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DOE OSTI2020

Materials Data on MgCO3 by Materials Project

MgCO3 is Calcite structured and crystallizes in the trigonal R-3c space group. The structure is three-dimensional. Mg2+ is bonded to six equivalent O2- atoms to form corner-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles are 59°. All Mg–O bond lengths are 2.13 Å. C4+ is bonded in a trigonal planar geometry to three equivalent O2- atoms. All C–O bond lengths are 1.30 Å. O2- is bonded in a trigonal planar geometry to two equivalent Mg2+ and one C4+ atom.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on MgCO3 by Materials Project

MgCO3 crystallizes in the orthorhombic C222_1 space group. The structure is three-dimensional. Mg2+ is bonded in a 4-coordinate geometry to eight O2- atoms. There are a spread of Mg–O bond distances ranging from 2.12–2.68 Å. C4+ is bonded to four O2- atoms to form corner-sharing CO4 tetrahedra. There is two shorter (1.35 Å) and two longer (1.46 Å) C–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a 3-coordinate geometry to three equivalent Mg2+ and one C4+ atom. In the second O2- site, O2- is bonded in a bent 120 degrees geometry to two equivalent Mg2+ and two equivalent C4+ atoms.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on MgCo3 by Materials Project

MgCo3 is beta Cu3Ti-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Mg is bonded to twelve Co atoms to form MgCo12 cuboctahedra that share corners with six equivalent MgCo12 cuboctahedra, corners with twelve equivalent CoMg4Co8 cuboctahedra, edges with eighteen CoMg4Co8 cuboctahedra, faces with eight equivalent MgCo12 cuboctahedra, and faces with twelve CoMg4Co8 cuboctahedra. There are six shorter (2.61 Å) and six longer (2.62 Å) Mg–Co bond lengths. There are two inequivalent Co sites. In the first Co site, Co is bonded to four equivalent Mg and eight Co atoms to form distorted CoMg4Co8 cuboctahedra that share corners with four equivalent MgCo12 cuboctahedra, corners with fourteen equivalent CoMg4Co8 cuboctahedra, edges with six equivalent MgCo12 cuboctahedra, edges with twelve CoMg4Co8 cuboctahedra, faces with four equivalent MgCo12 cuboctahedra, and faces with sixteen CoMg4Co8 cuboctahedra. There are a spread of Co–Co bond distances ranging from 2.49–2.72 Å. In the second Co site, Co is bonded to four equivalent Mg and eight equivalent Co atoms to form distorted CoMg4Co8 cuboctahedra that share corners with four equivalent MgCo12 cuboctahedra, corners with fourteen CoMg4Co8 cuboctahedra, edges with six equivalent MgCo12 cuboctahedra, edges with twelve equivalent CoMg4Co8 cuboctahedra, faces with four equivalent MgCo12 cuboctahedra, and faces with sixteen CoMg4Co8 cuboctahedra.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Precipitation of low-temperature disordered dolomite induced by extracellular polymeric substances of methanogenic Archaea Methanosarcina barkeri : Implications for sedimentary dolomite formation

Abstract A correlation between methanogenesis and dolomite formation has been reported; however, the mechanism underlying this association is not fully understood. In this study, we conducted forced carbonate precipitation experiments at room temperature in calcite-seeded Ca/Mg carbonate solutions containing either purified non-living biomass or bound extracellular polymeric substances (EPS) of the methanogen Methanosarcina barkeri. Purified non-living biomass and bound EPS was used so as to avoid the possible influence of the complex components of the growing microbial culture on carbonate crystallization. Our results demonstrated that non-living biomass of M. Barkeri can enhance the Mg incorporation into calcitic structure and induce the crystallization of disordered dolomite. In the presence of ~113 mg L–1 of non-living biomass, disordered dolomite with ~41 and 45 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. A systematic increase in the MgCO3 contents of the precipitated Ca-Mg carbonates was also observed with the increased non-living biomass concentration. Bound EPS was shown to be the component of non-living biomass that catalyzed the precipitation of disordered dolomite. At only ~25 mg L–1 of bound EPS, disordered dolomite with ~47 and 48 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. We propose that adsorption of bound EPS to growing carbonate surfaces through hydrogen bonding is the key to catalyzing disordered dolomite crystallization, and that this mechanism is also applicable to natural EPS-induced dolomite formation. This study provides significant insight into the formation mechanism of microbial-induced dolomite with high δ13C values.

Geochemistry & Geophysics↗

DOE OSTI2021

Thin Water Films Enable Low-Temperature Magnesite Growth Under Conditions Relevant to Geologic Carbon Sequestration

Injecting supercritical CO2 (scCO2) into basalt formations for long-term storage is a promising strategy for mitigating CO2 emissions. Mineral carbonation can result in permanent entrapment of CO2; however, carbonation kinetics in thin H2O films in humidified scCO2 is not well understood. We investigated forsterite (Mg2SiO4) carbonation to magnesite (MgCO3) via amorphous magnesium carbonate (AMC; MgCO3·xH2O, 0.5 < x < 1), with the goal to establish the fundamental controls on magnesite growth rates at low H2O activity and temperature. Experiments were conducted at 25, 40, and 50 °C in 90 bar CO2 with a H2O film thickness on forsterite that averaged 1.78 ± 0.05 monolayers. In situ infrared spectroscopy was used to monitor forsterite dissolution and the growth of AMC, magnesite, and amorphous SiO2 as a function of time. Geochemical kinetic modeling showed that magnesite was supersaturated by two to three orders of magnitude and grew according to a zero-order rate law. The results indicate that the main drivers for magnesite growth are sustained high supersaturation coupled with low H2O activity, a combination of thermodynamic conditions not attainable in bulk aqueous solution. This improved understanding of reaction kinetics can inform subsurface reactive transport models for better predictions of CO2 fate and transport.

Kerisit, Sebastien N.↗