Nucleation Of Spinel Research Articles

The role of mineral nanoparticles at a fluid-magnetite interface: Implications for trace-element uptake in hydrothermal systems

Abstract The migrating fluid-mineral interface provides an opportunity for the uptake of trace elements as solid solutions in the newly formed crystal lattice during the non-equilibrium growth of the crystal. However, mineral nanoparticles could precipitate directly from the interfacial fluid when it evolves to a supersaturated situation. To better understand the role of mineral nanoparticles in this scenario, this study focuses on a well-documented magnetite with oscillatory zoning from a skarn deposit by using high-resolution transmission electron microscopy (TEM). Our results show that the Al concentration in magnetite measured on a micrometer-scale is caused by three different effects: Al solid solution, Al-rich nanometer-sized lamellae, and zinc spinel nanoparticles in the host magnetite. Here, we propose a genetic relationship among the three different phases mentioned above. At first, a continuous increase of the Al concentration in the interfacial fluid can be incorporated into the crystal lattice of magnetite forming a solid solution. During cooling in a later stage, aluminum in magnetite is oversaturated and exsolution of hercynite (Al-rich lamellae) occurs from the host magnetite. If the Al concentration at the fluid-magnetite interface still increases during further growth of magnetite, the substitution of Fe by Al has gradually reached saturation so that aluminum cannot be incorporated in the magnetite crystal structure any longer. Using the magnetite lattice as a template, nucleation of abundant zinc spinel nanoparticles occurs. This will, in turn, lead to a gradual depletion of Al concentration in the interfacial fluid until the available ions for zinc spinel nucleation and growth have been used up. As a result, the migrating fluid-magnetite interface will enrich the Al concentration in the interfacial fluid until the available ion concentration is sufficient for nucleation of zinc spinel phase again. The fluid-mineral interface in this mechanism has been repeatedly utilized during crystal growth, providing an efficient way for the uptake of trace element from a related undersaturated bulk fluid.

The Mechanisms of Nickel-Iron Spinel Phase Nucleation in Aquous Solutions: Crystal Quasichemical Approach

The phenomenological model of nikel-iron spinel nucleation with spinel structure based on the partial charge theory and analysis of hydrolysis and polycondensation processes at the interaction of Fe3+-, Fe2+- and Ni2+- hydroxocomplexes at different pH values of reaction medium was proposed. UV-vis optical spectroscopy was used for verification of the obtained results about the regularities of hydroxocomplexes formation. Crystal quasichemistry approach was applied for nucleation of nikel-iron spinel under the conditions of predomination of different defect types in NixFe3-xO4-δ (δ is oxygen non-stoichiometry) spinel lattice. The analysis of changes in electrical conductivity and lattice parameter of defect nickel-iron spinel as a function of Ni2+ atomic concentration was done for different values of d parameter.

Characterizing the residual glass in a MgO/Al2O3/SiO2/ZrO2/Y2O3 glass-ceramic.

The non-isochemical crystallization of glasses leads to glass-ceramics in which the chemical composition of the amorphous matrix differs from that of the parent glass. It is challenging to solely analyse the properties of these residual glassy phases because they frequently contain finely dispersed crystals. In this study, the composition of the residual glass matrix after the crystallization of a glass with the mol% composition 50.6 SiO2 · 20.7 MgO · 20.7 Al2O3 · 5.6 ZrO2 · 2.4 Y2O3 is analysed by scanning transmission electron microscopy (STEM) including energy dispersive X-ray analysis (EDXS). A batch of the residual glass with the determined composition is subsequently melted and selected properties are analysed. Furthermore, the crystallization behaviour of this residual glass is studied by X-ray diffraction, scanning electron microscopy including electron backscatter diffraction and STEM-EDXS analyses. The residual glass shows sole surface crystallization of indialite and multiple yttrium silicates while bulk nucleation does not occur. This is in contrast to the crystallization behaviour of the parent glass, in which a predominant bulk nucleation of spinel and ZrO2 is observed. The crystallization of the residual glass probably leads to different crystalline phases when it is in contact to air, rather than when it is enclosed within the microstructure of the parent glass-ceramics.

Microstructural evidence for a disequilibrium condensation origin for hibonite‐spinel inclusions in the ALHA77307 CO3.0 chondrite

AbstractTwo hibonite‐spinel inclusions (CAIs 03 and 08) in the ALHA77307 CO3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique combined with transmission electron microscopy. These hibonite‐spinel inclusions are irregularly shaped and porous objects and consist of randomly oriented hibonite laths enclosed by aggregates of spinel with fine‐grained perovskite inclusions finally surrounded by a partial rim of diopside. Melilite is an extremely rare phase in this type of CAI and occurs only in one inclusion (CAI 03) as interstitial grains between hibonite laths and on the exterior of the inclusion. The overall petrologic and mineralogical observations suggest that the hibonite‐spinel inclusions represent high‐temperature condensates from a cooling nebular gas. The textural relationships indicate that hibonite is the first phase to condense, followed by perovskite, spinel, and diopside. Texturally, melilite condensation appears to have occurred after spinel, suggesting that the condensation conditions were far from equilibrium. The crystallographic orientation relationships between hibonite and spinel provide evidence of epitaxial nucleation and growth of spinel on hibonite surfaces, which may have lowered the activation energy for spinel nucleation compared with that of melilite and consequently inhibited melilite condensation. Hibonite contains abundant stacking defects along the (001) plane consisting of different ratios of the spinel and Ca‐containing blocks within the ideal hexagonal hibonite structure. This modification of the stacking sequence is likely the result of accommodation of excess Al in the gas into hibonite due to incomplete condensation of corundum from a cooling gas under disequilibrium conditions. We therefore conclude that these two hibonite‐spinel inclusions in ALHA77307 formed by high‐temperature condensation under disequilibrium conditions.

Single Crystal (Mn,Co)3O4 Octahedra for Highly Efficient Oxygen Reduction Reactions

Single-crystal (Mn,Co)3O4 octahedra were synthesized through a novel precipitation-aging method. Well-defined single-crystal octahedra can be formed by carefully controlling the precipitation-dissolution equilibrium, oxygen concentration, and solution temperature during synthesis to match the rates of spinel nucleation and octahedra growth. The single-crystal (Mn,Co)3O4 octahedra expose {111} and {011} facets of cubic and tetragonal phases, respectively, depending on the Mn/Co ratio. However, only single-crystal octahedra of Mn2CoO4 and Mn2.5Co0.5O4 with exposed {011} facets show the highest selectivity towards 4e− and 2e− oxygen reduction reactions (ORR) in alkaline solution, respectively. Furthermore, the single-crystal Mn2CoO4 octahedra with exposed {011} facets shows ∼30 times higher area specific activity for ORR than that of nanoparticles with random/mixed facets. The high electrocatalytic activity and selectivity towards ORR are correlated with the octahedral shape, the exposed facet, and the ratio of cations on the exposed facets (especially the octahedrally coordinated Mn4+ cations). This facet dependent catalytic performance provides a new route for obtaining highly selective and active ORR electrocatalysts.

The influence of Li sources on physical and electrochemical properties of LiNi0.5Mn1.5O4 cathode materials for lithium-ion batteries

LiNi0.5Mn1.5O4 cathode materials were successfully prepared by sol–gel method with two different Li sources. The effect of both lithium acetate and lithium hydroxide on physical and electrochemical performances of LiNi0.5Mn1.5O4 was investigated by scanning electron microscopy, Fourier transform infrared, X-ray diffraction, and electrochemical method. The structure of both samples is confirmed as typical cubic spinel with Fd3m space group, whichever lithium salt is adopted. The grain size of LiNi0.5Mn1.5O4 powder and its electrochemical behaviors are strongly affected by Li sources. For the samples prepared with lithium acetate, more spinel nucleation should form during the precalcination process, which was stimulated by the heat released from the combustion of extra organic acetate group. Therefore, the particle size of the obtained powder presents smaller average and wider distribution, which facilitates the initial discharge capacity and deteriorates the cycling performance. More seriously, there exists cation replacement of Li sites by transition metal elements, which causes channel block for Li ion transference and deteriorates the rate capability. The compound obtained with lithium hydroxide exhibits better electrochemical responses in terms of both cycling and rate properties due to higher crystallinity, moderate particle size, narrow size distribution and lower transition cation substitute content.

Nanoscopic nickel aluminate spinel (NiAl 2O 4) formation during NiAl(1 1 1) oxidation

Oxidation of a NiAl(1 1 1) single crystal surface was investigated using high resolution soft X-ray photoelectron spectroscopy (HRSXPS), high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray spectroscopy (EDS), X-ray mapping, and atomic force microscopy (AFM). After repeated oxygen exposure, annealing, and cleaning cycles under ultrahigh vacuum conditions, a new oxide phase in the form of tiny 3-dimensional surface structures was detected. These features are several micrometers long and ∼300 nm high and oriented along low index 〈 1 ¯ 0 1 〉 directions in the plane of the substrate; they have nickel aluminate spinel (NiAl 2O 4) stoichiometry. We propose that repeated cycles of oxygen dosing and annealing of the NiAl(1 1 1) surface leads to oxygen diffusion into the bulk and nucleation of spinel below the surface.

On the Spinel Precipitation in Al-Doped Ni1−xO

The Ni1–xO/NiAl2O4(69:1 molar ratio) composite fired at 1873 K for 1–80 h and then slowly cooled or water quenched in air was studied by X-ray diffraction and electron microscopy to clarify the stoichiometry, microstructures, and formation mechanism of spinel precipitates in Al-doped Ni1–xO. Expulsion of Al3+during slow cooling caused the formation of stoichiometric NiAl2O4precipitates which contained {110} domain boundaries of spinelloid nature and were of the same size regardless of the firing time at 1873 K. Instead of growing at NiAl2O4seeds, the spinel precipitate nucleated from dislocations (line vector parallel to 〈100〉), hence with {100} rather than close-packed {111} as the habit plane. The {100} interface is coherent given that the lattice parameter for NiAl2O4is almost exactly twice that of Ni1–xO. On the basis of diffusion data reported for Al-doped Ni1–xO single crystals, we suggest that below 1473 K, moving of some Al3+dopant from octahedral to interstitial tetrahedral sites caused the spinel nucleation to preferentially occur at dislocation cores with beneficial higher diffusivity and lower activation energy for defect clustering.

In situ formation of palisade bodies in calcium, aluminum‐rich refractory inclusions

Abstract— It has been suggested that palisade bodies—shells of spinel found within some calcium, aluminum‐rich inclusions (CAIs) and the phases the shells enclose—are intact mini‐CAIs that predate and were captured by their current hosts while the latter were still molten. We present new data and observations that indicate that most palisade bodies formed instead in situ while their host inclusions were crystallizing. The evidence includes observations of spinel‐lined cavities and glass‐filled, circular structures outlined by spinel in experimental run products crystallized from melts; a partially formed palisade body in an inclusion; a fassaite crystal that is optically continuous across a palisade wall; and similarity of unusual mineral compositions in some palisade bodies and their hosts. Our observations can be used to refute arguments for exotic origin and are most consistent with a model for in situ formation involving: (1) formation of vesicles in a largely molten inclusion; (2) nucleation of spinel upon and/or adherence to vapor‐melt interfaces, forming spinel shells around vesicles; (3)leakage of vesicles and filling with melt while spinel shells remain largely intact; and (4) crystallization of melt inside shells. This model is similar to one proposed for formation of segregation vesicles, which are partially‐ to completely‐filled vesicles found in some terrestrial basalts. In addition, we interpret framboids (i.e., dense clusters of spinel with little material between grains, found in most inclusions that contain palisade bodies) as polar or near‐polar sections through palisade bodies and therefore do not make a genetic distinction between the two features.

Catalysis of the Olivine to Spinel Transformation by High Clinoenstatite

Although enstatite is a major constituent of the Earth's upper mantle and subducting lithosphere, most kinetic studies of olivine phase transformations have typically involved single-phase polycrystalline aggregates. Transmission electron microscopy investigations of olivine to spinel and modified spinel (beta phase) reactions in the (Mg, Fe)(2)SiO(4)-(Mg,Fe)SiO(3) system show that transformation of olivine in the stability field of spinel plus phase begins with coherent nucleation of spinel on high-clinoenstatite grains. These observations demonstrate that high clinoenstatite can catalyze the transformation by enhancing nucleation kinetics and therefore imply that secondary phases can influence reaction kinetics during high-pressure mineral transformations.

Interaction between dislocations and NiFe2O4 precipitates in a NiO matrix

Dislocations have been introduced into NiO containing particles of NiFe2O4 by room temperature deformation. The interaction of the dislocations with the particles has been examined using weak-beam imaging in the TEM. The dislocations have a a2〈011〉 Burgers vectors and {011} glide planes. By heating a TEM sample which contained spinel particles surrounded by dislocation loops, the effect of dislocations on the particle morphology could be studied. An individual dislocation loop can cause a cusp in the precipitate which can act as a “noose” around the particle causing it to shrink. Dislocations can act as preferential sites for spinel nucleation as was observed for different nucleation rates. For slow nucleation rates, nucleation only occurred at the dislocations while nucleation also occurred elsewhere for a larger nucleation rate. Spinel precipitates can grow at dislocation to form a stacking fault in the precipitate—perfect dislocations in NiO are only partial dislocations in the spinel. The position of the stacking fault indicates the location of the original nuclei.

Direct observation of nucleation embryos during NiFe2O4 precipitation in NiO

Very small NiFe2O4 particles between 1 and 4 nm in size have been observed in an Fe-doped NiO sample quenched during early stages of spinel precipitation. The differences between the crystal structure of NiFe2O4 and NiO made it possible to use three different TEM imaging techniques to observe these very small particles. These very small particles are distributed throughout the volume of the sample and hence were not produced by specimen preparation. The large concentration of the very small precipitates compared with the much larger but less numerous growing precipitates indicates that the very small precipitates are actually embryos which form due to compositional fluctuations. The critical size for spinel nucleation was estimated to be 4.3 nm based on the distribution in the size of very small particles. The embryos are bounded by 〈111〉 planes with an octahedral morphology which is the minimum energy morphology.

Chemistry-crystallization relations in molecular mullite gels

Sol-gel chemistry profoundly affects the crystallization of molecular aluminosilicate gels. Segregation of transition alumina-yielding species during gel synthesis results in the epitaxial nucleation of spinel at 980°C. When synthesis conditions leading to alumina segregation are avoided, t-mullite crystallizes at 980°C. However, it is shown, for the first time, that these gels are phase-separated prior to crystallization by a hydrothermal reaction known as dealumination. When the molecular gels are synthesized and heated to avoid this mechanism of phase separation, the gels crystallize directly to o-mullite at 700°C. This crystallization temperature for o-mullite in aluminosilicate gels is ∼ 500°C lower than those previously reported.

Epitactic Nucleation of Spinel in Aluminosilicate Gels and Its Effect on Mullite Crystallization

Control over the structure of hybrid (colloidal + molecular) aluminosilicate gels was utilized to demonstrate that precursor chemistry has a direct and controllable effect on the ∼1000°C crystallization of spinel and mullite in molecular precursor systems. Synthesis or preparation conditions leading to the development of a cubic, transition alumina result in the epitactic nucleation of spinel at ∼1000°C in gels that otherwise crystallize directly to mullite at ∼1000°C. Thus, the preference for spinel nucleation in gels derived from solution precursor systems whose chemistries promote formation of transition alumina readily explains the reported inability to obtain substantial mullite yields at ∼1000°C. Isothermal transformation kinetics of colloidal and hybrid gels show that in the absence of direct mullite formation at ∼1000°C, the release of alumina from the spinel‐type crystal structure becomes the rate‐controlling step in the transformation. This necessitates higher temperatures for mullite formation and limits the kinetic enhancement possible with extrinsic increases in mullite nucleation frequency.

Devitrification of defense nuclear waste glasses: Role of melt insolubles

Time-temperature-transformation (TTT) curves have been determined for simulated nuclear waste glasses bounding the compositional range in the Defense Waste Processing Facility (DWPF). Formulations include all of the minor chemical elements such as ruthenium and chromium which have limited solubility in borosilicate glasses. Heterogeneous nucleation of spinel on ruthenium dioxide and subsequent nucleation of acmite on spinel is the major devitrification path. Heterogeneous nucleation on melt insolubles causes more rapid growth of crystalline devitrification phases, and subsequent changes in leaching and mechanical behavior than in glass free of melt insolubles. These studies point out the importance of simulating waste glass composition and processing as accurately as possible to obtain reliable estimates of glass performance.