Carbon Dioxide, Bicarbonate and Carbonate Ions in Aqueous Solutions at Deep Earth Conditions

How life started on Earth is a long-time unsolved mystery. There are various hypotheses ranging from outer space to seabed. Here, we applied extensive ab initio molecular dynamics (AIMD) simulations to study chemical reactions of NH3, H2O, H2, and CO at pressures (P) and temperatures (T) approximating the conditions of Earth's upper mantle (i.e. 10 - 13 GPa, 1000 -1400 K). Contrary to the previous assumption that larger organic molecules might readily dissociate in aqueous so...

Calcium and magnesium carbonates are believed to be the host compounds for most of the oxidized carbon in the Earth's mantle. Here, using evolutionary crystal structure prediction method USPEX, we systematically explore the MgO-CO2 and CaO-CO2 systems at pressures ranging from 0 to 160 GPa to search for thermodynamically stable magnesium and calcium carbonates. While MgCO3 is the only stable magnesium carbonate, three calcium carbonates are stable under pressure: well-known C...

The stability, structure and properties of carbonate minerals at lower mantle conditions has significant impact on our understanding of the global carbon cycle and the composition of the interior of the Earth. In recent years, there has been significant interest in the behavior of carbonates at lower mantle conditions, specifically in their carbon hybridization, which has relevance for the storage of carbon within the deep mantle. Using high-pressure synchrotron X-ray diffrac...

Carbon is an essential element for the existence and evolution of life on Earth. Its abundance in Earth's crust and mantle (the Bulk Silicate Earth, BSE) is surprisingly high given that carbon is strongly siderophile (metal-loving) at low pressures and temperatures, and hence should have segregated almost completely into Earth's core during accretion. Estimates of the concentration of carbon in the BSE lie in the range 100-260 ppm and are much higher than expected based on si...

The physicochemical behavior of elements and compounds is heavily altered by high pressure. The occurrence of pressure-induced reactions and phase transitions can be revealed by crystal structure prediction approaches. In this work, we explore the C-H-O phase diagram up to 400 GPa exploiting an evolutionary algorithm for crystal structure predictions along with ab initio calculations. Besides uncovering new stable polymorphs of high-pressure elements and known molecules, we p...

Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in Earth's crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here, we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally...

Calcium carbonate plays a crucial role in the global carbon cycle, and its phase diagram has always been of significant scientific interest. In this study, we used molecular dynamics (MD) to investigate several structural phase transitions of calcium carbonate. Using the Raiteri potential model, we explored the structural transitions occurring at a constant pressure of 1 bar with temperatures ranging from 300 K to 2500 K, and at a constant temperature of 1600 K with pressures...

Superionic hydrogen was previously thought to be an exotic state predicted and confirmed only in pure H2O ice. In Earth's deep interior, H2O exists in the form of O-H groups in ultra-dense hydrous minerals, which have been proved to be stable even at the conditions of the core-mantle boundary (CMB). However, the superionic states of these hydrous minerals at high P-T have not been investigated. Using first-principles calculations, we found that pyrite structured FeO2Hx (0 <= ...

CO2 has attracted considerable attention in the recent years due to its role in the greenhouse effect and environmental management. While its reaction with water has been studied extensively, the same cannot be said for reactivity in supercritical CO2 phase, where the conjugate acid/base equilibria proceed through different mechanisms and activation barriers. In spite of the apparent simplicity of the CO2 + H2O reaction, the collective effect of different environments has dra...

We present a theoretical study of solid carbon dioxide up to 50GPa and 1500K using first-principles calculations. In this pressure-temperature range, interpretations of recent experiments have suggested the existence of CO2 phases which are intermediate between molecular and covalent-bonded solids. We reexamine the concept of intermediate phases in the CO2 phase diagram and propose instead molecular structures, which provide an excellent agreement with measurements.