Structures and stability of calcium and magnesium carbonates at mantle pressures

The origin of water on the Earth is a long-standing mystery, requiring a comprehensive search for hydrous compounds, stable at conditions of the deep Earth and made of Earth-abundant elements. Previous studies usually focused on the current range of pressure-temperature conditions in the Earth's mantle and ignored a possible difference in the past, such as the stage of the core-mantle separation. Here, using ab initio evolutionary structure prediction, we find that only two m...

First principles linear response calculations are used to investigate the lattice dynamics of what is thought to be the third most abundant phase in the lower mantle, CaSiO_3 perovskite. The commonly assumed cubic structure (Pm3m) is found to be dynamically unstable at all pressures, exhibiting unstable modes along the Brillouin zone edge from the M-point to the R-point. Based on these results, we predict that the ground state structure of CaSiO_3 perovskite is a distorted ph...

The Earth's lower mantle is believed to be composed mainly of (Mg,Fe)SiO3 perovskite, with lesser amounts of (Mg,Fe)O and CaSiO3). But it has not been possible to explain many unusual properties of the lowermost 150 km of the mantle (the D" layer) with this mineralogy. Here, using ab initio simulations and high-pressure experiments, we show that at pressures and temperatures of the D" layer, MgSiO3 transforms from perovskite into a layered CaIrO3-type post-perovskite phase. T...

Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc \rightarrow bcc \rightarrow simple cubic \rightarrow Ca-IV \rightarrow Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is simila...

Atomistic simulations provide a meaningful way to determine the physico-chemical properties of liquids in a consistent theoretical framework. This approach takes on particular usefulness for the study of molten carbonates, in a context where thermodynamic and transport data are crucially needed over a large domain of temperatures and pressures (to ascertain the role of these melts in geochemical processes) but are very scarce in the literature, especially for the calco-magnes...

We investigated the stability of polymeric CO2 over a wide range of pressures, temperatures, and chemical environments. We find that the I-42d polymeric structure, consisting of a three-dimensional network of corner sharing CO4 tetrahedra, forms at 40-140 GPa and from a CO-N2 mixture at 39 GPa. An exceptional stability field of 0 to 286 GPa and 100 to 2500 K is documented for this structure, making it a viable candidate for planetary interiors. The stability of the tetrahedra...

The distribution, recycling and storage of carbon in the Earth are of fundamental importance to understand the global carbon cycle between the deep Earth and near surface reservoirs. Degassing of CO2 at mid-ocean ridges may give information on the source region but the very low solubility of CO2 in tholeitic basalts has for consequence that near all Mid-Ocean Ridge Basalts glasses exsolve their CO2 rich vapor at shallow depth as they approach the ocean floor. Hence their CO2 ...

The solid inner core of the Earth is predominantly composed of iron alloyed with several percent Ni and some lighter elements, Si, S, O, H, and C being the prime candidates. There have been a growing number of papers investigating C and H as possible light elements in the core, but the results are contradictory. Here, using ab initio simulations, we study the Fe-C and Fe-H systems at inner core pressures (330-364 GPa). Using the evolutionary structure prediction algorithm USP...

Xenon, which is quite inert under ambient conditions, may become reactive under pressure. The possibility of formation of stable xenon oxides and silicates in the interior of the Earth could explain the atmospheric missing xenon paradox. Using the ab initio evolutionary algorithm, we predict the thermodynamical stabilization of Xe-O compounds at high pressures (XeO, XeO2 and XeO3 at pressures above 83, 102 and 114 GPa, respectively). Our calculations indicate large charge tra...

We have developed an efficient and reliable methodology for crystal structure prediction, merging ab initio total-energy calculations and a specifically devised evolutionary algorithm. This method allows one to predict the most stable crystal structure and a number of low-energy metastable structures for a given compound at any P-T conditions without requiring any experimental input. Extremely high success rate has been observed in a few tens of tests done so far, including i...