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.

Many organisms use amorphous calcium carbonate (ACC) during crystalline calcium carbonate biomineralization, as a means to control particle shape/size and phase stability. Here, we present an in situ small-and wide-angle X-ray scattering (SAXS/WAXS) study of the mechanisms and kinetics of ACC crystallization at rapid time scales (seconds). Combined with offline solid and solution characterization, we show that ACC crystallizes to vaterite via a threestage process. First, hydrated and disordered ACC forms, then rapidly transforms to more ordered and dehydrated ACC; in conjunction with this, vaterite forms via a spherulitic growth mechanism. Second, when the supersaturation of the solution with respect to vaterite decreases sufficiently, the mechanism changes to ACC dissolution and vaterite crystal growth. The third stage is controlled by Ostwald ripening of the vaterite particles. Combining this information with previous studies, allowed us to develop a mechanistic understanding of the abiotic crystallization process from ACC to vaterite and all the way to calcite. We propose this is the underlying abiotic mechanism for calcium carbonate biomineralization from ACC. This process is then augmented or altered by organisms (e.g., using organic compounds) to form intricate biominerals. This study also highlights the applicability of in situ time-resolved SAXS/WAXS to study rapid crystallization reactions.

Calcium sulfate minerals such as gypsum play important roles in natural and industrial processes, but their precipitation mechanisms remain largely unexplored. We used time-resolved sample quenching and high-resolution microscopy to demonstrate that gypsum forms via a three-stage process: (i) homogeneous precipitation of nanocrystalline hemihydrate bassanite below its predicted solubility, (ii) self-assembly of bassanite into elongated aggregates co-oriented along their c axis, and (iii) transformation into dihydrate gypsum. These findings indicate that a stable nanocrystalline precursor phase can form below its bulk solubility and that in the CaSO(4) system, the self-assembly of nanoparticles plays a crucial role. Understanding why bassanite forms prior to gypsum can lead to more efficient anti-scaling strategies for water desalination and may help to explain the persistence of CaSO(4) phases in regions of low water activity on Mars.

The formation pathways of gypsum remain uncertain. Here, using truly in situ and fast time-resolved small-angle X-ray scattering, we quantify the four-stage solution-based nucleation and growth of gypsum (CaSO4·2H2O), an important mineral phase on Earth and Mars. The reaction starts through the fast formation of well-defined, primary species of <3 nm in length (stage I), followed in stage II by their arrangement into domains. The variations in volume fractions and electron densities suggest that these fast forming primary species contain Ca–SO4-cores that self-assemble in stage III into large aggregates. Within the aggregates these well-defined primary species start to grow (stage IV), and fully crystalize into gypsum through a structural rearrangement. Our results allow for a quantitative understanding of how natural calcium sulfate deposits may form on Earth and how a terrestrially unstable phase-like bassanite can persist at low-water activities currently dominating the surface of Mars.

The presence of alcohol in binary alcohol− water mixtures can affect the precipitation pathways of anhydrous crystalline CaCO 3 polymorphs and their morphology. We explored the formation pathways and the effects of several parameters on calcite, vaterite, and aragonite: concentration of simple alcohols, time, and shaking speed, and we derived a multiparameter model for predicting what phase is preferred. We found that shaking speed and alcohol concentration are the most important parameters for affecting the stability of vaterite and aragonite and for changing vaterite morphology, from cauliflower-shaped, spherical aggregates, to dendritic, flatter structures. In all our experiments, the precipitated aragonite was twinned, and both the vaterite and aragonite can be interpreted to form through spherulitic growth. Classical growth theory fully describes their formation; there is no need to invoke the popular hypothesis for nonclassical growth by self-assembly of nanocrystals. These studies, and future work with solutions of low water activity, are paving the way to a better understanding of how organisms select their preferred polymorph and engineer CaCO 3 morphology during biomineralization.

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Monohydrocalcite is a member of the carbonate family which forms in Mg-rich environments at a wide range of Mg Ca ratios Mg aq Ca aq Although found in modern sedimentary deposits and as a product of biomineralization, there is a lack of information about its formation mechanisms and about the role of Mg during its crystallization. In this work we have quantitatively assessed the mechanism of crystallization of monohydrocalcite through in situ synchrotron-based small and wide angle X-ray scattering (SAXS/WAXS) and off-line spectroscopic, microscopic and wet chemical analyses. Monohydrocalcite crystallizes via a 4-stage process beginning with highly supersaturated solutions from which a Mg-bearing, amorphous calcium carbonate (ACC) precursor precipitates. This precursor crystallizes to monohydrocalcite via a nucleation-controlled reaction in stage two, while in stage three it is further aged through Ostwald-ripening at a rate of 1.8±0.1 nm/h1/2. In stage four, a secondary Ostwald ripening process (66.3±4.3 nm/h1/2) coincides with the release of Mg from the monohydrocalcite structure and the concomitant formation of minor hydromagnesite. Our data reveal that monohydrocalcite can accommodate significant amounts of Mg in its structure MgCO and that its Mg content and dehydration temperature are directly proportional to the saturation index for monohydrocalcite (SIMHC) immediately after mixing the stock solutions. However, its crystallite and particle size are inversely proportional to these parameters. At high supersaturations (SIMHC=3.89) nanometer-sized single crystals of monohydrocalcite form, while at low values (SIMHC=2.43) the process leads to low-angle branching spherulites. Many carbonates produced during biomineralization form at similar conditions to most synthetic monohydrocalcites, and thus we hypothesize that some calcite or aragonite deposits found in the geologic record that have formed at high Mg/Ca ratios could be secondary in origin and may have originally formed via a metastable monohydrocalcite intermediate. This manuscript describes an experimental study in which we elucidated the formation mechanism of monohydrocalcite from a poorly-ordered precursor and the role of Mg in its crystallization. Combining in situ synchrotron-based with various off-line laboratory characterizations allowed us to derive complementary quantitative data that explain the monohydrocalcite crystallization via a multiple stage process. School of Earth and EnvironmentWe believe that our paper is of interest to a broad geochemical community and that our results may help explain a number of important biogeochemical processes (including biomineralization and their link to past variations in ocean chemistry).All authors have read and accepted the manuscript in its current format and we all confirm that this paper represents original work from which no part has been published, nor is being considered for publication, elsewhere. Dear Frank, Thank you very much for the comments and suggestions to improve our manuscript.P...

understanding of how additives modify CaCO 3 growth kinetics and the mechanisms that control precipitation is therefore a topic of considerable interest. [ 1-3 ] The initial steps of CaCO 3 crystallization can occur via the formation of a poorly ordered amorphous calcium carbonate (ACC) phase. Synthetically formed ACC is often short lived and quickly transforms to more stable crystalline CaCO 3 polymorphs such as vaterite, calcite, and aragonite. [ 4,5 ] In contrast, biogenic ACC can be stable for much longer, even over the entire lifetime of an organism. [ 6,7 ] The enhanced stabilization of ACC has been explained by the incorporation of considerable amounts of magnesium, phosphate, and silicate, as well as the occlusion of associated proteins and other macromolecules. [ 8,9 ] For example, ACC aggregates in the intestinal tract of the seabream, Sparus aurata , are stabilized by the incorporation of up to 54 mol% Mg. [ 10 ] Similarly, ACC formed in the exoskeleton and gastroliths of the crayfi sh, P. clarkii , contain phosphoenolpyruvate and 3-phosphoglycerate (intermediates of the glycolytic pathway), which were shown to be responsible for ACC stabilization. [ 11 ] While numerous laboratory studies have quantifi ed the effects of inorganic additives on ACC formation, stability, and crystallization, [ 9,12,13 ] the role of biomolecules is less well constrained, mainly because of the vast diversity of organic compounds, their variety of composition, chain length, and structure. Highly carboxylated species have been shown to extend ACC lifetime, [ 3,14-16 ] whereas molecules with a lower number of carboxyl groups, such as single unit amino acids, do not exert much control on ACC stability. [ 3,17 ] However, a mechanistic understanding of how these organic molecules affect ACC composition, structure, and lifetime is still lacking. This study reports on investigations of the role of citrate (CIT) in ACC formation and crystallization. CIT is an intermediate in the tricarboxylic acid cycle and can form during glycolysis. It is used in various industrial processes, for example, as a fl avoring additive in food, as a cleaning and chelating agent, and as a scale inhibitor in pipes, boreholes, and subsurface reservoirs. [ 18 ] More recently, CIT has been shown to be an ideal coating agent to protect and stabilize metallic nanoparticles and to control their size. [ 19 ] The effect of CIT on CaCO 3 polymorph selection and crystal growth rates has been examined