The intrinsic twinning and enigmatic twisting of aragonite crystals.

It is generally accepted that aragonite crystals of biogenic origin are characterized by significantly higher twin densities compared to samples formed during geological processes. Based on our single crystal X-ray diffraction (SCXRD) and transmission electron microscopy (TEM) study of aragonite crystals from various localities, we show that in geological aragonites, the twin densities are comparable to those of the samples from crossed lamellar zones of molluscs shells. The high twin density is consistent with performed calculations, according to which the Gibbs free energy of twin-free aragonite is close to that of periodically twinned aragonite structure.

Ba(BO): the first example of dynamic disorder in a borate crystal.

Based on molecular dynamic simulations, dynamic disorder of [BO] groups in the Ba(BO) compound has been established. This is the first example of dynamic disorder in borates. It has been shown that static disorder of BO groups in the Ba(BO) crystals [Bekker , 2018, 101, 450] can be the result of quenching of dynamically disordered high-temperature modifications.

Phase relations, and mechanical and electronic properties of nickel borides, carbides, and nitrides from calculations.

Based on density functional theory and the crystal structure prediction methods, USPEX and AIRSS, stable intermediate compounds in the Ni-X (X = B, C, and N) systems and their structures were determined in the pressure range of 0-400 GPa. It was found that in the Ni-B system, in addition to the known ambient-pressure phases, the new nickel boride, NiB-, stabilizes above 202 GPa. In the Ni-C system, NiC- was shown to be the only stable nickel carbide which stabilizes above 53 GPa.

γ-BaBO: High-Pressure High-Temperature Polymorph of Barium Borate with Edge-Sharing BO Tetrahedra.

The α- and β-modifications of barium metaborate are important functional materials used in optoelectronic devices. A new theoretically predicted modification of BaBO has been synthesized under conditions of 3 GPa and 900 °C, using the DIA-type apparatus. The new high-pressure modification, γ-BaBO, crystallizes in a centrosymmetrical group of monoclinic syngony (2/ (#14), = 4.

Impact shock origin of diamonds in ureilite meteorites.

The origin of diamonds in ureilite meteorites is a timely topic in planetary geology as recent studies have proposed their formation at static pressures >20 GPa in a large planetary body, like diamonds formed deep within Earth's mantle. We investigated fragments of three diamond-bearing ureilites (two from the Almahata Sitta polymict ureilite and one from the NWA 7983 main group ureilite). In NWA 7983 we found an intimate association of large monocrystalline diamonds (up to at least 100 µm), nanodiamonds, nanographite, and nanometric grains of metallic iron, cohenite, troilite, and likely schreibersite.

Experimental and Ab Initio Studies of Intrinsic Defects in "Antizeolite" Borates with a Ba(BO) Framework and Their Influence on Properties.

The porous Ba(BO) framework of the so-called "antizeolite" borates with channels along the axis is capable of accommodating various guest anionic groups, e.g. [BO], [F], [F], and [(Li,Na)F].

(Fe,Ni)P allabogdanite can be an ambient pressure phase in iron meteorites.

An orthorhombic modification of (Fe,Ni)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)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)P polymorphs.

Hypervelocity collision and water-rock interaction in space preserved in the Chelyabinsk ordinary chondrite.

A comprehensive geochemical study of the Chelyabinsk meteorite reveals further details regarding its history of impact-related fragmentation and melting, and later aqueous alteration, during its transit toward Earth. We support an ∼30 Ma age obtained by Ar-Ar method (Beard et al., 2014) for the impact-related melting, based on Rb-Sr isotope analyses of a melt domain.

New high-pressure phases of FeN and FeC stable at Earth's core conditions: evidences for carbon-nitrogen isomorphism in Fe-compounds.

We carried out calculations on the crystal structure prediction and determination of - diagrams within the quasi-harmonic approximation for FeN and FeC. Two new isostructural phases FeN-2/ and FeC-2/ which are dynamically and thermodynamically stable under the Earth's core conditions were predicted. The FeC-2/ phase stabilizes preferentially to the known h-FeC at 253-344 GPa in the temperature range of 0-5000 K, and the FeN-2/ stabilizes preferentially relative to the β-FeN - at ∼305 GPa over the entire temperature range.

Temperature-induced oligomerization of polycyclic aromatic hydrocarbons at ambient and high pressures.

Temperature-induced oligomerization of polycyclic aromatic hydrocarbons (PAHs) was found at 500-773 K and ambient and high (3.5 GPa) pressures. The most intensive oligomerization at 1 bar and 3.

Incommensurately modulated twin structure of nyerereite NaKCa(CO).

The incommensurately modulated twin structure of nyerereite NaKCa(CO) has been first determined in the (3 + 1)-dimensional symmetry group Cmcm(α00)00s with modulation vector q = 0.383a*. Unit-cell values are a = 5.

High-Pressure-High Temperature (HP-HT) Stability of Polytetrafluoroethylene: Raman Spectroscopic Study Up to 10 GPa and 600 ℃.

The increasing demand for use of polymers at extreme conditions makes important the exploration of their behavior in a wide pressure and temperature range, which remains unknown for polytetrafluoroethylene (PTFE), one of the most common materials. An in situ Raman spectroscopic study of PTFE shows that it is stable within the range of 2-6 GPa at 500 ℃ and up to 12 GPa at 400 ℃. At T > 500 ℃ and P > 3.

Thermal equation of state of Molybdenum determined from in situ synchrotron X-ray diffraction with laser-heated diamond anvil cells.

Here we report that the equation of state (EOS) of Mo is obtained by an integrated technique of laser-heated DAC and synchrotron X-ray diffraction. The cold compression and thermal expansion of Mo have been measured up to 80 GPa at 300 K, and 92 GPa at 3470 K, respectively. The P-V-T data have been treated with both thermodynamic and Mie-Grüneisen-Debye methods for the thermal EOS inversion.

Synthesis of heavy hydrocarbons at the core-mantle boundary.

The synthesis of complex organic molecules with C-C bonds is possible under conditions of reduced activity of oxygen. We have found performing ab initio molecular dynamics simulations of the C-O-H-Fe system that such conditions exist at the core-mantle boundary (CMB). H2O and CO2 delivered to the CMB by subducting slabs provide a source for hydrogen and carbon.

Natural occurrence of pure nano-polycrystalline diamond from impact crater.

Consolidated bodies of polycrystalline diamond with grain sizes less than 100 nm, nano-polycrystalline diamond (NPD), has been experimentally produced by direct conversion of graphite at high pressure and high temperature. NPD has superior hardness, toughness and wear resistance to single-crystalline diamonds because of its peculiar nano-textures, and has been successfully used for industrial and scientific applications. Such sintered nanodiamonds have, however, not been found in natural mantle diamonds.

Jadeite in Chelyabinsk meteorite and the nature of an impact event on its parent body.

The Chelyabinsk asteroid impact is the second largest asteroid airburst in our recorded history. To prepare for a potential threat from asteroid impacts, it is important to understand the nature and formational history of Near-Earth Objects (NEOs) like Chelyabinsk asteroid. In orbital evolution of an asteroid, collision with other asteroids is a key process.

Thermal equation of state of solid naphthalene to 13 GPa and 773 K: in situ X-ray diffraction study and first principles calculations.

In a wide range of P-T conditions, such fundamental characteristics as compressibility and thermoelastic properties remain unknown for most classes of organic compounds. Here we attempt to clarify this issue by the example of naphthalene as a model representative of polycyclic aromatic hydrocarbons (PAHs). The elastic behavior of solid naphthalene was studied by in situ synchrotron powder X-ray diffraction up to 13 GPa and 773 K and first principles computations to 20 GPa and 773 K.

Carbon precipitation from heavy hydrocarbon fluid in deep planetary interiors.

The phase diagram of the carbon-hydrogen system is of great importance to planetary sciences, as hydrocarbons comprise a significant part of icy giant planets and are involved in reduced carbon-oxygen-hydrogen fluid in the deep Earth. Here we use resistively- and laser-heated diamond anvil cells to measure methane melting and chemical reactivity up to 80 GPa and 2,000 K. We show that methane melts congruently below 40 GPa.

The post-stishovite phase transition in hydrous alumina-bearing SiO2 in the lower mantle of the earth.

Silica is the most abundant oxide component in the Earth mantle by weight, and stishovite, the rutile-structured (P4(2)/mnm) high-pressure phase with silica in six coordination by oxygen, is one of the main constituents of the basaltic layer of subducting slabs. It may also be present as a free phase in the lower mantle and at the core-mantle boundary. Pure stishovite undergoes a displacive phase transition to the CaCl(2) structure (Pnnm) at approximately 55 GPa.