Alkali ion migration in albite and K-feldspar

As part of a study of ion migration mechanisms in feldspars, the dynamical behaviour of the alkali metal cations ions in albite and K-feldspar has been investigated using a combination of dielectric spectroscopy and atomistic computer simulation techniques. The low-frequency dielectric properties of these minerals 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 spectra was observed for both albite and K-feldspar; the activation energy for these processes was determined to be 1.33 eV in both albite and K–feldspar. Atomistic simulation techniques were used to elucidate the mechanism and energetics of the cation migration processes. Mechanisms involving the conventional hopping of Na⁺ and K⁺ ions between cation sites in the (010) plane were found to give calculated energy barriers in good agreement with the experimentally determined activation energies. These results assist in understanding the nature of the processes responsible for the observed dielectric behaviour.     

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     

阳离子对斜发沸石物理化学性能的影响

利用离子交换方法可以制备出K-型、Na-型和Ca-型斜发沸石。斜发沸石所含的阳离子类型不同,其物理化学性能有明显的差异。本文就不同类型的阳离子对斜发沸石的某些物理化学性能的影响进行了研究。     

Diagenetic transformation of clinoptilolite to analcime in silicic tuffs of Hokkaido, Japan

Diagenetic clinoptilolite and analcime are present in the diagenesis of Tertiary marine silicic tuffs in the MITI-Toyokoro borehole, Hokkaido, Japan. Chemical analysis, SEM and petrographic observations show the mechanism of clinoptilolite to analcime transformation. Na-K clinoptilolite (NaK=6:4), an alteration product of volcanic glass, is detected at depths of 1230 to 2710m. A more Na-rich clinoptilolite (Na: K = 9:1) discovered at depths between 2590 and 2710 m is a result of a cation exchange mechanism. The formation of analcime at greater depths is due to a microdissolution-precipitation mechanism, that is Na-rich clinoptilolite dissolved and analcime precipitated as a result of increasing burial. At 2710 m analcime occurs next to the Na-rich clinoptilolite and it is assumed that the Na-rich clinoptilolite is an intermediate phase.     

斜发沸石岩的阳离子交换性能——对钾离子选择吸附的研究

斜发沸石是天然沸石中较常见和具有实际应用价值的一种。其化学分子式为:Ca（Na·K）₄Al₆Si₃₀O₇₂·24H₂O。关于斜发沸石的吸附、阳离子交换等性能,国外已有很多研究,并已将天然的斜发沸石、丝光沸石应用于从空气中富集氧、石油化工、原子能放射性废水和工业污水的处理、干燥剂,土壤改良剂、造纸充填剂等许多方面。     

Oil decolorization properties of the Emirler clinoptilolite (Bigadiç, Turkey)

Oil decolorization properties of the Emirler clinoptilolite and its acid-activated forms were studied using neutralized cottonseed oil. Although the oil decolorization capacity of natural clinoptilolite is low, it can be improved by a factor of 1.8 after HCl activation under the optimized conditions (1M HCl, solid/liquid ratio = 1/20, T = 25°C, t = 1 hour). Acid activation increases the total specific surface area from 40 to 70 m²/g which is mostly realized with a pore diameter range smaller than 15 nm. The bleaching capacity of acid-activated clinoptilolite is comparable to that of natural bentonitic clays but it is 1.6 times lower than the capacity of activated bleaching earths such as Tonsil and others. However, the amount of oil absorbed by clinoptilolite during the bleaching process is almost half that compared with the commonly used natural and acid-activated clays.     

Mineralogical Characteristics and Cationic Exchange Properties of a Zeolite Mineral from Romana, NW Sardinia, Italy

An investigation was conducted into the Tertiary ignimbrite formations in the Romana area in NW Sardinia. Scanning electron microscopy–energy-dispersive spectrometry indicated the presence of clinoptilolite and subordinate smectite, quartz, mordenite and calcite. Laboratory analyses of representative samples collected from the deposit indicated a cation exchange capacity ranging from 85 to 135 mEq per 100 g. Ion exchange experiments conducted in flasks containing different initial cation concentrations and in columns supplied with solution having constant cation concentration, showed different selectivity towards the cations examined, in the order: NH₄⁺> Pb²⁺> Cd²⁺> Cu²⁺> Zn²⁺> Mn²⁺> Ni²⁺. Compared to the untreated material, under the experimental conditions used and considering the eluate, and the Italian regulatory limits for discharge into surface waters, the zeolite treated with a 1-N NaCl solution yielded a 1.3–5-fold increase in the volume of treated water, depending on the cation examined. The experimental results obtained show that the cationic exchange properties of the zeolite rock from the Romana deposit are comparable with the average quality of zeolites currently imported into Italy from non-European countries.     

Temperature-jump induced cation exchange kinetics in (Co<Subscript>0.1</Subscript>Mg<Subscript>0.9</Subscript>)<Subscript>2</Subscript>SiO<Subscript>4</Subscript> olivine: an in situ optical spectroscopic study

In the olivine crystal structure, cations are distributed over two inequivalent octahedral sites, M1 and M2. Kinetics of cation exchange between the two octahedral sites in (Co_0.1Mg_0.9)₂SiO₄ single crystal have been studied in the temperature range from 600 to 800°C by monitoring the time evolution of the absorbance of Co²⁺ ions in M1 or M2 sites using optical spectroscopy after rapid temperature jumps. It was found from such temperature-jump induced relaxation experiments that with increasing temperature the absorbance of Co²⁺ ions in the M1 site decreases while that in the M2 site increases. This indicates a tendency of Co²⁺ cations to populate the M2 site with increasing temperatures and vice versa. The experimental relaxation data can be modeled using a triple exponential equation based on theoretical analysis. Activation energies of 221 ± 4 and 213 ± 10 kJ/mol were derived from relaxation experiments on the M2 site and M1 site, respectively, for the cation exchange processes in (Co_0.1Mg_0.9)₂SiO₄ olivine. Implications for cation diffusion at low temperatures are discussed.     

Phase transitions in leucite: Dielectric properties and transition mechanism

To investigate the mechanisms for the cubic-tetragonal phase transition in leucite (KAlSi₂O₆), the frequency dependence of the dielectric constant (), and of the electric conductivity (), have been measured as a function of temperature. The dielectric loss function, tan , contains two main features: (i) a classical Debye peak, with activation energy of 0.77 eV, ascribed to hopping of K-atoms between their channel (W) sites, via the vacant side-channel (S) sites; (ii) a heavily-overdamped relaxational mode which softens when the crystal is cooled towards the phase transition temperature. The latter relaxational mode shows a critical behaviour, and is thus directly correlated with the transition mechanism. As it is only the potassium ions that could relax at frequencies well below the optical phonon branches, it appears that their movement is relaxational (i.e. heavily overdamped) rather than phonon-like. At temperatures above the transition point, the relaxation of K⁺ in an electric field is analysed in terms of collective motions within tetragonal domains. Direct evidence for the existence of such domains follows from the presence of diffuse intensity in single-crystal X-ray diffraction experiments.     

江苏镇江沸石矿石特征研究

通过对江苏镇江沸石矿石某钻孔样品和开采露头捡块样进行X衍射分析、化学成分分析、吸铵量分析、吸蓝量分析、阳离子交换容量、酸碱度分析等,与样品的颜色相结合分析沸石特征,该钻孔沸石岩矿石的矿物组成主要为斜发沸石、蒙脱石等。     

On the driving force of cation exchange in clays: Insights from combined microcalorimetry experiments and molecular simulation

We study the origin of the ionic exchange enthalpy in montmorillonite clays using microcalorimetry measurements and molecular simulation. We first determine the standard reaction enthalpy for well-defined interlayer water contents. We then show by a detailed analysis based on thermodynamic cycles that replacing Na⁺ ions by Cs⁺ in the interlayer of montmorillonite clays is an endothermic process, and that the overall exchange is exothermic only because it is dominated by the exothermic replacement of Cs⁺ by Na⁺ in the aqueous phase. This conclusion from ionic exchange enthalpies supports the one of a recent study of the ionic exchange free energy by Teppen and Miller [Teppen B. J. and Miller D. M. (2006) Hydration energy determines isovalent cation exchange selectivity by clay minerals. Soil Sci. Soc. Am. J.70(1), 31-40] and contradicts long-held views on the role of ion-clay interactions in determining the ionic exchange thermodynamics. This calls for a paradigm shift for the origin of this exchange: The driving force is the “hydrophobicity” of Cs⁺ compared to Na⁺ and not its affinity for clay surfaces.     

Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation

The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO₂²⁺) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10⁻³ M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH)₆] octahedral sites over [Al(O,OH)₆] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO₂. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO₂.     

Spectroscopic study of heavy metals sorption on clinoptilolite

Sorption of heavy metal cations (Pb(II), Cr(III), Cd(II), Ni(II)) from aqueous solutions on natural Na-clinoptilolite was studied using atomic absorption spectrometry (AAS) and FT-IR spectroscopy. It was found that the sorption capacity of clinoptilolite decreases in the following order: Pb(II) (22,600 mg/kg), Cr(III) (21,200 mg/kg), Cd(II) (10,400 mg/kg) and Ni(II) (6,200 mg/kg). In the FT-IR spectra of the samples, in the region of pseudolattice vibrations (500–800 cm^–1), systematic changes connected with the type of cation and its concentration in the initial solution were observed. The proportions of ion exchange and chemisorption in the whole process of sorption were also estimated. It was found that the amount of cations sorbed on clinoptilolite depended on the concentrations and pH of the solutions used as well as on the contact time of zeolite-solution system. After 120 min of the reaction, despite the metal type, 90–100% of the total amount of cations were immobilized.     

Ion mobility and dynamic disordering of water in analcime

Ions and water molecules mobilities were studied by the ⁷Li, ²³Na, ¹H nuclear magnetic resonance in the framework aluminosilicate mineral analcime and in its cation-exchanged (lithium) form. It has been found that cation exchange as well as dehydration alter radically dynamics of particles interposed in channels of the zeolite framework. The highest mobility of alkali cations with the diffusion activation energy of 0.52 eV was detected in hydrated Li-analcime. The ion diffusion increase with temperature is accompanied by an increase in the dynamic orientational disordering of water sublattice. The energy of formation of structural defects responsible for ion diffusion and water disordering was deduced to be 0.2 eV. As a possible defect, the occupation of water positions by cations is discussed (model of bridging-type ion diffusion).     

河北省赤城县独石口沸石岩矿物学特征及其性能研究

独石口沸石岩共生矿物组合为斜发沸石、蒙脱石、α-方石英和少量丝光沸石。斜发沸石呈微晶产于玻璃质岩屑和火山尘中,也呈板状产于玻屑和珍珠岩溶解形成的孔洞中,沸石总量大于70％;化学成分显示高Si、高Al、富K、富Ca、贫Na的特征;阳离子交换容量高,交换性阳离子以Ca为主,其次为K,Na,Mg;阳离子选择交换顺序为NH_4~+>K~+>Ca~(2+)>Na~+>Mg~(2+);不同阳离子类型的沸石岩其热稳定性和吸附性有差异,K型和Na型沸石岩可稳定至700℃,而Ca型沸石岩在500℃时结构被破坏;Ca型沸石岩吸附性好,而K型差。     

我国东部白垩纪蚀变火山岩中的斜发沸石和丝光沸石

沸石是一种含水的碱和碱土金属的骨架状铝硅酸盐。由于沸石具有分子筛的作用,在应用中通常又将沸石称为分子筛。沸石作为催化剂、干燥剂、吸附剂和离子交换剂广泛地应用于工农业各个方面。天然沸石作为一种新型的矿物资源越来越为人们所重视。     

Study of the electrical charge transport in ilvaite using impedance spectroscopy and thermopower data

The electrical conduction in the mineral ilvaite was studied between ≈170 and 450 K. A natural ilvaite from Elba (Italy) was found to be semiconducting with a DC conductivity 1.8×10^–3 (Ωcm)^–1 at 300 K, measured parallel (∥) to the [001] direction; perpendicular (⊥) to [001] it was 1.4×10^–5 (Ωcm)^–1, i.e. the conductivity is highly anisotropic. The conduction is effected by a hopping charge transport between localized levels in the energy gap associated with activation energies E _A =0.3–0.5 eV. It is concluded that impurities (Mg,Al,Mn) may play a decisive role in the charge hopping transport ∥ [001] that is basically governed by Fe²⁺-Fe³⁺ pairs on A-sites of the lattice as the source of electrons. Although the E_A-values were similar for both measured directions, the sign of thermopower is different which points to different charge transfer mechanisms. The bulk DC conductivity σ_DC ^AC for measurements ∥ [001], obtained by extrapolation of AC data using impedance spectroscopy, could only be determined at T<300 K owing to sample–electrode interfacial effects. In contrast, the bulk σ_DC ^AC⊥ [001] showed a slight break at ≈380 K that may reflect the structural phase transition monoclinic→orthorhombic at ≈345 K. From AC conductivity measurements in the frequency range 20 Hz–1 MHz at T<300 K, a dispersive character of electronic relaxation was found, resembling that of amorphous semiconductors and of impurity conduction in crystalline semiconductors where it was ascribed to charge hopping processes of electrons between localized levels of cation pairs or clusters of limited lengths. Received: 4 August 1997 / Revised, accepted: 12 January 1998     

Magic angle spinning NMR observation of sodium site exchange in nepheline at 500° C

Dynamics in minerals at time scales from seconds to microseconds are important in understanding mechanisms of displacive phase transitions, diffusion, and conductivity. High resolution, magic-angle-spinning (MAS) NMR spectroscopy can directly show the rates of exchange among sites, potentially providing less model-dependent information than more traditional NMR relaxation time measurements. Here we use a newly developed high temperature MAS probe (Doty Scientific, Inc.) to observe the exchange of Na⁺ among the alkali sites in a high-Na nepheline at temperatures as high as 500° C. Observed exchange rates are consistent with correlation times derived from cation diffusivity.     

Ligand field theory and inter- and intracrystalline cation distribution of transition elements in minerals and related inorganic compounds

Many independent investigations have shown, that the ligand field stabilization energy LFSE has, in addition to other factors such as ionic size, site distortion, covalency, respectively, an important influence on the distribution of transition elements between different coexisting minerals as well as between different, crystallographic sites within the same mineral. A transition element prefers or is enriched in that mineral or that lattice position within a mineral, in which its LFSE has its maximum value. Principally, the LFSE of transition metal ions can be evaluated from the absorption spectra in the visible and near infrared parts of the electromagnetic spectrum. In the present paper, the influence of the LFSE on the polyhedral distortion, the phase stability, the intra-as well as the intercrystalline cation distribution of transition metal ions in some well established examples is presented and discussed. The Gibbs free energies of the exchange reactions can be evaluated from the site occupancies and used as a measure for the site preference. Deviations from a pure ionic model are explained on the basis of MO-theory and covalent bonding and other crystal chemical effects.     

Lithium and boron isotopes in illite-smectite: The importance of crystal size

Clay minerals record chemical data about the past, acting like natural computer memory chips. To retrieve the data we must understand how they are stored. To achieve this we have examined the isotopic information revealed by two trace elements, lithium and boron, that are incorporated into the common clay minerals illite-smectite (I-S) during diagenesis. We used hydrothermal experiments at 300°C, 100 MPa, to speed up the reaction of smectite to illite that normally occurs during slow (10-100 Ma) sediment burial. During illitization, Li substitutes into the octahedral sites and B enters the tetrahedral sites of the silicate framework. Both Li and B are also adsorbed in the interlayer of smectite, but Li is preferred over B in the exchange sites. To determine the equilibrium isotope fractionation of the two trace elements it is important to remove these adsorbed interlayer species. By measuring the isotopic composition of Li and B in the silicate framework during reaction, we can address the relative timing of element exchange in the different crystallographic sites. Furthermore, because illitization of smectite is a crystal growth process (not an isomorphous replacement) we have examined the effect of crystal size on the isotope fractionation.The results show that Li and B approach an isotopic steady state when R1 ordering occurs, long before oxygen isotopes equilibrate with the fluid. The isotopic fractionation (α_{mineral-water}) for Li (0.989) is similar to that for B (0.984) at 300°C. However, when separated into <0.2, 0.2-2.0, and >2.0 μm fractions, there are significant differences in measured isotope ratios by as much as 9‰. Crystal growth mechanisms and surface energy effects of nanoscale crystals may explain the observed isotopic differences. The fact that different crystals equilibrate at different rates (based on size) may be applied to natural samples to reveal the changing paleofluid history, provided we understand the conditions of equilibrium. This has very important implications for the interpretation of diagenetic environments, fluid flow, and surficial geochemical cycling.