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The speciation and mobility of rare earth elements (REE) strongly depends on pH, which controls the formation of charged aqueous hydroxyl species. The latter potentially play an important role in controlling heavy REE adsorption on clay minerals in near-neutral to alkaline waters such as in regolith-hosted REE mineral deposits. However, accurate REE hydrolysis constants are needed for developing geochemical models that can predict the role of these charged species in natural systems. Here, we develop a robust experimental UV-Vis spectrophotometric method using m-cresol purple to determine in situ pH from 25 to 75 °C. This method is used to derive the average ligand number and hydrolysis constants of erbium (Er) at 25 °C in aqueous solutions with low ionic strength (≤0.001 mol L−1) at pH from ∼7 to 9.5 and in the presence of Er concentrations from 0 to 0.057 mM. The average ligand number ranges between 1 and 3 indicating that Er(OH)2+, Er(OH)2+ and Er(OH) 03 control speciation in the experiments. The logarithm of the Er hydrolysis constants (, n = 1 to 3) derived at infinite dilution for the reaction Er3+ + nH2O = Er(OH)n3−n + nH+ are: , ,. Implementation of these experimental data into a geochemical model indicates that the Er(OH)2+ and Er(OH) 03 species are both stable in a much wider pH range than previously predicted. Consequently, the positively charged REE hydroxyl complexes can potentially control the fractionation of light vs. heavy REE via adsorption as observed in the formation of certain regolith-hosted REE deposits. [ABSTRACT FROM AUTHOR]

Rare earth elements (REE) include the lanthanides (La–Lu), Y, and Sc which are critical elements for the green energy transition. The REE show a decrease in ionic radii with increased atomic numbers, which results in a so-called lanthanide contraction systematically affecting crystal structures and mineral properties. Here we present a compilation of reference Raman spectra of ten REE sesquioxides (A-, B- and C-type), five REE hydroxides, eight xenotime-structured REE phosphate endmembers and two solid solutions, seven monazite-structured REE phosphate endmembers and two solid solutions and seven rhabdophane endmembers with up to five Ce1−xLREEx rhabdophane solid solutions (LREE = La–Gd). Raman mode assignment is based on a detailed literature review summarizing existing analytical work and theoretical calculations and systematic trends observed in this study by analyzing different REE-bearing solids. The wavenumbers of the main REE-O Raman band systematically increase with decreasing ionic radii forming discrete linear trends within isostructural mineral groups, that can be used to estimate the REE-O mode in other solids with known REE-O coordination numbers. Photoluminescence using 266 nm, 532 nm and 633 nm excitation laser wavelengths for REE-bearing oxides, hydroxides, anhydrous and hydrous phosphates is also presented providing a new framework for identifying REE-phases in phosphate-bearing natural mineral deposits. [ABSTRACT FROM AUTHOR]

Apatite is a common accessory phase in igneous and metamorphic rocks. Its stability in magmatic-hydrothermal and hydrothermal systems is known to be a key control on the mobility of rare earth elements (REE). To better constrain how apatite is altered during fluid-rock interaction at comparably low temperatures, batch-type apatite dissolution experiments were conducted at 150 and 250 °C at saturated water vapor pressure in acidic to mildly acidic (pH of 2–4) aqueous fluids having variable salinities (0, 0.5, and 5 wt% NaCl). The study reveals the dominance of apatite dissolution textures with the formation of micrometer-scale etch pits and dissolution channels developing prominently along the c-axis of the apatite crystals. Backscattered electron imaging shows an increase in apatite dissolution with increasing temperature and upon reacting the crystals with more acidic and higher salinity starting fluids. This study also demonstrates an increase in dissolved REE in the experimental fluids corroborating with the observed apatite dissolution behavior. Backscattered electron imaging of secondary minerals formed during apatite dissolution and scanning electron microscopy-based energy dispersive spectrometry peaks for Ca, P, and REE support the formation of monazite-(Ce) and minor secondary apatite as deduced from fluid chemistry (i.e., dissolved P and REE concentrations). The studied apatite reaction textures and chemistry of the reacted fluids both indicate that the mobility of REE is controlled by the dissolution of apatite coupled with precipitation of monazite-(Ce), which are enhanced by the addition of NaCl in the starting fluids. This coupled process can be traced by comparing the REE to P ratios in the reacted fluids with the stoichiometry of the unreacted apatite crystals. Apatite metasomatized at temperatures <300 °C is therefore controlled by dissolution rather than dissolution-reprecipitation reactions commonly observed in previous experiments conducted above 300 °C. Furthermore, this study demonstrates that the presence of NaCl plays a crucial role in increasing the solubility of apatite, which controls the availability of REE to form secondary phosphates even in mildly acidic aqueous fluids. This implies that both the effects of acidity/alkalinity of the fluids and the role of dissolved alkalis (NaCl and KCl), need to be considered for understanding the controls on REE in magmatic-hydrothermal systems. Lastly, the experiments of this study expand the known conditions at which apatite is susceptible to be overprinted by hydrothermal alteration from 900 °C down to 150 °C and highlights the necessity of appropriately screening apatite grains using backscattered electron and cathodoluminescence imaging for signs of hydrothermal alteration textures in igneous apatite. [ABSTRACT FROM AUTHOR]

Numerical simulations were carried out for determining the chemical reactions relevant for the sequestration of CO2 in basaltic rock formations. The mineralogy of natural geological systems consists of silicate minerals such as the phyllosilicates and zeolites that form complex solid solutions. Using the GEMS code package based on Gibbs energy minimization, combined with the new MINES database, we can now simulate the solubility of these multicomponent and multisite mineral solid solutions in basaltic rocks. This study explores the varying effects of CO2 partial pressures, basaltic glass dissolution kinetics and reaction time on the complex chemistry of the overall CO2-water-basalt reaction path. The simulations indicate four reaction progress stages with the competing reactions between smectites (di- and trioctahedral) and Ca-Fe- Mg-carbonates controlling the amount of CO2 mineralized. A better understanding of these key mineral-fluid reactions and improvement of their thermodynamic models is critical for making more acurate predictive calculations. This comprises the basis for extending the simulations to reactive transport models, and for the assessment of the feasibility of long-term CO2 storage in basaltic rock formations. [ABSTRACT FROM AUTHOR]

Abstract: The alteration of basaltic glass was studied experimentally at elevated CO2 pressures (~10–25bar) and hydrothermal conditions (75–250°C) to determine the effects of temperature and extent of reaction (ξ) on the secondary mineral formation and compositional evolution of alteration assemblages resulting from CO2–water–basalt interaction. At <100°C, the alteration products consisted of concentric layers of ankerite and dolomite–ankerite solid solutions (Ca–Mg–Fe carbonates) and amorphous silica. At ≥150°C, mixed Ca–Mg–Fe smectites and chlorite, calcite, amorphous silica and zeolites formed instead of Ca–Mg–Fe carbonates. Competing reactions between carbonates and clays for major divalent cations (Ca, Mg and Fe) were affected by the extent of reaction with smectites formed initially and progressively being replaced by calcite and chlorite. The basaltic glass dissolution rate and mechanism were also affected by temperature and reaction time. At lower temperatures (≤150°C), a hydrated leached layer formed on the glass surface and the mass fluxes in the system were largely controlled by the dissolution rate and mechanism of the glass, whereas at higher temperature (250°C) the dissolution rate of the basaltic glass was fast with the reactions primarily driven by secondary mineral replacement and growth. We conclude that the driving force for carbonate mineralization in basaltic glass at elevated CO2 conditions is linked to the geochemical behavior and mobility of Ca, Mg and Fe together with the availability of Si for Al–Si-mineral formation. At <100°C, the divalent cations are available for Ca–Mg–Fe carbonate formation, whereas at ≥150°C abundant clays limit the availability of Mg and Fe resulting in calcite being the only carbonate formed. This implies a quantitatively more effective fixation of CO2 at <100°C associated with the formation of Ca–Mg–Fe carbonates. [Copyright &y& Elsevier]

Abstract: CO2-water–basaltic glass batch experiments were performed in order to study the feasibility of low temperature CO2 sequestration into basalts including the key reactions and chemical mass transfer associated to progressive water–rock interaction. The experiments were carried out at 40°C for up to 260days with initial dissolved CO2 concentrations ranging from 24 to 305mmol/kg. Alteration minerals were identified on the basaltic glass grain surfaces and in the matrix, consisting of poorly crystalline Ca–Mg–Fe carbonates, Fe-hydroxides and/or oxy-hydroxides and Ca–Mg–Fe clays. Other cryptocrystalline phases were identified by variable amounts of Al, Si and Fe reflected by the secondary mineral compositions. The water chemistry was monitored during the experiments and was characterized by an increase in Si, Ca, Mg and Na with time, whereas Al, Fe and CO2 decreased. The dissolution of basaltic glass in CO2-rich waters was observed to be incongruent with the overall water composition and secondary mineralogy depending on reaction progress and pH. The pH was determined primarily by the initial CO2 concentration and its ionization constants, the amount of dissolved basaltic glass and by the mass and composition of secondary minerals formed. Initially, the pH increased rapidly from <4.5 to ∼4.5–6. Under these conditions, Mg and Ca were observed to be mobile and dissolved stoichiometrically relative to Na, whereas Si, Al and Fe were immobile. Upon quantitative CO2 mineralization, the pH increased to >6.5 and the mobility of most elements consequently decreased including Mg and Ca. The experimental results indicate that increased aqueous CO2 concentrations modify considerably the natural water–basalt reaction path. The mineralization of CO2 into carbonates was rapid and controlled by the initial CO2 concentration and rock to water ratio, with the composition of the carbonates depending on the availability of Ca, Mg and Fe and the oxidation state of Fe. At pH <5.5, Fe was in the ferrous oxidation state resulting in the formation of Fe-rich carbonates with the incorporation of Ca and Mg as well as the formation of Ca–(Mg)–Fe smectites. At pH >5.5, the mobility of Fe decreased due to oxidation of Fe(II) to Fe(III) and subsequent formation of ferrihydrite. At pH >6.5, the experimental solutions became progressively supersaturated with calcite, zeolites and Mg-rich clays limiting the mobility of Ca and Mg. The results indicate that reactions between clays (Ca–(Mg)–Fe smectites) and carbonates at pH <6.5, and zeolites, clays (Ca–Mg–Fe smectites) and carbonates at pH >6.5, control together the availability of Ca, Mg and Fe, playing a key role for low temperature CO2 mineralization and sequestration into mafic rocks. [Copyright &y& Elsevier]

Abstract: The interaction between CO2-rich waters and basaltic glass was studied using reaction path modeling in order to get insight into the water–rock reaction process including secondary mineral composition, water chemistry and mass transfer as a function of CO2 concentration and reaction progress (ξ). The calculations were carried out at 25–90°C and pCO2 to 30bars and the results were compared to recent experimental observations and natural systems. A thermodynamic dataset was compiled from 25 to 300°C in order to simulate mineral saturations relevant to basalt alteration in CO2-rich environment including revised key aqueous species for mineral dissolution reactions and apparent Gibbs energies for clay and carbonate solid solutions observed to form in nature. The dissolution of basaltic glass in CO2-rich waters was found to be incongruent with the overall water composition and secondary mineral formation depending on reaction progress and pH. Under mildly acid conditions in CO2 enriched waters (pH <6.5), SiO2 and simple Al–Si minerals, Ca–Mg–Fe smectites and Ca–Mg–Fe carbonates predominated. Iron, Al and Si were immobile whereas the Mg and Ca mobility depended on the mass of carbonate formed and water pH. Upon quantitative CO2 mineralization, the pH increased to >8 resulting in Ca–Mg–Fe smectite, zeolites and calcite formation, reducing the mobility of most dissolved elements. The dominant factor determining the reaction path of basalt alteration and the associated element mobility was the pH of the water. In turn, the pH value was determined by the concentration of CO2 and extent of reaction. The composition of the carbonates depended on the mobility of Ca, Mg and Fe. At pH <6.5, Fe was in the ferrous oxidation state resulting in the formation of Fe-rich carbonates with the incorporation of Ca and Mg. At pH >8, the mobility of Fe and Mg was limited due to the formation of clays whereas Ca was incorporated into calcite, zeolites and clays. Competing reactions between clays (Ca–Fe smectites) and carbonates at low pH, and zeolites and clays (Mg–Fe smectites) and carbonates at high pH, controlled the availability of Ca, Mg and Fe, playing a key role for low temperature CO2 mineralization and sequestration into basalts. Several problems of the present model point to the need of improvement in future work. The determinant factors linking time to low temperature reaction path modeling may not only be controlled by the primary dissolving phase, which presents challenges concerning non-stoichiometric dissolution, the leached layer model and reactive surface area, but may include secondary mineral precipitation kinetics as rate limiting step for specific reactions such as retrieved from the present reaction path study. [Copyright &y& Elsevier]