering dominated relative to absorption—b(PAR)/a(PAR) = 12.3. Results strongly indicated that the high values in the optical properties were related to the very fine particle sizes (mean = 2–6 μm) of the suspended sediment. Data and results are discussed and compared to similar studies from both temperate and tropical estuaries.     

High-pressure behaviour of Ca-rich C 2 /c clinopyroxenes along the join diopside-enstatite (CaMgSi2O6-Mg2Si2O6)

 An in situ high-pressure (HP) X-ray diffraction investigation of synthetic diopside and of the Ca_0.8Mg_1.2Si₂O₆ clinopyroxene (Di₈₀En₂₀) was performed up to respectively P=40.8 and 15.1 GPa, using high brilliance synchrotron radiation. The compression of the cell parameters is markedly anisotropic, with β_b ⋙ β_c > β_a > β_asinβ for any pressure range and for both diopside and Di₈₀En₂₀. The compressibility along the crystallographic axes decreases significantly with pressure and is higher in Di₈₀En₂₀ than in diopside. The β cell parameter decreases as well with pressure, at a higher rate in Di₈₀En₂₀. The cell volume decreases at almost the same rate for the two compositions, since in diopside a higher compression along a* occurs. A change in the mechanism of deformation at P higher than about 5–10 GPa is suggested for both compositions from the analysis of the strain induced by compression. In diopside at lower pressures, the deformation mainly occurs, at a similar rate, along the b axis and at a direction 145° from the c axis on the (0 1 0) plane. At higher pressures, instead, the deformation occurs mostly along the b axis. In Di₈₀En₂₀ the orientation of the strain axes is the same as in diopside. The substitution of Ca with Mg in the M2 site induces at a given pressure a higher deformation on (0 1 0) with respect to diopside, but a similar change in the compressional behaviour is found. Changes in the M2 polyhedron with pressure can explain the above compressional behaviour. A third-order Birch-Murnaghan equation of state was fit to the retrieved volumes, with K=105.1(9) GPa, K′=6.8(1) for diopside and K=107.3(1.4) GPa, K′=5.7(3) for Di₈₀En₂₀; the same equation can be applied for any pressure range. The elasticity of diopside is therefore not significantly affected by Mg substitution into the M2 site, in contrast to the significant stiffening occurring for Ca substitution into Mg-rich orthopyroxenes. Received: 3 January 2000 / Accepted: 21 May 2000     

Bioavailability of arsenic in the soil horizon: a laboratory column study

Groundwater contaminated with arsenic (As), when extensively used for irrigation, causes potentially long-term detrimental effects to surface soils. Such contamination can also directly affect human health when irrigated crops, such as rice, vegetable and fruits, are used for human consumption. Therefore, an understanding of the leaching behavior of As in surface soils is of high importance, because such behavior may increase the bioavailability of As in the soil horizon. In this study, we have investigated the role of phosphate ions in leaching and bioavailability of As in the soil horizon, where drinking groundwater contains elevated levels of As (≥50 μg/L). Soil and groundwater samples were characterized in the laboratory and measured for physical and chemical constituents. The soils are generally neutral to slightly alkaline in character (pH range 7.5–8.1) with low to moderate levels of free Fe₂O₃, Al₂O₃, CaCO₃, organic carbon, and clay content. The measured electrical conductivity (mean 599 μS/cm) of the soils demonstrates their non-saline nature. The Eh values (range −37 to −151 mV) of the groundwater indicate anoxic condition with low to moderate levels of bicarbonate (range 100–630 mg/L) and phosphate (range 0.002–4.0 mg/L). The arsenic content (range 50–690 μg/L; mean 321 μg/L) in groundwater has exceeded both WHO recommended guideline values (10 μg/L) and the National safe drinking water limit (50 μg/L). Regression analyses demonstrate that the bioavailability of As in the soil horizon is mainly controlled by the composition of free Fe₂O₃ and CaCO₃ content of the soils. However, application of P could increase bioavailability of As in the soil horizon and become available to plants for uptake.     

Thermal expansion and structural transformations of stuffed derivatives of quartz along the LiAlSiO4–SiO2 join: a variable-temperature powder synchrotron XRD study

 The structural behavior of stuffed derivatives of quartz within the Li_{1− x} Al_{1− x} Si_{1+ x} O₄ system (0 ≤ x ≤ 1) has been studied in the temperature range 20 to 873 K using high-resolution powder synchrotron X-ray diffraction (XRD). Rietveld analysis reveals three distinct regimes whose boundaries are defined by an Al/Si order-disorder transition at x=∼0.3 and a β–α displacive transformation at x=∼0.65. Compounds that are topologically identical to β-quartz (0 ≤ x < ∼0.65) expand within the (0 0 1) plane and contract along c with increasing temperature; however, this thermal anisotropy is significantly higher for structures within the regime 0 ≤ x < ∼0.3 than for those with compositions ∼0.3 ≤ x < ∼0.65. We attribute this disparity to a tetrahedral tilting mechanism that occurs only in the ordered structures (0 ≤ x < ∼0.3). The phases with ∼0.65 ≤ x ≤ 1 adopt the α-quartz structure at room temperature, and they display positive thermal expansion along both a and c from 20 K to their α–β transition temperatures. This behavior arises mainly from a rotation of rigid Si(Al)-tetrahedra about the <100> axes. Landau analysis provides quantitative evidence that the charge-coupled substitution of Li+Al for Si in quartz dampens the α–β transition. With increasing Li+Al content, the low-temperature modifications exhibit a marked decrease in spontaneous strain; this behavior reflects a weakening of the first-order character of the transition. In addition, we observe a linear decrease in the α–β critical temperature from 846 K to near 0 K as the Li+Al content increases from x=0 to x=∼0.5. Received: 26 June 2000 / Accepted: 1 December 2000     

Synthetic MgGa2O4-Mg2GeO4 spinel solid solution and β-Mg3Ga2GeO8: chemistry, crystal structures, cation ordering, and comparison to Mg2GeO4 olivine

 Structural parameters and cation ordering are determined for four compositions in the synthetic MgGa₂O₄-Mg₂GeO₄ spinel solid solution (0, 8, 15 and 23 mol% Mg₂GeO₄; 1400 °C, 1 bar) and for spinelloid β-Mg₃Ga₂GeO₈ (1350 °C, 1 bar), by Rietveld refinement of room-temperature neutron diffraction data. Sample chemistry is determined by XRF and EPMA. Addition of Mg₂GeO₄ causes the cation distribution of the MgGa₂O₄ component to change from a disordered inverse distribution in end member MgGa₂O₄, ^[4]Ga = x = 0.88(3), through the random distribution, toward a normal cation distribution, x = 0.37(3), at 23 mol% Mg₂GeO₄. An increase in a_o with increasing Mg₂GeO₄ component is correlated with an increase in the amount of Mg on the tetrahedral site, through substitution of 2 Ga³⁺⇄ Mg²⁺+Ge⁴⁺. The spinel exhibits high configurational entropy, reaching 20.2 J mol⁻¹ (four oxygen basis) near the compositional upper limit of the solid solution. This stabilizes the spinel in spite of positive enthalpy of disordering over the solid solution, where ΔH _D  = αx + βx ², α = 22(3), β = −21(3) kJ mol⁻¹. This model for the cation distribution across the join suggests that the empirically determined limit of the spinel solid solution is correlated with the limit of tetrahedral ordering of Mg, after which local charge-balanced substitution is no longer maintained. Spinelloid β-Mg₃Ga₂GeO₈ has cation distribution ^M1[Mg_0.50(2)Ga_0.50(2)] ^M2[Mg_0.96(2)Ga_0.04(2)] ^M3[Mg_0.77(2) Ga_0.23(2)]₂ (Ge_0.5Ga_0.5)₂O₈ (tetrahedral site occupancies are assumed). Octahedral site size is correlated to Mg distribution, where site volume, site distortion, and Mg content follow the relation M1<M3<M2. The disordered cation distribution provides local electrical neutrality in the structure, and stabilization through increased configurational entropy (27.6 J mol⁻¹; eight oxygen basis). Comparison of the crystal structures of Mg_{1+ N} Ga_{2−2 N} Ge _N O₄ spinel, β-Mg₃Ga₂GeO₈, and Mg₂GeO₄ olivine reveals β-Mg₃Ga₂GeO₈ to be a true structural intermediate. Phase transitions across the pseudobinary are necessary to accommodate an increasing divergence of cation size and valence, with addition of Mg₂GeO₄ component. Octahedral volume increases while tetrahedral volume decreases from spinel to β-Mg₃Ga₂GeO₈ to olivine, with addition of Mg and Ge, respectively. Furthermore, M-M distances increase regularly across the join, suggesting that changes in topology reduce cation-cation repulsion. Received: 9 November 1998 / Revised, accepted: 3 August 1999     

Thermodynamic data from redox reactions at high temperatures. VI. Thermodynamic properties of CoO–MnO solid solutions from emf measurements

Activities of CoO in (Co,Mn)O solid solutions in contact with metallic Co have been determined on ten compositions ranging from 0.12 to 0.84 X_CoO in order to calibrate the divariant equilibrium between (Co,Mn)O oxide solutions and Co metal as an oxygen fugacity sensor for application in experimental petrology. Experiments were conducted over the temperature range 900–1300 K at 1 bar, using an electrochemical technique with oxygen-specific calcia-stabilized zirconia (CSZ) electrolytes. Co + CoO or Fe + FeO was used as the reference electrode. Compositions of the (Co,Mn)O solid solutions were measured after each run by electron microprobe, and these were checked for internal consistency by measuring the lattice parameter by X-ray diffraction. Activity–composition relations were fitted to the Redlich–Kister formalism. (Co,Mn)O solid solutions exhibit slight positive deviations from ideality, which are symmetrical (corresponding to a regular solution mixing model) across the entire composition range with A₀ ^G = 3690(±47) Jmol⁻¹. Excess entropies and enthalpies were also derived from the emf data and gave S^ex=0.77(±0.08) JK⁻¹mol⁻¹ and H^ex=4558(±90) Jmol⁻¹ respectively. The experimental data from this study have been used to formulate the (Co,Mn)O/Co oxygen fugacity sensor to give an expression: where μO₂ ^CoCoO=−492,186 + 509.322 T − 53.284 T lnT + 0.02518 T², taken from O'Neill and Pownceby (1993). Received: 10 September 1999 / Accepted: 4 April 2000