The impact of stoichiometry on the initial steps of crystal formation: Stability and lifetime of charged triple-ion complexes.

Minerals form in natural systems from solutions with varying ratios of their lattice ions, yet non-stoichiometric conditions have generally been overlooked in investigations of new formation (nucleation) of ionic crystals. Here, we investigated the influence of cation:anion ratio in the solution on the initial steps of nucleation by studying positively and negatively charged triple ion complexes and subsequent particle size evolution. Our model systems are carbonates and sulfates of calcium and barium, as it was recently shown that solution stoichiometry affects the timing and rate of their nucleation. Molecular dynamics (MD) simulations and dynamic light scattering (DLS) flow experiments show that nucleation correlates with the stability and lifetime of the initial complexes, which were significantly impacted by the cation:anion stoichiometry and ion type. Specifically, B a S O 4 2 2 - ${{\rm B}{\rm a}{\left({{\rm S}{\rm O}}_{4}\right)}_{2}^{2-}}$ was found to have higher association constants and its lifetime was twofold longer than B a 2 S O 4 2 + ${{{\rm B}{\rm a}}_{2}{{\rm S}{\rm O}}_{4}^{2+}}$ . Similar trends were observed for B a C O 3 ${{{\rm B}{\rm a}{\rm C}{\rm O}}_{3}}$ and C a S O 4 ${{{\rm C}{\rm a}{\rm S}{\rm O}}_{4}}$ . Contrastingly, for C a C O 3 ${{{\rm C}{\rm a}{\rm C}{\rm O}}_{3}}$ , C a C O 3 2 2 - ${{\rm C}{\rm a}{\left({{\rm C}{\rm O}}_{3}\right)}_{2}^{2-}}$ was found to have lower association constants and its lifetime was shorter than C a 2 C O 3 2 + ${{{\rm C}{\rm a}}_{2}{{\rm C}{\rm O}}_{3}^{2+}}$ . These trends in stability and lifetime follow the same asymmetrical behaviour as observed experimentally for particle formation using techniques like DLS. This suggests a causal relationship between the stability and lifetime of the initial charged complexes and the nucleation under non-stoichiometric conditions.

Impact of Solution {Ba2+}:{SO42-} on Charge Evolution of Forming and Growing Barite (BaSO4) Crystals: A ζ-Potential Measurement Investigation.

The impact of solution stoichiometry on formation of BaSO4 (barite) crystals and the development of surface charge was investigated at various predefined stoichiometries (raq = 0.01, 0.1, 1, 10, and 100, where raq = {Ba2+}:{SO42-}). Synthesis experiments and zeta potential (ζ-potential) measurements were conducted at a fixed initial degree of supersaturation (Ωbarite = 1000, where Ωbarite = {Ba2+}{SO42-}/Ksp), at circumneutral pH of ∼6, 0.02 M NaCl, and ambient temperature and pressure. Mixed-mode measurement-phase analysis light scattering (M3-PALS) showed that the particles stayed negative for raq < 1 during barite crystal formation and positive for raq > 1. At raq = 1, two populations with a positive or negative ζ-potential prevailed for ∼2.5 h before a population with a circumneutral ζ-potential (-10 to +10 mV) remained. We relate the observations of particle charge evolution to particle size and morphology evolution under the experimental conditions. Furthermore, we showed that the ζ-potential became more negative when the pH was increased for every raq. In addition, our results demonstrated that the type of monovalent background electrolyte did not influence the ζ-potential of barite crystals significantly, although NaCl showed slightly different behavior compared to KCl and NaNO3. Our results show the important role of surface charge (evolution) during ionic crystal formation under nonstoichiometric conditions. Moreover, our combined scanning electron microscopy and ζ-potential results imply that the surface charge during particle formation can be influenced by solution stoichiometry, besides the pH and ionic strength, and may aid in predicting the fate of barite in environmental settings and in understanding and improving industrial barite (surface chemistry) processes.

Asymmetrical Dependence of {Ba2+}:{SO4 2-} on BaSO4 Crystal Nucleation and Growth in Aqueous Solutions: A Dynamic Light Scattering Study.

The impact of solution stoichiometry, upon formation of BaSO4 crystals in 0.02 M NaCl suspensions, on the development of particle size was investigated using dynamic light scattering (DLS). Measurements were performed on a set of suspensions prepared with predefined initial supersaturation, based on the quotient of the constituent ion activity product {Ba2+}{SO4 2-} over the solubility product K sp (Ωbarite = {Ba2+}{SO4 2-}/K sp = 100, 500, or 1000-11,000 in steps of 1000), and ion activity solution stoichiometries (r aq = {Ba2+}:{SO4 2-} = 0.01, 0.1, 1, 10 and 100), at circumneutral pH of 5.5-6.0, and ambient temperature and pressure. DLS showed that for batch experiments, crystal formation with varying r aq was best investigated at an initial Ωbarite of 1000 and using the forward detection angle. At this Ωbarite and set of r aq, the average apparent hydrodynamic particle size of the largest population present in all suspensions increased from ∼200 to ∼700 nm within 10-15 min and was independently confirmed by transmission electron microscopy (TEM) imaging. Additional DLS measurements conducted at the same conditions in flow confirmed that the BaSO4 formation kinetics were very fast for our specifically chosen conditions. The DLS flow measurements, monitoring the first minute of BaSO4 formation, showed strong signs of aggregation of prenucleation clusters forming particles with a size in the range of 200-300 nm for every r aq. The estimated initial bulk growth rates from batch DLS results show that BaSO4 crystals formed fastest at near-stoichiometric conditions and more slowly at nonstoichiometric conditions. Moreover, at extreme SO4-limiting conditions, barite formation was slower compared to Ba-limiting conditions. Our results show that DLS can be used to investigate nucleation and growth at carefully selected experimental and analytical conditions. The combined DLS and TEM results imply that BaSO4 formation is influenced by solution stoichiometry and may aid to optimize antiscalant efficiency and regulate BaSO4 (scale) formation processes.

Recalibrating the calcium trap in amino acid carboxyl groups via classical molecular dynamics simulations.

In order to use classical molecular dynamics to complement experiments accurately, it is important to use robust descriptions of the system. The interactions between biomolecules, like aspartic and glutamic acid, and dissolved ions are often studied using standard biomolecular force-fields, where the interactions between biomolecules and cations are often not parameterized explicitly. In this study, we have employed metadynamics simulations to investigate different interactions of Ca with aspartic and glutamic acid and constructed the free energy profiles of Ca2+-carboxylate association. Starting from a generally accepted, AMBER-based force field, the association was substantially over and under-estimated, depending on the choice of water model (TIP3P and SPC/fw, respectively). To rectify this discrepancy, we have replaced the default calcium parameters. Additionally, we modified the σij value in the hetero-atomic Lennard-Jones interaction by 0.5% to further improve the interaction between Ca and carboxylate, based on comparison with the experimentally determined association constant for Ca with the carboxylate group of L-aspartic acid. The corrected description retrieved the structural properties of the ion pair in agreement with the original biomolecule - Ca2+ interaction in AMBER, whilst also producing an association constant comparable to experimental observations. This refined force field was then used to investigate the interactions between amino acids, calcium and carbonate ions during biogenic and biomimetic calcium carbonate mineralisation.

Enhanced phosphorus recycling during past oceanic anoxia amplified by low rates of apatite authigenesis.

Enhanced recycling of phosphorus as ocean deoxygenation expanded under past greenhouse climates contributed to widespread organic carbon burial and drawdown of atmospheric CO2. Redox-dependent phosphorus recycling was more efficient in such ancient anoxic marine environments, compared to modern anoxic settings, for reasons that remain unclear. Here, we show that low rates of apatite authigenesis in organic-rich sediments can explain the amplified phosphorus recycling in ancient settings as reflected in highly elevated ratios of organic carbon to total phosphorus. We argue that the low rates may be partly the result of the reduced saturation state of sediment porewaters with respect to apatite linked to ocean warming and acidification and/or a decreased availability of calcium carbonate, which acts as a template for apatite formation. Future changes in temperature and ocean biogeochemistry, induced by elevated atmospheric CO2, may similarly increase phosphorus availability and accelerate ocean deoxygenation and organic carbon burial.

Experimental and Theoretical Constraints on Amino Acid Formation from PAHs in Asteroidal Settings.

Amino acids and polycyclic aromatic hydrocarbons (PAHs) belong to the range of organic compounds detected in meteorites. In this study, we tested empirically and theoretically if PAHs are precursors for amino acids in carbonaceous chondrites, as previously suggested. We conducted experiments to synthesize amino acids from fluoranthene (PAH), with ammonium bicarbonate as a source for ammonia and carbon dioxide under mimicked asteroidal conditions. In our thermodynamic calculations, we extended our analysis to additional PAH-amino acid combinations. We explored 36 reactions involving the PAHs naphthalene, anthracene, fluoranthene, pyrene, triphenylene, and coronene and the amino acids glycine, alanine, valine, leucine, phenylalanine, and tyrosine. Our experiments do not show the formation of amino acids, whereas our theoretical results hint that PAHs could be precursors of amino acids in carbonaceous chondrites at low temperatures.

Aragonite dissolution protects calcite at the seafloor.

In the open ocean, calcium carbonates are mainly found in two mineral forms. Calcite, the least soluble, is widespread at the seafloor, while aragonite, the more soluble, is rarely preserved in marine sediments. Despite its greater solubility, research has shown that aragonite, whose contribution to global pelagic calcification could be at par with that of calcite, is able to reach the deep-ocean. If large quantities of aragonite settle and dissolve at the seafloor, this represents a large source of alkalinity that buffers the deep ocean and favours the preservation of less soluble calcite, acting as a deep-sea, carbonate version of galvanization. Here, we investigate the role of aragonite dissolution on the early diagenesis of calcite-rich sediments using a novel 3D, micrometric-scale reactive-transport model combined with 3D, X-ray tomography structures of natural aragonite and calcite shells. Results highlight the important role of diffusive transport in benthic calcium carbonate dissolution, in agreement with recent work. We show that, locally, aragonite fluxes to the seafloor could be sufficient to suppress calcite dissolution in the top layer of the seabed, possibly causing calcite recrystallization. As aragonite producers are particularly vulnerable to ocean acidification, the proposed galvanizing effect of aragonite could be weakened in the future, and calcite dissolution at the sediment-water interface will have to cover a greater share of CO2 neutralization.

Magnetic structure and exchange interactions in pyrrhotite end member minerals: hexagonal FeS and monoclinic Fe7S8.

Iron mono-sulphides, or pyrrhotites, are minerals present in the Earth's crust and mantle as well as major magnetic constituents of several classes of meteorites, thus are of interest to a wide range of disciplines including geology, geophysics, geochemistry, and material science. Despite displaying diverse magnetic properties as a result of iron vacancy ordering, the underlying exchange mechanism has not been quantified. This study presents an examination of the electronic and magnetic properties for the two pyrrhotite group end members, hexagonal FeS and monoclinic Fe7S8(4C superstructure) by means of density functional theory coupled with a Heisenberg magnetic model. The easy magnetization axes of FeS and Fe7S8are found to be positioned along the crystallographicc-direction and at an angle of 56° to thec-direction, respectively. The magnetic anisotropy energy in Fe7S8is greatly increased as a consequence of the vacancy framework when compared to FeS. The main magnetic interaction, in both compounds, is found to be the isotropic exchange interaction favouring antiferromagnetic alignment between nearest-neighbouring spins. The origin of the exchange interaction is elucidated further following the Goodenough-Kanamori-Anderson rules. The antisymmetric spin exchange is found to have a minor effect in both compounds. The theoretical findings presented in this work thus help to further resolve some of the ambiguities in the magnetic features of pyrrhotites.

Controlling CaCO3 Particle Size with {Ca2+}:{CO3 2-} Ratios in Aqueous Environments.

The effect of stoichiometry on the new formation and subsequent growth of CaCO3 was investigated over a large range of solution stoichiometries (10-4 < r aq < 104, where r aq = {Ca2+}:{CO3 2-}) at various, initially constant degrees of supersaturation (30 < Ωcal < 200, where Ωcal = {Ca2+}{CO3 2-}/K sp), pH of 10.5 ± 0.27, and ambient temperature and pressure. At r aq = 1 and Ωcal < 150, dynamic light scattering (DLS) showed that ion adsorption onto nuclei (1-10 nm) was the dominant mechanism. At higher supersaturation levels, no continuum of particle sizes is observed with time, suggesting aggregation of prenucleation clusters into larger particles as the dominant growth mechanism. At r aq ≠ 1 (Ωcal = 100), prenucleation particles remained smaller than 10 nm for up to 15 h. Cross-polarized light in optical light microscopy was used to measure the time needed for new particle formation and growth to at least 20 µm. This precipitation time depends strongly and asymmetrically on r aq. Complementary molecular dynamics (MD) simulations confirm that r aq affects CaCO3 nanoparticle formation substantially. At r aq = 1 and Ωcal ≫ 1000, the largest nanoparticle in the system had a 21-68% larger gyration radius after 20 ns of simulation time than in nonstoichiometric systems. Our results imply that, besides Ωcal, stoichiometry affects particle size, persistence, growth time, and ripening time toward micrometer-sized crystals. Our results may help us to improve the understanding, prediction, and formation of CaCO3 in geological, industrial, and geo-engineering settings.

Reconsidering Calcium Dehydration as the Rate-Determining Step in Calcium Mineral Growth.

The dehydration of cations is generally accepted as the rate-limiting step in many processes. Molecular dynamics (MD) can be used to investigate the dynamics of water molecules around cations, and two different methods exist to obtain trajectory-based water dehydration frequencies. Here, these two different post-processing methods (direct method versus survival function) have been implemented to obtain calcium dehydration frequencies from a series of trajectories obtained using a range of accepted force fields. None of the method combinations reproduced the commonly accepted experimental water exchange frequency of 10-8.2 s-1. Instead, our results suggest much faster water dynamics, comparable with more accurate ab initio MD simulations and with experimental values obtained using neutron scattering techniques. We obtained the best agreement using the survival function method to characterize the water dynamics, and we show that different method combinations significantly affect the outcome. Our work strongly suggests that the fast water exchange kinetics around the calcium ions is not rate-limiting for reactions involving dissolved/solvated calcium. Our results further suggest that, for alkali and most of the earth alkali metals, mechanistic rate laws for growth, dissolution, and adsorption, which are based on the principle of rate-limiting cation dehydration, need careful reconsideration.

Coupled dynamics of iron and phosphorus in sediments of an oligotrophic coastal basin and the impact of anaerobic oxidation of methane.

Studies of phosphorus (P) dynamics in surface sediments of lakes and coastal seas typically emphasize the role of coupled iron (Fe), sulfur (S) and P cycling for sediment P burial and release. Here, we show that anaerobic oxidation of methane (AOM) also may impact sediment P cycling in such systems. Using porewater and sediment profiles for sites in an oligotrophic coastal basin (Bothnian Sea), we provide evidence for the formation of Fe-bound P (possibly vivianite; Fe3(PO4)2(·)8H2O) below the zone of AOM with sulfate. Here, dissolved Fe(2+) released from oxides is no longer scavenged by sulfide and high concentrations of both dissolved Fe(2+) (>1 mM) and PO4 in the porewater allow supersaturation with respect to vivianite to be reached. Besides formation of Fe(II)-P, preservation of Fe-oxide bound P likely also contributes to permanent burial of P in Bothnian Sea sediments. Preliminary budget calculations suggest that the burial of Fe-bound P allows these sediments to act as a major sink for P from the adjacent eutrophic Baltic Proper.

Mg/Ca partitioning between aqueous solution and aragonite mineral: a molecular dynamics study.

We have calculated the concentrations of Mg in the bulk and surfaces of aragonite CaCO(3) in equilibrium with aqueous solution, based on molecular dynamics simulations and grand-canonical statistical mechanics. Mg is incorporated in the surfaces, in particular in the (001) terraces, rather than in the bulk of aragonite particles. However, the total Mg content in the bulk and surface of aragonite particles was found to be too small to account for the measured Mg/Ca ratios in corals. We therefore argue that most Mg in corals is either highly metastable in the aragonite lattice, or is located outside the aragonite phase of the coral skeleton, and we discuss the implications of this finding for Mg/Ca paleothermometry.

Adsorption of arsenite and arsenate onto muscovite and biotite mica.

Arsenite and arsenate sorption was studied on two silt-sized phyllosilicates, namely muscovite and biotite, as a function of solution pH (pH 3-8 for muscovite, and 3-11 for biotite) at an initial As concentration of 13 microM. The amount of arsenic adsorbed increases with increasing pH, exhibiting a maximum value, before decreasing at higher pH values. Maxima correspond to 3.22+/-0.06 mmol kg-1 As(V) at pH 4.6-5.6 and 2.86+/-0.05 mmol kg-1 As(III) at pH 4.1-6.2 for biotite, and 3.08+/-0.06 mmolkg-1 As(III) and 3.13+/-0.05 mmol kg-1 As(V) at pH 4.2-5.5 for muscovite. The constant capacitance surface complexation model was used to explain the adsorption behavior. Biotite provides greater reactivity than muscovite toward arsenic adsorption. Isotherm data obeyed the Freundlich or Langmuir equation for the arsenic concentration range 10(-7)-10(-4) M. Released total Fe, Si, K, Al, and Mg in solution were analyzed. Calculation of saturation indices by PHREEQC indicated that the solution was undersaturated with respect to aluminum arsenate (AlAsO42H2O), scorodite (FeAsO42H2O), and claudetite/arsenolite (As4O6).