Chemical Textures on Rare Earth Carbonates: An Experimental Approach to Mimic the Formation of Bastnäsite.

The interaction between multi-component rare earth element (REE) aqueous solutions and carbonate grains (dolomite, aragonite, and calcite) are studied at hydrothermal conditions (21-210 °C). The effect of ionic radii of five REEs (La, Ce, Pr, Nd, Dy) on solid formation are analyzed using two solution types: equal REE concentrations and concentrations normalized to Post Archean Australian Shale Standard (PAAS). The interaction replaces the host Ca-Mg carbonate grains with a series of REE minerals (lanthanite → kozoite → bastnäsite → cerianite). At 165 °C, equal concentration solutions promote kozoite crystallization, maintaining similar REE ratios in solids and solution. PAAS solutions result in zoned REE-bearing crystals with heterogeneous elemental distributions and discreet REE phases (e.g., cerianite). Chemical signatures indicate metastable REE-bearing phases transforming into more stable polymorphs, along with symplectite textures formed by adjacent phase reactions. Overall, experiments highlight the dependence of polymorph selection, crystallization pathway, mineral formation kinetics, and chemical texture on REE concentrations, ionic radii, temperature, time, and host grain solubility.

The role of fluocerite in the genesis of bastnäsite: mechanistic insights and transformation pathways.

Fluocerite is a rare earth element (REE) fluoride found as an accessory mineral in magmatic-hydrothermal REE ore deposits, including alkaline complexes and carbonatites, where it is often associated with REE-fluorocarbonates. This study investigates the crystallisation kinetics, mechanisms and energetics of fluocerite (REEF3) and its role as a precursor phase of bastnäsite, one of the key minerals used for the extraction of REE. Fluocerite was synthesized by reacting pure fluorite (CaF2) with La-, Ce- and Nd-bearing solutions at temperatures ranging from ambient to low hydrothermal (30-90 °C). The synthetic fluocerites were then placed in contact with Na2CO3 solutions at temperatures up to 200 °C. A combined approach using powder X-ray diffraction and scanning electron microscopy with energy dispersive spectroscopy was used to determine the nature and quantify the crystallising solids. Our findings reveal a temperature-dependent fluorite-fluocerite transformation, with completion observed within 1 hour at 90 °C and extending to around 30 days at 30 °C. The rate of crystallisation decreases proportionally with the atomic number of the rare earth elements. On the other hand, the carbonation reaction of fluocerite exhibits a significantly slower rate, by ∼3 orders of magnitude, in comparison to the fluorite-fluocerite transformation, regardless of temperature conditions. The synthetic La, Ce and Nd fluocerites transformed into bastnäsite at all temperatures, also forming cerianite (CeO2) in the Ce-bearing experiments and metastable kozoite, NdCO3OH(orth), in the Nd-bearing experiments. Activation energies of fluocerite nucleation increase proportionally with the ionic radii of the REE (81 ± 6 (La); 84 ± 5 (Ce), 96 ± 10 (Nd) kJ mol-1), while the activation energies associated with fluocerite crystallisation are slightly higher for La (90 ± 12 kJ mol-1) and similar for Ce and Nd (76 ± 12 and 72 ± 8 kJ mol-1, respectively).

Utilization of Eggshell Waste Calcite as a Sorbent for Rare Earth Element Recovery.

The green energy transition requires rare earth elements (REE) for the permanent magnets used in electric cars and wind turbines. REE extraction and beneficiation are chemically intensive and highly damaging to the environment. We investigated the use of eggshell waste as a sustainable alternative sorbent for the capture and separation of REE from aqueous solutions. Hen eggshell calcite was placed in multi-REE (La, Nd, Dy) solutions at 25 to 205 °C for up to 3 months. A pervasive diffusion of the REE inside the eggshell calcite was observed along pathways formed by the intracrystalline organic matrix and calcite crystal boundaries. At 90 °C, kozoite (REECO3OH, orthorhombic) spherulites precipitate on the surface of the dissolving calcite. At 165 and 205 °C, an interface-coupled dissolution-precipitation mechanism is observed, resulting in the complete dissolution of the calcite shell and its pseudomorphic replacement by polycrystalline kozoite. At 205 °C, kozoite is slowly replaced by hydroxylbastnäsite (REECO3OH, hexagonal), the stable form of the rare earth hydroxycarbonate polymorphs. Our results demonstrate two potential applications of eggshell waste for the uptake of rare earth elements in solution: at low temperatures, as a mixed organic-inorganic adsorbent and absorbent, given sufficient sorption time; and at higher temperatures, as an efficient sacrificial template for the precipitation of rare earth hydroxycarbonates.

Impact of Rare Earth Elements on CaCO3 Crystallization: Insights into Kinetics, Mechanisms, and Crystal Morphology.

This study investigated the crystallization kinetics and mechanisms of calcium carbonate (CaCO3) in the presence of rare earth elements (REEs) including lanthanum (La), neodymium (Nd), and dysprosium (Dy). Through a comprehensive approach utilizing UV-vis spectrophotometry, powder X-ray diffraction, and high-resolution electron microscopy, we examined the effects of REEs on CaCO3 growth from solution at varying concentrations and combinations of REEs. Our findings highlight that even trace amounts of REEs significantly decelerate the rate of CaCO3 crystallization, also leading to alterations in crystal morphology and mechanisms of growth. The impact of REEs becomes more pronounced at higher concentrations and atomic mass, although the potential formation of poorly ordered REEs carbonate precursor phases can result in a decrease in the REE3+/Ca2+ ratio, influencing the crystallization rate of CaCO3. Vaterite and calcite were identified as the main crystallized polymorphs, with vaterite exhibiting distinct growth defects and calcite developing complex morphologies at higher REEs concentrations and an internal architecture suggesting a nonclassical growth route. We propose that REEs ions selectively adsorb onto different calcite surfaces, impeding growth on specific sites and resulting in intricate morphologies.

The role of nanocerianite (CeO2) in the stability of Ce carbonates at low-hydrothermal conditions.

The formation of cerianite (CeO2) was investigated at low hydrothermal conditions (35-205 °C) via two experimental settings: (1) crystallisation from solution experiments, and (2) replacement of Ca-Mg carbonates (calcite, dolomite, aragonite) mediated by Ce-bearing aqueous solutions. The solid samples were studied with a combination of powder X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy. The results revealed a multi-step crystallisation pathway: amorphous Ce carbonate → Ce-lanthanite [Ce2(CO3)3·8H2O] → Ce-kozoite [orthorhombic CeCO3(OH)] → Ce-hydroxylbastnasite [hexagonal CeCO3(OH)] → cerianite [CeO2]. We found that Ce carbonates can decarbonise in the final stage of the reaction, forming cerianite which significantly increases the porosity of the solids. The redox behaviour of Ce combined with the temperature, and the availability of CO2 3- govern this crystallisation sequence, the sizes, morphologies, and crystallisation mechanisms of the solid phases. Our results explain the occurrence and behaviour of cerianite in natural deposits. These findings also present a simple, environmental-friendly, and cost-efficient method for the synthesis of Ce carbonates and cerianite with tailored structures and chemistries.

Targeted Crystallization of Rare Earth Carbonate Polymorphs at Hydrothermal Conditions via Mineral Replacement Reactions.

The interaction between rare earth element (REE)-rich (La, Pr, Nd, Dy) aqueous solutions, dolomite (CaMg(CO3)2), and aragonite (CaCO3) at low temperature hydrothermal conditions (25-220 °C) is studied. The experiments result in the solvent-mediated surface precipitation and subsequent pseudomorphic mineral replacement of the dolomite and aragonite seeds by newly formed REE-carbonates. The host grains are replaced from periphery inward. The newly formed REE-bearing carbonates in La-, Pr-, and Nd-doped systems follow the crystallization sequence: lanthanite [REE2(CO3)3·8H2O] → kozoite [orthorhombic REECO3(OH)] → hydroxylbastnasite [hexagonal REECO3(OH)]. The interaction of Dy-bearing solutions with dolomite results only in the crystallization of kozoite [orthorhombic DyCO3(OH)]. However, experiments with aragonite reveal a two-step crystallization pathway: tengerite [Dy2(CO3)3·2-3(H2O)] → kozoite [orthorhombic DyCO3(OH)]. The temperature, the dissolution rate of the host mineral, and the ionic radii of the REE3+ in question are found to control the kinetics of the replacement reaction, the polymorph selection, and the crystallization pathways toward bastnasite. The findings allow to gain a more in-depth understanding of the formation REE-bearing carbonates, particularly the mineral bastnasite, which is the main source of REEs for industry. This knowledge can be used to improve REE separation, exploration, exploitation methods, as well to produce carbonate minerals with tailored structures.

Organic synthesis on Mars by electrochemical reduction of CO2.

The sources and nature of organic carbon on Mars have been a subject of intense research. Steele et al. (2012) showed that 10 martian meteorites contain macromolecular carbon phases contained within pyroxene- and olivine-hosted melt inclusions. Here, we show that martian meteorites Tissint, Nakhla, and NWA 1950 have an inventory of organic carbon species associated with fluid-mineral reactions that are remarkably consistent with those detected by the Mars Science Laboratory (MSL) mission. We advance the hypothesis that interactions among spinel-group minerals, sulfides, and a brine enable the electrochemical reduction of aqueous CO2 to organic molecules. Although documented here in martian samples, a similar process likely occurs wherever igneous rocks containing spinel-group minerals and/or sulfides encounter brines.

The role of amorphous precursors in the crystallization of La and Nd carbonates.

Crystalline La and Nd carbonates can be formed from poorly-ordered nanoparticulate precursors, termed amorphous lanthanum carbonate (ALC) and amorphous neodymium carbonate (ANC). When reacted in air or in aqueous solutions these precursors show highly variable lifetimes and crystallization pathways. We have characterized these precursors and the crystallization pathways and products with solid-state, spectroscopic and microscopic techniques to explain the differences in crystallization mechanisms between the La and Nd systems. ALC and ANC consist of highly hydrated, 10-20 nm spherical nanoparticles with a general formula of REE2(CO3)3·5H2O (REE = La, Nd). The stabilities differ by ∼2 orders of magnitude, with ANC being far more stable than ALC. This difference is due to the Nd(3+) ion having a far higher hydration energy compared to the La(3+) ion. This, together with temperature and reaction times, leads to clear differences not only in the kinetics and mechanisms of crystallization of the amorphous precursor La- and Nd-carbonate phases but also in the resulting crystallite sizes and morphologies of the end products. All crystalline La and Nd carbonates developed spherulitic morphologies when crystallization occurred from hydrous phases in solution at temperatures above 60 °C (La system) and 95 °C (Nd system). We suggest that spherulitic growth occurs due to a rapid breakdown of the amorphous precursors and a concurrent rapid increase in supersaturation levels in the aqueous solution. The kinetic data show that the crystallization pathway for both La and Nd carbonate systems is dependent on the reaction temperature and the ionic potential of the REE(3+) ion.

The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, via vaterite.

The kinetics and mechanisms of nanoparticulate amorphous calcium carbonate (ACC) crystallization to calcite, via vaterite, were studied at a range of environmentally relevant temperatures (7.5-25 °C) using synchrotron-based in situ time-resolved Energy Dispersive X-ray Diffraction (ED-XRD) in conjunction with high-resolution electron microscopy, ex situ X-ray diffraction and infrared spectroscopy. The crystallization process occurs in two stages; firstly, the particles of ACC rapidly dehydrate and crystallize to form individual particles of vaterite; secondly, the vaterite transforms to calcite via a dissolution and reprecipitation mechanism with the reaction rate controlled by the surface area of calcite. The second stage of the reaction is approximately 10 times slower than the first. Activation energies of calcite nucleation and crystallization are 73±10 and 66±2 kJ mol(-1), respectively. A model to calculate the degree of calcite crystallization from ACC at environmentally relevant temperatures (7.5-40 °C) is also presented.

Removal of cadmium from wastewaters by aragonite shells and the influence of other divalent cations.

The effect of dissolved Zn, Co, Pb, Mg, and Ca on the uptake of cadmium by biogenic aragonite was investigated. Experiments were performed in batch-reactors using metal-cadmium-bearing solutions and shell fragments with diameters in different ranges, the solid/liquid ratio being 10 grams per liter. Different initial concentrations of cadmium and metals (1.0-0.005 mM) were used. Uptake takes place via heterogeneous nucleation of metal-bearing crystallites onto the shell surfaces. Cadmium removal occurs by surface precipitation of otavite. Under the conditions used here, Co and Ca as well as Pb < or = 0.3 mM and Zn < or = 0.3 mM do not have a significant effect on the removal of cadmium. At higher concentrations, Pb and Zn outcompete Cd for the dissolving carbonate ions and thus decrease significantly the Cd removal rates. In contrast, Mg has a slight enhancing effect. Pb and Zn are removed faster than Cd, precipitating as PbCO3, Pb3(CO3)2(OH)2, and Zn5(CO3)2(OH)6. Within 24-72 h, the concentrations of lead, cadmium, and zinc decrease until approximately 0.5 microM, and the presence of aragonite buffers the solution to a pH above 8 avoiding redissolution. The study demonstrates the high effectiveness of biogenic aragonite in removing Cd and other metals from polluted waters.