Location and quantitative analysis of OH in coesite

 The incorporation of hydrogen (deuterium) into the coesite structure was investigated at pressures from 3.1 to 7.5 GPa and temperatures of 700, 800, and 1100 °C. Hydrogen could only be incorporated into the coesite structure at pressures greater 5.0 GPa and 1100 °C . No correlation between the concentration of trace elements such as Al and B and the hydrogen content was observed based on ion probe analysis (1335 ± 16 H ppm and 17 ± 1 Al ppm at 7.5 GPa, 1100 °C). The FTIR spectra show three relatively intense bands at 3575, 3516, and 3459 cm⁻¹ (ν₁ to ν₃, respectively) and two very weak bands at 3296 and 3210 cm⁻¹ (ν₄ and ν₅, respectively). The band at 3516 cm⁻¹ is strongly asymmetric and can be resolved into two bands, 3528 (ν_2a) and 3508 (ν_2b) cm⁻¹, with nearly identical areas. Polarized infrared absorption spectra of coesite single-crystal slabs, cut parallel to (0 1 0) and (1 0 0), were collected to locate the OH dipoles in the structure and to calibrate the IR spectroscopy for quantitative analysis of OH in coesite (ɛ _{i ,tot}=190 000 ± 30 000 l mol⁻¹ _H2O cm⁻²). The polarized spectra revealed a strong pleochroism of the OH bands. High-pressure FTIR spectra at pressures up to 8 GPa were performed in a diamond-anvil cell to gain further insight into incorporation mechanism of OH in coesite. The peak positions of the ν₁, ν₂, and ν₃ bands decrease linearly with pressure. The mode Grüneisen parameters for ν₁, ν₂, and ν₃ are −0.074, −0.144 and −0.398, respectively. There is a linear increase of the pressure derivatives with band position which follows the trend proposed by Hofmeister et al. (1999). The full widths at half maximum (FWHM) of the ν₁, ν₂, and ν₃ bands increase from 35, 21, and 28 cm⁻¹ in the spectra at ambient conditions to 71, 68, and 105 in the 8 GPa spectra, respectively. On the basis of these results, a model for the incorporation of hydrogen in coesite was developed: the OH defects are introduced into the structure by the substitution Si^4+(Si2)+4O²⁻= ^[4]□^(Si2) + 4OH⁻, which gives rise to four vibrations, ν₁, ν_2a, ν_2b, and ν₃. Because the OH(D)-bearing samples do contain traces of Al and B, the bands ν₄ and ν₅ may be coupled to Al and/or B substitution. Received: 19 December 2000 / Accepted: 23 April 2001     

In-situ IR measurements of OH species in quartz at high temperatures

The nature of OH species in natural clear quartz was investigated by means of in-situ IR measurements over the temperature range –185 to 1000 °C. Reversible thermal behavior of OH species was examined for a sample pre-heated to 1000 °C for 1 hour. At room temperature, the IR spectrum of the quartz sample examined includes an intense absorption peak at 3379 cm^–1 which has been assigned to an OH stretching vibration associated with Al substituting for Si (OH(Al)). The major spectral changes of the OH(Al) bond involve a systematic shift of its peak position and a decrease in its integral absorbance with temperature. A quasi-linear increase of the peak position from –185 to 400 °C is interpreted to be due to the change in the vibrational frequency of OH(Al) with hydrogen bond (H bond) distance. At higher temperatures, the IR frequency shows only a slight change, indicating a small influence of the H bond. On the other hand, the gradual decrease of the integral absorbance of OH(Al) with temperature indicates a decrease of this defect’s molar absorptivity without any reduction in defect concentration. This is interpreted to result from a decrease in dipole moment of OH(Al) with temperature. A sudden shift of the vibrational frequency from 3396 to 3386 cm^–1 between 550 and 560 °C and a constant value of the integral absorbance from 535 to 570 °C were considered to be related to the change in H bond distance during the structural transformation of α-quartz to its β-form. The local environment of OH(Al) begins to change at temperatures below 570 °C, where the crystallographic α–β transition occurs. Received: 18 February 1998/ Accepted: 10 July 1998     

Atmospheric oxidation of the pyritic waste rock in Maardu, Estonia. 1 field study and modelling

 A field survey and modelling of the oxidation and carbonate buffering reactions inside the alum-shale-containing waste rock dumps located in Maardu, Estonia, was accomplished. In the slope areas, the shale has been altered at high temperatures due to the spontaneous combustion and the pyritic acidity has been eliminated through migration of SO_x gases out from the dump. In the central parts of the waste rock plateaus, low temperature pyrite oxidation fronts develop towards the dump depth and towards the centres of individual shale lumps. The main secondary phases precipitating in the weathering profile are gypsum, ferric oxyhydroxide, K-jarosite and smectite. The respective field data made it possible to calibrate the two-stage oxygen diffusion model and the characteristic pyrite oxidation rate 0.06–0.08 mol of pyrite reacted per kg of available water (pyrox/H₂O value) was estimated to describe the first tens of years of dump performance. The model is capable to compare different shale disposal strategies that are illustrated with two case scenarios. The buffering of sulphuric acid by Mg-calcite appears to be an incongruent reaction with gypsum precipitating that leads to the build-up of the high Mg/Ca ratio in the leachate. Application of the Mg/Ca method estimates the pyrox/H₂O value in the range of 0.05–0.14 mol/kg. Received: 26 January 1999 · Accepted: 23. February 1999     

Atmospheric oxidation of the pyritic waste rock in Maardu, Estonia, 2: an assessment of aluminosilicate buffering potential

 The assessment of the aluminosilicate buffering potential during acid weathering of the Estonian alum shale is provided. It is found that the stoichiometric interaction between dissolved pyrite oxidation products and illite of the shale best describe the buffering process and are consistent with earlier field studies. The scheme includes incongruent dissolution of illite with smectite and K-jarosite precipitating. This complex mechanism involves buffering of 8% of the acidity by K⁺ and temporary precipitation of 25% of the acidity as K-jarosite. Dissolution proceeds at a low pH (1.5–3) until all pyrite in the shale particle is oxidised. Hence, if the total amount of illite present is larger than needed for stoichiometric interactions, only part of it is involved in a buffering process, neutralising a certain percentage of acidity. The next stage in shale weathering is the incongruent dissolution of K-jarosite with the release of the precipitated acidity and the formation of ferric oxyhydroxide. Received: 3 August 1998 · Revised paper: 26 January 1999 · Accepted: 23 February 1999     

Analysis and speciation of selenium ions in mine environments

 Aqueous extracts of five mine soil samples and a set of selenite (SeO₃ ^2–)–selenate (SeO₄ ^2–) solutions (0.5, 1, 5, 10 and 25 mg/l) were speciated using atomic absorption spectroscopy with hydride generation (AAS-HG) and ion chromatography (IC) to compare these methods for Se speciation. In another experiment, a SeO₃ ^2––SeO₄ ^2––SO₄ ^2– solution (25 mg/l) was reacted with CaO, MgO, MnO₂, CuO, La₂O₃, and WO₃ to evaluate the relative distribution of the Se species and SO₄ ^2– in the SORB (sorbed ions that were desorbed by NaOH), SOLN (equilibrium concentrations), CMPX (irreversibly sorbed and neutral ion pair complexes) fractions. The AAS-HG method was capable of analyzing Se as low as 0.002 mg/l, which was below the detection limit of IC. High concentration of SO₄ ^2– affected the chromatographic Se speciation either by shifting or overlapping Se peaks, in which AAS-HG was more useful. However, IC was capable of speciating aqueous SeO₃ ^2––SeO₄ ^2– directly without any sample pretreatment, whereas AAS-HG measured SeO₃ ^2–+SeO₄ ^2–, and SeO₃ ^2– in separate runs and SeO₄ ^2– was calculated from the difference, i.e., spectrophotometric speciation was an indirect method. For both Se species, AAS-HG and IC data were comparable within detection standard deviations. Ratios of different Se species at measured soil pH and pe indicated that SeO₃ ^2– or SeO₄ ^2– would be the dominant Se species; the p(SeO₄ ^2–/SeO₃ ^2–) values further suggested SeO₄ ^2– would be the major solution species. Except for CaO and MgO treatments, the %SeO₃ ^2– in the SORB fraction was ≥%SeO₄ ^2–. In the SOLN fraction %SeO₄ ^2– was ≥%SeO₃ ^2– for all oxides but CaO, whereas in the CMPX fraction this order was observed for only CaO and MnO₂. The %SeO₃ ^2– was highest in the SOLN fraction for all oxides but MgO and La₂O₃ for these two oxides SeO₃ ^2– dominated in CMPX and SORB fractions, respectively. The SOLN fraction also contained the highest %SeO₄ ^2– for all oxides but MgO which retained SeO₄ ^2– primarily in the SORB fraction. The %SeO₃ ^2– and %SeO₄ ^2– in the CMPX fraction were highest for MgO, thus, suggesting a possible formation of MgSeO₃ ⁰ and MgSeO₄ ⁰. A similar trend of SORB %SO₄ ^2– and SORB %SeO₄ ^2– was attributed to the analogous adsorption mechanisms (outer-sphere complexation). For all oxides but MgO, %SO₄ ^2– was <%SeO₄ ^2– in the SOLN fraction and ≥%SeO₄ ^2– in the CMPX fraction. Comparative ionic distributions provide an overall picture of the relative abundance of different Se species in various fractions associated with different oxides present in micro- and macrolevels in soils and geological materials. Received: 27 December 1995 · Accepted: 29 April 1997     

Dehydration dynamics of Ba-phillipsite: an in situ synchrotron powder diffraction study

 The stepwise dehydration process of the Ba-exchanged form of the zeolite phillipsite was studied by in situ synchrotron X-ray powder diffraction. A series of structure refinements were performed using the Rietveld method on powder diffraction data measured in the interval between 332 and 712 K. At 482 K, more than half of the water molecules were lost. The continuous water loss causes the Ba cations to migrate inside the zeolite channels in order to achieve a stable coordination with the framework oxygens. The dehydration process was completed at 663 K, where a new, completely dehydrated stable phase was detected. The temperature range of stability of this phase was more than 100 K, thanks to the stable coordination of the Ba cations with the framework oxygens. This phase is the first example of completely dehydrated zeolite containing divalent (barium) cations. Received: 8 January 2001 / Accepted: 1 November 2001     

Dehydration process and structural development of cordierite ceramic precursors derived from FTIR spectroscopic investigations

 Cordierite precursors were prepared by a sol-gel process using tetraethoxysilane, aluminum sec.-butoxide, and Mg metal flakes as starting materials. The precursors were treated by 15-h heating steps in intervals of 100 °C from 200 to 900 °C; they show a continuous decrease in the analytical water content with increasing preheating temperatures. The presence of H₂O and (Si,Al)–OH combination modes in the FTIR powder spectra prove the presence of both H₂O molecules and OH groups as structural components, with invariable OH concentrations up to preheating temperatures of 500 °C. The deconvolution of the absorptions in the (H₂O,OH)-stretching vibrational region into four bands centred at 3584, 3415, 3216 and 3047 cm⁻¹ reveals non-bridging and bridging H₂O molecules and OH groups. The precursor powders remain X-ray amorphous up to preheating temperatures of 800 °C. Above this temperature the precursors crystallize to μ-cordierite; at 1000 °C the structure transforms to α-cordierite. Close similarities exist in the pattern of the 1400–400 cm⁻¹ lattice vibrational region for precursors preheated up to 600 °C. Striking differences are evident at preheating temperatures of 800 °C, where the spectrum of the precursor powder corresponds to that of conventional cordierite glass. Bands centred in the “as-prepared” precursor at 1137 and 1020 cm⁻¹ are assigned to Si–O-stretching vibrations. A weak absorption at 872 cm⁻¹ is assigned to stretching modes of AlO₄ tetrahedral units and the same assignment holds for a band at 783 cm⁻¹ which appears in precursors preheated at 600 °C. With increasing temperatures, these bands show a significant shift to higher wavenumbers and the Al–O stretching modes display a strong increase in their intensities. (Si,Al)–O–(Si,Al)-bending modes occur at 710 cm⁻¹ and the band at 572 cm⁻¹ is assigned to stretching vibrations of AlO₆ octahedral units. A strong band around 440 cm⁻¹ is essentially attributed to Mg–O-stretching vibrations. The strongly increasing intensity of the 872 and 783 cm⁻¹ bands demonstrates a clear preference of Al for a fourfold-coordinated structural position in the precursors preheated at high temperatures. The observed band shift is a strong indication for increasing tetrahedral network condensation along with changes in the Si–O and Al–O distances to tetrahedra dimensions similar to those occurring in crystalline cordierite. These structural changes are correlated to the dehydration process starting essentially above 500 °C, clearly demonstrating the inhibiting role of H₂O molecules and especially of OH groups. Received: 1 March 2002 / Accepted: 26 June 2002     

Adsorption, desorption and oxidation of arsenic affected by clay minerals and aging process

 Adsorption/desorption and oxidation/reduction of arsenic at clay surfaces are very important to the natural attenuation of arsenic in the subsurface environment. Although numerous studies have concluded that iron oxides have high affinities for the adsorption of As(V), very little experimental work has addressed the arsenic attenuation capacities of different clay minerals and aging process affecting the transformation of arsenic. The abundance of clay minerals in a variety of geochemical environments and their influence on adsorption of contaminants suggests a need for more experimental work to characterize the adsorption desorption, and oxidation of arsenic on clay minerals. In this investigation three types of clay mineral were studied: the 1 : 1 layer clays [halloysite (IN), sedimentary M-kaolinite, and weathered EPK-kaolinite]; the 2 : 1 layer clays [illite (MT) and illite/montmorillonite (MT)]; the 2 :>: 1 layer clay [chlorite (CA)]. The halloysite and the chlorite had much greater As(V) adsorption (25–35 folds) than the other clay minerals. The clay minerals had lower As(III) adsorption than As(V) adsorption, and the adsorption was affected by pH. Desorption of arsenic from the clay minerals was significantly influenced by the aging process. The quantities of extractable As(III) and As(V) decreased with increasing aging time. The results demonstrated that oxidation of As(III) to As(V) occurred on the clay surfaces, whereas reduction of As(V) to As(III) was not found in any of the clay minerals studied. The oxidation of As(III) was affected by the types of clay and aging time. Received: 22 March 1999 · Accepted: 15 April 1999     

Groundwater processes and sedimentary uranium deposits

 Hydrologic processes are fundamental in the emplacement of all three major categories of sedimentary uranium deposits: syngenetic, syndiagenetic, and epigenetic. In each case, the basic sedimentary uranium-enrichment cycle involves: (1) leaching or erosion of uranium from a low-grade provenance; (2) transport of uranium by surface or groundwater flow; and (3) concentration of uranium by mechanical, geochemical, or physiochemical processes. Although surface flow was responsible for lower Precambrian uranium deposits, groundwater was the primary agent in upper Precambrian and Phanerozoic sedimentary uranium emplacement. Meteoric or more deeply derived groundwater flow transported uranium in solution through transmissive facies, generally sands and gravels, until it was precipitated under reducing conditions. Syndiagenetic uranium deposits are typically concentrated in reducing lacustrine and swamp environments, whereas epigenetic deposits accumulated along mineralization fronts or tabular boundaries. The role of groundwater is particularly well illustrated in the bedload fluvial systems of the South Texas uranium province. Upward migration of deep, reducing brines conditioned the host rock before oxidizing meteoric flow concentrated uranium and other secondary minerals. Interactions between uranium-transporting groundwater and the transmissive aquifer facies are also reflected in the uranium mineralization fronts in the lower Tertiary basins of Wyoming. Similar relationships are observed in the tabular uranium deposits of the Colorado Plateau. Received, May 1998 · Revised, July 1998 · Accepted, September 1998     

Modelling of hillslope runoff processes

 The present study is aimed at modelling hillslope flows with emphasis on subsurface stormflows that involve macropores. The physical processes connected with the runoff process on a hillslope are identified. The components which are considered in modelling the hillslope flow are the nature of flows in the macropore and micropore domains, the spatial and temporal characteristics of the macropore network, the interaction between the domains, and the initiation of flow in the macropores. Both Horton and Dunne's variable source area generation mechanisms are explicitly incorporated in the model. The dominant physical processes governing hillslope runoff are conceptualized in terms of parameters which are derived from the physical properties of the soil, the nature of macropores, and hillslope geometry. The conceptualization of the model is then used to examine infiltration and runoff production. This helps to compute the development of the groundwater table, runoff hydrograph, and soil moisture profile. Received: 5 October 1996 · Accepted: 25 June 1997     

Modeling microbial processes in porous media

 The incorporation of microbial processes into reactive transport models has generally proceeded along two separate lines of investigation: (1) transport of bacteria as inert colloids in porous media, and (2) the biodegradation of dissolved contaminants by a stationary phase of bacteria. Research over the last decade has indicated that these processes are closely linked. This linkage may occur when a change in metabolic activity alters the attachment/detachment rates of bacteria to surfaces, either promoting or retarding bacterial transport in a groundwater-contaminant plume. Changes in metabolic activity, in turn, are controlled by the time of exposure of the microbes to electron acceptors/donor and other components affecting activity. Similarly, metabolic activity can affect the reversibility of attachment, depending on the residence time of active microbes. Thus, improvements in quantitative analysis of active subsurface biota necessitate direct linkages between substrate availability, metabolic activity, growth, and attachment/detachment rates. This linkage requires both a detailed understanding of the biological processes and robust quantitative representations of these processes that can be tested experimentally. This paper presents an overview of current approaches used to represent physicochemical and biological processes in porous media, along with new conceptual approaches that link metabolic activity with partitioning of the microorganism between the aqueous and solid phases. Received, January 1999 · Revised, June 1999, July 1999 · Accepted, October 1999     

Thermodynamic data of the high-pressure phase Mg5Al5Si6O21(OH)7 (Mg-sursassite)

 Calorimetric and P–V–T data for the high-pressure phase Mg₅Al₅Si₆O₂₁(OH)₇ (Mg-sursassite) have been obtained. The enthalpy of drop solution of three different samples was measured by high-temperature oxide melt calorimetry in two laboratories (UC Davis, California, and Ruhr University Bochum, Germany) using lead borate (2PbO·B₂O₃) at T=700 ^∘C as solvent. The resulting values were used to calculate the enthalpy of formation from different thermodynamic datasets; they range from −221.1 to −259.4 kJ mol⁻¹ (formation from the oxides) respectively −13892.2 to −13927.9 kJ mol⁻¹ (formation from the elements). The heat capacity of Mg₅Al₅Si₆O₂₁(OH)₇ has been measured from T=50 ^∘C to T=500 ^∘C by differential scanning calorimetry in step-scanning mode. A Berman and Brown (1985)-type four-term equation represents the heat capacity over the entire temperature range to within the experimental uncertainty: C _P (Mg-sursassite) =(1571.104 −10560.89×T ^−0.5−26217890.0 ×T ⁻²+1798861000.0×T ⁻³) J K⁻¹ mol⁻¹ (T in K). The P V T behaviour of Mg-sursassite has been determined under high pressures and high temperatures up to 8 GPa and 800 ^∘C using a MAX 80 cubic anvil high-pressure apparatus. The samples were mixed with Vaseline to ensure hydrostatic pressure-transmitting conditions, NaCl served as an internal standard for pressure calibration. By fitting a Birch-Murnaghan EOS to the data, the bulk modulus was determined as 116.0±1.3 GPa, (K ^′=4), V _T,0 =446.49 ³ exp[∫(0.33±0.05) × 10⁻⁴ + (0.65±0.85)×10⁻⁸ T dT], (K _T/T) _P  = −0.011± 0.004 GPa K⁻¹. The thermodynamic data obtained for Mg-sursassite are consistent with phase equilibrium data reported recently (Fockenberg 1998); the best agreement was obtained with Δ_f H ⁰ ₂₉₈ (Mg-sursassite) = −13901.33 kJ mol⁻¹, and S ⁰ ₂₉₈ (Mg-sursassite) = 614.61 J K⁻¹ mol⁻¹. Received: 21 September 2000 / Accepted: 26 February 2001     

Raman scattering and X-ray diffraction study of the thermal decomposition of an ettringite-group crystal

 A Raman scattering and X-ray diffraction study of the thermal decomposition of a naturally occurring, ettringite-group crystal is presented. Raman spectra, recorded with increasing temperature, indicate that the thermal decomposition begins at ≈55 °C, accompanied by dehydration of water molecules from the mineral. This is in contrast to previous studies that reported higher temperature breakdown of ettringite. The dehydration is completed by 175 °C and this results in total collapse of the crystalline structure and the material becomes amorphous. The Raman scattering results are supported by X-ray diffraction results obtained at increasing temperatures. Received: 9 July 2001 / Accepted: 14 August 2002     

Incorporation of Cr<Superscript>3+</Superscript> in dickite: a spectroscopic study

 We have investigated a well-ordered sample of natural Cr-bearing dickite from Nowa Ruda (Lower Silesia, Poland) using electron paramagnetic resonance (EPR) at X- and Q-band frequencies (9.42 and 33.97 GHz, respectively) and optical diffuse reflectance spectroscopy. The observation of the spin-forbidden transitions at 15500 and 14690 cm⁻¹ allows us to unambiguously identify the major contribution of octahedrally coordinated Cr³⁺ ions in the optical spectrum. The X- and Q-band EPR spectra show two superposed Cr³⁺ signals. The corresponding fine-structure parameters were determined at room temperature and 145 K. These results suggest the substitution of Cr³⁺ for Al³⁺ in equal proportions in the two unequivalent octahedral sites of the dickite structure. In kaolin group minerals, the distortion around Cr³⁺ ions (λ≈ 0.2–0.4) in Al sites is significantly less rhombic than that observed around Fe³⁺ ions (λ≈ 0.6–0.8). Received: 29 June 2001 / Accepted: 22 October 2001     

The dry oxidation of tetragonal FeS1- x mackinawite

The gradual oxidation of dry mackinawite (tetragonal FeS_{1? x} ) has been studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), transmission Mössbauer spectroscopy (TMS) and X-ray photoelectron spectroscopy (XPS). The initial material and samples exposed to the air (5?min to 6?months) have been analysed. Diffraction patterns showed the slow disappearance of mackinawite with time with concomitant appearance of greigite (Fe₃S₄) and elemental sulphur (S(0)) as well as iron (oxyhydr)oxides, i.e. magnetite (Fe₃O₄) and probably goethite (α-FeOOH). After 6 months' air exposure, mackinawite and also greigite were entirely converted into elemental sulphur and iron (oxyhydr)oxide(s), indicating that greigite was an intermediate reaction product. Mössbauer spectra of samples oxidized in air appeared rather complex for interpreting what was easily conceivable in view of the association of several phases, as revealed by the diffraction patterns. The low-temperature Mössbauer spectrum obtained after 6?months air exposure was attributed to magnetite, although a mixture of magnetite and goethite was not completely excluded. XPS iron and oxygen data confirmed the formation of Fe(III) (oxyhydr)oxides at the surface after an induction period. Sulphur spectra demonstrated various oxidation states from S(-II) (monosulphide) to S(VI) (sulphate) for the longest experiments. Mackinawite in these experiments reacted mainly with adsorbed O₂ to form elemental sulphur and magnetite. Additionally, sufficient sulphur was generated to react stoichiometrically with mackinawite to produce greigite. Finally, greigite, in the longest experiments, was transformed into elemental sulphur and magnetite.     

Pressure dependence of hydroxyl solubility in coesite

 The solubility of hydroxyl in coesite was investigated in multianvil experiments performed at 1200 °C over the nominal pressure range 5–10 GPa, at an f _O2 close to the Ni-NiO buffer. The starting material for each experiment was a cylinder of pure silica glass plus talc, which dehydrates at high P and T to provide a source of water and hydrogen (plus enstatite and excess SiO₂). Fourier-transform infrared (FTIR) spectra of the recovered coesite crystals show five sharp bands at 3606, 3573, 3523, 3459, and 3299 cm⁻¹, indicative of structurally bonded hydrogen (hydroxyl). The concentration of hydrogen increases with pressure from 285 H/10⁶ Si (at 5 GPa) to 1415 H/10⁶ Si (at 10 GPa). Assuming a model of incorporation by (4H)_Si defects, the data are fit well by the equation C _OH=Af ² _H2<\INF>Oexp(−PΔV/RT), with A=4.38 H/10⁶ Si/GPa, and ΔV=20.6 × 10⁻⁶ m³ mol⁻¹. An alternative model entailing association of hydrogen with cation substitution can also be used to fit the data. These results show that the solubility of hydroxyl in coesite is approximately an order of magnitude lower than in olivines and pyroxenes, but comparable to that in pyropic garnet. However, FTIR investigations on a variety of ultrahigh pressure metamorphic rocks have failed in all cases to detect the presence of water or hydrogen in coesite, indicating either that it grew in dry environments or lost its hydrogen during partial transformation to quartz. On the other hand, micro-FTIR investigations of quartz crystals replacing coesite show that they contain varying amounts of H₂O. These results support the hypothesis that preservation of coesite is not necessarily linked to fast exhumation rates but is crucially dependent on limited fluid infiltration during exhumation. Received: 23 August 1999 / Accepted: 10 April 2000     

Groundwater capture processes under a seasonal variation in natural recharge and discharge

 "Capture" is the increase in recharge and the decrease in discharge that occurs when pumping is imposed on an aquifer system that was in a previous state of approximate dynamic equilibrium. Regional groundwater models are usually used to calculate capture in a two-step procedure. A steady-state solution provides an initial-head configuration, a set of flows through the boundaries for the modeled region, and the initial basis for the capture calculation. The transient solutions provide the total change in flows through the boundaries. A difference between the transient and steady-state solutions renders the capture calculation. When seasonality is a modeling issue, the use of a single initial hydraulic head and a single set of boundary flows leads to miscalculations of capture. Instead, an initial condition for each season should be used. This approach may be accomplished by determining steady oscillatory solutions, which vary through the seasons but repeat from year to year. A regional groundwater model previously developed for a portion of the San Pedro River basin, Arizona, USA, is modified to illustrate the effect that different initial conditions have on transient solutions and on capture calculations. Received, September 1996 · Revised, October 1997 · Accepted, October 1997     

Hydrogeochemical analysis of salinization processes in the coastal aquifer of Oropesa (Castellón, Spain)

 The coastal aquifer of Oropesa is affected by salinization processes undoubtedly associated with intense groundwater exploitation for agriculture supply. The aquifer corresponds geologically to a tectonic depression with Plioquaternary fill. Hydrogeologically, this aquifer is detrital, with intergranular porosity, which receives substantial recharge from adjacent Mesozoic aquifers. Contact with the sea, in addition to the presence of cultivated soil requiring extreme exploitation of groundwater, frequently give rise to processes of seawater intrusion. The present research is an attempt to understand the saltwater intrusion in this aquifer, using hydrochemical analyses of the behavior of certain minor ions that could help in the characterization process. In the case of the Oropesa sector, groundwater salinization does not appear to be attributable solely to the intrusion of seawater, but there are also anomalies related largely to the geology of the sector and its surroundings, the type of recharge, the hydrodynamic conditions in the specific area, etc. Received: 23 January 1995 · Accepted: 12 September 1995     

Feasibility and cost of creating an iron-phosphate coating on pyrrhotite to prevent oxidation

 Acid mine drainage (AMD) occurs when sulfide minerals are exposed to an oxidizing environment. Most of the methods for preventing AMD are either short-term or high cost solutions. Coating with iron phosphate is a new technology for the abatement of AMD. It involves treating the sulfide with a coating solution composed of H₂O₂, KH₂PO₄, and sodium acetate as a buffer agent. The H₂O₂ oxidizes the sulfide surface and produces Fe³⁺ so that iron phosphate precipitates as a coating on the sulfide surface. Experiments performed under laboratory conditions prove that an iron phosphate coating can be established on pyrrhotite surfaces with optimal concentrations of the coating solution in the range of: 0.2M/0.01M H₂O₂, 0.2M KH₂PO₄, and 0.2M sodium acetate NaAc, depending on the experimental scale. Iron phosphate coating may be a long-term solution to the problem of AMD. The method would be easy to implement; the reagent cost, however, is not low enough, although it is lower than the conventional treatment with lime. Received: 30 March 1995 · Accepted: 6 September 1995     

The role of geomorphic processes in the transport and fate of mercury in the Carson River basin, west-central Nevada

 The historic processing of precious metal ores mined from the Comstock Lode of west-central Nevada resulted in the release of substantial, but unquantified amounts of mercury-contaminated mill tailings to the Carson River basin. Geomorphic and stratigraphic studies indicate that the introduction of these waste materials led to a period of valley-floor aggradation that was accompanied by lateral channel instability. The combined result of these geomorphic responses was the storage of large volumes of mercury-enriched sediment within a complexly structured alluvial sequence located along the Carson River valley. Much of the contaminated sediment is associated with filled paleochannels produced by the cutoff and abandonment of meander loops, and their subsequent infilling with contaminated particles. Geochemically, these deposits are characterized by variations in mercury levels that exceed three orders of magnitude. Continued lateral instability, coupled with an episode of channel-bed incision, followed the decline of Comstock mining, and has reexposed contaminated debris within the banks of the river. Erosion of bank sediments reintroduces mercury-enriched particles to the modern channel bed. It is suggested on the basis of geochemical and sedimentological data that during the bank erosion process, much of the mercury associated with fine (<63 μ) valley-fill deposits are carried downstream without being incorporated to any appreciable extent within the channel-bed sediments. In contrast, mercury associated with larger and denser particles, particularly mercury-gold-silver amalgam grains, are accumulated in the channel-bed sediments as the river traverses polluted reaches of the Carson River valley. Concentration patterns developed along the modern channel indicate that the valley fill is the primary source of mercury to the river today. Thus, these data imply that efforts to reduce the influx of mercury to the aquatic environment should examine methods for reducing bank erosion rates. Received: 13 December 1996 · Accepted: 15 April 1997