Equilibrium Crystallization Research Articles

Temperature and pressure dependence of the alpha relaxation in ortho-terphenyl.

Molecular dynamics (MD) simulations of ortho-terphenyl using an all-atom model with the optimized potentials for liquid simulations (OPLS) force field were performed both in the high temperature Arrhenian region and at lower temperatures that include the onset of the super-Arrhenian region. From the MD simulations, the internal energy of both the equilibrium liquid and crystal was determined from 300 K to 600 K and at pressures from 0.1 MPa to 1 GPa. The translational and rotational diffusivities were also determined at these temperatures and pressures for the equilibrium liquid. It is shown that within a small offset, the excess internal energy Ūx from the MD simulations is consistent with the experimentally determined excess internal energy reported earlier [Caruthers and Medvedev, Phys. Rev. Mater. 2, 055604, (2018)]. The MD mobility data {including extremely long-time 1 atm simulations from the study by Eastwood et al. [J. Phys. Chem. B 117, 12898, (2013)]} were combined with experimental data to form a unified dataset, where it was shown that in both the high temperature Arrhenian region and the lower temperature super-Arrhenian region, the mobility is a linear function of 1/Ūx(T,p), albeit with different proportionality constants. The transition between the Arrhenian and super-Arrhenian regions is relatively sharp at a critical internal energy Ūx α. The 1/Ūx(T,p) model is able to describe the mobility data over nearly 16 orders-of-magnitude. Other excess thermodynamic properties such as excess enthalpy and excess entropy (i.e., the Adam-Gibbs model) are unable to unify the pressure dependence of the mobility.

Construction of Gallium–Tin Nonequilibrium State Diagram and Its Analysis

Overcooling of gallium–tin alloys under normal conditions has been studied by thermal analysis. The following samples have been analyzed: Ga (I); two hypoeutectic alloys: 95% Ga + 5% Sn (II), 90% Ga + 10% Sn (III); eutectic alloy: 96.3% Ga + 13.7% Sn (IV); and five hypereutectic alloys with Sn content of 20% (V), 35% (VI), 50% (VII), 80% (VIII), and pure tin (IX). A nonequilibrium state diagram of this system is constructed. Herewith, the eutectic composition does not vary and the eutectic temperature decreases to 5.5°C, that is, 26°C below that of three-phase eutectic equilibrium. The eutectic temperature does not actually vary upon a variation of cooling rates of eutectic alloy from 0.06 to 60°C/min. It has been detected that a slight decrease in overcooling is expected in the hypoeutectic region, whereas in the hypereutectic region overcooling increases while the alloy composition approaches eutectic. Activities and activity coefficients of components on the lines of equilibrium and nonequilibrium liquidus have been calculated. It is demonstrated that the activities on the lines of both equilibrium and nonequilibrium liquidus decrease in a predictable manner, and the activity coefficients increase while the composition approaches eutectic. Concentration paths of equilibrium and nonequilibrium crystallization are shown in the state diagrams.

Analysis of the Results of Pulsed Processing of Melts

The paper presents the results of a relatively new method of external action on melts developed by the authors. The essence of the technology is the impact on melts before casting with electromagnetic pulses (EMP) of short duration (1 ns) and with high pulse power (2 kW). To create electromagnetic pulses, a generator with the following characteristics was used: pulse amplitude of 10 kV, the leading edge of the pulse was 0.1 ns, pulse duration of 1 ns, pulse repetition rate at 1 kHz, and a calculated pulse power of 2 MW. A distinctive feature of the generator used was its low power consumption of 50 Watts. The results of processing low-melting melts of the Al–50Pb, Bi–38Pb, and Bi–18Sn–32Pb systems presented in the work indicated that EMP treatment led to the occurrence of equilibrium crystallization of the metal, increasing its density. In addition to the experimental results, a theory is provided to explain the mechanism of the influence of pulse processing on the properties of metals of these and other systems previously studied by the authors.

Collective Motion of Atoms in a Superheated Crystal and a Supercooled Melt of a Simple Metal

Stable and metastable states of a crystal and a melt have been studied near the equilibrium crystal–melt phase transition point using a four-point correlation coefficient previously proposed to reveal collective motions of atoms in a liquid. The embedded copper atom model has been considered as an example with the molecular dynamics method. Strong hysteresis of the correlation coefficient values is detected along the paths of isochoric superheating of the crystal until its melting near the spinodal and the inverse supercooling of the melt to its crystallization. The capabilities of this coefficient for a quantitative estimate of the character of the near-range order, the degree of correlation of the motion of neighboring atoms, and the radius of action of dynamic collective effects in condensed matter are revealed.

Struvite crystallization by using raw seawater: Improving economics and environmental footprint while maintaining phosphorus recovery and product quality

Seawater, as an alternative magnesium source, has the potential to improve the overall economics and environmental footprint of struvite production compared to the use of pure magnesium salts. However, the dilution effect and the presence of other ions in seawater can reduce the phosphorus recovery potential and the simultaneous precipitation of other compounds may reduce the quality of the produced struvite. This work presents a comparative study of seawater and MgCl2 by performing a series of thermodynamic equilibrium modeling and crystallization experiments. The results revealed that acceptable phosphorus recovery (80–90%) is achievable by using seawater as the magnesium source for struvite precipitation. Further, the simultaneous precipitation of calcium phosphates was successfully controlled and minimized by optimum selection of reaction pH and seawater volume (i.e. Mg:P and Mg:Ca molar ratios). The increase of temperature from 20 °C to 30 °C reduced the phosphorus recovery by 15–20% while it increased the particle size by 30–35%. The presence of suspended solids in reject water did not have significant effects on phosphorus recovery but it made the struvite separation difficult as the obtained struvite was mixed with suspended solids. The experimental results and economic evaluation showed that the use of seawater can reduce the chemical costs (30–50%) and the CO2-footprint (8–40%) of struvite production. It was concluded that seawater is a potential alternative to pure magnesium sources in struvite production, while studies in larger scale and continuous mode are needed for further verification before full-scale applications.

The role of Te, As, Bi, Sn and Sb during the formation of platinum-group-element reef deposits: Examples from the Bushveld and Stillwater Complexes

The distribution of platinum-group element (PGE) and Te, As, Bi, Sb and Sn (TABS) in whole-rock samples, and in disseminated base metal sulfides (BMS) pentlandite, pyrrhotite and chalcopyrite from the Bushveld and Stillwater Complexes are reported. The samples are from: the Merensky Reef (Bushveld), the J-M Reef (Stillwater), Picket Pin deposit (Stillwater), and also barren sulfide-bearing samples, from outside the reef intervals from both intrusions. The objective of the study was to document the distribution of PGE and TABS in PGE-reef deposits, and to investigate whether TABS play a significant role during the formation of PGE-reef deposits.The whole-rock concentrations of PGE and TABS (except for As) correlate with S and PGE, and thus their distribution appear to be controlled by BMS. The distribution of As, and to a lesser extent Sb, correlate with incompatible elements and with changes in K-phologopite compositions, suggesting that these elements are controlled both by the amount of trapped liquid in cumulate rocks, and the amount of sulfides. The possible role of TABS in forming pre-nucleation clusters (nanonuggets) to enrich the reefs in PGE is considered and discarded, because the ratio of TABS/PGE < 0.3 is too low for TABS to form pre-nucleation clusters with PGE. A model where both the PGE and TABS are initially collected by a base metal sulfide liquid is favoured. During equilibrium crystallization of the sulfide liquid, Pd, Pt and the TABS were incompatible with the crystallizing BMS, and concentrated in the fractionated liquid eventually crystallizing as TABS-rich PGM. However, a portion of Pd, Pt and TABS also partitioned into the crystallizing BMS. During cooling and S-loss they exsolved from the BMS as PGM included in the BMS. In the reefs, the ratio of PGE/TABS is very high, and thus during exsolution all of the TABS could be accommodated in PGM, and consequently the BMS of the reefs have very low TABS concentrations. In contrast, outside the reefs the PGE/TABS ratios are low, and thus some TABS remained in the BMS, resulting in the BMS outside the reefs having higher TABS concentrations than those from within the reefs.

Coherent phase decomposition in the Pd–H system

We have used molecular dynamics and Monte Carlo methods to simulate the structure and phase stability of a Pd crystal in thermodynamic equilibrium with molecular hydrogen gas at temperature T and pressure $$ P_{g}^{H2} $$. The pressure–composition–temperature (PCT) curves were deduced under the extreme conditions of an open system (Pd crystal in equilibrium with a large-volume H2 gas reservoir) and a closed system (Pd crystal in equilibrium with H2 gas reservoir of infinitesimal volume). The PCT curves from the open simulations reproduce the experimental observations, including the ubiquitous hysteresis. The PCT curves from the closed-system simulations are continuous curves. Below a tri-critical point, the Pd–H system decomposes into two coherent phases. We find excellent agreement between the present simulation results and the predictions of the Schwarz–Khachaturyan theory for the decomposition of a Pd–H alloy into two coherent hydride phases.

Shapes of epitaxial gold nanocrystals on SrTiO3 substrates.

Morphological control of gold nanocrystals is important as their catalytic and optical properties are highly shape dependent. In this paper we report the shapes of gold nanocrystals which deviate from the equilibrium Wulff shape due to the influence of the SrTiO3 single crystal substrates. The gold crystals are characterized by scanning tunneling microscopy (STM) and scanning electron microscopy (SEM). The nanocrystals have an equilibrium shape of a truncated octahedron with {111} and {001} facets. On all three substrate surfaces, i.e., SrTiO3(001)-(2 × 1), SrTiO3(001)-c(4 × 2), and SrTiO3(111)-(4 × 4) + (6 × 6), the height-to-width ratio of the gold crystals is not a constant as would be expected for equilibrium crystals, but instead it increases with crystal height. We propose that as the crystals increase in size, their aspect ratio heightens to relax the interfacial strain. The ratio between the {111} and {001} surface areas of our gold crystals is found to differ on the three substrates, which we speculate is due to the selective adsorption of surfactants on the {111} and {001} gold facets resulting from the different substrate surfaces. Reentrant facets of gold crystals that should be present according to their Wulff shape are not observed because these concave sites typically grow out due to kinetic considerations. This study demonstrates the significant effect of the crystal facet termination and surface reconstruction of an oxide substrate on the shape of supported gold nanocrystals.

Crystallization Behavior of Amorphous Si Nanoinclusions Embedded in Silicon Oxide Matrix

Herein, crystallization behavior of amorphous Si (a‐Si) nanoparticles embedded in Si oxide matrix is studied thermodynamically. The change of the Gibbs free energy of the Si nanoinclusion/Si oxide matrix system upon crystallization of the Si nanoinclusion core is calculated as a function of normalized core radius. Minimum values of the Gibbs free energy determine equilibrium crystallization states of Si nanoinclusions. The equilibrium radii of the crystallized Si parts normalized with respect to the Si particle radii, as well as the barriers for crystallization, are calculated as functions of Si size and annealing temperature. The degree of Si crystallization is shown to increase with Si nanoparticle size. The existence of minimum critical radius of Si nanoinclusions for the onset of crystallization is demonstrated. Obtained results have a good correlation with available experimental data and contribute to the understanding of the formation of crystalline Si structures in the nanosized Si/Si oxide matrix composites formed by phase separation of nonstoichiometric silicon oxide films during high‐temperature annealing.

Casimir stresses in active nematic films

We calculate the Casimir stresses in a thin layer of active fluid with nematic order. By using a stochastic hydrodynamic approach for an active fluid layer of finite thickness L, we generalize the Casimir stress for nematic liquid crystals in thermal equilibrium to active systems. We show that the active Casimir stress differs significantly from its equilibrium counterpart. For contractile activity, the active Casimir stress, although attractive like its equilibrium counterpart, diverges logarithmically as L approaches a threshold of the spontaneous flow instability from below. In contrast, for small extensile activity, it is repulsive, has no divergence at any L and has a scaling with L different from its equilibrium counterpart.

Ages and glass compositions for paired large-volume eruptions from the Acigöl volcanic complex, Cappadocia (Turkey)

Acigol volcanic complex is a bimodal volcanic field located at the western margin of the Neogene–Quaternary Cappadocian volcanic province in Central Anatolia (Turkey). Its most voluminous eruptions are preserved as a pair of widespread tuffs known as Lower Acigol Tuff (LAT) and Upper Acigol Tuff (UAT). Both are nearly aphyric high-silica rhyolites that are compositionally identical; they can be distinguished, however, by the presence of obsidian lithics in UAT and the absence thereof in LAT. Here, we report the discovery of medial deposits of these tuffs at ~ 60–65 km distance from their source, and re-estimate the volume for LAT in the range of 21.4–26.7 km3 (similar constraints are lacking for UAT, but a subequal volume is assumed). Development of paleosol between LAT and UAT provides evidence for a hiatus between both events which is further supported by radiometric dating using combined U–Th and (U–Th)/He zircon geochronology. The revised age for LAT based on sequence modeling of U–Th zircon crystallization and (U–Th)/He zircon eruption ages is 190 ± 11 ka, whereas new and published (U–Th)/He zircon ages indicate eruption of UAT at 164 ± 4 ka (uncertainties generally stated at 1σ). The compositional similarity between both eruptions previously known from whole-rock analyses also extends to glass major, minor, and trace element compositions. Moreover, U–Th zircon crystallization ages are also indistinguishable between LAT and UAT in proximal and medial locations. U–Pb zircon ages for crystals in secular equilibrium indicate a provenance from Cretaceous granitic basement and Miocene volcanic overburden typical for Cappadocia, and such xenocrysts appear to be slightly more common in LAT compared to UAT. The compositional similarity between LAT and UAT along with the apparent recycling of zircon from the non-erupted LAT magma during the UAT event indicates sequential tapping of rhyolite melt from a magma system that maintained thermal and compositional uniformity over a protracted time interval. This can be explained by rhyolite melt extraction from a thermally buffered crystal mush that remained viable to produce voluminous high-silica eruptions for few 10s of ka. The subsequent migration of eruptive activity in the Acigol volcanic complex from east to west also led to a change in magma chemistry and triggered a new pulse of zircon crystallization, indicating abandonment of the previous magmatic focus, and a shift of magma input to a new location. The identification of sizable medial deposits of LAT and UAT implies that both tephra units are potentially useful marker horizons for marine isotope stage (MIS) 6 sedimentary sequences in and around Anatolia. The recent discovery of UAT in a Black Sea sediment core supports this view.

Crystallization and Cooling of the Noril’sk Intrusions According to the Pyroxen’s Geothermometry Data

Composition of clinopyroxenes in vertical sections of some ore-bearing basic intrusions in the Noril’sk area has been studied by EPMA. These data formed the basis for the calculation of the pyroxene crystallization temperatures using the El Negro monopyroxene geothermometer (Fe-Mg exchange between M1-M2 positions). The results of the calculations allowed estimating the variations of temperature crystallization within the magmatic chambers from early to the late stages. The temperature varies from 1200 for idiomorphic crystals in equilibrium to liquid and up to 400 degrees (in solid intra exchange) for xenomorphic grains and marginal zones of pyroxenes. We reconstructed the regimes of crystallization of the magmatic chamber by this data. The vertical distribution of paleotemperatures of the intrusive chamber there is the evidence of absence vertical move crystals into such magmatic chambers.

Mathematical modelling of heat-resistant nickel alloys high temperature corrosion

The mathematical modelling of the average corrosion rate () or nickel alloys with different alloying systems (for mono-, directed- and equilibrium crystallization) under synthetic ash conditions at different temperatures has been carried out. Since modern heat-resistant nickel alloys have complex multicomponent compositions, in which all the elements influence on corrosion complexly, the high temperature corrosion (HTC) stability was evaluated by the value of the known parameter Pks(surface resistance coefficient). This made it possible to compare the level of stability of alloys with different alloying schemes. However, this parameter does not cover all the alloying elements that are present in the heat-resistant nickel alloys. Therefore, as a result of analysis and processing of experimental data, the ratio of alloying elements to assess the corrosion resistance has been proposed; this ratio considers the complex influence of the main components of the alloy. Since high temperature corrosion (HTC) is related to the presence of certain elements in the alloy and their concentration, the ratio Kps (surface resistance coefficient) makes it possible to evaluate more adequately the average corrosion rate at different temperatures for multi-component nickel systems. The dependence of the average corrosion rate on Kps for monocrystalline alloys is straightforward, due to the specific character of the alloying systems of this class materials. Characteristic for them is a significantly less amount of chromium (up to 10% by weight) is characteristic of them and a decrease in the content (and in some alloys, absence) of titanium as well, which results in a significant reduction in the high temperature corrosion (HTC)-stability of the material. It has been established that to ensure the required level of high temperature corrosion (HTC) stability of alloys of directed and equilibrium crystallization, the value of the ratio should be not less than Kps ≥ 2, which will provide a non-destructive dense film of corrosion products. So, for the alloys with Kps ≤ 2 the formation of a thick layer of corrosion products, which is easily separated during the operation is characteristic. The obtained regression models give an opportunity to predict the average speed of corrosion depending on the alloying system, both for the development of new heat- resistant nickel alloys for directed crystallization, and in improving the composition of the known industrial alloys within the brand composition. The obtained correlation dependencies are exponential in nature

Prediction of high temperature corrosion of nickel-based superalloys

Purpose. Establish a correlation between the rate of corrosion and the ratio of alloying elements based on the mathematical treatment of experimental values of foundry heat resisting nickel alloys (ZNS) under synthetic ash conditions.Methods of research. Experimental values were processed in the Microsoft Office program suite in the EXCEL package with the least squares method, with the result of correlation dependencies of the “parameter-property” type with the obtaining of mathematical equations of regression models that optimally describe these dependencies and the construction of trend lines.Results. Since modern ZNS have complex multicomponent compositions, in which there is a complex influence of elements on corrosion, the evaluation of the stability of the corrosion was carried out per the size of the known parameter Кпс. This allowed to compare the level of stability of alloys with different doping schemes. However, this parameter does not cover all the doping elements that are present in the ZNS. Therefore, because of the analysis and processing of experimental data, the ratio of alloying elements to assess corrosion resistance is proposed, which considers the complex influence of the main components of the alloy. Since VTK is related to the presence of certain elements in the alloy and their concentration, the ratio Kпс allows more adequately to evaluate, for multi-component nickel systems, the average rate of corrosion at different temperatures. The dependence of the average corrosion rate on Kпс for monocrystalline alloys is straightforward, due to the specificity of the doping systems of the materials of this class. Characteristic for them is a significantly smaller amount of chromium (up to 10% by weight) and lowering of the content (and in some alloys, absence of) titanium, which leads to a significant reduction in the VTK-stability of the material. It is established that in order to ensure the required level of VTK-stability of alloys of directed and equilibrium crystallization, the value of the ratio should be not less than Кпс ≥ 2, which will provide a nondestructive dense film of corrosion products. So, for alloys with Кпс ≤ 2, the formation of a thick layer of corrosion products is characteristic, which is easily evaporated during the operation. The obtained regression models give an opportunity to predict the average speed of corrosion depending on the alloy alloy system, as in the development of new ZNS for directed crystallization, and in improving the composition of known industrial compositions within the branded composition. The obtained correlation dependencies are exponential in nature.Scientific novelty. For the first time, the ratios of Kпс are proposed, by which one can adequately predict the average corrosion rate for different temperatures of the multicomponent ZNS compositions.Practical value. An effective solution is proposed for predicting the average corrosion rate for different temperatures of alloys, both in the development of new formulations of ZNS, and in the development of well-known industrial brands.

Petrography, relationships, and petrogenesis of the gabbroic lithologies in Northwest Africa 773 clan members Northwest Africa 773, 2727, 3160, 3170, 7007, and 10656

AbstractThe Northwest Africa (NWA) 773 clan of lunar meteorite stones are coarse‐grained breccias that provide an opportunity to examine a lunar igneous system that includes inferred intrusive and extrusive lithologies, possibly related through a common liquid line of descent from a single source region. Such extensive sampling of a single very low‐Ti (VLT) magmatic system on the Moon is unprecedented among the lunar samples. This study focuses on the olivine gabbro (OG), anorthositic gabbro (AG), and ferroan gabbro (FG) lithologies variably contained in NWA 773, NWA 2727, NWA 3160, NWA 3170, NWA 7007, and NWA 10656. Mineral compositions in the three gabbros indicate the crystallization sequence OG → AG → FG. Petrologic modeling of these three lithologies, and an olivine phyric basalt that also occurs in the NWA 773 clan, however, suggests that the relationship among the lithologies is more complex. The OG and basalt can be modeled as originating from a VLT KREEP‐bearing parental melt similar to the Apollo 14 Green Glass b1 composition through mainly equilibrium crystallization. The AG and FG, however, do not fit this simple model and require either a more complex crystallization sequence involving fractional crystallization, magma chamber recharge, or perhaps heterogeneity in the source region.

Nanoscale Study of Titanomagnetite from the Panzhihua Layered Intrusion, Southwest China: Multistage Exsolutions Record Ore Formation

Titanomagnetite from Fe-Ti-V ores of the Lanjiahuoshan deposit, Panzhihua layered intrusion, Southwest China, was investigated at the nanoscale. The objectives were to establish the composition of exsolution phases and their mutual relationships in order to evaluate the sequence of exsolution among oxide phases, and assess mechanisms of ore formation during magma emplacement. At the micron-scale, titanomagnetite shows crosscutting sets of exsolutions with ilmenite and Al-Mg-Fe-spinel (pleonaste), as well as overprint, both in terms of phase re-equilibration and remobilization of trace elements. Most complex textures were found in titanomagnetite surrounded by ilmenite and this was selected for high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging and STEM energy-dispersive X-ray spectrometry (EDS) spot analysis and mapping on a thin foil prepared in situ on a focused ion beam scanning electron microscope platform. Titanomagnetite revealed two sequential sets of exsolutions, {111} crosscutting {100}, which are associated with changes in phase speciation and trace element distribution patterns. Qandilite is the dominant spinel phase inside titanomagnetite; magnesioferrite is also identified. In contrast, Fe-poor, Al-rich, Mg-bearing spinel is present within ilmenite outside the grain. Vanadium enrichment in newly-formed magnetite lamellae is clear evidence for trace element remobilization. This V-rich magnetite shows epitaxial relationships with ilmenite at the contact with titanomagnetite. Two-fold super-structuring in ilmenite is evidence for non-redox re-equilibration between titanomagnetite and ilmenite, supporting published experimental data. In contrast, the transformation of cubic Ti-rich spinel into rhombohedral ilmenite imaged at the nanoscale represents the “oxy-exsolution” model of titanomagnetite–ilmenite re-equilibration via formation of a transient ulvöspinel species. Nanoscale disorder is encountered as vacancy layers in Ti-rich spinel, and lower symmetry in the Fe-poor, Al-Mg phase, suggesting that slow cooling rates can preserve small-scale phase equilibration. The cooling history of titanomagnetite ore can be reconstructed as three distinct stages, concordant with published models for the magma plumbing system: equilibrium crystallization of Al-rich, Mg-bearing titanomagnetite from cumulus melts at ~55 km, with initial exsolutions occurring above 800 °C at moderate fO2 conditions (Stage 1); crosscutting {111} exsolutions resulting in formation of qandilite, attributable to temperature increase due to emplacement of another batch of melt affecting the interstitial cumulus during uplift. Formation of 2-fold superstructure ilmenite + V-rich magnetite exsolution pairs representing non-redox equilibration indicates resetting of the cooling path at this stage (Stage 2); and ilmenite formation from pre-existing Ti-rich spinel and ulvöspinel, illustrative of redox-driven cooling paths at &lt;10 km (Stage 3). HAADF STEM provides direct imaging of atomic arrangements, allowing recognition of processes not recognizable at the micron-scale, and can thus be used to constrain exsolution models during ore formation.

Monazite and xenotime solubility in hydrous, boron-bearing rhyolitic melt

Abstract We conducted a series of monazite and xenotime dissolution experiments in a boron-bearing silicic melt at 1000–1400 °C and 800 MPa in a piston-cylinder apparatus. We present new measurements of monazite and xenotime solubility in hydrous (~3 wt% water), boron-bearing rhyolitic melts, as well as the diffusivities of the essential structural constituents of those minerals (LREE, P, and Y). We compare our results to the previous studies and discuss the implications of this study on the understanding of natural, silicic (granitic/rhyolitic) systems. We propose one equation describing the relationship between the solubility of xenotime and temperature in hydrous rhyolitic melts: ln ⁡ Y = 18.3 ± 0.3 − 125499 ± 3356 R T and another for monazite: ln ⁡ Σ LREE = 18.6 ± 1.5 − 129307 ± 18163 R T . In the presence of sufficient phosphorous, the concentration of LREE needed for monazite saturation is within the uncertainty of the Y concentration needed from xenotime saturation and indicates that in the case of equilibrium crystallization the mineral that forms will only depend on the availability of LREE and Y and HREE. Given the similarity of the solubility of xenotime to that of monazite, we propose that previously published models of monazite solubility in silicic melts can potentially be applied to xenotime, and could, like monazite, serve as a geothermometer. In the case of disequilibrium crystallization in front of rapidly growing crystals, Y will diffuse faster than LREE and xenotime will only crystallize when LREE are depleted. We also found that the diffusion of Y is greater than the diffusion of P from dissolving xenotime, unlike the similar diffusivities of LREE and P during monazite dissolution. The significant difference between Y and P diffusivities suggests that the components forming xenotime diffuse as separate entities rather than molecular complexes. The dissolution of phosphates (monazite, xenotime, apatite) in hydrous, silicic melts with the addition of boron leads to liquid-liquid immiscibility at high temperatures where the saturation values of P and either light rare earth elements or Y are in the weight percent range. Immiscibility is not observable at low, magmatic temperatures, most probably due to the lower concentrations of P2O5 necessary for phosphate saturation at these conditions; however addition of other components, notably F, may result in liquid-liquid immiscibility at magmatic temperatures.

On the source of diogenites and olivine diogenites: Compositional diversity from variable fO2

Abstract The genesis of diogenites and olivine-bearing diogenites has been debated for decades, with current models favouring formation via either mineral settling in a homogeneous magma ocean, or as late stage intrusions into the crust of asteroid 4 Vesta. Using pMELTS, both equilibrium and fractional crystallisation modelling was conducted on a large range of melt compositions generated by varied extent of batch melt extraction from 11 bulk Vesta starting compositions at a range of fO2 conditions to simulate the magma ocean concept. The resulting mineral compositions were compared with those of 200 diogenite meteorites in an attempt to resolve this debate. Models that involve

Equilibrium crystallization modeling of Venusian lava flows incorporating data with large geochemical uncertainties

Our understanding of the Venusian surface composition is limited to the in-situ bulk rock chemical analyses collected by the Vega and Venera missions. However, these analyses have exceedingly large analytical uncertainties – up to 50% by weight (1σ) for certain oxide components. In this study, we use the Venera 14 lander data and apply a Monte Carlo approach to assess how significant uncertainties affect modeling of lava solidification. Thermodynamic modeling of mineral-melt equilibria is conducted over 1,000 iterations of simulated bulk composition within the Gaussian probabilistic bounds of the reported analytical uncertainty along a cooling path from 1350°C to 950°C, with a fixed pressure of 90 bars. The results are used to calculate melt and apparent viscosity (ηmelt and ηapp, respectively). Despite the significant analytical uncertainty of the lander data, lava viscosity is tightly constrained. Mean log ηmelt increases from 1.65 ± 0.46 Pas at 1350°C to 8.57 ± 1.42 Pas at 950°C (1σ). Log ηapp is less well-defined due to increased scatter in crystal fraction with solidification, increasing to 17.7 ± 1.3 Pas at 950°C (γ=10−6 s−1). A significant increase in ηapp occurs between 1240°C and 1080°C due to a rapid increase in crystal mass fraction (Φ increases from ∼0.05 at 1250°C to 0.8 at ∼1100°C). Slow cooling through this 150°C window must occur so as to not drastically increase lava viscosity and impede flow. These results show that despite limited geochemical data and large analytical uncertainties, reasonable constraints for the physical and chemical evolution of lava solidification can be obtained. Most of the Venusian surface is composed of volcanic plains and rises, containing abundant landforms characteristic of fluid basaltic lava. Our results provide new insights into the crystallization processes in Venusian lava flows, which are fundamental for understanding Venusian igneous processes, geodynamics, and resurfacing. This work provides a necessary framework for future thermorheological flow models to determine flow volume and effusion rates.

Equilibrium crystal shape of GaAs and InAs considering surface vibration and new (111)B reconstruction: ab-initio thermodynamics

This work reports on the theoretical equilibrium crystal shapes of GaAs and InAs as a function of temperature and pressure, taking into account the contribution of the surface vibration, using ab-initio thermodynamic calculations. For this purpose, new (111)B reconstructions, which are energetically stable at a high temperature, are suggested. It was found that there was a feasible correspondence between the calculated equilibrium shapes and the experimental shapes, which implied that the previous experimental growth was performed under conditions that were close to equilibrium. In this study, GaAs and InAs were selected as prototype compound semiconductors, but the developed calculation methodology can also be applied to other III–V compound semiconductor materials.