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Diamond-hosted inclusions provide information on the composition of the upper mantle. Many researchers found molecular CO2 inclusions, along with mineral and melt ones, in diamonds. Available experimental data on simplified systems suggest that CO2 fluid cannot occur in equilibrium with rock-forming mantle minerals. However, it was suggested that the complex composition of minerals contributes to their stabilization. This paper presents thermodynamic calculations for reactions of diopside–jadeite and pyrope–grossular solid solutions with CO2. It is shown that the formation of the solid solutions expands the stability field of CO2 fluid with eclogite minerals to lower temperatures, corresponding to the geothermal parameters of the continental lithosphere. The obtained relationships are confirmed by the results of preliminary experiments with model mixtures at high pressures and temperatures. [ABSTRACT FROM AUTHOR]

Phase relations in one of the key petrological systems, CaSiO3, have been comprehensively investigated for the first time in the pressure range 0–100 GPa and temperatures 0–2500 K within the density functional theory using the method of lattice dynamics in the quasi-harmonic approximation. The results showed that at atmospheric pressure and 0 K CaSiO3 is stable in the wollastonite structure, which above 1250 K transforms to the high-temperature pseudowollastonite modification. Above a pressure of 4 GPa, CaSiO3 is stable in the breyite structure. The phase equilibrium curve has a negative slope of dP/dT = –0.6 MPa/K. At 8 GPa, CaSiO3 decomposes into an assemblage of Ca2SiO4-larnite and titanite-structured CaSi2O5. The phase equilibrium curve has a positive slope of dP/dT = 1.35 MPa/K. At a pressure of 13 GPa, Ca2SiO4-larnite reacts with CaSi2O5, forming a phase with a perovskite-like structure – CaSiO3-perovskite. The pressure of this phase transition is practically independent of temperature. In the low-temperature region, Ca-perovskite is stable in the tetragonal modification CaSiO3-I4/mcm. Above 340 K at 13 GPa, Ca-perovskite is stable in the cubic modification CaSiO3- The phase transition temperature increases to 755 K with pressure increase to 100 GPa. The thermodynamic parameters were also calculated for the first time for wollastonite, pseudowollastonite, and titanite-structured CaSi2O5. [ABSTRACT FROM AUTHOR]

Based on first-principle calculations in the framework of the density functional theory and structure prediction algorithms, we have determined iron carbide phases stable at the Earth's core pressures and temperatures. It is shown that Fe7C3 is unstable and decomposes into the mixture Fe2C + Fe3C over the entire range of pressures and temperatures specific to the Earth's inner core. Subsequent decomposition of Fe3C into the mixture Fe + Fe2C is unfavorable. We also predict a new low-temperature modification Fe3C-C2/m-II dynamically and thermodynamically stable over the pressure range 290-305 GPa. [ABSTRACT FROM AUTHOR]

High-pressure phases (ringwoodite, maskelynite, and jadeite) present as coarse-grained fragments in the shock-melt vein of the Dhofar 922 L6 chondrite were studied by scanning electron microscopy and Raman spectroscopy. Ringwoodite and jadeite crystallized from melted plagioclase and olivine fragments, respectively, at the cooling--decompression stage. We assume that the high contents of Fe and Mg in maskelynite--jadeite aggregates are due to a miscibility gap in the hedenbergite--jadeite and diopside--jadeite joins at ~600 and ~700 °C, respectively. The P-T conditions of the formation of shock-melt veins are estimated at >19 GPa and >2150-2300 °C. [ABSTRACT FROM AUTHOR]

It has been demonstrated that the high-pressure (15 GPa) treatment of shungite samples (Karelia, Russia) at a temperature of 1600°C for 2 h transforms them to diamond. The comparison of the Raman spectra of the diamond material prepared from shungite and carbonado, which is a rare variety of cubic diamond sometimes called black diamond, has revealed their significant similarities. Our data indicate the possible growth of carbonado from the shungite-like natural material, which may be one of the ways carbonado appears under terrestrial conditions. [ABSTRACT FROM AUTHOR]

The paper reports results of an experimental study of decarbonation and melting reactions in the MgCO3–SiO2 system at pressures up to 32 GPa, using multianvil technique and in-situ X-ray diffraction with synchrotron radiation. At 3–7 GPa and 1400–1700 K, the reaction proceeds with the release of carbon dioxide and the formation of enstatite. At 9–13 GPa and 1850–1930 K, clinoenstatite, carbonate–silicate melt, and CO2 were found among the reaction products. At 16 GPa and 1825 K, the reaction is associated with the formation of wadsleyite and, at a higher temperature, by the generation of carbonated melt (with Mg/Si ratio close to wadsleyite) stishovite, and CO2 fluid. At this pressure, which coincides with the stability field of the wadsleyite–stishovite assemblage in the MgSiO3 phase diagram, the reaction temperature decreases by about 100 K. At higher pressures, the reaction proceeds with the formation of the MgSiO3 (akimotoite or bridgmanite) + melt assemblage. The reaction temperature at 25–35 GPa does not change and is about 2000 K. With a further increase in temperature to 2100 K, bridgmanite melts incongruently, reacting with the carbonate–silicate melt to form stishovite. The composition of the eutectic mixture shifts towards MgCO3 with increasing pressure. The reaction marks the upper temperature limit for the stability of magnesite and a free SiO2 phase in the Earth's mantle and generally coincides with the mantle adiabat at depths of 300–900 km. [ABSTRACT FROM AUTHOR]

Abstract—This paper reports the synthesis of double K–Ca carbonates at atmospheric pressure in closed graphite capsules. The mixtures of K2CO3 and CaCO3 corresponding to the stoichiometry K2Ca(CO3)2 and K2Ca2(CO3)3 were used as starting materials. The low-temperature modification of K2Ca(CO3)2 was synthesized by a solid-state reaction at 500°C during 96 h. The high-temperature modification of K2Ca(CO3)2 as well as K2Ca2(CO3)3 were synthesized by a solid-state reaction at 600°C for 72 h and by cooling of the melt from 830 to 650°C for 30 min. The obtained carbonates were studied by Raman spectroscopy. The Raman spectrum of bütschliite is characterized by the intense band at 1093 cm–1 and several bands at 1402, 883, 826, 640, 694, 225, 167 and 68 сm–1. The Raman spectrum of fairchildite has characteristic intense bands at 1077 and 1063 cm–1, and several bands at 1760, 1739, 719, 704, 167, and 100 сm–1. In the Raman spectrum of K2Ca2(СO3)3, the intense lines were found at 1078 and 1076 cm–1 and several lines at 1765, 1763, 1487, 1470, 1455, 1435, 1402, 711, 705, 234, 221, 167, 125 and 101 сm–1. The collected Raman spectra can be used to identify carbonate phases entrapped as microinclusions in phenocrysts and xenoliths from kimberlites and other alkaline rocks. [ABSTRACT FROM AUTHOR]

The minerals of the Maslyanino iron meteorite and their trace-element composition are described in detail for the first time, and the meteorite classification is substantiated. The meteorite is a fine-structural octahedrite. Its metallic matrix consists of kamacite, taenite, and schreibersite. Large troilite segregations are associated with silicate inclusions; in addition, rare minerals altaite and dobreelite are found. The silicate inclusions contain olivine, orthopyroxene, clinopyroxene, plagioclase, apatite, merrillite, chromite, and graphite. A detailed trace-element analysis of the metallic matrix permits the Maslyanino meteorite to be assigned to the narrow Pitts subgroup of the IAB group. It is also similar to meteorites of the Udei Station subgroup. Both subgroups include meteorites with silicate inclusions and are intermediate between the sLL (low Au and Ni contents) and sLM (low Au and medium Ni contents) subgroups. According to the metal-lographic data, the cooling rate of the Maslyanino meteorite is 30-60 °C/Myr. The data obtained are consistent with the formation of the meteoritic material under impact of a parent asteroid resulting in the removal of its outer chondrite-winonaite shell. Subsequent weaker impacts led to the formation of IAB group meteorites (including meteorites with silicate inclusions) and winonaites from the asteroid remnant. [ABSTRACT FROM AUTHOR]

Lavas of the Kamchatka volcanoes store cubic-octahedral diamonds identical in morphology and structure to synthetic crystals, and their natural origin is doubted. Judging by published data, the diamonds discovered in the Tolbachik lavas are similar to synthetic diamonds made by different producers, and the analyzed samples rather result from contamination with synthetic material. Ophiolite-hosted diamonds reported from Europe, China, Mongolia, and Polar Urals look like the Tolbachik diamonds and are of the same type. The similarity between crystals coming from geologically dissimilar objects indicates that contamination may occur in those cases as well. Thus, diamonds found in unusual hosts or geologic settings require careful checking. These findings have to be reproduced repeatedly in other in situ samples and approved by independent experts; with all respect to the priority of the first finders, the sampling sites should be open to many researchers, especially the respective specialists. The inevitable disproval of false diamond findings is discouraging and discredits the true discoveries. Possible contamination with synthetic or natural material from cutting tools has to be excluded in all newly found diamonds before claiming their natural origin. [ABSTRACT FROM AUTHOR]

The structure and properties of high-pressure phases of iron nitrides Fe7N3 in the pressure range of 50-150 GPa have been studied with ab initio calculations within the electron density functional theory. A new phase Amm2-Fe7N3, which is the most energetically favorable in the pressure range of 43-128 GPa, has been found using the USPEX (Universal Structure Predictor: Evolutionary Xtallography) algorithms. It has been thermodynamically shown that another high-pressure phase β-Fe7N3 is isostructural to a similar phase of iron carbide. The elastic properties have been calculated for all modifications ε-, β-, and Amm2-Fe7N3 stable at high pressures. [ABSTRACT FROM AUTHOR]

The parameters of equations of state of stoichiometric and nonstoichiometric phases of the γ′-FeN, ɛ-FeN, ɛ-FeN, ɛ-FeN, and ɛ-FeN iron nitrides in a pressure range up to 500 GPa have been determined using ab initio calculations. The points of the sharp drop and disappearance of the magnetic moment on iron atoms have been found. It has been shown that certain changes in the magnetic moment are accompanied by a change in the volume of a unit cell of the nitrides. The calculated parameters of equations of state demonstrate that the compressibility of both magnetic and nonmagnetic iron nitrides decreases monotonically with an increase in the content of nitrogen. [ABSTRACT FROM AUTHOR]