Solubility investigations in the amorphous calcium magnesium carbonate system

Formation and properties of spindle-shaped aragonite mesocrystals from Mg-bearing solutions

The formation of aragonite under ambient conditions is typically linked to Mg-rich aqueous environments. The grains that form in such environments show peculiar properties such as aggregate-like appearance and mesocrystalline character. We tested the effect of dissolved Mg2+ ions on the formation of aragonite mesocrystals by synthesizing aragonite with an automatic titrator at constant pH and at different dissolved Mg : Ca ratios, and by studying the properties of the precipitated material with various scanning transmission electron microscopy (STEM) techniques. At all studied Mg : Ca ratios the firstly condensed carbonate phase was Mg-bearing amorphous calcium carbonate (Mg-ACC) that transformed into aragonite during the synthesis experiments. The aragonite grains had typically aggregate-like appearance and spindle shapes, with the external morphologies of the spindles unaffected by variation in solution chemistry. The alignment of the nanocrystals within the aggregates was crystallographically highly coherent, the [001] directions of nanocrystals showing only a small misorientation with respect to one another; however, both parallel and twin assembly of neighbouring crystals occurred. An increase in the dissolved Mg concentration decreased the crystallographic coherence between the aragonite nanocrystals, suggesting an important role of Mg2+ ions in the assembly of aragonite spindles. Whereas the mesoscale-ordered arrangement of nanocrystals implies a particle-mediated assembly, the observed differences in particle size and composition between the amorphous precursor and the crystalline end-product suggest that the crystallization includes at least partial dissolution and re-precipitation. These findings provide insight into the formation of aragonite and could contribute to the understanding of important aspects of the formation of mesocrystals and hierarchically structured biogenic minerals.

Glass transition temperatures and crystallization kinetics of a synthetic, anhydrous, amorphous calcium-magnesium carbonate

We report the first calorimetric observations of glass transition temperatures and crystallization rates of anhydrous, amorphous calcium-magnesium carbonate using fast scanning differential scanning calorimetry. Hydrous amorphous Ca0.95Mg0.05CO3 · 0.5H2O (ACMC) solid was precipitated from a MgCl2-NaHCO3 buffered solution, separated from the supernatant, and freeze-dried. An aliquot of the freeze-dried samples was additionally dried at 250°C for up to 6 h in a furnace and in a high-purity N2 atmosphere to produce anhydrous ACMC. The glass transition temperature of the anhydrous Ca0.95Mg0.05CO3 was determined by applying different heating rates (1000-6000 K s-1) and correcting for thermal lag to be 376°C and the relaxational heat capacity was determined to be Cp = 0.16 J/(g K). Additionally, the heating rate dependence of the temperature that is associated with the corrected crystallization peaks is used to determine the activation energy of crystallization to be 275 kJ mol-1. A high-resolution transmission electron microscopy study on the hydrous and anhydrous samples provided further constraints on their compositional and structural states. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

CO2 Mineralization by MgO Nanocubes in Nanometric Water Films

Water films formed by the adhesion and condensation of air moisture on minerals can trigger the formation of secondary minerals of great importance to nature and technology. Magnesium carbonate growth on Mg-bearing minerals is not only of great interest for CO2 capture under enhanced weathering scenarios but is also a prime system for advancing key ideas on mineral formation under nanoconfinement. To help advance ideas on water film-mediated CO2 capture, we tracked the growth of amorphous magnesium carbonate (AMC) on MgO nanocubes exposed to moist CO2 gas. AMC was identified by its characteristic vibrational spectral signature and by its lack of long-range structure by X-ray diffraction. We find that AMC (MgCO3·2.3-2.5H2O) grew in sub-monolayer (ML) to 4 ML thick water films, with formation rates and yields scaling with humidity. AMC growth was however slowed down as AMC nanocoatings blocked water films access to the reactive MgO core. Films could however be partially dissolved by exposure to thicker water films, driving AMC growth for several more hours until nanocoatings blocked the reactions again. These findings shed new light on a potentially important bottleneck for the efficient mineralization of CO2 using MgO-bearing products. Notably, this study shows how variations in the air humidity affect CO2 capture by controlling water film coverages on reactive minerals. This process is also of great interest in the study of mineral growth in nanometrically thick water films.

Heat-Induced Dolomitization of Amorphous Calcium Magnesium Carbonate in a CO2-Filled Closed System

We report a method to synthesize dolomite [CaMg(CO₃)₂] from amorphous calcium magnesium carbonate (ACMC) via solid-state transformation. When ACMC is heated in air, it does not crystallize into dolomite but decomposes into Mg calcite, magnesium oxide, and CO₂. Hence, we heated ACMC in a closed system filled with CO₂ gas (pCO₂ >1.2 bar at 420 °C) and produced submicron-sized dolomite. Single-phase dolomite was obtained after dissolving impurities in the run products, such as northupite [Na₃Mg(CO₃)₂Cl] and eitelite [Na₂Mg(CO₃)₂], in water. Also, we investigated the crystallization process of dolomite by changing the heating temperature and heating time. Despite crystallization by solid-state transformation, the heated samples crystallized to dolomite via Ca-rich protodolomite with no ordering reflection of X-ray diffraction as previously observed for hydrothermal synthesis. The results demonstrated that this crystallization pathway is kinetically favored even in solid-state transformation and that the Ca-rich protodolomite phase preferentially crystallizes during heating, leading to phase separation from the amorphous phase. Therefore, the crystallization process via protodolomite as a precursor is a common mechanism in dolomite crystallization, suggesting the presence of kinetic barriers other than hydration of cations.

Molecular-scale mechanisms of CO2 mineralization in nanoscale interfacial water films

The calamitous impacts of unabated carbon emission from fossil-fuel-burning energy infrastructure call for accelerated development of large-scale CO₂ capture, utilization and storage technologies that are underpinned by a fundamental understanding of the chemical processes at a molecular level. In the subsurface, rocks rich in divalent metals can react with CO₂, permanently sequestering it in the form of stable metal carbonate minerals, with the CO₂-H₂O composition of the post-injection pore fluid acting as a primary control variable. In this Review, we discuss mechanistic reaction pathways for aqueous-mediated carbonation with carbon mineralization occurring in nanoscale adsorbed water films. In the extreme of pores filled with a CO₂-dominant fluid, carbonation reactions are confined to angstrom to nanometre-thick water films coating mineral surfaces, which enable metal cation release, transport, nucleation and crystallization of metal carbonate minerals. Although seemingly counterintuitive, laboratory studies have demonstrated facile carbonation rates in these low-water environments, for which a better mechanistic understanding has come to light in recent years. The overarching objective of this Review is to delineate the unique underlying molecular-scale reaction mechanisms that govern CO₂ mineralization in these reactive and dynamic quasi-2D interfaces. We highlight the importance of understanding unique properties in thin water films, such as how water dielectric properties, and consequently ion solvation and hydration behaviour, can change under nanoconfinement. We conclude by identifying important frontiers for future work and opportunities to exploit these fundamental chemical insights for decarbonization technologies in the twenty-first century.

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 2 to 3 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.

Patterns of Element Incorporation in Calcium Carbonate Biominerals Recapitulate Phylogeny for a Diverse Range of Marine Calcifiers

Elemental ratios in biogenic marine calcium carbonates are widely used in geobiology, environmental science, and paleoenvironmental reconstructions. It is generally accepted that the elemental abundance of biogenic marine carbonates reflects a combination of the abundance of that ion in seawater, the physical properties of seawater, the mineralogy of the biomineral, and the pathways and mechanisms of biomineralization. Here we report measurements of a suite of nine elemental ratios (Li/Ca, B/Ca, Na/Ca, Mg/Ca, Zn/Ca, Sr/Ca, Cd/Ca, Ba/Ca, and U/Ca) in 18 species of benthic marine invertebrates spanning a range of biogenic carbonate polymorph mineralogies (low-Mg calcite, high-Mg calcite, aragonite, mixed mineralogy) and of phyla (including Mollusca, Echinodermata, Arthropoda, Annelida, Cnidaria, Chlorophyta, and Rhodophyta) cultured at a single temperature (25°C) and a range of pCO₂ treatments (ca. 409, 606, 903, and 2856 ppm). This dataset was used to explore various controls over elemental partitioning in biogenic marine carbonates, including species-level and biomineralization-pathway-level controls, the influence of internal pH regulation compared to external pH changes, and biocalcification responses to changes in seawater carbonate chemistry. The dataset also enables exploration of broad scale phylogenetic patterns of elemental partitioning across calcifying species, exhibiting high phylogenetic signals estimated from both uni- and multivariate analyses of the elemental ratio data (univariate: λ = 0-0.889; multivariate: λ = 0.895-0.99). Comparing partial R ² values returned from non-phylogenetic and phylogenetic regression analyses echo the importance of and show that phylogeny explains the elemental ratio data 1.4-59 times better than mineralogy in five out of nine of the elements analyzed. Therefore, the strong associations between biomineral elemental chemistry and species relatedness suggests mechanistic controls over element incorporation rooted in the evolution of biomineralization mechanisms.

Malonate utilization by Pseudomonas aeruginosa affects quorum-sensing and virulence and leads to formation of mineralized biofilm-like structures

Pseudomonas aeruginosa is an opportunistic pathogen that uses malonate among its many carbon sources. We recently reported that, when grown in blood from trauma patients, P. aeruginosa expression of malonate utilization genes was upregulated. In this study, we explored the role of malonate utilization and its contribution to P. aeruginosa virulence. We grew P. aeruginosa strain PA14 in M9 minimal medium containing malonate (MM9) or glycerol (GM9) as a sole carbon source and assessed the effect of the growth on quorum sensing, virulence factors, and antibiotic resistance. Growth of PA14 in MM9, compared to GM9, reduced the production of elastases, rhamnolipids, and pyoverdine; enhanced the production of pyocyanin and catalase; and increased its sensitivity to norfloxacin. Growth in MM9 decreased extracellular levels of N-acylhomoserine lactone autoinducers, an effect likely associated with increased pH of the culture medium; but had little effect on extracellular levels of PQS. At 18 hr of growth in MM9, PA14 formed biofilm-like structures or aggregates that were associated with biomineralization, which was related to increased pH of the culture medium. These results suggest that malonate significantly impacts P. aeruginosa pathogenesis by influencing the quorum sensing systems, the production of virulence factors, biofilm formation, and antibiotic resistance.

The gammaproteobacterium Achromatium forms intracellular amorphous calcium carbonate and not (crystalline) calcite

Achromatium is a long known uncultured giant gammaproteobacterium forming intracellular CaCO₃ that impacts C and S geochemical cycles functioning in some anoxic sediments and at oxic-anoxic boundaries. While intracellular CaCO₃ granules have first been described as Ca oxalate then colloidal CaCO₃ more than one century ago, they have often been referred to as crystalline solids and more specifically calcite over the last 25 years. Such a crystallographic distinction is important since the respective chemical reactivities of amorphous calcium carbonate (ACC) and calcite, hence their potential physiological role and conditions of formation, are significantly different. Here, we analyzed the intracellular CaCO₃ granules of Achromatium cells from Lake Pavin using a combination of Raman microspectroscopy and scanning electron microscopy. Granules in intact Achromatium cells were unequivocally composed of ACC. Moreover, ACC spontaneously transformed into calcite when irradiated at high laser irradiance during Raman analyses. Few ACC granules also transformed spontaneously into calcite in lysed cells upon cell death and/or sample preparation. Overall, the present study supports the original claims that intracellular Ca-carbonates in Achromatium are amorphous and not crystalline. In that sense, Achromatium is similar to a diverse group of Cyanobacteria and a recently discovered magnetotactic alphaproteobacterium, which all form intracellular ACC. The implications for the physiology and ecology of Achromatium are discussed. Whether the mechanisms responsible for the preservation of such unstable compounds in these bacteria are similar to those involved in numerous ACC-forming eukaryotes remains to be discovered. Last, we recommend to future studies addressing the crystallinity of CaCO₃ granules in Achromatium cells recovered from diverse environments all over the world to take care of the potential pitfalls evidenced by the present study.

Effect of temperature on the transformation of amorphous calcium magnesium carbonate with near-dolomite stoichiometry into high Mg-calcite

Amorphous calcium magnesium carbonate (ACMC) transformation into high Mg-calcite (HMC) proceeds via dissolution and re-precipitation at the ACMC-solution interface.

Low temperature and limited water activity reveal a pathway to magnesite via amorphous magnesium carbonate

Forsterite carbonated in thin H2O films to magnesite via amorphous magnesium carbonate during reaction with H2O-bearing liquid CO2 at 25 °C. This novel reaction pathway contrasts with previous studies that were carried out at higher H2O activity and temperature, where more highly hydrated nesquehonite was the metastable intermediate.

Intracellular amorphous Ca-carbonate and magnetite biomineralization by a magnetotactic bacterium affiliated to the Alphaproteobacteria

Bacteria synthesize a wide range of intracellular submicrometer-sized inorganic precipitates of diverse chemical compositions and structures, called biominerals. Their occurrences, functions and ultrastructures are not yet fully described despite great advances in our knowledge of microbial diversity. Here, we report bacteria inhabiting the sediments and water column of the permanently stratified ferruginous Lake Pavin, that have the peculiarity to biomineralize both intracellular magnetic particles and calcium carbonate granules. Based on an ultrastructural characterization using transmission electron microscopy (TEM) and synchrotron-based scanning transmission X-ray microscopy (STXM), we showed that the calcium carbonate granules are amorphous and contained within membrane-delimited vesicles. Single-cell sorting, correlative fluorescent in situ hybridization (FISH), scanning electron microscopy (SEM) and molecular typing of populations inhabiting sediments affiliated these bacteria to a new genus of the Alphaproteobacteria. The partially assembled genome sequence of a representative isolate revealed an atypical structure of the magnetosome gene cluster while geochemical analyses indicate that calcium carbonate production is an active process that costs energy to the cell to maintain an environment suitable for their formation. This discovery further expands the diversity of organisms capable of intracellular Ca-carbonate biomineralization. If the role of such biomineralization is still unclear, cell behaviour suggests that it may participate to cell motility in aquatic habitats as magnetite biomineralization does.

A novel lurasidone hydrochloride–shikimic acid co-amorphous system formed by hydrogen-bonding interaction with the retained pH-dependent solubility behavior

The current study was aimed at investigating the lurasidone hydrochloride–shikimic acid co-amorphous system using a new type of organic acid.