No abstract

Orthocarbonates are a newly discovered class of compounds that are stable at high pressures. The presence of sp3-hybridized carbon, having structural similarity to orthosilicates, and their potential participation in the global planetary carbon cycle have triggered intensive theoretical and experimental investigations into these compounds. Here, based on the density functional theory and crystal structure prediction calculations, we predict new stable crystal structures of the orthocarbonates Sr3CO5-Cmcm, Sr3CO5-I4/mcm, Ba2CO4-Pnma, and Ba3CO5-I4/mcm. Summarizing the obtained data, we show that orthocarbonates of alkaline-earth metals are isotypic to ambient-pressure orthosilicates with only rare exceptions. The lower-pressure stability limit for Ba-orthocarbonates is around 5 GPa. However, the stability limit increases with decreasing cation radius and reaches 13 GPa for Ca-orthocarbonates. Based on the calculations of Gibbs free energies with the quasi-harmonic approximation, the reaction 2M2CO4 = M3CO5 + MCO3 (M = Sr and Ba) is established. At 20 GPa, this reaction is realized at temperatures above 1080 K for Sr2CO4 and above 740 K for Ba2CO4, and the Clapeyron slope is positive in both cases. The obtained P–T diagrams for SrCO3 and BaCO3 show that equilibrium between the structures of aragonite and postaragonite is observed at 15–17 GPa for SrCO3 and 5–7 GPa for BaCO3. The transition pressure is almost independent of temperature. No other more favorable structures than postaragonite have been found for these compounds in the considered pressure range, up to 200 GPa. Thus, in contrast to CaCO3 and MgCO3, the transition from sp2 to sp3 hybridization is not realized for these compounds. Two of the found structures, Sr2CO4-Pnma and Sr3CO5-Cmcm, are dynamically stable at ambient pressure. This indicates the possibility of recovering the crystals from a high-pressure environment and conducting further laboratory investigation.

Orthocarbonates of alkaline earth metals are the newly discovered class of compounds stabilized at high pressures. Mg-orthocarbonates are the potential carbon host phases, transferring oxidized carbon in the Earth’s lower mantle up to the core–mantle boundary. Here, we demonstrate the possibility for the formation of Mg2CO4 in the lower mantle at pressures above 50 GPa by ab initio calculations. Mg2CO4 is formed by the reaction MgCO3 + MgO = Mg2CO4, proceeding only at high temperatures. At 50 GPa, the reaction starts at 2200 K. The temperature decreases with pressure and drops down to 1085 K at the pressure of the Earth’s core–mantle boundary, approximately 140 GPa. Two stable structures, Mg2CO4-Pnma and Mg2CO4-P21/c, were revealed using a crystal structure prediction technique. Mg2CO4-Pnma is isostructural to mineral forsterite (Mg2SiO4), while Mg2CO4-P21/c is isostructural to mineral larnite (β-Ca2SiO4). Transition pressure from Mg2CO4-Pnma to Mg2CO4-P21/c is around 80 GPa. Both phases are dynamically stable on decompression down to the ambient pressure and can be preserved in the samples of natural high-pressure rocks or the products of experiments. Mg2CO4-Pnma has a melting temperature more than 16% higher than the melting temperature of magnesite (MgCO3). At 23.7, 35.5, and 52.2 GPa, Mg2CO4-Pnma melts at 2661, 2819, and 3109 K, respectively. Acoustic wave velocities V p and V s of Mg2CO4-Pnma are very similar to that of magnesite, while universal anisotropy of Mg2CO4-Pnma is stronger than that of magnesite, as well as the coefficient A U is larger for orthocarbonate. The obtained Raman spectra of Mg2CO4-Pnma would help its identification in high-pressure experiments.

CaC2O5-I4̅2d was obtained by reacting CO2 and CaCO3 at lower Earth mantle pressures and temperatures ranging between 34 and 45 GPa and between 2000 and 3000 K, respectively. The crystal structure was solved by single-crystal X-ray diffraction and contains carbon atoms tetrahedrally coordinated by oxygen. The tetrahedral CO4 4– groups form pyramidal [C4O10]4– complex anions by corner sharing. Raman spectroscopy allows an unambiguous identification of this compound, and the experimentally determined spectra are in excellent agreement with Raman spectra obtained from density functional theory calculations. CaC2O5-I4̅2d persists on pressure release down to ∼18 GPa at ambient temperature, where it decomposes into calcite and, presumably, CO2 under ambient conditions. As polymorphs of CaCO3 and CO2 are believed to be present in the vicinity of subducting slabs within Earth’s lower mantle, they would react to give CaC2O5-I4̅2d, which therefore needs to be considered instead of end-member CaCO3 in models of the mantle mineralogy.

Over the past few years, the concept of carbonates, as the salts of MCO3 or M'2CO3 composition with [CO3] triangles in the crystal structures was sufficiently extended. In addition to...

The search of the stable intermediate compounds in the system PbO−CO2 have been performed using crystal structure prediction technique in the pressure range of 0–50 GPa. Stable structures were found for the compositions Pb2CO4 and PbC2O5. The earlier known oxy‐carbonate Pb2CO4‐P212121 was shown to transform at 10 GPa into another oxy‐carbonate Pb2CO4‐Pnma and than at 22 GPa – into the sp3‐hybridized orthocarbonate with the same symmetry. PbC2O5 became thermodynamically stable at almost 10 GPa in the form of the pyrocarbonate structure P21/c. At 35–45 GPa, PbC2O5‐P21/c transforms into PbC2O5‐Fdd2 with the framework of [CO4] tetrahedra. Another energetically favorable, although dynamically unstable, pyrocarbonate structure PbC2O5‐Ptrue1‾ was found in the narrow pressure interval around 10 GPa. We suggest that some analogue of this structure can appear in experiment. One structure, sp3‐hybridized Pb2CO4‐Pnma, is dynamically and thermally stable at ambient pressure, and therefore can be decompressed and extracted from the high‐pressure environment.

An orthorhombic modification of (Fe,Ni) 2 P, allabogdanite, found in iron meteorites was considered to be thermodynamically stable at pressures above 8 GPa and temperatures of 1673 K according to the results of recent static high-pressure and high-temperature experiments. A hexagonal polymorphic modification of (Fe,Ni) 2 P, barringerite, was considered to be stable at ambient conditions. Experimental investigation through the solid-state synthesis supported by ab initio calculations was carried out to clarify the stability fields of (Fe,Ni) 2 P polymorphs. Both experimental and theoretical studies show that fe 2 P-allabogdanite is a low-temperature phase stable at ambient conditions up to a temperature of at least 773 K and, therefore, is not necessarily associated with high pressures. This is consistent with the textural relationships of allabogdanite in iron meteorites. Fe 2 P plays a substantial role in the mineralogy of iron meteorites. Two polymorphic modifications of Fe 2 P−Ni 2 P solid-solution currently known are hexagonal barringerite (C22) 1,2 and orthorhombic allabogdanite (C23) 3,4. Barringerite was discovered in the Ollague pallasite found in Bolivia 1. The composition determined by electron probe microanalysis was (Fe 0.58 Ni 0.42 Co 0.003) 1.95 P. It is isostructural with the hexagonal modification of Fe 2 P with lattice parameters a = 5.87(7) Å and c = 3.44(4) Å. In a short time after its discovery, it was shown that the Ollague meteorite is a fragment of a very large, up to several hundred kilograms, Imilac pallasite, the numerous fragments of which were found in the province of Antofagasta, Chile. It turned out that the Ollague meteorite was artificially reheated, which could cause high-temperature changes in the chemical and mineral compositions 5. It is noteworthy that barringerite has never been observed in other fragments of the Imilac meteorite or other investigated pallasites. Some barringerite findings are associated with placers 6 , but their sources are probably cosmogenic 2. Detailed mineralogical and crystal-chemical characterization of terrestrial barringerite from pyrometamorphic rocks of the Hatrurim formation, Israel is reported by Britvin et al. 2. The mineral occurs in associations of the so-called «paralavas» − initially silicate-carbonate sedimentary rocks that remelted during pyrometamorphic processes at about 1300 K, but under low pressure. Barringerite from the Hatrurim formation is almost pure Fe 2 P, the exact composition is (Fe 1.95 Ni 0.03 Cr 0.02) 2.00 P. The unit cell parameters determined by single crystal X-ray diffraction are a = 5.867(1) Å and c = 3.464(1) Å with Z = 3. The orthorhombic modification of Fe 2 P−Ni 2 P solid-solution, allabogdanite was first detected in anomalous Onello high-Ni ataxite found in 1997 in the alluvium of the Onello River, Yakutia, Russia 3. Earlier this mineral in the Onello ataxite was considered barringerite 7-9. The unit cell parameters of allabogdanite refined from single-crystal data are a = 5.792(7) Å, b = 3.564(4) Å, and c = 6.69...

We carried out ab initio calculations on the crystal structure prediction and determination of P–T diagrams within the quasi-harmonic approximation for Fe7N3 and Fe7C3.