Peritectic Point Research Articles

The lithium oxide solubility in molten fluoride system CeF3 - FLiNaK

Molten systems based on alkali metal halides with lithium oxide additives are promising as a working medium on pyrochemical reprocessing of nuclear waste. A mixture of alkali metal fluorides of eutectic composition (FLiNaK) is a suggesting solvent due to the high solubility of actinide oxides, low viscosity, high boiling points, low vapor pressure, and resistance to radiation damage. Thermal analysis, XRD, Raman spectroscopy and thermodynamic simulations were used to obtain evidences on the phase equilibria and liquidus points of the system (0.85 FLiNaK - 0.15 CeF3) - Li2O, containing up to 8.8 mol % lithium oxide. The solubility of lithium oxide in the fluoride melt FLiNaK - CeF3 and the thermodynamic parameters of dissolution were obtained. The eutectic point (the Li2O content is 3.1 mol %, Tm = 489??) and two peritectic points (lithium oxide content are 3.2 and 4.2 mol %, and liquidus points are 497 and 549??, respectively) were found. Thermodynamic simulation results show an exothermic effect due to interaction between lithium oxide and fluoride melt. The interaction product oxyfluoride CeOF was detected by XRD analysis and Raman spectroscopy.

Optimizing supercooling and phase stability by additives in calcium chloride hexahydrate for cyclical latent heat storage

Calcium chloride hexahydrate (CaCl2·6H2O) based phase change material (PCM) is promising for latent heat storage due to its favorable thermal properties, but supercooling and thermal cyclic instability limits its wider application. We examine the role of nucleation agents, in particular strontium chloride hexahydrate (SrCl2·6H2O), as an additive to the PCM, on the degree of supercooling and the latent heat. Utilizing a water bath and a cooling chamber, we systematically evaluate the degree of supercooling in various PCM samples, while X-Ray diffraction and microscopic analysis are employed to characterize the role of the additive. The results suggest that particle shapes of the additives and their crystallographic phase contribute significantly to supercooling suppression. Optimizing a tradeoff between supercooling and latent heat reduction for long-term nucleation performance, we identify designer PCM composition with an optimal 3 wt% of SrCl2·6H2O for an automated thermal cycling system by differential scanning as well as drop calorimetry. Incorporating 2 wt% potassium chloride (KCl) shifts the composition at the peritectic point away from the unfavorable CaCl2·4H2O phase, enhancing the freeze-thaw cycle stability of CaCl2·6H2O. Notably, the latent heat variation is ∼0.08% over 1000 freeze-thaw cycles.

Insights into dendrite growth and phase transition during Cs2LiLaBr6 crystal growth

The lanthanum crystal Cs2LiLaBr6 (CLLB) has excellent scintillation properties for gamma/neutron discrimination. The initial melt with hyper-peritectic ratios (55 < LiBr/mol.% < 61) was found beneficial for CLLB crystal growth. The hyper-peritectic melt would pass though the peritectic point. However, the detailed process was still unknown. Herein, the dendrite growth and peritectic reaction were discovered near the peritectic point in CLLB single crystal ingots for the first time. The mechanism of dendrite growth was analyzed. The Cs2LaBr5-LiBr phase diagram was modified according to these results. The understanding of phase transition and dendrite evolution can guide crystal growth of CLLB and other similar elpasolite crystals.

High-accuracy realization of temperature fixed and reference points.

The harmonization of international temperature measurements requires the high-accuracy realization of many different temperature reference points. This results from the feature of the intensive measurand temperature that temperatures cannot simply be divided or multiplied. Thus, the points must cover the whole range of interest, at present from 1 mK to a few 1000K. Furthermore, instruments are necessary for the interpolation between the non-continuous guide values. This led to the establishment of International Temperature Scales (ITS). The ITS prescribe interpolation instruments and assign fixed temperature values to suitable phase transitions without uncertainty. The large temperature range can only be covered by applying very different phase transitions. This includes the classical transitions, namely triple, melting, and freezing points, but also second-order transitions, as superfluid and superconducting ones, and the very new eutectic or peritectic points of metal-carbon compositions. A high-accuracy realization requires a reliable uncertainty estimation. This is, therefore, the central topic of this review. Since a given non-ideal condition of a sample, especially the impurity content, cannot be reproduced as accurate as necessary, the fixed- and reference-point temperatures are defined for ideal substances under ideal conditions. Thus, the estimation of the uncertainty of the realizations must be based on estimating the magnitude of all physical effects influencing the observed phase-transition temperature. The application of this methodology is discussed in the paper as unifying topic independent of the individual problems to be solved. Furthermore, recommendations of the Consultative Committee for Thermometry are summarized, and own experiences are supplemented.

The Polyol Process and the Synthesis of ζ Intermetallic Compound Ag5Sn0.9.

The present work concerns the intermetallic compound (IMC) existing in the Ag-Sn system and its potential use in electronics as attachment materials allowing the adhesion of the chip to the substrate forming the power module. First, we present the synthesis protocol in polyol medium of a compound with the chemical formula Ag5Sn0.9 belonging to the solid solution of composition located between 9 and 16 at.% Sn, known as solid solution ζ (or ζ-Ag4Sn). This phase corresponds to the peritectic invariant point at 724 °C. Differential thermal analysis and X-ray dispersive analysis confirm the single-phased (monocrystalline) nature of the Ag5Sn0.9 powder issued after synthesis. Scanning electron microscopy shows that Ag5Sn0.9 particles are spherical, and range in submicronic size of around 0.18 μm. X-ray diffraction analysis reveals that the ζ phase mostly exists under the two allotropic varieties (orthorhombic symmetry and hexagonal symmetry) with however a slight excess of the hexagonal variety (60% for the hexagonal variety and 40% for the orthorhombic variety). The lattice parameters resulting from this study for the two allotropic varieties are in good agreement with the Hume-Rothery rules.

Structural, thermal and superconducting properties of Ag2O-doped YBa2Cu3O7-x composite materials

A composite material with superconducting properties, consisting of YBa2Cu3O7-x doped with Ag2O, was obtained after 233 h of sintering at 960 °C and followed by a special cooling program in oxygen atmosphere. Preferential orientation of YBa2Cu3O7-x (YBCO-123) crystallites was evidenced through X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Small increase of c parameter for orthorhombic YBCO-123 crystalline unit cell together with a slight decrease of oxygen content (7-x) versus Ag content, were encountered. The dimensions of YBCO-123 crystallites are dependent on Ag content. Thermogravimetric analysis (TGA) during heating in flowing air atmosphere from room temperature (RT) to 1100 °C, showed that total mass loss is depending on Ag content. The cooling (from 1100 °C to RT, in air atmosphere) endows optimal oxygen accumulation only for 0 and 5 wt% Ag, while final masses for 10 and 20 wt% of Ag content, are more than 10% less. Differential scanning calorimetry (DSC) showed peritectic melting points for YBa2Cu3O7-x dependent on Ag percentage. DSC graphics also highlighted the endothermic effects: silver melting; melting and decomposition of Ag2O (in 225.81–345.12 °C temperature range) only for 10 and 20 wt%, but completely missing in the case of 5 wt% Ag content. We concluded that some of Cu sites from YBCO-123 superconductor were substituted by Ag ions. Quasi de Almeida-Thouless lines in the range of magnetic field amplitudes from 0.4 to 800 A/m, in the temperature range from 77 K to RT, were also investigated. Two cases are found concerning intergranular critical currents (Jci): depinning mechanism available for 5 wt% Ag content; the decoupling mechanism for 10, and 20 wt% Ag content.

Phase equilibria in the Na2O-WO3 system

The Na2O-WO3 binary system is an important chemical system with various applications, such as catalysts, energy and corrosion resistant materials. In this study, phase equilibria and liquidus temperatures in the Na2O-WO3 binary system have been experimentally determined in air. The system was investigated at the temperature range from 550 to 830 °C using a high-temperature equilibration and quenching technique. The microstructure and compositions of the phases obtained in the quenched samples were then analyzed by an electron probe micro-analyzer (EPMA). Binary compounds Na2W2O7, Na2W4O13 and Na2W6O19 have been identified and the liquidus temperature temperatures in these primary phase fields were determined. Two peritectic points in the system were confirmed and compared with previous data. The present study provided useful thermodynamic data including liquidus and solid solutions. Accurate phase equilibria data will be used as the base for the development of thermodynamic modelling.

Investigation of the Quasi-Binary Phase Diagram FLiNaK-NdF3.

The NdF3 solubility in molten eutectic FLiNaK, which is a conceivable medium for a molten salt reactor (MSR), was determined by the quasi-binary phase diagram FLiNaK-NdF3. The eutectic mixture FLiNaK was prepared by direct melting of components LiF, NaF and KF·HF. The acidic anhydrous salt (KF·HF) was used instead of the hygroscopic KF. The NdF3 was sintered by hydrofluorination of Nd2O3. The oxygen impurity in the prepared eutectic FLiNaK, determined by an oxygen analyzer LECO OH836, was 0.036 wt.%, whereas the NdF3 contained 0.04 wt.% of oxygen. A part of the FLiNaK-NdF3 quasi-binary phase diagram was obtained using two thermal analysis techniques: differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The FLiNaK-NdF3 phase diagram in the region of 0–30 mol.% NdF3 contains one eutectic at 2 mol.% NdF3 and 450 °C and two peritectic points: 8 mol.% NdF3 at 500 °C and 22 mol.% NdF3 at 575 °C. The region of the FLiNaK-NdF3 phase diagram below the liquidus line is rather complicated due to the complex structure of the multicomponent system in its molten state, as in its solid state. The NdF3 solubility in FLiNaK is about 5 mol.% at 490 °C and 29 mol.% at 700 °C; this means that the process of the MA transmutation in the MSR can be carried out in molten FLiNaK with a content of actinides as high as 15–20 mol.% in the temperature range of 550–650 °C.

Phase equilibrium studies in V2O5‐Fe2O3 and V2O5‐TiO2 systems

AbstractV2O5‐Fe2O3 and V2O5‐TiO2 systems represent two important chemical systems with various applications, including energy, catalysts, and high‐performance materials. In the present study, high‐temperature phase equilibrium experiments were conducted at the temperature range of 670–1000°C in air. Electron probe X‐ray micro‐analyzer (EPMA) was used to analyze the microstructure and composition of the phases presented in quenched samples. Systematic experiments demonstrated that V2O5‐containing systems should not be quenched by water‐based quenching media. Phase diagrams in both systems were constructed, and the eutectic and peritectic points of the systems were confirmed and compared with previous studies. The present study improved the previous results and could be used as the base for thermodynamic modelings and further applications of the two systems.

How do secondary iron enrichments form within basaltic eucrites? An experimental approach

AbstractSome basaltic eucrites and basaltic lithologies in howardites derived from the asteroid 4 Vesta exhibit unusual secondary veinlet textures consisting mostly of fayalitic olivine and Fe‐enrichments within pyroxenes. Recent studies discussed the formation of these Fe‐rich phases either by interaction with a vapor and/or liquid phase (metasomatism), or by a high‐temperature melting process. We therefore performed a series of heating and hydrothermal experiments with liquids of different compositions on natural pyroxene crystals (augite and orthopyroxene) to evaluate these contrasting hypotheses. The results of the heating experiments show that incongruent melting of pyroxenes at about 1070 °C causes textures that are very similar to those observed in the meteorites. We conclude that a part of the natural secondary veins might be explained by heating processes at similar temperatures. The hydrothermal experiments with aqueous liquids of different Fe‐enriched compositions clearly indicate ion exchange reactions resulting in partial Fe‐enrichments of the pyroxene. Interestingly, these Fe‐enrichments occurred independent of the Fe content of the liquid, which can be explained by an internal origin of Fe from the pyroxenes. In one hydrothermal experiment of augite with Fe‐oxalate solution, deposition of fayalitic olivine was observed. From our experimental observations, we conclude that aqueous liquids are plausible candidates for explaining the deposition of Fe‐enrichments and fayalitic olivine inside the fractures of pyroxene. However, we cannot rule out a high‐temperature melting process slightly above the peritectic point of pyroxene to explain a fraction of observed secondary Fe‐enrichments.

Mannitol Crystallization at Sub-Zero Temperatures: Time/Temperature-Resolved Synchrotron X-ray Diffraction Study and the Phase Diagram.

Mannitol, a common pharmaceutical ingredient, exhibits complex polymorphism even in simple binary mannitol/water mixtures, with four crystalline forms observed. In this investigation, time/temperature-resolved synchrotron X-ray diffraction measurements are performed during freezing and thawing of mannitol/water mixtures. Mannitol crystallization depends strongly on the cooling rate and is initiated during cooling, if the cooling rate is lower than the critical cooling rate; otherwise, mannitol remains amorphous during freezing and crystallizes during subsequent heating above -30 °C. A temperature-composition phase diagram is constructed, reflecting eutectic and peritectic points and lower-temperature equilibria involving mannitol hemihydrate, hexagonal ice, and β-mannitol. Comparison of the experimental data with the phase diagram reveals that the mannitol crystallization behavior does not follow the equilibrium but appears to obey the Ostwald crystallization rule. Novel insights on equilibrium and kinetics phase relationships in mannitol/water systems could lead to improved formulations and manufacturing processes for pharmaceuticals and biopharmaceuticals.

Thermodynamic Properties of CaCl&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-CaF&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-CaO System: Phase Diagram Investigation

The CaCl2-CaF2-CaO phase diagram was investigated in the CaO low region (<40 mol.%). CaCl2-CaF2 and CaCl2-CaO binary diagrams, constituting the ternary system, were first studied by Differential Scanning Calorimetry (DSC) measurements and X-Ray Diffraction (XRD) characterization; a good agreement was obtained between the phase diagram models calculated with FactSage® software (FTsalt database) and present experimental data. As the CaF2-CaO liquidus could not be measured by DSC due to the high melting temperature, this diagram was calculated using FTsalt database combined with FToxid database of FactSage® software. The ternary phase diagram was obtained by calculations and exhibits an eutectic at the composition CaCl2-CaF2-CaO (78.2-15.7-6.1 mol.%) melting at 637°C, and five peritectic points. Measurements of relevant vertical cross-sections for three CaCl2-CaF2 compositions (50-50, 40-60 and 30-70 mol.%) up to 18 mol.% CaO are in agreement with the ternary phase diagram model. For each section, the liquidus temperature is constant up to around 11 mol.% CaO and then sharply increases. Moreover, an increase of CaF2 content in CaCl2-CaF2 melt leads to a decrease of the CaO solubility in isothermal condition.

Experimental study and thermodynamic modeling of solid-liquid equilibrium of binary and ternary mixtures formed by C11H24, C12H26 and C14H30 for cryogenic thermal energy storage

Abstract In this study, experimental investigations of solid-liquid phase diagram of binary and ternary mixtures formed by the C11H24, C12H26 and C14H30 were carried out to employ the potential phase change materials (PCMs) for cryogenic applications using the differential scanning calorimetry (DSC). In addition, the thermodynamic equilibrium was modeled employing the ideal model, and the Wilson and NRTL model were applied to correlate the liquidus line. The results show that the C11-C12 system exhibits a peritectic transition and the composition of possibly peritectic point is 60wt% C12 with the temperature of 251.65 K. The compositions of the eutectic points appear at 10wt% C14 for C11-C14 system and 18wt% C14 for C12-C14 system with eutectic temperatures of 246.85 K and 260.45 K, respectively. The C11-C12-C14 ternary system presents two possibly eutectic points at compositions of 67wt% C11/5wt% C12 and 86wt% C11/10wt% C12 with eutectic temperatures of 246.45 K and 248.05 K, and has the slight supercooling (1.7 ˚C and 0.5 ˚C), indicating miscibility in liquid phase and immiscibility in solid phase. Moreover, the liquidus lines calculated by the theoretical models are in good agreement with the experimental results. The best predicted results of the ternary mixture and C11-C12 binary subsystem are obtained by the ideal model, with deviations of 0.75% and 0.58% respectively.

Analysis of Solid-State Interactions of Ketoprofen-Coformer Binary Mixtures by DSC and Hot Stage Microscopy

Ketoprofen is a non-steroidal anti-inflammatory drug with poor water solubility, so the absorption is less than optimal. One method to improve the solubility of ketoprofen is through the formation of multicomponent solid forms. The success of the formation of the multicomponent solid forms is strongly influenced by interactions between components in their solids. In this study, the analysis of the interactions in solid form of ketoprofen-coformers was carried out using the differential scanning calorimetry (DSC) and hot stage microscopy (HSM) with adipic acid and isonicotinamide as coformers. From the experimental results, the mixtures of ketoprofen-adipic acid show a solid-liquid phase diagram that indicates a simple eutectic system with eutectic points on the molar fraction of ketoprofen 0.9 and temperature at 92.9 °C. The ketoprofen-isonicotinamide mixtures have a eutectic system with the peritectic point. The solid-liquid phase diagram has indicated that the ketoprofen-adipic acid in eutectic composition forms a miscible liquid phase without interaction in its solid form, whereas the ketoprofen-isonicotinamide forms a miscible liquid phase accompanied by interaction with the excess component. The results of the HSM analysis showed the same phenomenon as the result of the DSC experiment and have confirmed with the FTIR analysis

Liquidus Surface and Spinodal of Fe-B-C Alloys

In this work the study is performed for the specimens of Fe-B-C alloys with boron content of 0.005–7.0 wt. % and carbon content of 0.4–6.67 wt. %, the rest is iron. According to the findings of microstructure analysis, XRD and differential thermal analyses, the primary phases and the temperatures of their formation are determined. Depending on boron content (in the range of 1.5–8.80 wt. %) and carbon content (0.5–6.67 wt. %) in the Fe-B-C alloys, the primary phases in the process of crystallization are γ-Fe, boron cementite Fe3(CB) and boride Fe2В. The outcomes of the experiment carried out in this work determine the phase composition and phase transformations occurring in the alloys and the liquidus surface is constructed. The findings show that the liquidus temperature for Fe-B-C system alloys is low compared to binary Fe-B and Fe-C alloys. At the liquidus surface of the Fe-B-C alloys, there is a point at boron content of 2.9 wt. % and carbon content of 1.3 wt. % with the lowest temperature of 1375 K and it is the point of intersection of monovariant eutectics. This fact is in a good agreement with the results of other authors. The microstructure of alloys located at the curves of monovariant eutectics is represented by the γ–Fe+Fe2B and γ–Fe+Fe3(CB) eutectics and the primary crystals of Fe2B iron boride in the shell of Fe3(BC) boron cementite. In this paper it is shown experimentally the existence of a quasi-binary section and the coordinates of the peritectic point are fixed: the boron content is 5.0 wt. %, carbon content is 3.0 wt. % and the temperature is 1515 K. The free energy of the Fe-B-C melt is calculated for the first time by the quasi-chemical method and the surface of thermodynamic stability of the Fe-B-C melt is plotted, depending on temperature and boron and carbon content in the alloy. The results obtained in the paper show that in order to obtain a homogeneous Fe-B-C melt, which does not contain any microheterogeneous structure in the form of short-order microregions, it is necessary to perform the overheating more than to 180 K for the region where the primary phase is iron, and no less than to 200 K for the regions with boron cementite and boride.

Ionic liquid formation with deep eutectic forces at an atypical ratio (2:1) of naproxen to lidocaine in the solid-state, thermal characterization and FTIR investigation.

This study aimed to determine if an ionic liquid resulted from the combination of naproxen and lidocaine and the impact that varying the molar percentage of each component had on thermal behavior. Samples were prepared by mixing naproxen and lidocaine in tetrahydrofuran (THF), followed by solvent removal using a vacuum oven. Thermal events were investigated using differential scanning calorimetry (DSC) and the extent of proton transfer was probed using Fourier-Transform Infrared Spectroscopy (FTIR). The binary samples that had a higher molar percentage of naproxen formed a 2:1 naproxen to lidocaine complex, with a melting temperature in the range of 82-85°C. This complex was determined to be a peritectic composition of naproxen and lidocaine and spectroscopic analysis confirmed the complex had ionic liquid character as well as hydrogen bonding interactions. Binary samples that contained a greater molar percentage of lidocaine formed high-order complexes of varying stoichiometry with melting temperatures below 50°C. Spectroscopic analysis determined that a greater molar concentration of lidocaine drove more eutectic behavior, although some ionization was also present. The conclusion from this study was that naproxen and lidocaine form an ionic liquid material at a peritectic point corresponding to 2mol of naproxen to 1mol of lidocaine. At molar concentrations of greater lidocaine than naproxen, higher order complexes form which are largely driven by eutectic behavior.

Performance of WC–C peritectic and Ru–C eutectic fixed points

Two types of high-temperature fixed points (HTFPs) were evaluated by VNIIOFI, NIM, KRISS and NMIJ. WC–C peritectic point cells manufactured independently in different National Metrology Institutes (NMIs) were compared for the first time, and agreement of the melting temperatures for three high-quality cells at the level of 0.05 °C was demonstrated. This confirms, in conjunction with previous results that verified their long-term stability, high repeatability and reproducibility, the high potential of the WC–C cell as the highest-temperature reference point for radiation thermometry.The performance of the Ru–C eutectic fixed point was verified by evaluating Ru–C cells manufactured from 99.999% purity Ru materials from different manufacturers. These cells used considerably purer materials than those used in previous studies. New Ru–C cells were constructed from the best performing materials, and the t90 values of these cells were measured at the four NMIs on their locally-realized ITS-90 scale. The values agreed within 0.25 °C among the four NMIs, and the t90 value of Ru–C was determined to be 1953.64 °C with an expanded uncertainty of 0.20 °C. Both results confirmed that performance of WC–C and of Ru–C is comparable to or exceeds that of the HTFPs evaluated in the Consultative Committee for Thermometry’s HTFP project, and both fixed points have capability as a reference fixed points to be included in the future MeP-K in terms of thermodynamic temperature T.

The System K2CO3–CaCO3–MgCO3 at 3 GPa: Implications for Carbonatite Melt Compositions in the Shallow Continental Lithosphere

Potassic dolomitic melts are believed to be responsible for the metasomatic alteration of the shallow continental lithosphere. However, the temperature stability and range of compositions of these melts are poorly understood. In this regard, we performed experiments on phase relationships in the system K2CO3–CaCO3–MgCO3 at 3 GPa and at 750–1100 °C. At 750 and 800 °C, the system has five intermediate compounds: Dolomite, Ca0.8Mg0.2CO3 Ca-dolomite, K2(Ca≥0.84Mg≤0.16)2(CO3)3, K2(Ca≥0.70Mg≤0.30)(CO3)2 bütschliite, and K2(Mg≥0.78Ca≤0.22)(CO3)2. At 850 °C, an additional intermediate compound, K2(Ca≥0.96Mg≤0.04)3CO3)4, appears. The K2Mg(CO3)2 compound disappears near 900 °C via incongruent melting, to produce magnesite and a liquid. K2Ca(CO3)2 bütschliite melts incongruently at 1000 °C to produce K2Ca2(CO3)3 and a liquid. K2Ca2(CO3)3 and K2Ca3(CO3)4 remain stable in the whole studied temperature range. The liquidus projection of the studied ternary system is divided into nine regions representing equilibrium between the liquid and one of the primary solid phases, including magnesite, dolomite, Ca-dolomite, calcite-dolomite solid solutions, K2Ca3(CO3)4, K2Ca2(CO3)3, K2Ca(CO3)2 bütschliite, K2Mg(CO3)2, and K2CO3 solid solutions containing up to 24 mol % CaCO3 and less than 2 mol % MgCO3. The system has six ternary peritectic reaction points and one minimum on the liquidus at 825 ± 25 °C and 53K2CO3∙47Ca0.4Mg0.6CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, the liquid transforms into three solid phases: K2(Mg0.78Ca0.22)(CO3)2, K2(Ca0.70Mg0.30)(CO3)2 bütschliite, and a K1.70Ca0.23Mg0.07CO3 solid solution. Since, at 3 GPa, the system has a single eutectic, there is no thermal barrier for liquid fractionation from alkali-poor toward K-rich dolomitic compositions, more alkaline than bütschliite. Based on the present results we suggest that the K–Ca–Mg carbonate melt containing ~45 mol % K2CO3 with a ratio Ca/(Ca + Mg) = 0.3–0.4 is thermodynamically stable at thermal conditions of the continental lithosphere (~850 °C), and at a depth of 100 km.

New data on the system Na2CO3–CaCO3–MgCO3 at 6 GPa with implications to the composition and stability of carbonatite melts at the base of continental lithosphere

Subsolidus and melting phase relationships in the system Na2CO3–CaCO3–MgCO3 have been studied at 6 GPa and 900–1250 °C using a Kawai-type multianvil press. At 900 and 1000 °C, the system has four intermediate compounds: Na2Ca4(CO3)5 burbankite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and Na2Mg(CO3)2 eitelite. The Na-Ca compounds dissolve noticeable amounts of Mg component, whereas eitelite dissolves a few percents of Ca component: Na2(Ca≥0.91Mg≤0.09)4(CO3)5, Na2(Ca≥0.94Mg≤0.06)3(CO3)4, Na4(Ca≥0.67Mg≤0.33)(CO3)3, and Na2(Mg≥.93Ca≤0.07)(CO3)2. At 1050 °C, the system is complicated by an appearance of dolomite. Na-Ca burbankite decomposes at 1075 ± 25 °C to aragonite plus Na2Ca3(CO3)4. Na4Ca(CO3)3 and eitelite disappear via congruent melting between 1200 and 1250 °C. Na2Ca3(CO3)4 remains stable through the whole studied temperature range. The liquidus projection of the studied ternary system has eight primary solidification phase regions for magnesite, dolomite, calcite-dolomite solid solutions, aragonite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and Na2CO3 solid solutions. The system has five ternary peritectic reaction points and one minimum on the liquidus at 1050 °C and 48Na2CO3·52(Ca0.75Mg0.25)CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which a liquid transforms into three solid phases upon cooling: Na2(Ca0.94Mg0.06)3(CO3)4, Na4(Ca0.67Mg0.33)(CO3)3, and Na2(Mg0.93Ca0.07)(CO3)2 eitelite. Since at 6 GPa, the system has a single eutectic, there is no thermal barrier preventing continuous liquid fractionation from alkali-poor toward Na-rich dolomitic compositions. Cooling of the Na-Ca-Mg carbonatite melt from 1400 to 1100 °C within the lherzolite substrate will be accompanied by magnesite crystallization and wehrlitization keeping calcium number of the melt at 40 and shifting the Na2CO3 content to ≥40 mol%. In the case of the eclogitic wall rock, the cooling will be accompanied by dolomite crystallization keeping calcium number of the melt at 60–65 and shifting the Na2CO3 content to ≥30 mol%.

Phase relations in the system Na2CO3–CaCO3–MgCO3 at 3 GPa with implications for carbonatite genesis and evolution

The phase relations in the system Na2CO3−CaCO3−MgCO3 have been studied at 3 GPa and 700–1285 °C using a Kawai-type multianvil press. At 700 °C, the system has five intermediate compounds: dolomite, Mg-bearing Na2Ca4(CO3)5 burbankite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and eitelite. As temperature increases to 800 °C, the system is complicated by an appearance of Ca-dolomite and Mg-bearing shortite, while Na2Ca4(CO3)5 disappears. At 850 °C, Na4Ca(CO3)3 decomposes to produce Na carbonate and nyerereite. The latter melts incongruently at 875 ± 25 °C to form Na2Ca3(CO3)4. Incongruent melting of eitelite to magnesite and liquid, occurs at 925 ± 25 °C. Mg-bearing shortite melts incongruently at 950 ± 50 °C, producing Na2Ca3(CO3)4 and liquid. Na2Ca3(CO3)4 disappears at 1000 °C via incongruent melting to calcite and liquid. The liquidus projection of the studied ternary system has seven primary solidification phase regions for magnesite, dolomite-calcite solid solutions, Na2Ca3(CO3)4, Mg-bearing shortite, nyerereite, eitelite, and Na carbonate. The primary solidification regions are separated by five peritectic and three cotectic monovariant lines. The system has six ternary peritectic points and one minimum on the liquidus at 850 °C and 52Na2CO3∙48(Ca0.62Mg0.38)CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, a liquid transforms into three solid phases: shortite, Na carbonate, and eitelite. Since the system has a single eutectic at 3 GPa, there is no thermal barrier preventing continuous liquid fractionation from Na-poor toward Na-rich dolomitic compositions more alkaline than eitelite and nyerereite. Considering the present results and previous data, a range of Na-Ca-Mg double carbonates changes in the following sequence upon pressure and temperature increase: Na2Ca2(CO3)3 (Amm2) shortite, Na2Ca(CO3)2 (P21ca) nyerereite, Na2Mg(CO3)2 (R3¯) eitelite (0.1 GPa) → Na2(Ca0.97–0.98Mg0.02–0.03)4(CO3)5 (P63mc), Na2(Ca≥0.91Mg≤0.09)3(CO3)4 (P1n1), Na2(Ca ≥ 0.81 Mg0≤0.19)(CO3)2 (R3¯) nyerereite, Na2(Ca0.77–0.93Mg0.07–0.23)2(CO3)3 (Amm2) shortite, Na4(Ca0.90–0.98Mg0.02–0.10)(CO3)3 (Ia3d), Na2(Mg≥0.9Ca0≤0.1)(CO3)2 (P21ca) eitelite (3 GPa) → Na2(Ca≥0.87Mg0≤0.13)4(CO3)5 (P63mc), Na2(Ca≥0.89Mg≤0.11)3(CO3)4 (P1n1), Na4(Ca ≥ 0.7 Mg0≤0.3)(CO3)3 (Ia3d), Na2(Mg≥0.92Ca0≤0.08)(CO3)2 (P21ca) eitelite (6 GPa). Using the present results at 3 GPa and previous data at 6 GPa in the Na2CO3−CaCO3−MgCO3 system, we constrained isopleths of the Na2CO3 content in melt coexisting with Ca-Mg carbonates. We found that the cratonic geotherms cross the isopleths so that the carbonatite melt percolating upward via the continental mantle lithosphere should become progressively enriched in Na, evolving from alkali-poor dolomitic composition at depths exceeding 200 km toward sodic dolomitic with the ~52 mol% Na2CO3 at 80–120 km depths.