Ab Initio Thermodynamics and the Relationship between Octahedral Distortion, Lattice Structure, and Proton Substitution Defects in Malachite/Rosasite Group Endmember Pokrovskite Mg₂CO₃(OH)₂

Abstract

Divalent metal hydroxycarbonates with MCO(OH) stoichiometry are widely used in industry and are abundant in nature as the malachite/rosasite group of minerals. Essential to their performance as catalytic precursors and in nanoelectronics, these materials and minerals exhibit a high degree of cation ordering in mixed metal systems due to differences in distortion of the octahedral metal sites. Density-functional theory (DFT) calculations on pokrovskite MgCO(OH) in the rosasite structure and Mg analogues of monoclinic and orthorhombic forms of malachite determine that the octahedral sites are innately distorted, and that d Cu(II) Jahn-Teller distortion accommodates this distortion rather than causes it, leading to the significant preference of Cu for the type I octahedral sites. This distortion also leads to a high propensity for formation of cation vacancies charge balanced by proton substitution. Ab initio thermodynamics is used to determine that there are conditions under which proton substitution defects are slightly more stable than the stoichiometric structure, consistent with the widespread observation of such defects in pokrovskite in nature. Pokrovskite itself is most likely to form under CO-rich/low water conditions, particularly those utilizing supercritical CO for carbon sequestration and is sufficiently thermodynamically stable to trap CO under geological conditions. Low temperature and high water concentration promotes the formation of proton substitution defects, which has implications for synthesis of any material where octahedral strain may be relieved by proton substitution defects.

Ab Initio Thermodynamics of Hydrated Calcium Carbonates and Calcium Analogues of Magnesium Carbonates: Implications for Carbonate Crystallization Pathways

“…There are two distinct types of metal sites, M1 and M2 shown in Figure k, with M1 exhibiting two apical hydroxyl groups and four η 1 carbonate groups, and M2 having two axial η 1 carbonate groups and four hydroxyl groups. The structure is described in detail in Chaka . In the Ca analogue of pokrovskite, the rigid coordination structure and metal placement does not allow for the carbonate groups to shift from η 1 to η 2 ; hence, Ca remains 6-fold coordinated.…”

“…The structure is described in detail in Chaka. 92 In the Ca analogue of pokrovskite, the rigid coordination structure and metal placement does not allow for the carbonate groups to shift from η 1 to η 2 ; hence, Ca remains 6-fold coordinated. Formation of this complicated structure would be expected to require a high concentration of cation and carbonate and low water, as well as a basic pH, and hence be kinetically more difficult to form.…”

Quantitative Review of Olivine Carbonation Kinetics: Reactivity Trends, Mechanistic Insights, and Research Frontiers

“…Furthermore, in situ experimental techniques for determining the time-resolved abundances of solids (including intermediate phases), gases, fluids, and solutes are required to delineate and quantify carbonation and serpentinization reactions under elevated-pressure and -temperature conditions. These techniques are continuing to be developed, as researchers are investigating olivine reactivity under subsurface-relevant conditions with in situ optical, ,,, nuclear magnetic resonance (NMR), − and X-ray spectroscopies that complement X-ray scattering and diffraction investigations. ,,,,− Experimental advances need to be partnered with computational studies to help unravel key mechanistic information, such as those that provide insight into processes at olivine–H 2 O–CO 2 interfaces ,− and the thermodynamic stabilities of Mg carbonates. − Coupled reactivity and transport during olivine carbonation is also being evaluated with flow-through − and diffusion-limited experimental setups ,− that simulate a range of realistic fluid:rock ratios, flow rates, and pore network , regimes. These studies and others − are beginning to clarify physicochemical feedback between olivine carbonation and porosity–permeability development, including how localized flow rates and crystallographic orientation influence preferential carbonate precipitation.…”

Potential additives for magnesia-based concrete with enhanced performance and propensity for CO2 sequestration

“…15 and 16, respectively. In addition to unhydrated MgO (42.9° and 58.7° 2θ), brucite (18.6°, 38.1° and 50.9° 2θ) and pokrovskite (Mg2CO3(OH)2; main reflection at 17.2° and 32.8° 2θ [57,58]) were observed in all samples after 2 days of hydration under ambient conditions (Fig. 13).…”