Aragonite Growth Research Articles

Investigation of the acicular aragonite growth behavior in AOD stainless steel slag during slurry-phase carbonation

This study presents a novel method for producing acicular aragonite using argon oxygen decarburization (AOD) slag while controlling the reaction temperature, reaction time, stirring speed, and the magnesium-to‑calcium stoichiometric ratio. This approach provides steel plants with an opportunity to decrease their CO2 emissions and promote efficient resource utilization and CO2 storage through the production of high-quality value-added products. The experimental results showed that reaction temperature was the most significant factor affecting the carbonation efficiency of AOD slag, followed by reaction time, stirring speed, CO2 partial pressure, and the liquid-to-solid ratio (L/S). The study also found that elevated temperature and prolonged reaction duration favored the preferential precipitation of aragonite. Additionally, raising the temperature and the magnesium-to‑calcium stoichiometric ratio was shown to enhance the formation of aragonite, affecting its crystal growth orientation and dimensions. The optimal combination of reaction parameters for the preparation of acicular aragonite was found to be the reaction time of 8 h, the magnesium-to‑calcium stoichiometric ratio of 0.8, the reaction temperature of 120 °C, and the stirring speed of 200 r·min−1. Under these conditions, the resulting acicular aragonite exhibited excellent overall uniformity, a large aspect ratio, and a smooth crystal surface, with a content of 91.49 %, a single crystal length ranging from 9.86 to 32.6 μm, and a diameter ranging from 0.63 to 2.15 μm. This study provides valuable insights into the efficient production of acicular aragonite from steel slag while reducing CO2 emissions and promoting the sustainable use of resources.

Experimental and theoretical investigations of stable Sr isotope fractionation during its incorporation in aragonite

Strontium partitioning and isotope fractionation between aragonite and fluid have been determined experimentally at low values of the fluid saturation state (Ω) with respect to this mineral (1.1 ≤ Ωaragonite ≤ 2.2), and the measured isotope fractionation has been compared with the results of first-principles simulations. For the latter, Density Functional Theory (DFT) was used for estimation of the equilibrium Sr isotope fractionation between aragonite and Sr2+(aq). The obtained results suggest that, for values of Ωaragonite close to unity, the apparent distribution coefficient of Sr in aragonite (DSr,aragonite=XSrCO3XCaCO3(CaSr)fluid) exhibits values higher than one that rapidly decrease at increasing aragonite growth rate. Under equilibrium conditions (i.e. Ωaragonite=1) a DSr,aragoinite value of 2.7 can be extrapolated. Additionally, for aragonite growth rates rp ≤ 10−8.0±0.2 (mol/m2/s) the Sr isotope fractionation between aragonite and the fluid (i.e. Δ88/86Sraragonite-fluid) shows a constant value of −0.1±0.05‰, whereas it decreases to −0.40‰ when the growth rate increases to 10−7.7(mol/m2/s). The surface reaction kinetic model (SRKM) developed by DePaolo (2011) has been used to describe the dependence of DSr,aragonite and Δ88/86Sraragonite-fluid on mineral growth rate. In this model the best fit for DSr,aragonite and Δ88/86Sraragonite-fluid were 4 and −0.01‰, respectively, whereas the kinetic isotope fractionation factor for Δ88/86Sraragonite-fluid was −0.6‰. The results of first-principles calculations yield an equilibrium Sr isotope fractionation factor of −0.04‰ which is in excellent agreement with the experimental value of the present study. These results are the first experimental measurements of Sr isotope fractionation during inorganic aragonite precipitation as a function of growth rate and the first DFT calculations of Sr equilibrium fractionation in the aragonite-fluid system. The results of this study provide new insight into the mechanisms controlling stable Sr isotope composition in aragonite, which has implications for using Sr isotopes for paleo-reconstructions of natural archives, particularly those of abiogenic origin.

Assembly of the Intraskeletal Coral Organic Matrix during Calcium Carbonate Formation.

Scleractinia coral skeleton formation occurs by a heterogeneous process of nucleation and growth of aragonite in which intraskeletal soluble organic matrix molecules, usually referred to as SOM, play a key role. Several studies have demonstrated that they influence the shape and polymorphic precipitation of calcium carbonate. However, the structural aspects that occur during the growth of aragonite have received less attention. In this research, we study the deposition of calcium carbonate on a model substrate, silicon, in the presence of SOM extracted from the skeleton of two coral species representative of different living habitats and colonization strategies, which we previously characterized. The study is performed mainly by grazing incidence X-ray diffraction with the support of Raman spectroscopy and electron and optical microscopies. The results show that SOM macromolecules once adsorbed on the substrate self-assembled in a layered structure and induced the oriented growth of calcite, inhibiting the formation of vaterite. Differently, when SOM macromolecules were dispersed in solution, they induced the deposition of amorphous calcium carbonate (ACC), still preserving a layered structure. The entity of these effects was species-dependent, in agreement with previous studies. In conclusion, we observed that in the setup required by the experimental procedure, the SOM from corals appears to present a 2D lamellar structure. This structure is preserved when the SOM interacts with ACC but is lost when the interaction occurs with calcite. This knowledge not only is completely new for coral biomineralization but also has strong relevance in the study of biomineralization on other organisms.

Utilization of green mussel shell waste for calcium carbonate synthesis through the carbonation method with temperature variation

In this study, PCC (precipitate calcium carbonate) was synthesized from green mussel shell waste via calcination and subsequent carbonation methods. Organic substances were removed from green mussel shell powder using a 5-hour calcination at 900 °C. Furthermore, the carbonation method was used in the Ca (NO3)2 solution at constant stirring speed with pH control by NH4OH, followed by the injection of carbon dioxide at 50, 70, and 90 °C temperature variations to precipitate calcium as CaCO3 (PCC). According to Rietveld’s quantitative XRD analysis, PCC products at 50 °C, 70 °C, and 90 °C exhibited primarily calcite and aragonite phases, with a significant needle-like morphology of aragonite growth during synthesis. Aragonite growth appears to have increased with increasing temperature. The results show that a simple, low-cost approach to green recycling works.

Site-specific interactions enhanced dissolution of natural aragonite (110) surfaces in succinic acid (SUC) solutions: Implications for the oceanic aragonite dissolution fluxes

Aragonite, one of the most common biological calcium carbonate minerals, is widespread in marine plankton and neritic sediments (e.g., accounting for 89% of pelagic calcification of CaCO3 in the surface ocean). Its dissolution directly affects the export fluxes of CaCO3 in seawater. The aragonite dissolution in the ocean correlates with the increase of partial pressure of carbon dioxide and is pertinent to acidic biomolecules. However, aragonite dissolution in seawater with acidic biomolecules has been overlooked, and the interaction mechanism is unclear. In-situ atomic force microscopy (AFM) was employed to observe the dissolution features of aragonite (110) growth surfaces in succinic acid (SUC, HOOC-CH2-CH2-COOH) solutions. The results demonstrate that (1) both the morphologies and spreading rates of the etch pits formed on aragonite (110) surfaces are altered by the interactions between SUC molecules and the surfaces; (2) the [11‾1] and [1‾11] steps of the etch pits on aragonite (110) surfaces in SUC solutions are kinetically controlled; (3) dissolution rates of aragonite (110) surfaces are proportional to SUC concentrations, which is attributed to the strong complexation between SUC molecules and surface-bounded Ca atoms (≡Ca+); (4) etch pits morphologies of aragonite (110) and calcite (10.4) surfaces are different in SUC solutions, and the spreading rates of the etch pits of the former are one to two orders of magnitude lower than those of the latter. Furthermore, aragonite is found to be more sensitive to SUC molecules than calcite, suggesting that dissolving aragonitic materials could be more easily affected by circumambient biomolecules than dissolving calcitic materials. In seawater that contains organic molecules, the dissolution fluxes of aragonite could be much slower than those of calcite. These findings not only reveal the site-specific interactions between SUC molecules and the ≡Ca+ on aragonite (110) surfaces but also emphasize its implication for the oceanic aragonite dissolution fluxes.

Controls of temperature and mineral growth rate on Mg incorporation in aragonite

The incorporation of Mg in aragonite was experimentally investigated as a function of mineral growth rate at 5, 15 and 25 °C using the constant addition technique. Aragonite growth rate was found to be the major parameter affecting the Mg partitioning coefficient (i.e. DMg=(MgCa)solid(MgCa)fluid) between aragonite and fluid, whereas the effect of temperature is smaller but measurable. At similar surface normalized growth rates, DMg values decrease as temperature increases from 5 to 25 °C. The magnitude of decrease as a function of temperature is similar for all the experiments of this study where growth rate varied in the range 10-8.6 ≤ rp ≤ 10-7.1 (mol/m2/s). The combined effect of aragonite growth rate and temperature on DMg can be described by the linear equation:Log DMg = 0.583(±0.020) Log rp − 0.026(±0.001) T + 0.863(±0.153); R2 = 0.97.where T is the temperature in degrees Celsius.The increase of DMg values at decreasing temperatures in experiments conducted at similar growth rates is consistent with the increase of fluid supersaturation with respect to aragonite. Thus, it can be inferred that increased Mg incorporation at higher supersaturation is associated with the greater presence of defect sites on the growing mineral surface, similar to the incorporation of other incompatible ions in carbonate minerals. Overall, the relationship between Mg content of aragonite with the degree of saturation of the fluid with respect to this mineral phase suggests that DMg values or Mg/Ca ratio in natural aragonites can be used as a proxy for saturation degree of the formation fluid with respect to CaCO3 minerals.

Effects of Preferential Incorporation of Carboxylic Acids on the Crystal Growth and Physicochemical Properties of Aragonite

The preferential incorporation of carboxylic acids into aragonite and its effects on the crystal growth and physicochemical properties of aragonite were systematically investigated using a seeded co-precipitation system with different carboxylic acids (citric, malic, acetic, glutamic, and phthalic). Aragonite synthesized in the presence of citric and malic acids showed a remarkable decrease in the crystallinity and size of crystallite, and the retardation of crystal growth distinctively changed the crystal morphology. The contents of citric acid and malic acid in the aragonite samples were 0.65 wt % and 0.19 wt %, respectively, revealing that the changes in the physicochemical properties of aragonite were due to the preferential incorporation of such carboxylic acids. Speciation modeling further confirmed that citric acid with three carboxyl groups dominantly existed as a metal–ligand, (Ca–citrate)−, which could have a strong affinity toward the partially positively charged surface of aragonite. This indicates why citric acid was most favorably incorporated among other carboxylic acids. Our results demonstrate that the number of carboxyl functional groups strongly affects the preferential incorporation of carboxylic acids into aragonite; however, it could be suppressed by the presence of other functional groups or the structural complexity of organic molecules.

Heterogeneous Nucleation and Growth of CaCO3 on Calcite (104) and Aragonite (110) Surfaces: Implications for the Formation of Abiogenic Carbonate Cements in the Ocean

Although near-surface seawater is supersaturated with CaCO3, only a minor part of it is abiogenic (e.g., carbonate cements). The possible reason for such a phenomenon has attracted much attention in the past decades. Substrate effects on the heterogeneous nucleation and growth of CaCO3 at various Mg2+/Ca2+ ratios may contribute to the understanding of the origin of abiogenic CaCO3 cements. Here, we used in situ atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy to study the heterogeneous nucleation and growth of CaCO3 on both calcite (104) and aragonite (110) surfaces. The results show that (1) calcite spiral growth occurs on calcite (104) surfaces by monomer-by-monomer addition; (2) the aggregative growth of aragonite appears on aragonite (110) surfaces through a substrate-controlled oriented attachment (OA) along the [001] direction, followed by the formation of elongated columnar aragonite; and (3) Mg2+ inhibits the crystallization of both calcite and aragonite without impacting on crystallization pathways. These findings disclose that calcite and aragonite substrates determine the crystallization pathways, while the Mg2+/Ca2+ ratios control the growth rate of CaCO3, indicating that both types of CaCO3 substrate in shallow sediments and aqueous Mg2+/Ca2+ ratios constrain the deposition of abiogenic CaCO3 cements in the ocean.

On the controls of mineral assemblages and textures in alkaline springs, Samail Ophiolite, Oman

Interactions between meteoric water and ultramafic rocks in the Oman Ophiolite generate waters of variable physicochemical characteristics. The discharge of these waters forms complex alkaline pool networks, in which mineral precipitation is triggered by mixing, evaporation, and uptake of atmospheric CO2. A systematic and co-localized sampling of waters and solids in two individual spring sites allowed us to determine the saturation state of a range of minerals and correlate them to the different water and precipitate types. We subdivided the waters of the spring sites into three distinctive types: i) Mg-type; moderately alkaline (7.9<pH<9.5), Mg2+–HCO3−-rich waters, ii) Ca-type; hyperalkaline (pH>11.6), Ca2+–OH−-rich waters, and iii) Mix-type; alkaline to hyperalkaline (9.6 < pH < 11.5) waters with intermediate chemical composition. We first report the occurrence of hydrated magnesium (hydroxy-) carbonate phases in Mg-type waters. Nesquehonite forms in these waters via evaporation and transforms into dypingite and hydromagnesite under CO2-rich conditions. In Ca-type waters, the coupling of atmospheric CO2 uptake with evaporation leads to the formation of a calcitic crystalline crust on the air-water interface. The crusts are aragonite- and brucite-bearing, where Mg-type and Ca-type waters discharge and vigorously mix at the same pool. Unlike the Mg-type and Ca-type waters, the pools of Mix-type waters host massive aragonite-dominated deposits due to high Mg/Ca ratio that favors the growth of aragonite over calcite. The hydrodynamics during mixing spatially control brucite precipitation and restrict its formation and accumulation around specific mixing zones, where a continuous supply of Mg of inflowing Mg-type waters takes place. Crystal morphologies record the effect on the values of supersaturation and supersaturation rates in the pools due to mixing processes, evaporation and CO2 uptake. In Ca-type waters, CO2 uptake and evaporation dictate the textural characteristics of calcite both in crystalline crusts and rock coatings. Textural evolution of aragonite from crystalline sheaves to spheroidal shapes underlines the different supersaturation rates of calcium carbonate crystallization in flocculent material of Mix-type waters. Geochemical models of mixing between Mg-type and Ca-type waters revealed the evolution of mineral saturation indices under various mixing proportions, and their relation to the observed mineralogy and geochemistry of the pool waters. The thorough documentation of mineral assemblages and crystal morphologies enabled us to provide a more detailed account of how water composition, mixing, and mineral precipitation co-evolve in the alkaline spring systems, where CO2 is sequestered.

Experimental and theoretical modelling of kinetic and equilibrium Ba isotope fractionation during calcite and aragonite precipitation

Barium isotope fractionation during calcite and aragonite inorganic precipitation was studied in mixed flow reactors as a function of precipitation rate at 25 °C and pH = 6.3 ± 0.1. The measured Ba isotope fractionation that occurs between calcite and the forming fluid in the investigated range of calcite growth rates (10-8.0 ≤ Rp(calcite) ≤ 10−7.3 mol/m2/s) is insignificant. Barium isotope fractionation between aragonite and the fluid decreases with increasing precipitation rate from Δ137/134Baaragonite-fluid = +0.25 ± 0.06‰ for Rp(aragonite) ≤ 10−8.7 mol/m2/s to −0.10 ± 0.08‰ for Rp(aragonite) = 10−7.6 mol/m2/s, thus reflecting preferential incorporation of either heavy or light Ba isotopes in aragonite at slow and fast growth rates, respectively. The dependence of Ba isotope fractionation on aragonite growth rate is well described by the surface reaction kinetic model developed by DePaolo (2011) when the values +0.27‰ and −2.0 ± 0.2‰ are used for the equilibrium and kinetic Ba isotope fractionation factor, respectively. The enrichment of aragonite in the heavier Ba isotopes is consistent with the equilibrium fractionation factor of +0.34‰, calculated here between Ba-substituted aragonite and Ba2+ (aq), from first-principles calculations. This positive fractionation is related to a shorter average BaO bond length in the aragonite structure while the coordination number does not change much (i.e. 9). The lack of isotope fractionation between the Ba aquo ions and the 6-coordinated Ba in calcite likely suggests that the coordination reduction required for the incorporation in the lattice of Ba adsorbed at calcite growing sites proceeds without isotope fractionation with the fluid. Otherwise the precipitated calcite should have been enriched in heavy isotopes by ∼0.17‰, as predicted by first-principles calculations. These results are the first experimental measurements of Ba isotope fractionation during inorganic calcite and aragonite mineral formation and set the basis for understanding the mechanisms controlling Ba isotope composition in CaCO3 minerals that is an essential perquisite for application of this isotopic system in natural samples.

Biomimetic Synthesis of Calcium Carbonate with the Assistance of Amino Acids

Two acidic amino acids (aspartic acid, glutamic acid) and four neutral amino acids (cysteine, serine, tyrosine, glycine) were used as inducers for calcium carbonate biomimetic synthesis. The results showed that aspartic acid introduced is more effective for the growth of aragonite, while glutamic acid used is more effective for the growth of vaterite. Compared to those acidic amino acids, neutral amino acids used lead to less aragonite or vaterite produced, and tyrosine and serine show stronger effects on the growth of aragonite and vaterite than cysteine and glycine. On the other hands, it was found that the increased concentration of amino acid shows positive effects on the growth of aragonite and vaterite. In addition, the enhanced concentration of calcium chloride results in the increased crystalline volume of calcite significantly, as well as the more orderly structure as shown in scanning electron microscopy investigation. Meanwhile, the high calcium chloride concentration indicates weak inhibitory effect on aragonite growth but no significant influence on the growth of vaterite.

Formation of $${\text {CaCO}}_3$$ varieties from a carbonated aqueous solution

Experimental studies have been conducted to clarify the paragenetic conditions of CaCO3 varieties, calcite and aragonite formed from the same wall rock. In addition, the solubility dependence of calcite, aragonite, and limestone formed from limestone cave in CO2 -saturated solutions was compared with the previous studies. First, the calcite and aragonite were powdered to below 200 meshes. In addition, the powders were, respectively, added into the CO2 -saturated distilled water at 8, 13, 18, and 22∘C and were kept constant for 15 days. The amount of dissolved CaCO3 was then measured. The solubility of aragonite was higher than that of calcite, and the solubility decreased exponentially with increasing temperature. Simultaneous production and growth of calcite and aragonite proceeded only when the saturation of aragonite was reached in aqueous carbonate solution. Conversely, when the dissolved amount of CaCO3 has not been reached, calcite is only formed and grown. In these solutions, aragonite crystals dissolve and calcite crystals grow. The degree of saturation of aragonite can be reached by rapid evaporation of solvent, CO2 gas degassing, and the presence of Mg. However, paragenesis of calcite and aragonite in natural caves can be explained by rapid evaporation of the solvent. A rapid evaporation of the solvent may occur at a place, where a small amount of karst solution flows and a large amount of air flows. In such places, paragenesis of calcite and aragonite is possible.

Magnesium-Rich Nanometric Layer in the Skeleton of Pocillopora damicornis With Possible Involvement in Fibrous Aragonite Deposition

The complex structures and morphologies of coral skeletons make it difficult to study the construction of individual skeleton components. This is especially true regarding micro-structures deposited during different stages of skeletogenesis. As such structures serve as the basis for subsequent skeleton development, they are often obscured by additional skeletal deposition and growth. Recently we described a new model system, coral-on-a-chip, for studying coral biology and skeletogenesis. This system utilizes micropropagates of the Indo-Pacific coral Pocillopora damicornis maintained within a microfluidic environment, allowing us to follow early stages of skeletogenesis over time and under controlled environmental conditions. Our findings reveal that, following settlement onto glass slides, the micropropagates initially form a thin, almost two dimensional skeleton, which subsequently develops into a robust three-dimensional form with features resembling those of the mother colony. Studying the early stages of skeleton accretion in our micropropagates using high-resolution scanning electron microscopy and Energy Dispersive X-ray Spectroscopy (EDS) revealed a magnesium-rich layer deposited directly on the glass surface. This layer, which has a typical Mg/Ca ratio of 1.43±0.78, forms a dense lattice with a typical pore size of 100 nm. Microscopic observations indicate that this lattice serves as the basis for subsequent growth of fibrous aragonite. Examination of the underside of a skeleton from a small P. damicornis colony growing on a glass surface revealed a similar high-magnesium lattice at the interface between the glass and aragonitic skeleton in association with fibrous aragonite deposition. These observations suggest a role for this magnesium-rich lattice in the deposition of the fibrous aragonite forming the bulk of the coral skeleton.

Barium partitioning in calcite and aragonite as a function of growth rate

Calcite and aragonite growth rate experiments have been performed in the presence of aqueous Ba at 25 °C using the constant addition technique as a function of CaCO3 mineral growth rate (mol/m2/s). The partitioning of Ba in calcite at −8.4 ≤ Logrcalcite ≤ −7.3 exhibits a weak dependence on growth rate that can be expressed as:LogDBa,calcite=0.2477±0.0543×Logrcalcite-0.2949±0.4222;R2=0.81In the case of aragonite, Ba partitioning at −9.0 ≤ Lograragonite ≤ −7.8, exhibits a much stronger dependence on growth rate that can be described as:LogDBa,aragonite=0.4458±0.0563×Lograragonite+3.3407±0.4668;R2=0.84The determined DBa,aragonite values are systematically lower than unity and come in contrast to previous experimental works where Ba partitioning studied during aragonite nucleation. They are, however, in excellent agreement with the theoretical free energy correlation model by Wang and Xu (2001) that considers the effect of ionic radii size on DBa,aragonite. The defined growth rate dependence of Ba incorporation in aragonite provides new insights on the role of ionic radii size during the incorporation of trace elements and the formation of solid-solutions from aqueous solutions at low oversaturation degrees. The data presented here shed light on the process controlling the elevated DBa,aragonite values occurring at high saturation degrees of natural fluids with respect to aragonite that have been recorded in natural samples. Furthermore, the trend experimentally obtained herein has the potential to record variations in growth rate regimes in natural occurring aragonites and to provide information on the environmental conditions of natural waters in the past.

Climate variability in the western Mediterranean between 121 and 67 ka derived from a Mallorcan speleothem record

The western Mediterranean region is exceptionally vulnerable to predicted climate changes of increasing temperature and aridity. Characterizing past climate oscillations since the last interglacial (LIG) is critical to understand climate patterns during the present warm period. Here we present an accurately dated speleothem record (CAM-1) of paleoclimate between 121 and 67 ka from the island of Mallorca. The growth history combined with the isotopic record and mineral changes, provides evidence of dramatic climatic shifts in the western Mediterranean. Isotopic equilibrium deposition was assessed by Hendy tests and by comparing the δ18O in modern drip water with newly precipitated calcite. We argue that variability of δ18O in CAM-1 is mainly related to changes in source of precipitation, whereas high δ13C values reflect a dry climate. Calcite rather than aragonite growth is related to cooler, drier climate. The MIS 5e/5d transition, C24, C23, and C21 cold events concomitantly recorded in CAM-1, ODP 984, and NGRIP highlights the teleconnection between high and mid-latitudes in the Northern Hemisphere. Our record reveals a prolonged aridity during MIS 5c and a sudden climate shift from drier to wetter conditions beginning with MIS 5b. A growth hiatus at 67 to 53 ka probably marks the driest period, after which slow calcite growth indicates markedly drier climate during the last glacial and Holocene than during MIS 5. Many of the significant changes in growth rates and stable isotopic composition are synchronous with events that are likely driven by regional climate and large circulation patterns linked to the North Atlantic.

Phosphogenesis, aragonite fan formation and seafloor environments following the Marinoan glaciation

Carbonates in the Sete Lagoas Formation (São Francisco craton, Brazil) preserve a record of chemical, biological, and oceanographic changes that occurred during the Ediacaran Period. The base of this formation constitutes a post-glacial cap carbonate, which contains seafloor precipitates (carbonate and barite crystal fans) as well as various authigenic and diagenetic minerals (apatite, pyrite, and barite). Here, we present petrographic and geochemical data on this unit, and discuss the significance of its mineral association for marine environments following the Marinoan (‘Snowball Earth’) glaciation. For the first time, we report well-developed apatitic cements in a Neoproterozoic cap carbonate. Isopachous and intergranular void-filling cements encrust and surround seafloor-precipitated fan crystals that precipitated as aragonite. We propose a model for the origin of this mineral association, which relates phosphogenesis and aragonite fan formation to a single set of environmental conditions. According to this model, the boundary between oxic and anoxic conditions was located at or just below the sediment-water interface. Burial of iron (oxyhydr)oxides below this boundary liberated phosphate to pore water and provided fuel for iron reduction. Iron reduction released Fe2+, which inhibited nucleation of carbonate and allowed for aragonite growth on the seafloor. Concurrently, ‘iron-pumping’ shuttled phosphate from the water column to the sediment, and perhaps in conjunction with organic phosphorus remineralization via anaerobic microbial pathways, created conditions conducive to phosphate mineralization. This model corroborates the hypotheses that aragonite crystal fan formation requires the presence of an inhibitor to carbonate nucleation, in addition to high alkalinity, and that Fe2+ serves as this inhibitor. Overall, our work documents a close association between aragonite crystal fan formation and phosphogenesis at the beginning of the Ediacaran, illuminates the paleoenvironments of cap carbonates with seafloor precipitates, and contributes to understanding of phosphogenesis following low latitude glaciations.

Calcium-Induced Molecular Rearrangement of Peptide Folds Enables Biomineralization of Vaterite Calcium Carbonate

Proteins can control mineralization of CaCO3 by selectively triggering the growth of calcite, aragonite or vaterite phases. The templating of CaCO3 by proteins must occur predominantly at the protein/CaCO3 interface, yet molecular-level insights into the interface during active mineralization have been lacking. Here, we investigate the role of peptide folding and structural flexibility on the mineralization of CaCO3. We study two amphiphilic peptides based on glutamic acid and leucine with β-sheet and α-helical structures. Though both sequences lead to vaterite structures, the β-sheets yield free-standing vaterite nanosheet with superior stability and purity. Surface-spectroscopy and molecular dynamics simulations reveal that reciprocal structuring of calcium ions and peptides lead to the effective synthesis of vaterite by mimicry of the (001) crystal plane.

Spherulitic Growth of Coral Skeletons and Synthetic Aragonite: Nature's Three-Dimensional Printing.

Coral skeletons were long assumed to have a spherulitic structure, that is, a radial distribution of acicular aragonite (CaCO3) crystals with their c-axes radiating from series of points, termed centers of calcification (CoCs). This assumption was based on morphology alone, not on crystallography. Here we measure the orientation of crystals and nanocrystals and confirm that corals grow their skeletons in bundles of aragonite crystals, with their c-axes and long axes oriented radially and at an angle from the CoCs, thus precisely as expected for feather-like or "plumose" spherulites. Furthermore, we find that in both synthetic and coral aragonite spherulites at the nanoscale adjacent crystals have similar but not identical orientations, thus demonstrating by direct observation that even at nanoscale the mechanism of spherulite formation is non-crystallographic branching (NCB), as predicted by theory. Finally, synthetic aragonite spherulites and coral skeletons have similar angle spreads, and angular distances of adjacent crystals, further confirming that coral skeletons are spherulites. This is important because aragonite grows anisotropically, 10 times faster along the c-axis than along the a-axis direction, and spherulites fill space with crystals growing almost exclusively along the c-axis, thus they can fill space faster than any other aragonite growth geometry, and create isotropic materials from anisotropic crystals. Greater space filling rate and isotropic mechanical behavior are key to the skeleton's supporting function and therefore to its evolutionary success. In this sense, spherulitic growth is Nature's 3D printing.

Paleoenvironmental reconstruction of a saline lake in the Tertiary: Evidence from aragonite laminae in the northern Tibet Plateau

The origin of aragonite has long been debated because it is precipitated and preserved under specific conditions. Aragonite laminae, first found from Eocene to Miocene strata in the western Qaidam Basin, northern Tibet Plateau, contain much information on paleolake signatures. Mineralogical and geochemical analyses were conducted on alternating yellowish and grayish aragonite layers. The yellowish layers are mainly composed of aragonite crystals, while the grayish layers contain less aragonite and fewer organic remnants that accumulate among debris with sporadic framboidal pyrite. The δ13C values of yellowish layers are remarkably positive by approximately 4.01‰ (VPDB), and the δ18O values are slightly negative compared with base data of the Qaidam Basin. Considering the 12CO2 absorption of algal blooms, positive excursions of δ13C shown in aragonite indicate high 13C values in depositional water. Therefore, a seasonal algal-influenced inorganic origin is proposed to explain the formation of aragonite laminae. During warm seasons, Mg/Ca ratios are elevated because of evaporation effects. The algal blooms decrease the CO2 content, leading to high pH values. These conditions promote the rapid crystal growth of aragonite instead of other carbonate minerals. Slightly negative δ18O values in yellowish layers are interpreted as the result of intense inflow during warm seasons, which leads to less precipitation of organic matter and debris. The grayish layers in cold seasons are the opposite. From the Eocene to Oligocene, the progressively decreasing δ18O values of aragonite reflect global cooling during this time. A conspicuously positive step in δ18O values indicates an arid environment coinciding with the uplift of the Himalaya, from the Oligocene to Lower Miocene. The results from this study show that understanding of aragonite in the Qaidam Basin is essential to reconstruct the high-resolution paleoenvironment and to reveal the Tertiary evolution of paleoclimates in the northern Tibet Plateau.

Nacre tablet thickness records formation temperature in modern and fossil shells

Nacre, the iridescent outer lining of pearls and inner lining of many mollusk shells, is composed of periodic, parallel, organic sheets alternating with aragonite (CaCO3) tablet layers. Nacre tablet thickness (TT) generates both nacre's iridescence and its remarkable resistance to fracture. Despite extensive studies on how nacre forms, the mechanisms controlling TT remain unknown, even though they determine the most conspicuous of nacre's characteristics, visible even to the naked eye.Thermodynamics predicts that temperature (T) will affect both physical and chemical components of biomineralized skeletons. The chemical composition of biominerals is well-established to record environmental parameters, and has therefore been extensively used in paleoclimate studies. The physical structure, however, has been hypothesized but never directly demonstrated to depend on the environment. Here we observe that the physical TT in nacre from modern and fossil shallow-water shells of the bivalves Pinna and Atrina correlates with T as measured by the carbonate clumped isotope thermometer. Based on the observed TT vs. T correlation, we anticipate that TT will be used as a paleothermometer, useful to estimate paleotemperature in shallow-water paleoenvironments. Here we successfully test the proposed new nacre TT thermometer on two Jurassic Pinna shells. The increase of TT with T is consistent with greater aragonite growth rate at higher T, and with greater metabolic rate at higher T. Thus, it reveals a complex, T-dependent biophysical mechanism for nacre formation.