Effect of impurities on grain growth in synthetic calcite aggregates

 We investigated grain growth of calcite aggregates fabricated from crushed natural single crystals with different impurity content. The total trace-element concentration of the starting powders varied from about 170 ppm to more than 930 ppm with Mn as the major component. Samples were produced by hot-isostatic pressing of the different powders at 300 MPa confining pressure at 600 °C for 2 h. The starting material for the anneals was dry and had a uniform microstructure with an average grain size of about 5 μm and a porosity of <2.1%. Three disks with Mn concentrations of 10, 350, and 670 ppm, respectively, were annealed in the same run at a confining pressure of 300 MPa, and temperatures between 700 and 900 °C for up to 20 h. Grain growth was fastest in samples with the highest Mn concentrations. A multivariable fit to the data yields grain-growth exponents of 2.0 ± 0.3 for samples with 10 ppm Mn and 2.3 ± 0.2 for those with 670 ppm Mn. The activation energies for grain growth vary from 99 ± 12 kJ mol⁻¹ to 147 ± 14 kJ mol⁻¹ for the respective calcite compositions. Received: 22 August 2000 / Accepted: 12 March 2001     

Synthesis of feldspar bicrystals by direct bonding

 We have produced synthetic feldspar bicrystals using a direct bonding technique. A gem-quality orthoclase crystal from Itrongay, Madagascar, was used for the bonding experiments. Microprobe analysis shows only minor concentrations of iron and sodium. Orthoclase single crystal plates oriented parallel (0 0 1) were cut and chemomechanically polished with silica slurry. From interferometry, final roughness of the square crystal plates was about 0.34 nm. Specimens were wet-chemically cleaned using deionised water. The bonding procedure produced an orthoclase bicrystal with an optically straight grain boundary-oriented parallel (0 0 1), which was investigated by HREM. Along the interface no amorphous layer was observed between lattice fringes of both crystals. We suggest that the bicrystals formed by initial hydrogen bonding and subsequent water loss and polymerisation of silanol and aluminol groups at elevated temperatures. Received: 19 February 2001 / Accepted: 16 May 2001     

Temperature dependent grain boundary migration in deformed-then-annealed material: Observations from experimentally deformed synthetic rocksalt

Grain boundary migration between strained, substructured grains and newly appearing, strain free grains has been observed during static in-situ annealing of pre-deformed rocksalt in the SEM. With increasing temperature (T) the migration velocity increases and the character of grain boundary migration changes. As temperature increases there is an increase in the length of individual migrating boundary segments that move at similar rates. In addition, the frequency of migrating boundaries that form traces of a {100} boundary plane of at least one of the crystals involved increases, and moving grain boundaries between new and old grains change from highly irregular to smooth, straight boundaries. At the same time there is a decrease in the influence of the substructure of pre-existing strained grains on the grain boundary movement. Resultant microstructures reflect these changes. At  325–350 °C, the deformed-then-annealed microstructure is characterized by very irregular grain boundaries, a high abundance of 5–50 m scale remnants of old, substructured grains within new grains, giving a poikilitic microstructure. At  350–400 °C, grain boundaries often exhibit elongate embayments into the strained grains and most remnants of old, strained grains are located at former grain boundaries. At > 400 °C, grain boundaries between new and old, strained grains are straight to smoothly curved.The grain boundary velocity observations are explained by the effect of temperature on mobility coupled with local driving force variations. Additionally, at low annealing temperature, impurity (solute) drag and driving-force variations are influential, while at high temperature the anisotropy in grain boundary energy with crystallographic orientation becomes more important. Transferring the knowledge from our experiments to geological samples enables us to recognize and interpret similar microstructures in rocks, thereby making it is possible to relate microstructural characteristics to the pre-annealing and post-deformational annealing history.     

Atomistic simulation of the effect of impurities on vacancy migration at the {4 1 0}/[0 0 1] tilt grain boundary of MgO

We present the results of atomistic simulations to calculate the effect of impurities on vacancy migration at the {4 1 0} tilt grain boundary of MgO. We show that impurities within the mantle could significantly modify vacancy migration within the mantle due to segregation to grain boundaries. This segregation increases with increasing impurity size and with increasing pressure. The impurities have little effect on vacancy migration at 0 GPa but at higher pressures the impurities alter the activation energies such that the vacancies prefer to migrate towards them. This suggests that at mantle conditions the vacancies will be pinned both by the compression of the boundary and also the impurities, slowing diffusion. Received: 22 February 1999 / Revised, accepted: 1 June 1999     

Grain misorientations in partially molten olivine aggregates: an electron backscatter diffraction study

In polycrystalline aggregates of olivine with mean grain sizes above 35 μm plus a low basaltic melt fraction, both wetted and melt-free grain boundaries are observed after equilibration times at high pressures and temperatures of between 15 and 25 days. In order to assess a possible dependence of the wetting behaviour on the relative orientation of neighbouring grains, a SEM based technique, electron backscatter diffraction (EBSD), is used to determine grain orientations. From the grain orientations relative orientations of neighbouring grains are calculated, which are expressed as misorientation axis/angle pairs. The distribution of misorientation angles and axes of melt-free grain boundaries differ significantly from a purely random distribution, whereas those of wetted grain boundaries are statistically indistinguishable from the random distribution. The relative orientation of two neighbouring grains therefore influences the character of their common grain boundary. However, no clustering towards special (coincident site lattice) misorientation axes is observed, with the inference that the energy differences between special and general misorientations are too small to lead to the development of preferred misorientations during grain growth. Received: 8 December 1997 / Revised, accepted: 6 April 1998     

Metals and grain size distributions in soil of the middle Rio Grande basin, Texas USA

 This paper deals with the problem of increased heavy metal constituents in agricultural soils due to the expanded use of fertilizers and elevated atmospheric deposition. It discusses the extent of contamination in soil and establishes an environmental monitoring program in the chosen area of concern in the southern coastal region of Texas. Grain size, pH, and metals (Cu, Cd, Zn, Pb, Ni, Ba, As, Cr, Mn, and Fe) were determined in soils of the middle Rio Grande basin. The soils were mainly of sand texture and alkaline in character. Fine sand constituted the major proportion of the soil, and clay and silt ranged from 8–30% of the soil. Correlations of metal concentrations to grain size and iron contents were performed. Metals, except Cd and Pb, gave positive to negative relationships with decreases in grain size. Silt gave no relationship with metal content while clay and silt had a positive relationship. All these metals had a positive correlation with iron in the soil. The results indicate metals are associated with coarse sand, clay, and iron hydroxides surfaces of the soil. The comparison of metal content in soil of the middle Rio Grande basin with metals from other areas of the world suggests that it is relatively uncontaminated. Received: 14 December 1998 · Accepted: 19 Jaunuary 1999     

The fractal shape of sutured quartz grain boundaries: application as a geothermometer

Along grain boundaries of quartz from metamorphic and igneous rocks complex interfingering (sutures) may occur. Under the light microscope the lengths of the sutures range from approximately 10–1000 m. The sutured grain boundaries are statistically self-similar over one to two orders of magnitude. They represent fractals. Their mathematical counterpart are Koch curves which are developed after two to four iterations. The fractal (Hausdorff-Besicovitch) dimensions D of sutured quartz grain boundaries from different grades of metamorphism are between ca. 1.05 and 1.30. The D-value decreases with increasing temperature during formation of the sutures. On a statistical basis, D can be used as a measure of this temperature and, therefore, as a deformation-related thermometer.     

An XRD, TEM and Raman study of experimentally annealed natural monazite

 The healing of radiation damage in natural monazite has been experimentally studied in annealing experiments using XRD, TEM, Raman microprobe and cathodoluminescence analysis. The starting material was a chemically homogeneous monazite from a Brazilian pegmatite with a concordant U–Pb age of 474 ± 1 Ma and a U–Th/He age of 479 Ma. The monazite shows nm-scale defects induced by radioactive decay. The X-ray pattern of the unheated starting material revealed two distinct monazite “phases” A and B with slightly different lattice parameters. Monazite A shows sharp reflections of high amplitudes and slightly expanded lattice parameters (1% in volume) compared to a standard monazite. Phase B exhibits very broad reflections of low amplitudes. Two sets of experiments were performed. First, dry monazite powder was annealed at 500, 800 and 1000 °C for 7 days. Each run product was analysed by X-ray diffractometry. Second, monazite grains were hydrothermally annealed at temperatures from 500 to 1200 °C for 5 to 15 days. TEM observations show that partial healing of the monazite lattice already occurred at 500 °C and increased gradually with temperature, so that after 10 days at 900 °C complete healing was achieved. The observations are interpreted accordingly: the starting material has a mosaic structure consisting of two domains, A and B, which are basically two monazite crystals with different lattice parameters. We suggest that the A domains correspond to well-crystallised areas where helium atoms are trapped. The accumulation of He causes expansion of the A monazite lattice. Diffraction domains B are interpreted as a helium-free distorted monazite crystal lattice, which can be referred to old alpha-recoil tracks. These B domains are composed of “islands” with an expanded lattice, induced by the presence of interstitials, and “islands” of a compressed monazite lattice, induced by presence of vacancies. Both the islands will pose stress on the lattice in the vicinity of the islands. The broadening of the B reflections is due to the expanded or compressed diffraction domains and to the different amount of the distortion. With increasing temperature the unit-cell volume of monazite A decreases, i.e. the position of the A reflections shifts towards smaller d _hkl values. This was interpreted as a relaxation of the monazite lattice due to helium diffusion out of the monazite lattice. Simultaneously, the nm-sized defect domains B are healed. At 900–1000 °C only a monazite with well-crystallised lattice and minimum unit-cell volume is observed. Received: 7 May 2001 / Accepted: 11 October 2001     

Channel constituents in synthetic beryl: ammonium

 The infrared spectra and electron paramagnetic resonance (EPR) of channel constituents in beryls synthesized hydrothermally in the presence of NH₄Cl were investigated. Two forms of ammonium ion were observed to be incorporated into the c -channel. IR-spectra show the double band at 3295 and 3232 cm⁻¹ and two broad bands between 2600 and 3000 cm⁻¹ which were assigned to the NH₃ molecule and NH₄ ⁺ ion, respectively. Similar N–H stretching vibrations are also observed in Regency hydrothermal synthetic beryls and can be used to separate these synthetic beryls from their natural counterparts. After γ-irradiation of hydrothermally grown samples at 77 K, the EPR of the NH₃ ⁺(I) radical was observed. The NH₃ ⁺(II) radical replaces the NH₃ ⁺(I) radical when the sample is heated to room temperature. Both the NH₃ molecule and the NH₃ ⁺ radical have their C₃ symmetry axes perpendicular to the crystal c-axis. The spin Hamiltonian parameters of the NH₃ ⁺(I) are axial-symmetric due to the rapid rotation of the radical about the c-axis. The NH₃ ⁺(II) radical has a low symmetry and shows a hindered rotation because of its shift from the c-axis position and an interaction with the proton in the near neighbourhood. Possible models for the paramagnetic centres are discussed. Received: 16 May 2000 / Accepted: 5 July 2001     

Time evolution of twin domains in cordierite: a computer simulation study

The time and temperature evolution of twinning in cordierite is simulated using three computer models. The orientation of walls between twin domains in natural cordierite follows mainly the ferroelastic pattern which minimises the strain energy of the walls between twin-related domains. Such ferroelastic twinning is simulated in an elastic three-states Potts model in which each structural six-membered ring is represented by a three state pseudo-spin. The resulting twin pattern in a sample with 3169 structural rings shows sector trilling and fine scale ferroelastic wall patterns which coarsen with increasing annealing time. The poorly defined wall directions observed in cordierite were found to be related to twin walls which do not minimise the strain energy. Instead, these walls are located along the corners of pseudo-hexagonal rings and appear as the consequence of local rather than global interatomic interactions. Simulations using two-dimensional (38028 atoms) and three-dimensional (408 228 atoms) structural models show a predominance of these topological walls over the strain walls at early stages in the ordering process. The domain structure in the simulation is patchy rather than corresponding to repeated stripe structures found in other ferroelastic and co-elastic materials. In all models, a strong tendency for sector trilling is observed. In kinetic tweed patterns a novel 60o tweed is found at atomic length scales while the usual strain-mediated 90o tweed appears at mesoscopic length scales. An unusual surface tension effect in domain formation and ’writhing’, fluid-like motion was found in the three-dimensional structural model. This motion, along with the existence of non strain-mediated walls may contribute to cordierite’s poorly defined domain wall directions at the early stages of domain coarsening. Received: 11 March 1998 / Revised, accepted: 28 August 1998     

Pressure-induced phase transition in synthetic trioctahedral Rb-mica

 The crystal structure of a synthetic Rb analog of tetra-ferri-annite (Rb–TFA) 1M with the composition Rb_0.99Fe²⁺ _3.03(Fe³⁺ _1.04 Si_2.96)O_10.0(OH)_2.0 was determined by the single-crystal X-ray diffraction method. The structure is homooctahedral (space group C2/m) with M1 and M2 occupied by divalent iron. Its unit cell is larger than that of the common potassium trioctahedral mica, and similar lateral dimensions of the tetrahedral and octahedral sheets allow a small tetrahedral rotation angle α=2.23(6)°. Structure refinements at 0.0001, 1.76, 2.81, 4.75, and 7.2 GPa indicate that in some respects the Rb–TFA behaves like all other micas when pressure increases: the octahedra are more compressible than the tetrahedra and the interlayer is four times more compressible than the 2:1 layer. However, there is a peculiar behavior of the tetrahedral rotation angle α: at lower pressures (0.0001, 1.76, 2.81 GPa), it has positive values that increase with pressure [from 2.23(6)° to 6.3(4)°] as in other micas, but negative values −7.5(5)° and −8.5(9)° appear at higher pressures, 4.75 and 7.2 GPa, respectively. This structural evidence, together with electrostatic energy calculations, shows that Rb–TFA has a Franzini A-type 2:1 layer up to at least 2.81 GPa that at higher pressure yields to a Franzini B-type layer, as shown by the refinements at 4.75 and 7.2 GPa. The inversion of the α angle is interpreted as a consequence of an isosymmetric displacive phase transition from A-type to B-type structure between 2.81 and 4.75 GPa. The compressibility of the Rb–TFA was also investigated by single-crystal X-ray diffraction up to a maximum pressure of 10 GPa. The lattice parameters reveal a sharp discontinuity between 3.36 and 3.84 GPa, which was associated with the phase transition from Franzini-A to Franzini-B structure. Received: 21 October 2002 / Accepted: 25 February 2003     

A TEM and RUM study of the inherent displacive flexibility of the fresnoite framework structure type

 A careful electron diffraction study has been made of the incommensurately modulated room-temperature phases of the fresnoites Ba₂TiGe₂O₈ (BTG) and Ba₂TiSi₂O₈ (BTS) and used to determine their (3+1)- and (3+2)-dimensional superspace group symmetries. The primitive primary modulation wave vectors in both materials are found to occur close to the same position in the parent Brillouin zone, near ∼0.3〈110〉 _p ^*+1/2c _p ^st . A rigid unit mode (RUM) analysis of the inherent displacive structural flexibility of the ideal fresnoite framework structure type is then carried out in an attempt to understand the significance of this particular modulation wave vector. Six zero-frequency RUM modes and two close to zero frequency quasi-RUM (Q-RUM) modes are found to exist for any modulation wave vector. These RUM modes are all primarily associated with rotations of the constituent TO₄ (T=Si or Ge) tetrahedra and TiO₅ square pyramids around in-plane i.e. perpendicular to c rotation axes. A seventh RUM mode involving rotation of the constituent rigid polyhedra around c combined with shifts in the basal plane is found but only at a very specific modulation wave vector q∼0.30〈110〉 _p ^* , in close agreement with the condensed RUM mode found in the electron diffraction study. It is the condensation of just such a RUM mode that appears to play a major role in the various incommensurately modulated structures observed in Ba₂TiGe₂O₈, Ba₂TiSi₂O₈ and Sr₂TiSi₂O₈, respectively. Received: 26 November 2001 / Accepted: 25 May 2002 Present address: Y. Tabira Materials Characterization Laboratory Mitsui Mining & Smelting Co. Ltd. 1333-2 Haraichi, Ageo, Saitama 362-0021, Japan     

Rigid Unit Modes in disordered nepheline: a study of a displacive incommensurate phase transition

 Calculations of the Rigid Unit Modes (RUMs) allowed in the nepheline structure are used to explain the diffuse scattering previously seen in electron diffraction experiments. The RUM calculations also show that the modulation wavelength for incommensurate nephelines is essentially determined by the framework topology. X-ray diffraction is used to measure the intensity of the diffuse scattering as a function of temperature. The diffuse intensity increases sharply at 308 K. This effect is interpreted as being due to the softening of a phonon mode, indicating a phase transition. Measurements of this phase transition below the transition temperature are made using hard mode infrared spectroscopy. Received: 17 February 1999 / Revised, accepted. 15 October 1999     

High-temperature creep in a Ni2GeO4: a contribution to creep systematics in spinel

 High-temperature creep behavior in Ni₂GeO₄ spinel was investigated using synthetic polycrystalline aggregates with average grain sizes ranging from submicron to 7.4 microns. Cylindrical samples were deformed at constant load in a gas-medium apparatus at temperatures ranging from 1223 to 1523 K and stresses ranging from 40 to 320 MPa. Two deformation mechanisms were identified, characterized by the following flow laws: where σ is in MPa, d is in μm and T is in Kelvin. These flow laws suggest that deformation was accommodated by dislocation creep and grain-boundary diffusion (Coble) creep, respectively. A comparison with other spinels shows that an isomechanical group can be defined for spinels although some differences between normal and inverse spinels can be identified. When creep data for olivine and spinel are normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the dislocation creep regime and is considerably stronger than olivine in the diffusion creep regime at coarse grain size. However, when grain-size reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity (Coble creep behavior). Analysis of normalized diffusion creep data for olivine and spinel indicate that spinel is weaker than olivine at grain sizes less than 2 μm. Received: 18 June 2000 / Accepted: 3 April 2001     

The crystal chemistry of birefringent natural uvarovites. Part III. Application of the superposition model of crystal fields with a characterization of synthetic cubic uvarovite

Synthetic, flux-grown uvarovite, Ca₃Cr₂ [SiO₄]₃, was investigated by optical methods, electron microprobe analysis, UV-VIS-IR microspectrometry, and luminescence spectroscopy. The crystal structure was refined using single-crystal X-ray CCD diffraction data. Synthetic uvarovite is optically isotropic and crystallizes in the “usual” cubic garnet space group Ia3¯d [a=11.9973 Å, Z=8; 21524 reflections, R1=2.31% for 454 unique data and 18 variables; Cr–O=1.9942(6), Si–O=1.6447(6), Ca–O_a=2.3504(6), Ca–O_b= 2.4971(6) Å]. The structure of Ca₃Cr₂[SiO₄]₃ complies with crystal-chemical expectations for ugrandite group garnets in general as well as with predictions drawn from “cubically averaged” data of non-cubic uvarovite–grossular solid solutions (Wildner and Andrut 2001). The electronic absorption spectra of Cr³⁺ in trigonally distorted octahedra of synthetic uvarovite were analyzed in terms of the superposition model (SM) of crystal fields. The resulting SM and interelectronic repulsion parameters are =9532 cm^?1, =4650 cm^?1, power law exponent t ₄=6.7, Racah B₃₅=703 cm^?1 at 290 K (reference distance R ₀=1.995 Å; fixed power law exponent t ₂=3 and spin-orbit parameter ζ=135 cm^?1). The interelectronic repulsion parameters Racah B ₅₅=714 cm^?1 and C=3165 cm^?1 were extracted from spin-forbidden transitions. This set of SM parameters was subsequently applied to previously well-characterized natural uvarovite–grossular solid solutions (Andrut and Wildner 2001a; Wildner and Andrut 2001) using their extrapolated Cr–O bond lengths to calculate the energies of the spin-allowed bands. These results are in very good agreement with the experimentally determined band positions and indicate the applicability of the superposition model to natural 3d ^N prevailing systems in geosciences. Single-crystal IR absorption spectra of synthetic uvarovite in the region of the OH-stretching vibration exhibit one isotropic absorption band at 3508 cm^?1 at ambient conditions, which shifts to 3510 cm^?1 at 77 K. This band is caused by structurally incorporated hydroxyl groups via the (O₄H₄)-hydrogarnet substitution. The water content, calculated using an integral extinction coefficient ?=60417 cm^?2 l mol^?1, is c _H2O=33 ppm.     

The effect of distribution of iron-oxyhydroxide grain coatings on the transport of bacterial cells in porous media

 Among the demonstrated processes influencing the transport of bacteria through aquifers, the deposition of cells on mineral surfaces is one of the most important. For example, understanding the transport of introduced bacteria through aquifers is essential to designing some in situ bioremediation schemes. The impact of the presence and distribution of Fe(III)-oxyhydroxide-coated sand grains on bacterial transport through porous media was evaluated in column experiments in which bacteria (short rods; 1.2 μm length) were eluted through columns of quartz sand (0.5–0.6 mm in diameter) for several conditions of chemical heterogeneity of mineral substrate. Fe(III)-oxyhydroxide-coated sand was present as 10% of the mass, and it was arranged in three treatments: (1) homogeneously distributed, and present as a discrete layer (2) at the top and (3) at the bottom of 14-cm-long sand columns. A pulse input of 10⁸ cells ml^–1 was introduced in an artificial groundwater solution flowing at 14 cm h^–1 through the column, and eluted cells were counted. Peak breakthrough occurred at 1.0 pore volume. A large proportion of cells were retained; 14.7–15.8% of the cells were recovered after three pore volumes of solution had eluted through clean quartz sand, and only 2.1–4.0% were recovered from the Fe(III)-oxyhydroxide-coated sand mixtures. The three physical arrangements of the chemical heterogeneity resulted in essentially the same breakthrough of cells, indicating that the spatial distribution of iron coating does not affect the transport of bacteria. The results of the column transport experiments, which mimic hydrogeological conditions encountered in field problems, are consistent with our mechanistic understanding of bacterial sorption. Received: 10 April 1996 · Accepted: 17 February 1997     

Structural changes in sodium tetrasilicate glass around the liquid-glass transition: a neutron diffraction study

The structure of sodium tetrasilicate (Na₂Si₄O₉) glass and melt was studied in the range from 300 to 950?K by neutron diffraction. Increasing temperature leads to gradual decrease of the peak intensities in the static structure factors possibly with a change in the slope at the glass transition temperature (T _g?773?K), but no shift and broadening of the peaks is observed. Especially, the position of the first sharp diffraction peak (FSDP) at 1.6?Å^–1 remains constant in the whole temperature range studied. The corresponding pair correlation functions g(r) are very similar at all temperatures. Only a slight broadening of the Si-O and O-O first nearest-neighbour peaks with temperature is observed, which can be attributed to temperature enhanced dynamic distortions of the SiO₄ tetrahedra. All these results suggest that there is little change not only in the short- but also in the medium-range order of the sodium tetrasilicate glass and melt around the glass-liquid transition.     

Ion migration in nepheline: a dielectric spectroscopy and computer modelling study

 Using a combination of dielectric spectroscopy and atomistic computer simulation techniques, the dynamical behaviour of the loosely bound (Na⁺ and K⁺) channel ions in nepheline has been investigated. The low-frequency dielectric properties of a natural Bancroft nepheline have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation in the dielectric loss spectrum was observed; the activation energy for this process was determined to be 1.38 ± 0.02 eV. Atomistic simulation techniques were used to elucidate the mechanism and energetics of cation migration. A mechanism involving the hopping of Na⁺ ions between oval sites and partially occupied hexagonal (K⁺) sites, via a bottleneck consisting of a distorted sixfold ring of (Al,Si)O₄ tetrahedra, was found to give a calculated energy barrier in very good agreement with the experimentally determined activation energy. These results confirm the nature of the process responsible for the observed dielectric behaviour. Overall, this study demonstrates the intrinsic, microscopic control of cation diffusion processes in rock-forming minerals. Identifying specific energy barriers and preferred diffusion pathways is fundamental to the prediction of diffusion energetics. Received: 8 May 2000 / Accepted: 21 July 2000     

Crystal chemistry of iron in natural grandidierites: an X-ray absorption fine-structure spectroscopy study

 Fe–K edge XAFS spectra (pre-edge, XANES and EXAFS) were collected for eight grandidierites from Madagascar and Zimbabwe, as well as for Fe(II) and Fe(III) model compounds (staurolite, siderite, enstatite, berlinite, yoderite, acmite, and andradite). The pre-edge spectra for these samples are consistent with dominantly 5-coordinated ferrous iron. The analysis of the XANES and EXAFS spectra confirms that Fe(II) substitutes for Mg(II) in grandidierite, with a slight expansion of the local structure around Mg by ∼2%. In addition, ferric iron was also detected in some samples [5–10 mol% of the total Fe or 500–1100 ppm Fe(III)]. Based on theoretical calculations of the EXAFS region, Fe(III) appears to be located in the 5-coordinated sites of Mg(II) or in the most distorted 6-coordinated sites of Al (depending on the sample studied). Special attention is therefore required when using grandidierite as a model for ferrous iron in C_3v geometry, because of the possible presence of an extra contribution related to Fe(III). This additional contribution enhances significantly the Fe–K pre-edge integrated area [+40% for 1000 ppm Fe(III)]. Therefore, only a few grandidierite samples can be used as a robust structural model for the study of the Fe(II) coordination in glasses and melts. Received: 26 June 2000 / Accepted: 19 February 2001     

Factor analysis in hydrogeochemistry of coastal aquifers – a preliminary study

 This paper examines the results of R-mode factor analysis performed on major ion data from a hydrogeochemical survey over the coastal Quaternary deltaic aquifer of the Cauvery Basin, Tamil Nadu, India. Seven major ions (Ca, Mg, Na, K, HCO₃, Cl, and NO₃) were analyzed from each of the 126 water samples collected in two seasons (pre- and post-monsoon 63 each). A set of factors was found both in pre-monsoon and post-monsoon data which explained the source of the dissolved ions and the chemical processes which accompany the intrusion of seawater. Received: 4 March 1996 · Accepted: 28 August 1996