Octahedral Site Preference Energy Research Articles

Chromium Immobilization as Cr-Spinel by Regulation of Fe(II) and Fe(III) Concentrations

The complex environmental conditions at Cr-contaminated sites, characterized by uneven ion distribution, oxidants competition, and limited solid-phase mobility, lead to inadequate mixing of Fe-based reducing agents with Cr, posing significant challenges to the effectiveness of Cr remediation through Cr-spinel precipitation. This study investigates the distinct roles of Fe(II), Fe(III), and Cr(III) in Cr-spinel crystallization under ambient temperature and pressure. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray absorption near-edge structure spectroscopy, and Mössbauer spectroscopy were employed to elucidate the phase composition, microstructure, and ion coordination within the precipitates. Our findings indicate that Fe(II) acts as a catalyst in the formation of the spinel phase, occupying octahedral sites within the spinel structure. Under the catalytic influence of Fe(II), Fe(III) transitions into the spinel phase, occupying both the tetrahedral and the remaining octahedral sites. Meanwhile, Cr(III), due to its high octahedral site preference energy, preferentially occupies the octahedral sites. When Fe(II) or Fe(III) is present but does not meet the ideal stoichiometric ratio, a deficiency in Fe(II) leads to low yield and poor crystallinity of Cr-spinel, whereas a deficiency in Fe(III) can completely inhibit its formation. Conversely, when either Fe(II) or Fe(III) is in excess, the formation of Cr-spinel remains feasible. Furthermore, metastable Cr phases can be transformed into stable Cr-spinel by adjusting the Fe(II)/Fe(III)/Cr(III) ratio. These results highlight the broad range of conditions under which Cr-spinel mineralization can occur in environmental settings, enhancing our understanding of the mechanisms driving Cr-spinel formation in Cr-contaminated sites treated with Fe-based reducing agents. This research provides critical insights for optimizing Cr remediation strategies.

Cr dopant regulating d-orbital electronic configuration of NiFe spinel oxide to improve oxygen evolution reaction in Zn-air battery

Significant advancements towards enhancing the catalytic activity of spinel ferrites in oxygen evolution reaction (OER) have been achieved, however, further studies are required on this regard. In this study, Cr-doped spinel ferrite (NiFe2O4) embedded in N-doped C (Cr-NiFe2O4@NC) was synthesized via a high-temperature treatment using an NiFe Prussian blue analog as a precursor and K2Cr2O7 as a Cr source. Owing to the strong octahedral site preference energy, Cr3+ ions were successfully doped into octahedral sites and tailored the electronic configuration of NiFe2O4, as confirmed by XPS. The Cr-substituted Cr-NiFe2O4@NC exhibited the overpotential of 241 mV at 10 mA cm−2, which was smaller than that of NiFe2O4@NC (266 mV) at the same current density. Meanwhile, the Tafel slope of Cr-NiFe2O4@NC (65.32 mV dec−1) was also lower than that of NiFe2O4@NC (86.96 mV dec−1). Therefore, the Cr substitution can improve the electrocatalytic OER performance of NiFe2O4@NC. In particular, an assembled rechargeable Zn-air battery using Cr-NiFe2O4@NC as the cathode catalyst afforded a high power density (61.1 mA cm−2) together with a high specific capacity (740 mA g−1). Amazingly, it could power light-emitting diodes, demonstrating its real-world application. Density functional theory calculations revealed that Cr ion doping led to electron transfer in NiFe2O4, resulting in a density of electronic states close to the Fermi level, which probably regulated the adsorption/desorption behaviors of oxygenate intermediates (*O and *OOH), thus promoting the reaction kinetics of the OER. This study provides a helpful basis for the development of spinel oxides with high electrocatalytic OER activity and durability via cation doping.

Zinc- doped nickel ferrite nanoparticles for ESR, hyperhtermia and thier cytotoxicity in mouse muscle fibroblast (BLO-11) and human breast cancer (MDA-MB-231) cell lines

Spinel structured nickel ferrite (NiFe2O4) nanoparticles (NPs) are used in catalysis, magnetic, and microwave electronic devices. To study the magnetic interplay between half and above-filled d-orbitals in spinel structured transition metal cations: Fe3+ (3d5), Ni2+ (3d8), and Zn2+ (3d10), narrow size distributed, well crystallined zinc substituted NiFe2O4 magnetic nanoparticles (MNPs) were synthesized by solvothermal reflux method. Structural, morphological, magnetic, magnetic hyperthermia, and cytotoxicity properties of Zn0.15Ni0.85Fe2O4 (code: ZN1), Zn0.35Ni0.65Fe2O4 (code: ZN2), and Zn0.45Ni0.55Fe2O4 (code: ZN3) NPs were systematically studied. Both X-ray diffraction and electron diffraction profiles confirmed that all synthesized samples were crystallined in a cubic spinel structure. Further, the formation of octahedral and tetrahedral ligands in spinel structure is confirmed by Fourier transformed infrared (FTIR) spectra. In addition, metal cations and oxygen anions oxidation states were determined from X-ray photoelectron spectroscopy (XPS). Quantification of metal cations present in all samples was done by X-ray energy dispersive spectra. Macroscopic magnetic properties of NPs were studied by magnetometer and microscopic magnetic properties were studied by electron spin resonance (ESR) spectra. Superparamagnetic hyperthermia (SPMH) properties of NPs were studied by adiabatic hyperthermia curves using an induction heater. Biocompatibility properties of all NPs were studied by cell viability-based cytotoxicity assay on (BLO-11) mouse muscle fibroblast and (MDA-MB-231) human breast cancer cell lines. The above studies revealed that Zn2+ substitution initially enhances saturation mass magnetization (Ms) of NiFe2O4 and then reduces Ms when Zn2+ concentration is> 30 % due to zero octahedral site preference energy of Zn2+ and reduced superexchange strength in octahedral sublattice. This influences magnetic hyperthermia and cytotoxicity properties of NPs and is described here.

Understanding the structural behaviors and abrupt enhancement in magnetic ordering temperature of Fe-substituted Mn3O4

We report an increase in magnetic ordering temperature from 43 K to 425 K and a structural phase transition from tetragonal I41/amd (x = 0.00) to cubic Fd3¯m (x = 0.40) in (Mn1-xFex)3O4 (x = 0.00–0.40) systems. The phase diagram clearly demonstrates that lattice constants a√2 and c converge smoothly with Fe-substitution and become equal for x = 0.40, while the room temperature magnetic moments increase from 0.15 μB/f.u. (x = 0.10) to 1.1 μB/f.u. (x = 0.40). There are two possible cationic distribution scenarios for Fe cations; one leads Fe2+ to occupy the tetrahedral site, while the other leads Fe3+ to occupy the octahedral site. The octahedral site preference energy indicates that Fe3+ cations occupying the octahedral site have higher preference values. X-ray absorption spectra studies of MnL2,3-edge and FeL2,3-edge confirm that Fe3+ cations occupy octahedral sites by replacing Jahn-Teller active Mn3+ cations. OK-edge spectra revealed that Jahn-Teller distortion is reduced in the cubic x = 0.40 system. However, the higher structural symmetry is not the most significant factor in the abrupt enhancement of magnetic ordering temperature. The increase in magnetic ordering temperature could be understood on the basis of increased spin moment coupling between tetrahedral and octahedral cations due to substitution of Fe3+ cations at the octahedral site.

In situ mineral chemistry of chlorite in Donghua area, Dehua‐Youxi‐Yongtai ore district, Fujian Province, south‐east China: Elemental characteristics and their implications for exploration

As an ordinary alteration mineral in the process of fluid–rock interaction, chlorite (most especially its chemical composition) has the potential to become an effective tool to reveal the physicochemical conditions during alteration and help exploration. However, its elemental characteristics during the chloritization process are yet to be clearly understood. The Dehua‐Youxi‐Yongtai (DYY) ore district in south‐east China is one of the potential areas of gold and polymetallic deposits. Lying in the north‐west section of the DYY ore district, the Donghua area is notable for its remarkable metallogenic potential, and porphyry‐epithermal systems might have developed in the area. This study focuses on the elemental characteristics of chlorite in the Donghua area as footprints of the alteration process and mineralization vectoring. According to the geological features and occurrences, the chlorite from Donghua can be divided into two generations: (a) Chlorite I is closely related to chloritization developed in intrusive and volcanic rocks (Permian quartz monzobiorite and Jurassic volcanic rocks of Changlin Formation), and (b) chlorite II is accompanied by superimposed hydrothermal overprinting. The alteration process suggested by overprinting chlorite II can be regarded as almost coeval with the ca. 154–153 Ma magmatic event, and chlorite II is later than chlorite I generation. According to the geothermometry of the chlorite in Donghua, chlorite I might have crystallized on the temperature of 180–240°C with a peak of ~200°C, and the overprinting chlorite II might have experienced two episodes of hydrothermal/epithermal fluid pulsing. The mineral geochemistry of the trace elements in the two generations of the chlorite in Donghua shows different characteristics. The replacement of Mg2+ by Fe2+ plays an important role for ionic substitution in the octahedral position, especially for chlorite II. The other occasion might be Mg and Fe jointly entering the octahedral position as well as Mg‐ and Fe‐AlVI substitution mechanisms. The enrichment of Mg especially in chlorite II suggests low‐grade oxidation and acid conditions, which might be beneficial for the transportation of metallogenic substances. The obvious differences of Co and Ni are remarkable aspects of the trace elements of chlorite in Donghua, resulted by the ion substitution and function of different octahedral site preference energy. The mineral chemistry of chlorite II from Donghua can be helpful for targeting and exploration vectoring in the DYY ore district.

Formation mechanism of high-entropy spinel thin film and its mechanical and magnetic properties: Linking high-entropy alloy to high-entropy ceramic

High-entropy spinel thin film (HESTF) of (CrMnFeCoNi)3O4 was formed for the first time by heat-treating magnetron-sputtered thin film in Ar+O2 atmosphere. One of the most interesting observations is the formation of dual spinel structure, innermost Cr-rich spinel and outer Cr-lean spinel as the oxidation proceeds. For phase transformation from high entropy alloy (HEA) to dual high entropy spinel, the free energy minimization criterion was taken into consideration as well as the diffusivity of metal cations. The formation of innermost Cr-rich spinel is favored because of greater reduction of crystal field stable energy and octahedral site preference energy for the spinel with Cr3+ ions in the tetrahedral and octahedral sites. We propose that Cr-rich spinel forms first in the initial HEA film with the oxygen supply through the short circuit diffusion paths and remnant atoms/ions diffused outward to form the outer Cr-lean spinel layer. The hardness and elastic modulus of the thin film increased from 8.4 to 13.2 GPa and 190.8 to 255 GPa, respectively. HESTF was found to be a ferrimagnetic nanomaterial with its TC being greater than 873 K. HESTF of this study was observed to maintain its blocked spins up to higher temperatures with TB value of 225 K.

Tuning Transition Metal Oxides Towards Achieving Water-Stability and Electrochemical Stability As Electrode Materials for Sodium-Ion Batteries

Transition metal (TM) oxides are a fascinating class of materials, whose properties can be suitably tuned in a variety of ways; such as by selecting TM-ions/dopants having preferred electronic configurations, engineering the crystallographic site occupancy by dopants, controlling/modifying the degree of covalence of TM-O bonds, modifying lattice spacing(s), tuning phase assemblage etc. Such modifications done from the fundamental perspectives influence the performances of TM-oxides for a variety of applications, including their widespread usage as electrode-active materials in alkali metal-ion batteries.In the context of the upcoming Na-ion battery system, O3-type ‘layered’ Na-TM-oxides are promising as cathode-active materials due to their inherently high initial Na-content (as compared to the P2 counterparts). However, they suffer from instabilities caused due to multiple phase transformations during Na-removal/insertion and sensitivity to air/moisture. Against this backdrop, with the help of a dopant, having d 0 electronic configuration (viz., no octahedral site preference energy), we have been able to tune the composition and structural features to suppress the phase transitions upon Na-removal/insertion and also improve the air/water-stability in significant terms; so much so that long-term cyclic stability has been achieved with health/environment-friendly ‘aqueous processed’ electrodes (sans, usage of toxic/hazardous/expensive chemicals like NMP and PVDF) [1]. As will be explained in more elaborate terms during the talk, the changes in structural features, which have led to such outstanding water-stability, include differential contraction/dilation of the Na-‘inter-slab’/TM-‘slab’ spacing and partial occupancy of the dopant at tetrahedral sites of the structure.On the anode front, successful development of Na-ion battery system necessitates looking beyond hard carbon based anodes, which possess safety hazards due to the Na-insertion potential being a bit too close to the Na-plating potential. In this context, Na-titanates promise to be ‘safe’ anode materials; but suffer from cyclic instability [2]. Against this backdrop, we have developed carefully tuned bi-phase Na-titanate based electrodes, having Na2Ti3O7 and Na2Ti6O13 as the primary and secondary phases, respectively. The as-developed phase assemblage has been able to address the cyclic instability of single-phase Na-titanate, leading to long-term cyclic stability even at high current densities (up to 50C!) [3]. These are important steps towards the development of health/environment-friendly, cost-effective, safe and high-performance Na-ion batteries.Acknowledgement to RRCAT, Indore, India, for enabling the usage of Synchrotron facility.Parts of the concerned works have been published as (i.e., the associated publications); B. S. Kumar, A. Pradeep, A. Dutta, and A. Mukhopadhyay; J. Mater. Chem. A 8 (2020) 18064H. S. Bhardwaj, T. Ramireddy, A. Pradeep, M. K. Jangid, V. Srihari, H. K. Poswal, and A. Mukhopadhyay; ChemElectroChem 5[8] (2018) 1219A. Pradeep, B. S. Kumar, A. Kumar, V. Srihari, H. K. Poswal, and A. Mukhopadhyay; Electrochim. Acta 362 (2020) 137122

Oxidative Thermal Conversion of Hydrothermal Derived Precursors toward the Mixed-Metal Cobaltite Spinel Oxides (ZnCo2O4 and NiCo2O4): In-Situ Investigation by Synchrotron-Radiation XRD and XAS Techniques

In-situ investigations of structural transitions during the thermal-oxidative event of mixed-metal spinel oxide precursors, the so-called nickel- (NCO) and zinc-containing (ZCO) cobaltite spinel precursors, were investigated to understand the formations of the derived NiCo2O4 and ZnCo2O4 spinel oxides, respectively. In-situ XRD investigation revealed that emerged temperatures for spinel oxide phase were between 325 and 400 °C, depending on the cationic substituent. It indicated that the emerged temperature correlated with the absolute octahedral site preference energy (OSPE) of those cations that participated in the development of the spinel framework. Moreover, the incorporated nickel and zinc in the precursors was beneficial for inhibiting the occurrence of the undesired CoO phase. Time-resolved X-ray absorption spectroscopic (TRXAS) data suggested the local structure rearrangement of nickel and zinc throughout the calcination process, which differed from the behavior of single-metal cobalt system. The essential information reported herein provides a benefit to control the cationic distribution within spinel materials, leading to the tunable physical and chemical properties.

Experimental constraints on the partitioning of Ru, Rh, Ir, Pt and Pd between chromite and silicate melt: The importance of ferric iron

We have performed partitioning experiments to assess the role of chromium-rich spinel in controlling the behavior of the platinum-group elements (PGEs) during igneous differentiation. Spinels were equilibrated with natural and synthetic iron-bearing basalt at 0.1MPa and 2GPa at 1400–1900°C over an fO2 range of IW+1.6 to IW+7.Results from relatively reduced, graphite-encapsulated experiments done at 2GPa indicate that Ru is compatible in Cr-spinel (mineral/melt partition coefficient, D, of ~4), followed by Rh and Ir, which are moderately incompatible (D range of 0.04 to ~1), with Pt and Pd the most incompatible (D<0.2). Partition coefficients for Ir, Ru and Rh measured at more oxidizing conditions in this and previous studies are 10 to 1000 times higher than results from experiments using graphite capsules.We account for the variation in spinel–melt partitioning with a model which considers both the affinity of the PGE cation for a particular spinel lattice site, and the change in site occupancy accompanying the increase in ferric iron component with fO2. Assuming that Ir and Rh are present as divalent species, with a strong affinity for VI-fold coordination, DIr and DRh are predicted to rise rapidly with the ferric iron component, explaining the large D-values for magnetite-rich spinels. Model results indicate that DIr≤20 and DRh are ≤100 for ferric-iron poor, Cr-rich compositions, as would crystallize in komatiites, some layered intrusions, and ophiolites. The overall compatibility of Ru for chromite is consistent with the predominance of Ru3+ at experiment conditions and the similarity in the size of Ru3+ to Cr3+ and Fe3+. The increase in DRu with the ferric iron content of the spinel likely involves a strong effect of mineral composition superimposed on a change in melt speciation (Ru2+ to Ru3+) with increased fO2. The effect of mineral composition is a consequence of the difference in octahedral site preference energy (OSPE) between Ru3+, Fe3+ and Cr3+, with stronger partitioning of Ru into Fe3+-rich compositions due to the enhanced reduction in energy gained by the Ru3+ substitution. Ru partition coefficients for ferric-iron poor spinel are expected to be ~30, which is somewhat lower than values estimated from natural samples obtained from in situ chromite analyses.Results indicate that the ferric iron content of chromite exerts a strong control on the partitioning of some PGEs which should be taken into account in both future experimental work and in models of igneous differentiation.

Predicting the Spinel–Nepheline Liquidus for Application to Nuclear Waste Glass Processing. Part I: Primary Phase Analysis, Liquidus Measurment, and Quasicrystalline Approach

The crystal–melt equilibria in complex 15 component simulated high‐level waste (HLW) glass melts are modeled on the basis of quasicrystalline concepts. A pseudobinary phase diagram between a transition metal ferrite spinel (an incongruent melt product of transition metal iron‐rich acmite) and nepheline is defined in Part II on the basis of these concepts. The pseudobinary lies within the Al2O3–Fe2O3–Na2O–SiO2 quaternary system that defines the crystallization of basalt glass melts. Part I defines the complex transition metal species that crystallize in waste glasses, the role of the octahedral site preference energies, the evidence for quasicrystalline clustering in waste glasses, and the potential quasicrystalline exchange reactions at the liquidus. The liquidus model developed on the basis of these concepts has been used to prevent unwanted crystallization in the world's largest HLW melter for the past 4 years while allowing &gt;10 wt% higher waste loadings to be processed.

Predicting the Spinel–Nepheline Liquidus for Application to Nuclear Waste Glass Processing. Part II: Quasicrystalline Freezing Point Depression Model

The crystal‐melt equilibria in complex 15‐component‐simulated high‐level waste (HLW) glass melts are modeled based on quasicrystalline concepts that are discussed in Part I. A pseudobinary phase diagram between a transition metal ferrite spinel (an incongruent melt product of a transition metal iron‐rich acmite) and nepheline is defined based on calculation of the quasicrystalline melt precursors calculated by a freezing point depression approach. The pseudobinary lies within the Al2O3–Fe2O3–Na2O–SiO2 quaternary system that defines the crystallization of basalt glass melts. The modeling provides the partitioning of species between the melt and the primary liquidus phases. The medium‐range order of the melt and the melt‐crystal exchange equilibria are defined based on a constrained mathematical treatment that considers the crystallochemical coordination of the elemental species in acmite and nepheline. The liquidus phases that form are shown to be governed by the melt polymerization (Part II) and the thermodynamic octahedral site preference energies (Part I). The liquidus model developed based on these concepts has been used to prevent unwanted crystallization in the world's largest HLW melter for the past 4 years while allowing &gt;10 wt% higher waste loadings to be processed.

Spinel structure and liquidus temperature relationships in nuclear waste glass

The liquidus temperature of spinel (AB2O4) crystals frequently limits the waste loading of vitrified nuclear waste. In other studies, glass structure-spinel liquidus temperature relationships were discovered for the non-spinel forming cations in glass, but no such relationship was identified for the spinel-forming metal-ions. In this paper, coefficients from an empirical first order mixture model describing spinel liquidus temperature were correlated with the octahedral site preference energy (OSPE) of the spinel-forming metal ions. The OSPE was calculated using approximations of glass and spinel structure made by extrapolating from data on similar glasses and spinels. A literature review indicated that Cr(III), Ni(II), and Fe(II) are most likely to be in the octahedral sites in spinel, Mn(II) in the tetrahedral site, and that Fe(III) will be common in both sites. These assignments were used as starting assumptions to calculate the OPSE for each metal ion. A strong correlation between the OSPE and the liquidus temperature coefficients of spinel-forming metal ions was observed. Further correlations determined that the Crystal Field Stabilization Energy was the most important portion of the OSPE for these spinels. These results indicate that the thermodynamic properties of spinel could be predicted based on assumptions of spinel structure, underpinned by fundamental properties, when scaled empirically.

Equilibria involving the reciprocal spinel solid solution (Mg x Fe1−x ) (Al y Cr1−y )2O4: modeling and experiment

Developed in this article is a model for calculating cation distribution and activities in the reciprocal spinel solid solution (Mg X Fe1−X )(Al Y Cr1−Y )2O4 based on octahedral site preference energies of cations independent of composition and temperature, random distribution of ions on tetrahedral and octahedral sites, entropy of randomization of Jahn-Teller distortions associated with Fe2+ ions on the tetrahedral site, and the standard Gibbs energies of formation of the four pure spinel compounds. Enthalpy of mixing of this reciprocal solid solution caused by the large difference of ionic radii of Al3+ and Cr3+ present on the octahedral site was modeled based on experimental data on the binary systems. The tie-line compositions corresponding to the equilibria between the spinel solid solution and the sesquioxide solid solution (Al Z Cr1−Z )2O3 with corundum structure were computed. Values for activities in the corundum solid solution were taken from the literature. The oxygen potential corresponding to the three-phase equilibrium involving metallic iron, the spinel solid solution, and corundum solid solution was computed as a function of composition of the spinel solid solution. The computed results were verified by measurements on nine compositions inside the square representing the reciprocal system. The compositions of coexisting solid solutions were determined by electron-probe microanalysis (EPMA) and lattice parameter measurement using X-ray diffraction (XRD). The activities of FeAl2O4 and FeCr2O4 and oxygen potentials for three-phase equilibria were measured using two independent solid-state cells incorporating a bielectrolyte chain. Both cells gave consistent results within experimental error. The experimental results are in excellent agreement with the computed results, thus validating the model for the reciprocal spinel solid solution.

The systematics of trace-element partitioning between coexisting muscovite and biotite in metamorphic rocks from the Black Hills, South Dakota, USA

Coexisting muscovite and biotite in forty-nine staurolite- and sillimanite-zone schists from the southern Black Hills, South Dakota, USA, have been analyzed by ICP spectrometry for major and selected trace elements. This study represents the first comprehensive effort to document and explain trace-element partitioning behavior between coexisting micas in metamorphic rocks. Overall, the data reveal systematic element distributions across a wide thermal-compositional range. Mean Henry's Law partition coefficients [ K ∗ D (bio/mus)] are as follows: Mn (14 ± 6 (1 σ)), Ni (14 ± 6), Zn (13 ± 8), Li (4.0 ± 1.1), Ti(2.9 ± 0.7), Co (2.6 ± 1.6), Yb (2.0 ± 1.7), Cu (1.9 ± 1.2), Y (1.5 ± 1.5), Be (1.1 ± 1.5), Cr (1.0 ± 0.2), La (0.8 ± 0.7), V (0.7 ± 0.1), Zr (0.6 ± 0.2), Ba (0.5 ± 0.3), Sc (0.4 ± 0.1), Sr (0.4 ± 0.4), and Na (0.2 ± 0.1). This sequence is governed largely by the crystal structure of the micas and their major-element compositions. Several structural effects on K ∗ D have been identified. First, the observed K ast; D sequences Cr 3+ > V 3+ > Sc 3+ and Ni 2+ > Co 2+ > Cu 2+ are just as predicted from relative octahedral site preference energies, indicating that crystal-field effects influence the partitioning behavior of transition-metal cations. Second, the presence of smaller (i.e., more collapsed) interlayer sites in muscovite (relative to biotite) favors substitution in muscovite of cations smaller than K +, namely, Na +, Sr 2+, Ba 2+, and La 3+. Likewise, interlayer cations larger than K + (e.g., H 3O +, Rb +, and Cs +) are predicted to substitute preferentially into biotite. Third, tetrahedral Fe 3+ is predicted to favor biotite over muscovite because of larger tetrahedral sheets in biotite (due, in turn, to more Al and less Si). Fourth, the occurrence of heterovalent interlayer cations in micas suggests that their partitioning behavior is partly governed by charge-balance reactions. As a general compositional effect on K ∗ D , the preferences exhibited by biotite for Li + and divalent cations of first-row transition elements reflect its high abundance of comparably sized vi(Mg, Fe 2+) relative to coexisting muscovite. Likewise, relatively strong affinities of muscovite for Cr 3+, V 3+, and Sc 3+ reflect its high stoichiometric vi(Al, Fe 3+) abundance. Element-specific compositional effects on K ∗ D are less evident; but there are indications that Mg 2+ partitioning affects that of Li +, Ni 2+, and Mn 2+. Temperatures inferred from Mg-Tschermak (MgSiAL 2) exchange between coexisting muscovite and biotite ( Hoisch, 1989) provide a convenient datum by which to evaluate the thermal behavior of analogous vector components involving trace elements. Several of these components appear to possess exchange potentials ( ΔG rxn ) sufficiently large so that respective equilibrium constants approach unity with temperature increase. These components include the following: NiSiAl −2, MgSiCr −1Al −1, MgSiV −1Al −1, MgSiSc −1Al −1, CrAl −1, VAl −1, and Li 2 Si viAl −1, vi□ −1, ivAl −1. In contrast, any thermal sensitivity of other such components is masked by analytical scatter.

Cation distribution in magnetoplumbite and β′′-alumina structures

Atomistic computer simulation techniques have been used to investigate the structural features influencing relative cation enrichments in magnetoplumbite structure based on A1 2O 3 and β′′-alumina. The simulation results indicate that, like in spinel structure, Mg 2+, Co 2+, Mn 2+, Zn 2+, and Fe 2+ ions strongly prefer a tetrahedral coordinated environment in both magnetoplumbite and β′′-alumina structures. Due to its large octahedral site preference energy, Cr 3+ ion is enriched in octahedral sites. Therefore, in general, the normal site preferences of cations for tetrahedral or octahedral sites are paralleled in magnetoplumbite and β′′-alumina structures, but in the case of Ni 2+, the crystal structure has an effect on its distribution. There is a mixed Ni 2+ ion distribution among both tetrahedral and octahedral sites, with the tetrahedral site being more energetically favorable in both structures. More detailed site preferences of Ni 2+ ions in the presence of Zn 2+ in magnetoplumbite structure are also reported in this study.

Thermodynamics of Cobalt (II, III) Oxide (Co&lt;SUB&gt;3&lt;/SUB&gt;O&lt;SUB&gt;4&lt;/SUB&gt;): Evidence of Phase Transition

The Gibbs' energy change for the reaction, 3CoO (r.s.)+1/2O2(g)→Co3O4(sp), has been measured between 730 and 1250 K using a solid state galvanic cell: Pt, CuO+Cu2O|(CaO)ZrO2|CoO+Co3O4,Pt. The emf of this cell varies nonlinearly with temperature between 1075 and 1150 K, indicating a second or higher order phase transition in Co3O4around 1120 (±20) K, associated with an entropy change of ∼43 Jmol-1K-1. The phase transition is accompanied by an anomalous increase in lattice parameter and electrical conductivity. The cubic spinel structure is retained during the transition, which is caused by the change in CO+3 ions from low spin to high spin state. The octahedral site preference energy of CO+3 ion in the high spin state has been evaluated as -24.8 kJ mol-1. This is more positive than the value for CO+2 ion (-32.9 kJ mol-1). The cation distribution therefore changes from normal to inverse side during the phase transition. The transformation is unique, coupling spin unpairing in CO+3 ion with cation rearrangement on the spinel lattice, DTA in pure oxygen revealed a small peak corresponding to the transition, which could be differentiated from the large peak due to decomposition. TGA showed that the stoichiometry of oxide is not significantly altered during the transition. The Gibbs' energy of formation of Co3O4 from CoO and O2 below and above phase transition can be represented by the equations:ΔG0=-205,685+170.79T(±200) J mol-1(730-1080 K) and ΔG0=-157,235+127.53T(±200) J mol-1(1150-1250 K).

Interaction of dopant cations with 4:1 defect clusters in non-stoichiometric 3d transition metal monoxides: A theoretical study

The ASED-MO theory and a normalized ion energy model have been used to study the energetics of substitutional dopants in Mn, Fe, Co, and Ni non-stoichiometric monoxides; we find that certain dopants stabilize 4:1 defect clusters (4 octahedral M 2+ vacancies surrounding an M 3+ tetrahedral interstitial). For example, Fe 3+-based 4:1 clusters in Mn 1− x O and Mn 3+-based 4:1 clusters in Fe 1− x O have comparable stabilities to those in the undoped material, while Mn 3+- and Fe 3+-based 4:1 clusters in Co 1− x O and Ni 1− x O are significantly more stable than those in the undoped oxides. Cr 3+ has a strong octahedral site preference energy in all cases, which prevents formation of doped 4:1 defect clusters. Despite a strong tetrahedral site preference energy, Zn 2+ does not cause formation of 4:1 clusters because they would destabilize oxygen 2 p orbitals.

The stability of zincian spinels in sulfide systems and their potential as exploration guides for metamorphosed massive sulfide deposits

Zincian spinel [(Zn,Fe,Mg)Al 2 O 4 ] is found in a variety of geologic settings including metamorphosed massive sulfide deposits, aluminous metasediments, pegmatites, and marbles. Many spinels in and near metamorphosed massive sulfides have formed by desulfidation and oxidation reactions that involve members of the system Zn-Fe-Al-Si-S-O. Experimental reversals of the simplified reaction: ZnS + FeS + 2Al 2 O 3 + O 2 = ZnAl 2 O (sub 4/) + FeAl 2 O 4 + S 2 , using hydrothermal techniques, have been obtained at 2 and 5 kb and various temperatures using Fe (sub 1-x) S + FeS 2 + Fe 3 O 4 as a buffer. Calculated positions of the spinel-sphalerite equilibria in f (sub O 2 ) -f (sub S 2 ) -X space, for ideal behavior along the ZnAl 2 O 4 -FeAl 2 O 4 join as is suggested by octahedral site preference energies, agree with experimental results and theoretical calculations depending on the choice of the free energy of formation of hercynite. The compositions of spinels in equilibrium with sphalerite and either pyrite or pyrrhotite become more Zn-enriched with increasing f (sub S 2 ) and f (sub O 2 ) values and can be used as a measure of f (sub O 2 ) during metamorphism.Spinel associated with metamorphosed massive sulfide deposits, marbles, and aluminous metasediments is enriched in the gahnite (ZnAl 2 O 4 ), spinel (MgAl 2 O 4 ), and hercynite (FeAl 2 O 4 ) molecules, respectively, whereas spinel associated with pegmatites is depleted in the spinel molecule. Such variability in composition constitutes a potential exploration guide for metamorphosed massive sulfide deposits.

The crystal structure of CaFe3+SiAlO6 and the crystal chemistry of Fe3+?Al3+ substitution in calcium Tschermak's pyroxene

The crystal structure of a synthetic CaFe3+Al-SiO6 pyroxene (20 kb, 1,375° C) with unit cell dimensions a=9.7797(16), b=8.7819(14), c=5.3685(5) A, β=105.78(1)∘, space group C2/c has been refined by the method of least squares to an R-factor of 0.025 based on 812 reflections measured on an automatic single crystal diffractometer. The octahedral M1 site is occupied by 0.82 Fe3+ and 0.18 Al3+. Within the tetrahedral T site, Si4+ (0.50), Al3+ (0.41) and Fe3+ (0.09) ions are completely disordered, although submicroscopic domains with short-range order are very likely. The octahedral site preference energy of the Fe3+ ions with respect to Al3+ ions in CaFe3+AlSiO6 is about 10 kcal/mole, which is much higher than that found in Y3Al x Fe5−2O12 garnets. Topologically the structure of CaFe3+AlSiO6 is intermediate between that of diopside and calcium Tschermak's pyroxene, CaAlAlSiO6. For CaM3+ AlSiO6 clinopyroxenes an increase in the size of the M1 octahedron is accompanied by an increase in the average M2-0, bridging T-0 and 03-03′ distances and kinking of the tetrahedral chain.

Increase of chromium with fractionation in early formed pyroxene: Examples from boninite and Kasayama andesite.

Chromium in pyroxene generally decreases as magmatic fractionation proceeds because of its large octahedral site preference energy. Nevertheless, an increase in Cr with progressive fractionation was found in the early formed proto- and orthopyroxenes from the Bonin Island boninite and augites from the Kasayama olivine andesite. This unusual increase in Cr can be interpreted by the early removal of Cr from magma with Cr-spinel crystallization and the later reaction relations of the crystallized spinels with magma to form pyroxenes.