Razvan Caracas

Université de Paris

The abundance of volatile elements in the silicate Earth relative to primitive chondrites provides an important constraint on the thermochemical evolution of the planet. However, an overabundance of indium relative to elements with similar nebular condensation temperatures is a source of debate. Here we use ab initio molecular dynamics simulations...

Carbonate melts have been proposed to exist in the lower mantle, but their interaction with other lower mantle melt compositions is poorly understood. To understand miscibility in the carbonate-silicate-metal melt system, we simulate endmember, binary, and ternary melt mixtures and study how their Gibbs free energies of mixing evolve with pressure....

Jadeite is frequently reported in shocked meteorites, displaying a variety of textures and grain sizes that suggest formation by either solid-state transformation or by crystallization from a melt. Some-times, jadeite has been identified solely on the basis of Raman spectra. Here we argue that additional characterization is needed to confidently id...

Electrolytes play an important role in the internal structure and dynamics of water-rich satellites and potentially water-rich exoplanets. However, in planets, the presence of a large high-pressure ice mantle is thought to hinder the exchange and transport of electrolytes between various liquid and solid deep layers. Here we show, using first-princ...

Using particle swarm optimization with density functional theory, we identify the positions of hydrogen in a hypothetical Mg-end-member of phase egg (MgSiO4H2) and predict the most stable crystal structures with MgSiO4H2 stoichiometry at pressures between 0 and 300 GPa. The particle swarm optimization method consistently and systematically identifi...

Ab initio molecular dynamics calculations on a carbonate‐silicate‐metal melt were performed to study speciation and coordination changes as a function of pressure and temperature. We examine in detail the bond abundances of specific element pairs and the distribution of coordination environments over conditions spanning Earth’s present‐day mantle....

The position of the critical point determines the top of the liquid-vapor coexistence dome, and it is a physical parameter of fundamental importance in the study of high-energy shocks, including those associated with large planetary impacts. For most major planetary materials, such as oxides and silicates, the estimated position of the critical poi...

The magma ocean was a important reservoir for Earth’s primary volatiles. Understanding the volatile fluxes between the early atmosphere and the magma ocean is fundamental for quantifying the volatile budget of our planet. Here we investigate the vaporization of carbon and hydrogen at the boundary between the magma ocean and the thick, hot early atm...

We develop a Python-based open-source package to analyze the results stemming from ab initio molecular-dynamics simulations of fluids. The package is best suited for applications on natural systems, like silicate and oxide melts, water-based fluids, various supercritical fluids. The package is a collection of Python scripts that include two major l...

We characterize the structural and transport properties of fluid iron at the low densities and high temperature conditions characteristic to the outer parts of protolunar disks, which form after giant impacts. We observe that decreasing the density and increasing the temperature decluster the fluid, which is eventually dominated by isolated atoms a...

“Cousin” worlds—slightly bigger or slightly hotter than Earth—can help us understand planetary habitability, but we need more lab and numerical experiments to make the most of this opportunity.

Jadeite is frequently reported in shocked meteorites, displaying a variety of textures and grain sizes that suggest formation by either solid-state transformation or by crystallization from a melt. Sometimes, jadeite has been identified solely on the basis of Raman spectra. Here we argue that additional characterization is needed to confidently ide...

The early Earth was marked by at least one global magma ocean. Melt buoyancy played a major role for its evolution. Here we model the composition of the magma ocean using a six-component pyrolite melt, to which we add volatiles in the form of carbon as molecular CO or CO2 and hydrogen as molecular H2O or through substitution for magnesium. We compu...

The ab initio (ai) Gibbs ensemble (GE) Monte Carlo (MC) method coupled with Kohn–Sham density functional theory is successful in predicting the liquid–vapour equilibrium of insulating systems. Here we show that the aiGEMC method can be used to study also metallic systems, where the excited electronic states play an important role and cannot be negl...

We discuss major challenges in modeling giant impacts between planetary bodies, focusing on the equations of state (EOS). During the giant impact stage of planet formation, rocky planets are melted and partially vaporized. However, most EOS models fail to reproduce experimental constraints on the thermodynamic properties of the major minerals over...

Giant impacts are disruptive events occurring in the early stages of planetary evolution. They may result in the formation of a protolunar disk or of a synestia. A central planet and one or several moons condense upon cooling bearing the chemical signature of the silicate mantles of the initial bodies; the iron cores may partly vaporize, fragment a...

The position of the vapor-liquid dome and of the critical point determine the evolution of the outermost parts of the protolunar disk during cooling and condensation after the Giant Impact. The parts of the disk in supercritical or liquid state evolve as a single thermodynamic phase; when the thermal trajectory of the disk reaches the liquid-vapor...

The degree to which the Earth’s mantle stores and cycles water in excess of the storage capacity of nominally anhydrous minerals is dependent upon the stability of hydrous phases under mantle-relevant pressures, temperatures, and compositions. Two hydrous phases, phase D and phase H, are stable to the pressures and temperatures of the Earth’s lower...

To improve our understanding of the Earth's global carbon cycle, it is critical to characterize the distribution and storage mechanisms of carbon in silicate melts. Presently, the carbon budget of the deep Earth is not well constrained and is highly model‐dependent. In silicate melts of the uppermost mantle, carbon exists predominantly as molecular...

With ab initio molecular dynamics simulations on a Na-, Ca-, Fe-, Mg-, and Al-bearing silicate melt of pyrolite composition, we examine the detailed changes in elemental coordination as a function of pressure and temperature. We consider the average coordination as well as the proportion and distribution of coordination environments at pressures an...

We discuss major challenges in modeling giant impacts between planetary bodies, focusing on the equations of state (EOS). During the giant impact stage of planet formation, rocky planets are melted and partially vaporized. However, most EOS models fail to reproduce experimental constraints on the thermodynamic properties of the major minerals over...

The Raman Experiments for Astrobiology and Planetology REAP database of the Laboratoire de Géologie de Lyon is based on a collection of Raman spectra of representative minerals from the original “Handbook of Raman spectra for geology” web database. Now it is incremented with Raman spectra of organic compounds having an astrobiology interest. Eventu...

A formulation of the response of a system to strain and electric field perturbations in the pseudopotential-based density functional perturbation theory has been proposed by Hamann et al. [D. R. Hamann, X. Wu, K. M. Rabe, and D. Vanderbilt, Phys. Rev. B 71, 035117 (2005)], using an elegant formalism based on the expression of density functional the...

The crystallization of a magma ocean (MO) early in Earth's history shaped the entire evolution of our planet. The buoyancy relations between the forming crystals and the residual melt is the most important but also the most unknown parameter affecting the large-scale structure and evolution of the MO. The accumulation of crystals, near the depth of...

Current estimates of the carbon flux between the surface and mantle are highly variable, and the total amount of carbon stored in closed hidden reservoirs is unknown. Understanding the forms in which carbon existed in the molten early Earth is a critical step towards quantifying the carbon budget of Earth's deep interior. Here we employ first-princ...

Geometrical and vibrational characterization of magnesium hydroxide were performed using Density Functional Theory. Four possible crystal symmetries were explored: P\bar3 (No. 147, point group -3), C2/m (No. 12, point group 2), P3m1 (No. 156, point group 3m) and P\bar3m1 (No. 164, point group -3m) which are the currently accepted geometries found i...

All the different phases of water ice between 2 GPa and several megabars are based on a single body-centered cubic sub-lattice of oxygen atoms. They differ only by the behavior of the hydrogen atoms. In this study, we investigate the dynamics of the H atoms at high pressures and temperatures in water ice from first-principles molecular dynamics sim...

The Raman frequencies of quartz are used to evaluate deviatoric stresses in rocksalt-structure media in diamond-anvil cell experiments to pressures up to 20 GPa. The piezospectroscopic effect in quartz is modeled by first-principles calculations. Non-hydrostatic stresses measured in halogen salts give yield strength estimates of 3 GPa in the B1 str...

We study the hydrogen mobility in ringwoodite and wadsleyite considering multiple charge-balanced defects, including Mg < = > 2H, Si < = > Mg + 2H, and the hydrogarnet defect, Si < = > 4H, using molecular dynamics simulations based on the density functional theory at transition zone pressures and temperatures between 1500 and 2500 K. We determine t...

Terrestrial planets are thought to experience episode(s) of large-scale melting early in their history. Fractionation during magma-ocean freezing leads to unstable stratification within the related cumulate layers due to progressive iron enrichment upwards, but the effects of incremental cumulate overturns during MO crystallization remain to be exp...

In various shocked meteorites, low-pressure silica polymorph a-cristobalite is commonly found in close spatial relation with the densest known SiO 2 polymorph seifertite, which is stable above B80 GPa. We demonstrate that under hydrostatic pressure a-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 1...

Supplementary Figures, Supplementary Tables and Supplementary References

Water-rich planetary bodies including large icy moons and ocean exoplanets may host a deep liquid water ocean underlying a high-pressure icy mantle. The latter is often considered as a limitation to the habitability of the uppermost ocean because it would limit the availability of nutrients resulting from the hydrothermal alteration of the silicate...

The newly described hydrous phase H, MgSiO4H2, is stable to the pressures and temperatures of the Earth's lower mantle, suggesting that the Earth's lower mantle may participate in the cycling of water. We present the results of ab initio calculations on the stability of this phase within the phase H–δAlOOH–SiO2 system between 20 and 60 GPa, explori...

The newly described hydrous phase H, MgSiO4H2, is stable to the pressures and temperatures of the Earth's lower mantle, suggesting that the Earth's lower mantle may participate in the cycling of water. We present the results of ab initiocalculations on the stability of this phase within the phase H-delta AlOOH-SiO(2)system between 20 and 60GPa, exp...

Sound velocities of bridgmanite measured in the laboratory are a key to deciphering the composition of the lower mantle. Here, we report Debye sound velocities determined using nuclear inelastic scattering (NIS) for one majorite composition (Mg0.82Fe0.18SiO3) and five bridgmanite compositions (Mg0.82Fe0.18SiO3, Mg0.86Fe0.14Si0.98Al0.02O3, Mg0.88Fe0...

Knowledge of potential carbon carriers such as carbonates is critical for our understanding of the deep-carbon cycle and related geological processes within the planet. Here we investigated the high-pressure behavior of (Ca,Mn)CO3 up to 75 GPa by synchrotron single-crystal X-ray diffraction, laser Raman spectroscopy, and theoretical calculations. M...

The mechanical properties of C-doped hexagonal-close packed (hcp) iron are studied at high pressure from first-principles calculations. The energy required for doping with C as an interstitial impurity is 246 meV/1wt%C at 120 GPa for one unit cell of hcp Fe, and increases almost linearly with pressure. The density deficit of the inner core can be m...

In the present study single crystals of rhombohedral α-B were investigated under pressure to 60 GPa by means of single-crystal X-ray diffraction. The bulk modulus of α-B was found to be K = 224(7) GPa (K´ = 3.0(3)). Measurements of interatomic distances as a function of pressure revealed that the intericosahedral two-center two-electron (2c-2e) bon...

From first-principles molecular dynamics, we investigate the relation between the superionic proton conduction and the behavior of the O─H⋯O bond (ice VII′ to ice X transition) in body-centered-cubic (bcc) H2O ice between 1300 and 2000 K and up to 300 GPa. We bring evidence that there are three distinct phases in the superionic bcc stability field....

Iron isotopes constrain core chemistry The overall composition of Earth's core is an important constraint on the chemistry and evolution of our planet's interior. A longstanding problem has been determining the minor element contribution to its predominately iron-nickel alloy. Based on the iron isotope fractionation of various iron alloys with pres...

The Fe-based solid crystalline structure(s) present in Earth's core is(are) compact, whether hexagonal or cubic. The nature of the light element (or elements) has been the subject of considerable speculation because of its bearing on the overall bulk composition of Earth, the conditions under which the core formed, the temperature regime in the cor...

Abstract: Vibrational spectroscopic techniques, including Raman, infrared (IR), and inelastic neutron scattering (INS) or x-ray scattering (IXS) spectroscopies, are coupled with ab initio or first principles calculations to study the vibrational lattice dynamics and to predict and elucidate the thermodynamic behavior of minerals under high-pressure...

Raman spectra of magnesian phyllosilicates belonging to the serpentine, talc, and chlorite groups have been obtained at ambient conditions, and at high pressures and up to 200 °C in order to study high-pressure transformations in the 10 GPa range. The complex and distinct Raman spectra of these minerals allow straightforward identification, which m...

The origin of longstanding anomalies in experimental studies of the dense solid phases of H2O ices VII, VIII, and X is examined using a combination of first-principles theoretical methods. We find that a ferroelectric variant of ice VIII is energetically competitive with the established antiferroelectric form under pressure. The existence of domain...

We estimate the implications of having hydrogen dissolved in the solid core from a mineral physics perspective. Based on first-principles calculations, we observe that hydrogen has a strong effect on decreasing the density of the hexagonal close-packed structure of Fe. The structural effect is weakly dependent on the ordering pattern of the hydroge...

At lower mantle conditions, subducted mid oceanic ridge basalts (MORB) will crystallize more than 20 vol% of an aluminum-rich phase, which is referred to generally as the new aluminum (NAL) phase. Given that a significant proportion of the lower mantle may be comprised of subducted crust, the NAL phase may contribute to the bulk elastic properties...

Based on numerical results from density-functional perturbation theory calculations, we show that the magnetic spin transition in Fe2+-rich MgSiO3 perovskite can be identified as changes in the powder X-ray diffraction (XRD) pattern and the vibrational spectra. In particular theory predicts how the symmetry breaking and the volume reduction associa...

We study hydrated magnesium sulfate minerals from first-principles calculations based on density-functional theory. We determine the heat of hydration for MgSO4·nH2O, compute the Raman and infrared spectra for several phases and calculate the S isotope partitioning as a function of hydration. We find that epsomite and meridianiite with, respectivel...

Earth's core is an iron-rich alloy containing several weight percent of light element(s), possibly including silicon. Therefore the high pressure-temperature equations of state of iron–silicon alloys can provide understanding of the properties of Earth's core. We performed X-ray diffraction experiments using laser-heated diamond anvil cells to achi...

High-pressure behavior of coesite was studied on single crystals using diamond-anvil cells with neon as the pressure-transmitting medium by means of in situ Raman spectroscopy up to pressures of ~51 GPa. The experimental observations were complemented with theoretical computations of the Raman spectra under similar pressure conditions. We find that...

[1] The behavior of synthetic-powdered 57Fe-enriched enstatite (Mg0.980Fe0.020(5))(Mg0.760Fe0.240)Si2O6 has been explored by X-ray diffraction (XRD) and nuclear resonant inelastic X-ray scattering (NRIXS). The Pbca-structured enstatite sample was compressed in fine pressure increments for our independent XRD measurements. One structural transition...

Raman spectra were acquired on a series of natural and synthetic sulfide minerals, commonly found in enstatite meteorites: oldhamite (CaS), niningerite or keilite ((Mg,Fe)S), alabandite (MnS), troilite (FeS), and daubreelite (Cr2FeS4). Natural samples come from three enstatite chondrites, three aubrites, and one anomalous ungrouped enstatite meteor...

We compute the electrical conductivity of iron and iron alloys at Earth's core conditions and beyond from electron-phonon coupling in the ABINIT implementation. We find an excellent agreement with experimental results for pure hcp iron below 1 mbars. We confidently use our results up to core pressure conditions. We specifically look at saturation e...

We highlight applications of first-principles calculations based on density-functional theory, density-functional perturbation theory and molecular dynamics. We present intuitively the background of the various calculations. We exemplify with investigations of the physical behavior of major planetary materials under extreme conditions, currently un...

We perform first-principles molecular dynamics (MD) calculations to study C-bearing melts along the T=3000K isotherm up to high pressure. Then we use the melts to study the quenched glasses. We consider pure forsteritic compositions, atomic C-, CO2- and MgCO3- bearing compositions. We discuss in detail the behavior of the glass structure under comp...

We review specific aspects from computational condensed-matter physics with particular interest to the study of physical properties of the planetary ices. We describe the most widespread technique in use today, the density-functional theory and its derivative, the density-functional perturbation theory. We show the basic theoretical formalism and t...

Detailed investigation of single crystals of α-boron using Raman spectroscopy was performed under elevated pressures and temperatures. The behavior of the Raman modes under pressure was studied both theoretically and experimentally. The results were compared with the literature data.

The outer core of the Earth contains several weight percent of one or more unknown light elements, which may include silicon. Therefore it is critical to understand the high pressure–temperature properties and behavior of an iron–silicon alloy with a geophysically relevant composition (16 wt% silicon). We experimentally determined the melting curve...

Plate tectonics is the unifying paradigm of geodynamics yet the mechanisms and causes of its initiation remain controversial. Some models suggest that plate tectonics initiates when the strength of lithosphere is lower than 20-200MPa, below the frictional strength of lithospheric rocks (>700MPa). At present-day, major plate boundaries such as the s...

Understanding the physical and chemical properties of carbonate minerals at extreme conditions is important for modeling the deep carbon cycle, because they represent likely hosts for carbon in the lower mantle. Previous high-pressure studies have identified a structural and electronic phase transition in siderite using X-ray diffraction and X-ray...

Three single-crystals of magnesium silicate perovskite with differing chemical compositions have been studied by means of synchrotron X-ray diffraction in diamond anvil cells with He as pressure transmitting medium from room pressure up to 75 GPa. In addition to the end-member MgSiO3 composition, a perovskite containing 4 mol% of the Fe2+SiO3 compo...