Structures and Crystal Chemistry of Carbonate at Earth's Mantle Conditions

Abstract

We report an overview of the crystal structures of carbonates determined ab-initio with X-ray single crystal diffraction techniques at mantle conditions. The determined crystal structures of high-pressure polymorphs of CaCO 3 have revealed that structures denser than aragonite can exist at upper and lower mantle pressures. These results have stimulated the computational and experimental research of thermodynamically stable polymorphs. At lower mantle conditions, the carbonates transform into new structures featuring tetrahedrally coordinated carbon. The identification of a systematic class of carbonates, nesocarbonates, cyclocarbonates, and inocarbonates reveals a complex crystal chemistry, with analogies to silicates. They provide fundamental input for the understanding of deep carbonatite melt physical properties. The possible polymerization of carbonate units will affect viscosity, and the reduced polymerization in crystal structures as a function of oxidation state could suggest that also oxidation state may affect the mobility of deep carbonatitic magmas. Finally, we report two high-pressure structures of mixed alkali carbonates, which reveal that these compounds may form wide solid solutions and incorporate a sensible amount of vacancies, which would allow incorporation of high-strength elements and therefore play an important role for geochemical element partitioning in the mantle.

Thermal and compressional behavior of the natural borate kurnakovite, MgB3O3(OH)5·5H2O

“…The structural evolution of borates at high pressure may open a new window to understand the Pinduced trigonal-planar-to-tetrahedral transition of C coordination reported to occur at much higher pressures (e.g. [49]), which is of large relevance in Earth science and in materials science.…”

A mechanism for aragonite to post-aragonite transition in MCO3 (M = Ca, Sr and Ba) carbonates: evidence of a hidden metastable polymorph

“…Carbonates, and specially CaCO 3 , are minerals involved at least in three-fold relevant implications of Earth's carbon cycle: (i) global climate, (ii) carbon energetic reservoirs, and (iii) origin of life. [1][2][3][4][5] The presence of carbonates at different depths of Earth's interior depends on a variety of factors such as subduction processes, density and miscibility, chemical decomposition and reduction, or pressure-induced phase transitions. The thermodynamics and kinetics of carbonate polymorphic sequences are two fundamental aspects in many of these phenomena that deserve further investigation to quantitatively determine phase stability pressure regions and temperatures required to observe the transitions.…”

High-pressure and high-temperature structure and equation of state of Na<sub>3</sub>Ca<sub>2</sub>La(CO<sub>3</sub>)<sub>5</sub> burbankite

“…This result reveals the flexibility of this structure in a wide compositional range. Because of the important role of Ca-rich alkalinealkaline earth carbonates as candidate minerals in uppermantle carbonatitic systems and the successful synthesis of Na 3 Ca 2 La(CO 3 ) 5 at 5 GPa and 1073 K (Merlini et al, 2020), which proves the incorporation of light REE 3+ (LREE 3+ ) in upper-mantle conditions in a burbankite-like structure, we decided to further investigate the crystal chemical evolution and thermoelastic properties of this phase. The paper reports the characterization and the determination of thermoelastic properties of synthetic Na 3 Ca 2 La(CO 3 ) 5 at high pressure and temperature.…”