Non-equilibrium Crystallization Research Articles

Non-typical fractionation behaviors in a lepidolite-subtype pegmatite: Implications for the internal evolution of a naturally fluxed system

Despite their relatively rare occurrence, lepidolite-subtype pegmatites host abundant Li–Nb–Ta–Cs–Sn mineralization and represent a high-flux pegmatitic system with abnormally high F and Li activity. Characterization of highly fluxed melts and the impact of fluxes and exsolved fluids on fractionation of peraluminous melts have mainly been studied in experimental systems, with natural system correlations remaining poorly understood. Consequently, we conducted a systematic mineralogical study of a lepidolite-subtype pegmatite in the North Qinling orogenic belt, Central China. An abnormal “concave downward” fractionation trend for primary columbite-group minerals on the quadrilateral diagram is identified, and irregularly zoned columbite crystals coexist with F-rich minerals in one of the core zones have the highest Ta contents (normally 50.17–63.13 wt% Ta2O5) and Ta/(Nb + Ta) ratios (up to 0.65). Despite the consistently Ta-dominated B-site in the crystal lattice of microlite-group minerals, extreme compositional variations at the A- and Y-sites are observed. Compared with microlites in intermediate zones, the abrupt increase in U in microlite crystals in core zones and late units (up to 20.16 wt% UO2), is ascribed to the melt-fluid interaction with exsolved U-rich aqueous fluids. In addition, the fractional crystallization of F-bearing minerals resulted in a gradual decrease in F contents in microlite-group minerals from extremely F-rich (2.68–4.84 wt% F) in intermediate zones to low F species (mainly 0.82–1.71 wt% F) in core and late zones. Moreover, autometasomatism by a late fluxed melt and hydrothermal metasomatism by late aqueous fluids are identified in columbite- and microlite-group minerals. This work highlights that these non-typical fractionation behaviors related to the activity of fluxes (especially F) and the exsolution of aqueous fluids during the internal evolution of pegmatitic melts, are critical for the generation of lepidolite-subtype pegmatites. Fluorine was gradually enriched in the pegmatitic melt, and reached its highest level during crystallization of the (inner) intermediate and core zones. Non-equilibrium crystallization occurred throughout pegmatite evolution, and late units were most probably formed from aqueous fluid-enriched residual melts, rather than by hydrothermal replacement.

On the thermodynamic equilibrium of thermal casting processes

Based on thermodynamic calculations, it is shown that heating and cooling of metals and alloys are reversible, equilibrium processes. Based on thermodynamic calculations, it is shown that melting and crystallization of metals and alloys are also reversible, equilibrium processes. With an increase in the cooling rate of the metal, the temperature of its equilibrium crystallization decreases. At a high rate of metal cooling, a short‑term initial process of nonequilibrium crystallization is observed, which quickly passes into the main stage of equilibrium crystallization.

Nanoparticle Superlattices with Nonequilibrium Crystal Shapes.

Nanoparticle assembly is a material synthesis strategy that enables precise control of nanoscale structural features. Concepts from traditional crystal growth research have been tremendously useful in predicting and programming the unit cell symmetries of these assemblies, as their thermodynamically favored structures are often identical to atomic crystal analogues. However, these analogies have not yielded similar levels of influence in programming crystallite shapes, which are a consequence of both the thermodynamics and kinetics of crystal growth. Here, we demonstrate kinetic control of the colloidal crystal shape using nanoparticle building blocks that rapidly assemble over a broad range of concentrations, thereby producing well-defined crystal habits with symmetrically oriented dendritic protrusions and providing insight into the crystals' morphological evolution. Counterintuitively, these nonequilibrium crystal shapes actually become more common for colloidal crystals synthesized closer to equilibrium growth conditions. This deviation from typical crystal growth processes observed in atomic or molecular crystals is shown to be a function of the drastically different time scales of atomic and colloidal mass transport. Moreover, the particles are spherical with isotropic ligand grafts, and these kinetic crystal habits are achieved without the need for specifically shaped particle building blocks or external templating or shape-directing agents. Thus, this work provides generalizable design principles to expand the morphological diversity of nanoparticle superlattice crystal habits beyond the anhedral or equilibrium polyhedral shapes synthesized to date. Finally, we use this insight to synthesize crystallite shapes that have never before been observed, demonstrating the ability to both predict and program kinetically controlled superlattice morphologies.

Control of Interface Migration in Nonequilibrium Crystallization of Li2SiO3 from Li2O-SiO2 Melt by Spatiotemporal Temperature and Concentration Fields.

During liquid-solid transformation, bulk mass and thermal diffusion, along with the evolved interfacial latent heat, work in tandem to generate interfacial thermodynamic and kinetic forces, the interplay of which decides the solidification velocity and consequently the solidified phase attributes. Hence, access to interface dynamics information in dependence of bulk transfer processes is pivotal to tailor the desired quantity of solid phases of unique compositions. It finds particular application for engineering concentrated Lithium (Li) phases out of Li-ion battery slags, thus generating a high value-added product from a conventional waste process stream. However, considerable challenge exists to predict the impact of the diverse external cooling rates on the evolving internal transfer processes and thus tuning solidification routes for achieving phases of interest. Hence, in this work, a thermodynamically consistent nonequilibrium model, by considering spatiotemporal temperature and concentration fields, is developed and applied to study solidification of Li2SiO3 from a Li2O-SiO2 melt that constitutes an important subsystem of the Li containing battery-recycling slags. The approach treats the sharp solid/liquid interface as a moving heat source. In the presence of different heat extraction profiles, it evaluates the spatial temperature heterogeneity and its implicit correlation to internal material fluxes resulting from maximization of dissipation and consequently the interrelation to interface velocities. Model calculations revealed that irrespective of the external cooling rate, for an initial short time duration, the magnitude of which increased with decreasing cooling rates, the interface velocities show a reducing trajectory directly relatable to the reducing thermodynamic forces due to localized interfacial temperature rise from the generated latent heat of fusion from the initial solidification. This is followed by a thermodynamically controlled regime, whereby for each cooling rate, the interface velocities increase until a maxima, the magnitude of which decreases with decreasing cooling rates. Finally, the interface propagation speeds decrease as controlled by the kinetic regime.

Mechanism of beryllium mineralization in a granite-pegmatite system: Constraints from ore geology and beryl mineralogy of the large Arskartor Be-Nb-Mo deposit, southern Chinese Altai

The Arskartor Be-Nb-Mo deposit is the second largest Be deposit in the Chinese Altai, NW China, which hosts over 11,000 tons of BeO resource. The muscovite-albite granite and rare-metal pegmatite in the ore district are spatially and temporally coexisted and both are beryllium mineralized. The muscovite-albite granitic stock can be divided into a barren zone and a Be-mineralized zone. The pegmatite shows an well-developed internal zone including the layered muscovite-quartz-albite zone, (lower) beryl-muscovite-quartz zone, quartz core, (hanging) beryl-muscovite-quartz zone, and muscovite-quartz-microcline zone. Beryl is the dominant Be-bearing mineral and mainly occurs in the Be-mineralized granite, the layered muscovite-quartz-albite and the beryl-muscovite-quartz zones of the pegmatite. Subhedral to euhedral beryl in the Be-mineralized granite is interstitial to or intergrown with rock-forming minerals. In contrast, beryl crystals in the pegmatite are coarser in grain size and more euhedral in shape, and mainly coexist with coarse “booked” muscovite and blocky quartz. Concentrations of Li, Cs and Na/Li ratios of beryl are 184–760 ppm, 218–1996 ppm, and 2.13–21.3, respectively. The progressive variations of incompatible elements compositions and Na/Li ratios are consistent with the fractional crystallization mechanism of the granite-pegmatite system. Paragenesis and internal structure of beryl suggest nonequilibrium crystallization of a relatively incompatible elements and fluxes-enriched late granitic melt, generated the coarse-grained Be-mineralized granite. With the progressive enrichment of incompatible and fluxing elements and decreasing temperature, liquidus overcooling was achieved and generated the aplite and immediate zoned pegmatite that hosts abundant beryl. Moreover, the differentiating pegmatitic melt with varying amounts of aqueous fluids at different melt fractions, likely resulted in heterogeneous distribution of beryllium and subsequent variable amounts of beryl crystallization in different internal zones. The exsolution of aqueous fluids from the residual pegmatitic melt resulted in Mo-mineralization and related hydrothermal alteration. Consequently, both protracted fractional crystallization and subsolidus processes contributed the formation of the Arskartor granite-pegmatite system with Be-polymetallic mineralization.

Lower negative bounds on the static electric susceptibility of nonequilibrium cubic crystals

We use a classical, microscopic model of pointlike dipolarizable entities (a model that is standard in the case of positive polarizability) and investigate its behavior for simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc) crystals with one entity per primitive cell when the static polarizability of the entities is negative and the mutual electrostatic interaction between the entities is taken into account. We find that the static electric susceptibility is bounded below due to an instability towards self-polarization but negative values are possible in each case. The usual Clausius-Mossotti relation between the static polarizability and the static electric susceptibility remains valid in the case of negative parameters but is truncated at the lower bound; the value of the bound depends on the crystal structure and is always unrelated to the asymptote of the Clausius-Mossotti curve. The lower bounds of the static electric susceptibility are found to be −0.906 for sc and −1.00 for bcc and fcc. These results confirm that, although the magnitude of the static electric susceptibility does not diverge in the negative case (as it can in the positive case), the magnitudes attainable in the negative case for condensed media may, nevertheless, be many orders of magnitude greater than those predicted previously for inverted vapors and gases. This is a promising result in relation to the development of potential new technologies that exploit the phenomenon. Published by the American Physical Society 2024

Chiral domain dynamics and transient interferences of mirrored superlattices in nonequilibrium electronic crystals

Mirror symmetry plays a major role in determining the properties of matter and is of particular interest in condensed many-body systems undergoing symmetry breaking transitions under non-equilibrium conditions. Typically, in the aftermath of such transitions, one of the two possible broken symmetry states is emergent. However, synthetic systems and those formed under non-equilibrium conditions may exhibit metastable states comprising of both left (L) and right (R) handed symmetry. Here we explore the formation of chiral charge-density wave (CDW) domains after a laser quench in 1T-TaS2 with scanning tunneling microscopy. Typically, we observed transient domains of both chiralities, separated spatially from each other by domain walls with different structure. In addition, we observe transient density of states modulations consistent with interference of L and R-handed charge density waves within the surface monolayer. Theoretical modeling of the intertwined domain structures using a classical charged lattice gas model reproduces the experimental domain wall structures. The superposition (S) state cannot be understood classically within the correlated electron model but is found to be consistent with interferences of L and R-handed charge-density waves within domains, confined by surrounding domain walls, vividly revealing an interference of Fermi electrons with opposite chirality, which is not a result of inter-layer interference, but due to the interaction between electrons within a single layer, confined by domain wall boundaries.

Efficient leaching of manganese from electrolytic manganese residue and recovery of the leaching solution: Reuse and purification

Electrolytic manganese residue (EMR) is a hazardous solid waste, while it also contains resources of Mn over 7 %. In this study, sulfuric acid with high concentration was used to leach Mn from EMR, and the leaching solution was reused and purified to produce battery-grade manganese sulfate. The optimal leaching conditions for EMR were determined through single-factor experiments and response surface methodology. Under the conditions of H2SO4 concentration at 5.4 mol/L, liquid–solid ratio at 4.8 mL/g, and temperature at 80 °C, the highest Mn leaching efficiency of 99.12 % was achieved. The kinetics of leaching Mn from EMR follows the Avrami model. The leaching characteristics were described according to the basic hypothesis made by the Avrami equation and the fitted Avrami index. The kinetic process is macroscopically regarded as the inverse process of non-equilibrium crystallization. According to the Arrhenius equation, the leaching of Mn is controlled by the mixture of diffusion and reaction with an apparent activation energy of 19.8 kJ/mol. The EMR leaching solution was then reused for the leaching step of rhodochrosite ore to simulate actual production. The secondary leaching solution was purified using neutralization, sulfide precipitation, and fluoride precipitation, and manganese sulfate with high purity was produced from it by evaporative crystallization. This study provides a low-cost and high-efficiency process for leaching and recovering Mn from EMR, which can also reduce the environmental risk.

Investigation on nonequilibrium crystallization of highly undercooled Cu–Ni–Co alloys

Deep undercooling rapid solidification experiments of Cu–Ni–Co alloys were carried out using the molten glass purification cycle superheat method. 130 K, 177 K and 277 K undercooling degrees were obtained and analysed for Cu55Ni43Co2 alloy. The crystallographic information obtained by EBSD and the dynamics of the solidification front taken by high speed photography were analysed and it was concluded that both the formation of relatively rounded equiaxed grains during the first refinement and the subsequent formation of finer subcrystals were due to the release of a large amount of solidification energy during the solidification process. The re-glow stage releases a large amount of latent heat of solidification resulting in the remelting of the dendrites. TEM tests were carried out on Cu55Ni43Co2 alloy with a maximum undercooling of 277 K. It was found that part of the substructure showed a high density dislocation network. By analysing the effect of increasing undercooling on the microstructure evolution, combined with the EBSD and TEM characterisation data, it was found that the stress-induced recrystallization mechanism was the dominant factor in the grain refinement mechanism at large undercooling.

Pattern Formation under Deep Supercooling by Classical Density Functional-Based Approach.

Solidification patterns during nonequilibrium crystallization are among the most important microstructures in the natural and technical realms. In this work, we investigate the crystal growth in deeply supercooled liquid using the classical density functional-based approaches. Our result shows that the complex amplitude expanded phase-field crystal (APFC) model containing the vacancy nonequilibrium effects proposed by us could naturally reproduce the growth front nucleation (GFN) and various nonequilibrium patterns, including the faceted growth, spherulite, symmetric and nonsymmetric dendrites among others, at the atom level. Moreover, an extraordinary microscopic columnar-to-equiaxed transition is uncovered, which is found to depend on the seed spacing and distribution. Such a phenomenon could be attributed to the combined effects of the long-wave and short-wave elastic interactions. Particularly, the columnar growth could also be predicted by an APFC model containing inertia effects, but the lattice defect type in the growing crystal is different due to the different types of short-wave interactions. Two stages are identified during the crystal growth under different undercooling, corresponding to diffusion-controlled growth and GFN-dominated growth, respectively. However, compared with the second stage, the first stage becomes too short to be noticed under the high undercooling. The distinct feature of the second stage is the dramatic increments of lattice defects, which explains the amorphous nucleation precursor in the supercooled liquid. The transition time between the two stages at different undercooling is investigated. Crystal growth of BCC structure further confirms our conclusions.

SUBSTANTIATION OF THE CHOICE OF COATING MATERIAL WHEN RESTORING PARTS OF AGRICULTURAL MACHINERY BY COLD GAS-DYNAMIC SPRAYING

Based on the method of cold gas-dynamic spraying, technologies are being created to solve problems of energy and resource saving in various industries and non-traditional and effective methods of production, repair, restoration, anti-corrosion protection, obtaining electrical and heat-conductive, anti-friction and other coatings of functional units, and as well as elements of a wide variety of machinery and equipment. The main feature of cold gas-dynamic spraying is the absence of high temperatures during the formation of metal coatings, therefore, the absence of oxidation of particle and base materials, non-equilibrium crystallization processes, and high internal stresses in the workpieces. Coatings created in this way are a metal matrix with ceramic particles embedded in it. They are characterized by high strength, good performance properties and can be applied in any thickness. One of the important characteristics of coatings is the adhesion strength. Before spraying on any part, it is necessary to find out which coating material will give the best strength characteristics between the two materials. Therefore, before spraying, it is important to study the specifics of the formation of a durable coating for a given part, as well as to compare the strength characteristics of the cold gas-dynamic spraying method with other methods for restoring parts. In this paper, an analysis of various coating materials used in the restoration of parts by the method of cold gas-dynamic spraying is carried out. The strengths and weaknesses of various coating technologies are determined, as well as recommendations for choosing a material when restoring parts by cold gas-dynamic spraying.

Dynamics of cluster structure change in melts that forms a continuous series of solid solutions during equilibrium and nonequilibrium crystallization

The article describes the dynamics of changes in the melt structure during cooling of solid substitution solutions in the process of quasi-equilibrium (CRC) and non-equilibrium explosive crystallization (NRCC), taking into account clustering processes. The boundaries of the transition to the solid state are determined as the structure of clusters changes during crystallizations of the KRK and NRK types. The boundaries are determined taking into account the critical overheating and pre-crystallization supercooling relative to the liquidus line . It is determined that during quasi–equilibrium crystallization, the melt changes its structure in the following sequence , and during non-equilibrium explosive crystallization - in the sequence . The phase rule is used to calculate the first crystals that appeared during various types of crystallization. Alloy cooling curves describing various types of alloy crystallization.

Elimination of δ-ferrite in N50 steel and its effect on cryogenic mechanical properties

δ ferrite serves as a detrimental phase in TF (Toroidal Field) superconducting magnet jacket pipe materials. The distribution features of ferrite in N50 casting and forging were investigated, and the cryogenic mechanical properties with different ferrite content were studied accordingly. The results indicated that the non-equilibrium crystallization leads to the solidification mode gradually changing from AF (Austenitic-ferritic) at the surface to the FA (Ferritic-austenitic) at the center of the casting. The skeleton δ ferrites induced by interdendritic segregation could disappear almostly after annealing while the tangled lath δ ferrite at the center of casting caused by the more serious segregation seems to be stable. Based on thermodynamics calculations and through changing the Cr and Ni content according to the Espy diagram, the stable ferrites can be eliminated when the Creq/Nieq is less than 1.25. From fractography, the cryogenic impact toughness of N50 steel would be deteriorated because more secondary cracks were taken place by the mismatched deformation between the harder residual δ ferrites and matrix. Though the elimination of δ ferrites caused a slight decrease in cryogenic tensile strength, it greatly improved the cryogenic impact toughness.

Lithiation-Driven Crystallization of Ni-Based Cathode Materials during Calcination

Driven by the increasing need of energy storage for electric vehicles and electronic devices, the technological advancements of lithium-ion batteries (LIBs) are urgently in demand to increase energy density while reducing cost. LiNixMnyCo1-x-yO2 (NMC) is among the most promising low-cost, high-energy-density cathodes for next-generation LIBs.[i] ,[ii] NMC cathode materials are generally synthesized through high-temperature calcination of NMC and lithium hydroxides. Control of the calcination conditions has been found crucial to obtaining high-performance NMC cathodes, wherein structural ordering and morphology of the final products are finely tuned to maximize capacity while minimizing surface areas exposed to the electrolyte.[iii] ,[iv] However, making NMC cathode materials of desired structural properties has been challenging due to the complexity of the calcination process, involving phase transformation accompanied with non-equilibrium crystallization of intermediates prior to forming the layered oxides.[v] ,[vi] In this work, a combination of in situ X-ray spectroscopy and microscopy techniques is applied to track the calcination process during synthesis of the practical system LiNi0.8Mn0.1Co0.1O2 (NMC811) and the archetypal LiNiO2 (LNO). The reaction pathways and structural evolution of the involved phases were identified, revealing the strong compositional dependence of phase progression, structural ordering, and crystal growth during the calcination process. A strong correlation was found between the degree of lithiation in the rocksalt phase and the progression of the layered phase (see figure 1). In NMC811, the lithiation is facilitated by Co/Mn leading to the highly lithiated rocksalt and resulting in rapid layering at low temperatures. The nucleation of the layered phase is predominately driven by lithiation, while the growth process is kinetically limited by the sluggish Ni diffusion in LNO and further hindered in NMC811 due to the presence of Co/Mn. The findings from this study, with insights into crystallization thermodynamics and kinetics, may provide guidance to the design and synthesis of high-performance Ni-based NMC cathode materials. [i] N. Nitta, F. Wu, J. T. Lee, and G. Yushin, Materials today, 2015, 18(5), 252-264. [ii] M. S. Whittingham, Chemical reviews, 2014, 114(23), 11414-11443. [iii] J. C. Garcia, J. Bareño, J. Yan, G. Chen, A. Hauser, J. R. Croy, H. Iddir, The Journal of Physical Chemistry C, 2017, 121(15), 8290-8299. [iv] Y. Duan, L. Yang, M. J. Zhang, Z. Chen, J. Bai, K. Amine, F. Pan, F. Wang, Journal of Materials Chemistry A, 2019, 7(2), 513-519. [v] M. Bianchini, F. Fauth, P. Hartmann, T. Brezesinski, J. Janek, Journal of materials Chemistry A, 2020, 8(4), 1808-1820. [vi] F. Wang, J. Bai, Batteries & Supercaps, 2021. (https://doi.org/10.1002/batt.202100174) Figure 1

Numerical Analysis Nucleation and Growth Conditions of Single-Crystallinity Control during Laser Surface Modification of Ni-Based Single-Crystal Superalloy Part I: Thermal and Chemical Determinants

Nucleation and growth conditions of single-crystallinity control are convincingly elaborated by multi-scale mathematical modeling of heat and mass transport to totally abate undesirable weld defects, e.g. disoriented crystal and hot cracking inside molten pool of nonequilibrium crystallization, in order to illustrate the usefulness of predictive capability through theory and experiment procedures. Crystal growth is complicated by crystallinity-dependent thermal and chemical driving forces in front of dendrite tip during viable laser surface modification of Ni-based single-crystal superalloy. These two thermal metallurgical determinants play crucial role in crack-insusceptible columnar crystal growth, which is favorably oriented throughout weld depth. There is particular challenge in complete elimination of disoriented crystal, i.e. stray grain formation, for acceptable surface quality. Conservative (001)/[100] crystalline orientation is desired to diminish Al concentration and supersaturation, and morphologically satisfy epitaxial growth kinetics to successfully lessen central cracking with satisfactory variability of laser power and welding speed. Comparatively, (001)/[110] crystalline orientation is disadvantageous to asymmetrically augment Al concentration and supersaturation and aggressively increase interface instability, microstructure heterogeneity and hot cracking vulnerability along disoriented crystal boundaries. Disoriented crystal is increasingly withstood if the Al concentration and supersaturation in front of dendrite tip are low enough and crack-unsusceptible part is relatively large enough in case of attractive (001)/[100] crystalline orientation with optimal range of heat input to ameliorate microstructure homogeneity. Crystalline orientation region varies with diverse welding configurations, and epitaxy across solid/liquid interface is also sensitive to heat input of laser processing, which necessitate high efficient welding conditions optimization. Considerable effort is made to distinguish diffusion-driven crystal growth between a series of combinations of multiple welding conditions, such as critical welding configuration and heat input. Metallographically, the morphologies of crystal growth and hot cracking are experimentally observed to consistently support kinetics calculation result and well explain correlation between solidification behavior and crystal growth.

Two hydrothermal events associated with Au mineralization in the Youjiang Basin, southwestern China

The Youjiang Basin in southwestern China hosts remarkably large amounts of Carlin-type Au deposits. In such deposits, Au is mainly ionically bound to arsenian pyrite characterized by core and rim textures. Understanding the formation mechanism of zoned pyrite may improve our understanding of the genesis of massive Carlin-type Au deposits. This study examined the elemental content, mineral composition, and pyrite texture and chemistry of wall rock and ore from the Lannigou Au deposit, one of the largest Carlin-type Au deposits in the Youjiang Basin, to identify the genetic mechanisms of zoned pyrite.Five distinct types of pyrites, i.e., pyrite 1 (Py1), 2 (Py2), 3 (Py3), 4 (Py4), and ore pyrite (OPy), were identified in the Lannigou Au deposit. Py1, Py2, and Py3 were mainly observed in the wall rock, whereas Py4 and OPy, constituting the core and rim of zoned pyrite, respectively, were only discovered in the ore. Mineral mapping of the wall rock and ore indicated that pyrite is scarce in the wall rock (0.021 vol%). However, the amount of zoned pyrite significantly increased in the ore (1.275 vol%). Py1, Py2, and Py3, showing framboidal, nodular, and fine euhedral microcrystal textures, had high Pb, Co, Ni, Mn, Zn, and Mo contents (medians ranging from 10 to 1,000 ppm) and variable δ34S compositions ranging from −5.1 to 35.4‰, implying a sedimentary (syngenetic or diagenetic) origin. Py4 was homogeneous and contained numerous illite and quartz inclusions and fractures. In contrast, OPy showed multiple As-rich sub-bands and a lack of illite or quartz inclusions and fractures. Py4 had significantly lower Au, As, Tl, and Cu contents than OPy; however, Py4 had Pb, Co, Ni, Mn, Zn, and Mo contents similar to OPy. The range of δ34S values for Py4 and OPy were both narrow, but Py4 (7.6 to 13.3‰, with an average of 9.1‰) had lower δ34S values than OPy (9.4–14.1‰, with an average of 12.4‰). These texture and chemical features indicate that Py4 likely sedimented from a pre-ore hydrothermal fluid, but not at the early stage of the ore-forming fluid.Hydrothermal pyrite cores were also widespread and exhibited textural and chemical differences compared to Au-rich pyrite rims from other Au deposits in the Youjiang Basin. Based on the two geochronological clusters of Au deposits in the Youjiang Basin, we inferred the occurrence of at least two episodes of large-scale hydrothermal events. The two hydrothermal fluid episodes, which contributed to the zoned pyrite formation, likely originated from the mixing of initial magmatic fluid with δ34S values of ∼0 ± 5‰ and basin brine with elevated δ34S values (δ34S > 18‰). Pre-ore hydrothermal pyrite likely resulted in the rapid non-equilibrium crystallization of ore pyrite; thus, more Carlin-suite elements were captured by ore pyrite. Additionally, low-grade Au orebodies may have locally formed during the pre-ore hydrothermal event. These multiple hydrothermal events may have been crucial for the formation of massive Carlin-type Au deposits in the Youjiang Basin.

The Composition and Growth Mechanism of Coexisting 4M2 and 4A8 Biotite Polytypes from Rhyolite of Long Valley Caldera, California

AbstractPolytypism is common in micas, and the frequency of polytype occurrence is believed to be related closely to the crystallization conditions and chemical compositions of the corresponding fluids and melts. Coexisting multiple standard and complex/disordered polytypes in igneous rocks generally reflect a complicated magma evolution history. The purpose of the current study was to clarify the origin of coexisting biotite polytypes and their growth mechanism. Micro-X-ray diffraction (μXRD) and transmission electron microscopy (TEM) were used to investigate Fe-rich biotite phenocrysts in rhyolite from the Long Valley Caldera, California, USA. The μXRD analyses characterized various polytypes, and TEM observations revealed that common polytypes (e.g. 1M, 2M1, and 3T) and rare polytypes (e.g. 4M2 and 4A8) coexist within biotite monocrystals. The two 4-layer polytypes of Fe-rich biotite, 4M2 and 4A8, were identified via selected-area electron diffraction (SAED) and high-resolution scanning transmission electron microscopy (HRSTEM) at the atomic resolution, with unique stacking sequences ([0222] for 4M2 and [0022¯] for 4A8). Energy-dispersive X-ray spectroscopy (EDS) results showed differences in their chemical compositions, especially Fe and K. The 4A8 polytype is reported for the first time. The present study suggested that environmental changes, such as rapid cooling and inhomogeneous compositional distribution, led to chemical and structural oscillations and complex nucleation of the two 4-layer polytypes. Screw dislocations producing spiral growth enhance polytype stability and form ordered long-period/complex polytypes. These results are useful to understand the origin of long-period/complex polytypes and the intergrowths of diverse polytypes formed in non-equilibrium crystallization environments.

Undercooling and Non-Equilibrium Crystallization of Al-Mg Melts

Undercooling and Non-Equilibrium Crystallization of Al-Mg Melts

Thermodynamics of strain-induced crystallization in filled natural rubber under uni- and biaxial loadings, Part II: Physically-based constitutive theory

To understand thoroughly the strain-induced crystallization in natural rubbers, conventional mechanical measurements are inadequate because they only provide a macroscopic relation between stress and strain. In this second part, a physically-based constitutive model for filled natural rubbers is coupled with the infrared thermography-based quantitative surface calorimetry (Part I) to shed new light on multiaxiality of strain-induced crystallization. In contrast to previous works, the kinetics of phase transition outside thermodynamic equilibrium is discussed. By introducing only two additional parameters (compared to the equilibrium crystallization theory), underlying mechanisms of nonequilibrium strain-induced crystallization can be well interpreted. To capture multiaxiality of strain-induced crystallinity, the analytical network-averaging is utilized for the calculation of kinematic internal variables. Model predictions are then compared with comprehensive testing data (Part I) and demonstrate good agreement with these experiments.

Effect of carbon on the phase formation in Fe85−xCr15Cx (x = 10–17) melts at low cooling rates

The Fe-Cr-C alloys are promising hard facing materials with excellent wear resistance to address extreme technological environments which is provided by the presence of carbide chemical bonds stipulating high hardness. In this research, we study the influence of the carbon concentration from 10 to 17 at% on phase equilibria during heating, as well as crystallization processes of Fe-Cr-C alloys at 15 at% Cr and their relationship with the structure of the undercooled liquid state. The concentration dependence of undercoolability of Fe85−xCr15Cx (x = 10–17) melts revealed the minimum value in the region of the equilibrium eutectic near 14 at%C. Crystallization of Fe-Cr-C melts under cooling at a rate of 100 °C/min proceeds according to a nonequilibrium mechanism with the formation at the first stage of a solid solution based on fcc-Fe. Structure analysis of the undercooled Fe85−xCr15Cx melts (x = 10–17) showed that the nonequilibrium crystallization is related to peculiarities of interatomic interaction. Moreover, there is formation of structures with microhardness decreasing with growing carbon concentration in the alloy.