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     

Phase transition of potassium sulfate,K2SO4 (II); dielectric constant and electrical conductivity

The phase transition of K₂SO₄ has been investigated by measurements of the dielectric constant and electrical conductivity, correlated with the structural point of view. Using single crystals, the temperature dependences of the dielectric constants and electrical conductivities were measured at frequencies of 0.3, 1, 3, and 5 MHz in the temperature range from 20° to 640 °C. Within this range, the dielectric constant does not reach a maximum, but near the phase-transition temperature at 587° C, the dielectric constant along the c axis shows a larger discontinuity than those along the a and b axes. The temperature dependence of the dielectric constant is consistent with the disordered structure of the high temperature form. Based on the parabolic increase of the dielectric constant in the temperature range from 582° to 587° C, it is likely that the phase transition propagates through an intermediate state. The electrical conductivity coefficients of K₂SO₄ increase with increasing temperature, exhibiting semiconducting character above the phase-transition temperature. In the high-temperature form, the electrical conductivity along the a axis exceeds that along the c axis. Since the electrical conductivity of K₂SO₄ is mainly ionic in character, the migration of K⁺ ions makes a major contribution to the conduction process.     

Melting and premelting of silicates: Raman spectroscopy and X-ray diffraction of Li2SiO3 and Na2SiO3

The isostructural lithium (Li₂SiO₃) and sodium (Na₂SiO₃) metasilicates have been investigated from room temperature up to the melting point by single-crystal Raman spectroscopy and energy-dispersive X-ray powder diffraction. The unit-cell parameters and Raman frequencies of Li₂SiO₃ vary regularly with temperature up to the melting point, which is consistent with the lack of premelting effects in calorimetric measurements. In contrast, Na₂SiO₃ undergoes a transition at about 850 K from orthorhombic Cmc 2₁ symmetry, to a lower symmetry (possibly Pmc 2₁), and shows near 1200 K changes in the Raman spectra that correlate well with the premelting effects as determined from calorimetry observations. In both compounds, a high alkali mobility likely sets in several hundreds of degrees below the melting point. Premelting in Na₂SiO₃ is associated with extensive deformation of the silicate chains as evidenced near the melting point by similarities in the Raman spectra of the crystalline and liquid phases.     

The passivation of calcite by acid mine water. Column experiments with ferric sulfate and ferric chloride solutions at pH 2

Column experiments, simulating the behavior of passive treatment systems for acid mine drainage, have been performed. Acid solutions (HCl or H₂SO₄, pH 2), with initial concentrations of Fe(III) ranging from 250 to 1500 mg L⁻¹, were injected into column reactors packed with calcite grains at a constant flow rate. The composition of the solutions was monitored during the experiments. At the end of the experiments (passivation of the columns), the composition and structure of the solids were measured. The dissolution of calcite in the columns caused an increase in pH and the release of Ca into the solution, leading to the precipitation of gypsum and Fe–oxyhydroxysulfates (Fe(III)–SO₄–H⁺ solutions) or Fe–oxyhydroxychlorides (Fe(III)–Cl–H⁺ solutions). The columns worked as an efficient barrier for some time, increasing the pH of the circulating solutions from 2 to 6–7 and removing its metal content. However, after some time (several weeks, depending on the conditions), the columns became chemically inert. The results showed that passivation time increased with decreasing anion and metal content of the solutions. Gypsum was the phase responsible for the passivation of calcite in the experiments with Fe(III)–SO₄–H⁺ solutions. Schwertmannite and goethite appeared as the Fe(III) secondary phases in those experiments. Akaganeite was the phase responsible for the passivation of the system in the experiments with Fe(III)–Cl–H⁺ solutions.     

Ion migration in MgSiO3-perovskite and olivine by molecular dynamics calculations

We applied molecular dynamics (MD) simulations to finding likely paths of atomic migration of the Mg ion in forsterite (Mg₂SiO₄) and the oxygen ion in MgSiO₃-perovskite to better understand atomic diffusion in minerals. Our simulations show that there exist two routes of Mg migration in the forsterite structure, that is, paths between the M1 and M2 sites and between the M1 and M1 sites. In the MgSiO₃-perovskite structure, some oxygen ions migrate to the next sites all together through the O1 vacant site showing co-operative movements. The O ions are relatively mobile mainly along the b axis in the perovskite structure. Meta-stable sites are often present between a stable site and another stable site on atomic migration. In spite of many assumptions, our MD simulations may show likely paths of atomic migration in crystal.     

Determination of Fe3+ and Fe2+ concentrations in feldspar by optical absorption and EPR spectroscopy

Ferrous and ferric iron concentrations in feldspars with low total iron content (<0.32 wt% total Fe) were determined from optical and electron paramagnetic resonance (EPR) spectra to better than ±15 percent of the amount present. Optical spectra indicate that Fe²⁺ occupies two distorted M-sites in plagioclases of intermediate structural state. The linear dependence of the Fe²⁺/Fe total ratio on An content demonstrates that Fe²⁺ substitutes for Ca (not Na) so that the number of Ca-sites is a principal factor in iron partitioning in plagioclase. EPR powder spectra show that the number of sites for Fe³⁺ depends on structural state rather than on plagioclase chemistry. The observed linear correspondence of EPR double-integrated intensities with optical peak areas shows that all Fe³⁺ is tetrahedrally coordinated in both plagioclase and disordered potassium feldspar. Microcline perthites show, in addition to tetrahedral Fe³⁺, a signal due to axially coordinated ferric iron, which we associate with formation of hematite inclusions.     

Effect of pressure on sulfate ion association and ultrasonic absorption in sea water

Recent studies on sea water indicated that an increase in pressure of 1000 atm produces a small increase in the concentration of MgSO₄ ion-pairs; this is opposite to the effect observed in aqueous solutions of MgSO₄ alone. This increase has been assumed to imply a corresponding increase with pressure of ultrasonic absorption due to MgSO₄ in sea water, in contrast to the large decrease observed in aqueous solutions. A detailed calculation based on the Eigen and Tamm multistate dissociation model shows, however, that a decrease in ultrasonic absorption with pressure still occurs even though the concentration of ion-pairs increases     

A study of speciation of Sb in bisulfide solutions by X-ray absorption spectroscopy.

Direct evidence of the structure of thioantimonide species in alkaline aqueous solutions is provided by X-ray absorption spectroscopy. Twenty solutions containing thioantimonide species were prepared by dissolving stibnite (Sb₂S₃) in deoxygenated aqueous NaHS solutions; the solution pH range was 8–14, the [Sb_tot] 1–100 mM and the [HS⁻] 0.009–2.5 M. The structural environment of the dissolved Sb was determined by EXAFS analysis of the Sb K-edge over the temperature range 80–473 K.Many of the solutions contain a species with Sb bonded to four S atoms at 2.34 Å, consistent with the presence of a [Sb(V)S₄³⁻] species, demonstrating that oxidation of Sb(III) to Sb(V) has occurred on dissolution. There is evidence that the complementary reduced phase is H₂. In three solutions, the Sb has three nearest neighbor S atoms and two of these solutions have an additional S shell of two atoms at 2.9Å, with one showing evidence of an Sb shell at 4.15 Å. This provides evidence of the presence of multimeric Sb(V) thioantimonide species. Analysis of several solutions reveals the presence of a species with three Sb–S interactions of 2.41–2.42 Å, supporting the presence of a Sb(III) species such as Sb₂S₂(SH)₂. Six solutions have S coordination numbers from 2.7–4 Å and Sb–S distances of 2.37–2.39 Å, and are likely to contain mixtures of at least two species in concentrations such that each make a significant contribution to the EXAFS. There was no clear relationship between either [Sb_tot] or [HS⁻] and the type of species present, but Sb(III) species were only present in the solutions with high pH. The effect of temperature was most significant in one solution, where at 423 K partial hydrolysis occurred and the presence of a species such as Sb₂S₂(OH)₂, with an Sb–O distance of 1.91 Å, is indicated.The study provides new information on the coordination environment of thioantimonide species, complementary to previous studies and provides a basis for a better understanding of Sb speciation in aqueous solutions found in hydrothermal systems, anoxic basins and man-made, high pH environments. In particular it demonstrates the need for Sb(V) to be considered in theoretical and experimental studies of such systems. However, more definitive interpretation of some of the data is inhibited by the presence of mixtures of species and the lack of information on the outer coordination shells that would confirm the presence of multimeric species.     

Compositional diversity in insoluble organic matter in type 1, 2 and 3 chondrites as detected by infrared spectroscopy

Insoluble organic matter (IOM) isolated from 22 carbonaceous and ordinary chondrites spanning a wide range of groups and petrologic types were analyzed using Fourier transform infrared spectroscopy (FTIR). Based on common IR spectral features, it is observed that IOM falls into 4 molecularly distinct groups (designated here as A through D). Spectral group A includes type 1 and 2 chondrites and exhibits intense aliphatic C-H and carboxyl vibrational peaks. Spectral group B includes the least metamorphosed type 3 chondrites and Tagish Lake, and exhibits weaker aliphatic and carboxyl vibrational intensity. Spectral groups C and D include metamorphosed type ?3.1 chondrites and a heated CM chondrite. The carbonyl stretching features in spectral groups C and D differ from that in spectral groups A and B and from each other. In spectral group C, the carbonyl stretching is assigned to cyclic unsaturated lactones; in spectral group D carbonyl exists predominantly in the form of unsaturated ketone moieties. Both spectral groups C and D have a relatively narrow band structure around 1210 cm⁻¹ (assigned to aromatic skeletal modes) as compared with spectral groups A and B, which is consistent with the formation of more condensed aromatics by extensive thermal metamorphism. The differences in carbonyl structures in spectral groups C and D are not the result of different effective metamorphic temperatures, rather these differences likely result from variation in the activity of water and oxygen at different stages of parent body metamorphism. Such environmental variations must be local phenomena in the parent bodies as there is no correlation between spectral grouping and chondrite class or group.     

Interaction between fluorine and silica in quenched melts on the joins SiO2-AlF3 and SiO2-NaF determined by raman spectroscopy

The solubility mechanism of fluorine in quenched SiO₂-NaF and SiO₂-AlF₃ melts has been determined with Raman spectroscopy. In the fluorine abundance range of F/(F+Si) from 0.15 to 0.5, a portion of the fluorine is exchanged with bridging oxygen in the silicate network to form Si-F bonds. In individual SiO₄-tetrahedra, one oxygen per silicon is replaced in this manner to form fluorine-bearing silicate complexes in the melt. The proportion of these complexes is nearly linearly correlated with bulk melt F/(F+Si) in the system SiO₂-AlF₃, but its abundance increases at a lower rate and nonlinearly with increasing F/(F+Si) in the system SiO₂-NaF. The process results in the formation ofnonbridging oxygen (NBO), resulting in stabilization of Si₂O ₅ ^2? units as well as metal (Na⁺ or Al³⁺) fluoride complexes in the melts. Sodium fluoride complexes are significantly more stable than those of aluminum fluoride.     

Phase relations between Ca3Fe 2 3 +Si3O12-Fe 3 2 +Fe 2 3 +Si3O12 garnet and CaFeSi2O6-Fe2Si2O6 pyroxene solid solutions

Editorial responsibility: J. Hoefs     

Determination of Fluorine and Chlorine by Pyrohydrolysis and Ion Chromatography: Comparison with Alkaline Fusion Digestion and Ion Chromatography

A method to determine F and Cl in silicate materials by employing pyrohydrolysis and ion chromatography (IC) is described. Pyrohydrolysis involved mixing a pulverised sample (∼ 40 mg) with V₂O₅ (∼ 160 mg) and heating to 1100 °C under a wet oxygen flow in a quartz tube. Recovery yields of F and Cl were ∼ 97% using a NaF + NaCl standard solution. Detection limits of the pyrohydrolysis-IC method for silicate samples were 0.36 and 0.69 μg g^-1 for F and Cl, respectively. Fluorine and Cl concentrations were determined in the reference materials JB-2, JB-3 and JA-1 from the GSJ; BCR-2, BHVO-1, BHVO-2, AGV-1 and AGV-2 from the USGS; and NIST SRM 610, 612 and 614 glasses. Precisions (RSD) for determinations of F were 1–13% (except NIST SRM 614) and 2–19% for Cl, and were dependent on the concentration and blank correction. Most results obtained in this study were in good agreement with those of previous studies. In comparison, the Na₂CO₃ + ZnO fusion method at 900 °C showed that the yields of F and Cl by alkaline fusion systematically decreased with fusion duration time. The yields were 84% and 83% for JB-3, inferring that F and Cl were lost in this alkaline fusion.     

Ilmenite-type solid solutions between MgSiO3 and Al2O3 and some structural systematics among ilmenite compounds

Complete solid solutions with the ilmenite structure from pure MgSiO₃ to 75% MgSiO₃·25% Al₂O₃ have been synthesized in the pressure region between 240 and 300 kbar at 1000–1400°C in a diamond-anvil cell coupled with laser heating. The results suggest that complete solid solutions with the ilmenite structure might be formed between MgSiO₃ and Al₂O₃ under high pressure-high temperature conditions. The lattice parameters for the ilmenite solid solutions between MgSiO₃ and Al₂O₃ deviate from ideality in the same manner as those found by Berry and Combs along the FeVO₃–Fe₂O₃ join. For the ordered A²⁺ B⁴⁺O₃ ilmenite-type compounds, c_o is determined primarily by the size of the relatively large A²⁺ cation, whereas a_o depends strongly on the radii of both A²⁺ and B⁴⁺ cations. Such systematics might account for the fact that c_o and a_o for the ilmenite-type MgSiO₃ are respectively smaller and larger than those for Al₂O₃. The lattice parameters _ao and molar volumes for the A₂³⁺O₃ corundum-type compounds and the disordered A²⁺B⁴⁺O₃ ilmenites follow a different trend and are, therefore, readily distinguished from the ordered A²⁺B⁴⁺O₃ ilmenites.     

The replacement of anthophyllite by jimthompsonite: a model for hydration reactions in biopyriboles

Anthophyllite crystals found in ultramafic lenses of the Lepontine Alps (Switzerland) contain coherent, submicroscopic intergrowths of ordered and disordered biopyribole polysomes. The chain width distributions of disordered polysomes were analyzed using high resolution transmission electron microscopy (HRTEM). Chains wider than triple were interpreted as intermediate products in the transformation of anthophyllite to the triple chain silicate jimthompsonite. The concentration of individual chain types is strongly correlated with the reaction progress. Based on observed zipper terminations and the transformation rules given by Veblen and Buseck (1980) a scheme of possible reaction paths leading from anthophyllite to jimthomp sonite is proposed. The reaction scheme and a simple kinetic model for elementary reactions allow modeling of the observed chain width distributions. The model suggests that the complex reaction paths involving steps with increasing and decreasing chain width are more important in the formation of jimthompsonite than the direct transformation from anthophyllite. The wide chains (>triple) occurring as intermediate products of the multi-step paths are structurally closer to talc than jimthompsonite. The back-transformation of these wide chains to triple chains is, therefore, a strong argument that jimthompsonite is a stable phase and not only a metastable intermediate product in the transformation of anthophyllite to talc. Received: 8 July 1996 / Accepted: 13 December 1996     

Solubility of silica polymorphs in electrolyte solutions, II. Activity of aqueous silica and solid silica polymorphs in deep solutions from the sedimentary Paris Basin

Activity coefficient for aqueous silica in saline waters and brines from the Paris Basin was calculated using Pitzer's specific interaction model. Quartz and chalcedony are the only reported authigenic silica minerals in the Dogger aquifer of the Paris Basin (France). However, the measured silica concentrations fall between those of these two phases. The silica concentrations measured in Dogger fluids seem to be controlled by a microcrystalline quartz phase with a grain size computed to be about 20 nm. Studies have shown that pressure can preserve small grain size for a long time at the geological scale. The effective mechanism of pressure action is probably linked to the fact that pressure simultaneously favours dissolution at the grain-contact inducing a quartz supersaturation and prohibits the increase in size of reprecipitated microcrystalline quartz grains. This hypothesis is supported by other studies reported in the literature. The proposed model, which incorporates silica mineralogy and a precise calculation of aqueous silica activity, allows us to explain measured silica concentrations in the deep sedimentary solutions of the Dogger aquifers. In the Keuper brines, silica solubility can in most cases be explained by an equilibrium with either chalcedony or quartz. Another application of the present work is shown by an example, where we examined the importance of precisely evaluating the activity coefficient in basin characterisation, as the goal of reservoir characterisation is to describe the spatial distribution of petrophysical parameters such as porosity, permeability, and saturations.     

Electronic absorption spectroscopy of natural (Fe2+, Fe3+)-bearing spinels of spinel s.s.-hercynite and gahnite-hercynite solid solutions at different temperatures and high-pressures

Natural Fe²⁺, Fe³⁺-bearing spinel solid solutions from the spinel s.s.-hercynite and gahnite-hercynite series were analyzed and studied by electronic absorption spectroscopy in the spectral range 30000–3500 cm^–1 in the temperature and pressure ranges 77 T_K 600 and 10^–4 P_GPa 11.0. Two crystals were light-violet in color (type I) and six green or bluish-green (type II). The spectra of both types of spinels are dominated by an UV-absorption edge near 28000 to 24000 cm^–1, depending on the iron contents, and a very intense band system in the NIR centered around 5000 cm^–1, which is caused by spin-allowed dd-transition of tetrahedral Fe²⁺, derived from ⁵ E⁵ T₂. The strong band is in all spinels studied, split into four sub-bands, which can only be observed in very thin platelets. Between the UV-edge and the high-energy wing of the NIR-band there occur a number of very weak bands in type I spinels while the green type II spinels show some of these with significantly enhanced intensity. The intensity of the very weak bands is nearly independent from temperature. Such bands are attributed to spin-forbidden electronic transitions of ^IVFe²⁺. Temperature and pressure dependence of the intensity enhanced bands of spinels type II indicate that they are caused by ^IVFe²⁺ and ^VIFe³⁺. They are attributed to spin-forbidden transitions ⁶A_1g⁴A_1g, ⁴E_g, ⁴T_2g and ⁴T_1g of ^VIFe³⁺, the two latter being strongly intensified by exchange-coupling interaction with adjacent ^IVFe²⁺. The pressure dependence of ^IVFe²⁺ dd-band system in the NIR caused by spin-allowed ⁵ E⁵ T₂ transition noticeably differs from that of octahedral Fe²⁺, an effect which is attributed to a dynamic Jahn-Teller effect of ^IVFe²⁺ in the spinel structure.

Aluminum Enrichment in Silicate Melts by Fractional Crystallization: Some Mineralogic and Petrographic Constraints

The degree of aluminum saturation of an igneous rock may bedescribed by its Aluminum Saturation Index (ASI) defined asthe molar ratio Al₂O₃(CaO + K₂O + Na₂O). One suggested originfor mildly peraluminous granites (ASI between 1 and about 1.1)is by fractional crystallization of subaluminous (ASI < 1)magmas; hornblende, having ASI < 0.5, could be a major drivingforce in such a fractionation process. The efficacy of the processdepends not only on precipitation of hornblende and its effectiveremoval from the reacting system, but on the composition andnature of other coprecipitating phases, weighted by their modalabundances in the reactive system. Precipitation of feldspar(ASI = 1), for instance, would retard or even prevent aluminumenrichment in the melt if the ASI of melt is < 1, but wouldenhance such evolution if the ASI of the melt is > 1. Discussionof the efficacy of any mineral must be made in the context ofthe total reacting system. For hornblende to effectively cause a melt to evolve into aperaluminous composition, it must be able to coexist with peraluminousmagmas. Experimental phase equilibrium data show that at pressure> 5 kb hornblende can coexist with strongly peraluminousmelts (ASI {small tilde} 1.5). Scantily phyric volcanic rocksshow that hornblende can coexist with granitic magma havingASI {small tilde} 1.1 –1.2. The aggregate ASI of last-stageminerals of a typical granite is less than this value; therefore,even after hornblende has reacted out, the residual magma maybe expected to continue to evolve toward more aluminous compositions. Potentials and problems of using coarse-grained granitic rocksto probe courses of magmatic evolution are illustrated by asuite of samples from the Grayling Lake pluton of southwesternMontana. Such rocks probably always contain a large cumulatecomponent in their texture and should not be used as primarymeans to test the occurrence or efficacy of a fractionationprocess that might lead to peraluminous melts. The process isunlikely to give rise to peraluminous plutons of batholithicdimensions. A simple differential equation is presented thatallows the direct use of petrographic data (mineral chemistryand modal abundance) to predict the path of incremental evolutionof a given magma.     

Application of infrared spectroscopy to studies of silicate glass structure: Examples from the melilite glasses and the systems Na2O-SiO2 and Na2O-Al2O3-SiO2

Infrared (IR) and Raman spectroscopic methods are important complementary techniques in structural studies of aluminosilicate glasses. Both techniques are sensitive to small-scale (<15 Å) structural features that amount to units of several SiO₄ tetrahedra. Application of IR spectroscopy has, however, been limited by the more complex nature of the IR spectrum compared with the Raman spectrum, particularly at higher frequencies (1200–800 cm^?1) where strong antisymmetric Si-O and Si-O-Si absorptions predominate in the former. At lower frequencies, IR spectra contain bands that have substantial contributions from ‘cage-like’ motions of cations in their oxygen co-ordination polyhedra. In aluminosilicates these bands can provide information on the structural environment of Al that is not obtainable directly from Raman studies. A middle frequency envelope centred near 700 cm^?1 is indicative of network-substituted AlO₄ polyhedra in glasses with Al/(Al+Si)>0·25 and a band at 520–620cm^?1 is shown to be associated with AlO₆ polyhedra in both crystals and glasses. The IR spectra of melilite and melilite-analogue glasses and crystals show various degrees of band localization that correlate with the extent of Al, Si tetrahedral site ordering. An important conclusion is that differences in Al, Si ordering may lead to very different vibrational spectra in crystals and glasses of otherwise gross chemical similarity.     

Iron speciation in minerals and glasses probed by $$\hbox{M}_{2/3}$$-edge X-ray Raman scattering spectroscopy

We present a spectroscopic study of the iron \(\hbox{M}_{2/3}\)-edge for several minerals and compounds to reveal information about the oxidation state and the local coordination of iron. We describe a novel approach to probe the iron \(\hbox{M}_{2/3}\)-edge bulk sensitively using X-ray Raman scattering. Significant changes in the onset and shape of the Fe \(\hbox{M}_{2/3}\)-edge were observed on ferrous and ferric model compounds with Fe in octahedral and tetrahedral coordination. Simulation of the spectra is possible using an atomic multiplet code, which potentially allows determination of, e.g., crystal-field parameters in a quantitative manner. A protocol is discussed for determination of the Fe oxidation state in compounds by linear combination of spectra of ferric and ferrous end members. The presented results demonstrate the capabilities of Fe \(\hbox{M}_{2/3}\)-edge spectroscopy by X-ray Raman scattering to extract information on the ratio of trivalent to total iron \(\hbox{Fe}^{3+}/\sum \hbox{Fe}\) and local coordination. As X-ray Raman scattering is performed with hard X-rays, this approach is suitable for in situ experiments at high pressure and temperature. It thus may provide indispensable information on oxidation state, electronic structure and local structure of materials that are important for physical and chemical processes of the deep Earth.