Calcite Growth Research Articles

Bioprocessing-Inspired Fabrication of Calcite Prismatic Layers and Growth Mechanisms

Bioprocessing-Inspired Fabrication of Calcite Prismatic Layers and Growth Mechanisms

Temperature Effects on the Distribution of Aragonitic and Calcite‐Secreting Epifaunal Bivalves

ABSTRACTAimTo test if temperature significantly influences the global biogeographic distribution of marine epifaunal bivalves via their skeletal mineralogy.LocationGlobal.TaxaMarine, epifaunal bivalves.MethodsThe skeletal mineralogy of 45,789 epifaunal bivalve occurrences from 669 species from the Ocean Biodiversity Information System (OBIS) was related to sea surface temperatures from Bio‐ORACLE. Binomial regression was used to assess the influence of temperature and seasonality on the distribution of aragonitic and calcite‐secreting bivalve occurrences, aggregated in equal‐area grid cells.ResultsThe proportion of aragonitic bivalve occurrences significantly increases with mean annual temperature in our global analysis and most marine biogeographic realms. A greater prevalence of calcite‐secreting bivalves in seasonal climates could be shown at low mean annual temperatures at the global scale but not within biogeographic realms.Main ConclusionsThe global biogeographic distribution of epifaunal bivalves is significantly influenced by water temperature via their skeletal mineralogy. The mechanism driving this pattern is best explained by the temperature modulation of the effect of Mg2+ on calcite growth. Although this Mg2+ effect predicts an advantage for aragonite secretion at higher temperatures, poleward migration in response to higher temperature extremes will expose tropical taxa to cooler temperatures in the cold season, which may impede aragonite secretion in taxa not adapted to these climates.SignificanceOur results suggest that skeletal mineralogy is likely to influence ocean warming‐induced migration patterns.

A molecular view of peptoid-induced acceleration of calcite growth

The extensive deposits of calcium carbonate (CaCO3) generated by marine organisms constitute the largest and oldest carbon dioxide (CO2) reservoir. These organisms utilize macromolecules like peptides and proteins to facilitate the nucleation and growth of carbonate minerals, serving as an effective method for CO2 sequestration. However, the precise mechanisms behind this process remain elusive. In this study, we report the use of sequence-defined peptoids, a class of peptidomimetics, to achieve the accelerated calcite step growth kinetics with the molecular level mechanistic understanding. By designing peptoids with hydrophilic and hydrophobic blocks, we systematically investigated the acceleration in step growth rate of calcite crystals using in situ atomic force microscopy (AFM), varying peptoid sequences and concentrations, CaCO3 supersaturations, and the ratio of Ca2+/ HCO3-. Mechanistic studies using NMR, three-dimensional fast force mapping (3D FFM), and isothermal titration calorimetry (ITC) were conducted to reveal the interactions of peptoids with Ca2+ and HCO3- ions in solution, as well as the effect of peptoids on solvation and energetics of calcite crystal surface. Our results indicate the multiple roles of peptoid in facilitating HCO3- deprotonation, Ca2+ desolvation, and the disruption of interfacial hydration layers of the calcite surface, which collectively contribute to a peptoid-induced acceleration of calcite growth. These findings provide guidelines for future design of sequence-specific biomimetic polymers as crystallization promoters, offering potential applications in environmental remediation (such as CO2 sequestration), biomedical engineering, and energy storage where fast crystallization is preferred.

Constraining sulfur incorporation in calcite using inorganic precipitation experiments

The sulfur over calcium ratio (S/Ca) in foraminiferal shells was recently proposed as a new and independent proxy for reconstructing marine inorganic carbon chemistry. This new approach assumes that sulfur is incorporated into CaCO3 predominantly in the form of sulfate (SO42−) through lattice substitution for carbonate ions (CO32–), and that S/Ca thus reflects seawater [CO32–]. Although foraminiferal growth experiments validated this approach, field studies showed controversial results suggesting that the potential impact of [CO32–] may be overwritten by one or more parameters. Hence, to better understand the inorganic processes involved, we here investigate S/Ca values in inorganically precipitated CaCO3 (S/Ca(cc)) grown in solutions of CaCl2 − Na2CO3 − Na2SO4 − B(OH)3 − MgCl2. Experimental results indicate the dependence of sulfate partitioning in CaCO3 on the carbon chemistry via changing pH and suggest that faster precipitation rates increase the partition coefficient for sulfur. The S/Ca ratios of our inorganic calcite samples show positive correlation with modelled [CaSO40](aq), but not with the concentration of free SO42− ions. This challenges the traditional model for sulfate incorporation in calcite and implies that the uptake of sulfate potentially occurs via ion-ion pairs rather than being incorporated as single anions. Based on the [Ca2+] dependence via speciation, we here suggest a critical evaluation of this potential proxy. As sulfate complexation seems to control sulfate uptake in inorganic calcite, application as a proxy using foraminiferal calcite may be limited to periods for which seawater chemistry is well-constrained. As foraminiferal calcite growth is modulated by inward Ca2+ flow to the site of calcification coupled to outward H+ pumping, sulfate incorporation as CaSO40 ion-pair in the foraminifer’s shell also provides a mechanistic link for the observed relationship between S/Ca(cc) and [CO32–].

Exploring calcite Coprecipitation: Synthesis, Crystal Growth, and pH effect

In this study, the synthesis and crystal growth of calcite (CaCO3) was examined with a focus on the coprecipitation and hydrothermal methods; additionally, crystal nucleation and growth was monitored under acidic and alkaline conditions. In the experiments, different Ca2+ solutions (pH 1.4 and 8.1) were mixed with Na2CO3 (pH 11.6), and the rates for the addition of the reactants were adjusted. Using an acidic Ca2+ solution as the calcium source and the rapid addition of carbonate, larger crystals were precipitated and grown in a single-step coprecipitation process. The fast addition of carbonate in an initially alkaline calcium solution produced intermediate crystal sizes. However, the slow dropwise addition of carbonate led to smaller crystals due to localized supersaturation and rapid nucleation. Coprecipitation followed by hydrothermal treatment produced larger crystals with the best-defined morphology. Structural and compositional analyses were conducted using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed the formation of calcite with different crystal sizes and provided insight into the controlled synthesis of CaCO3 crystals; our study has relevance for various industrial applications and for the investigation of natural sources.

Testing the hypothesis that solvent exchange limits the rates of calcite growth and dissolution.

It is established that the rates of solvent exchange at interfaces correlate with the rates of a number of mineral reactions, including growth, dissolution and ion sorption. To test if solvent exchange is limiting these rates, quasi-elastic neutron scattering (QENS) is used here to benchmark classical molecular dynamics (CMD) simulations of water bound to nanoparticulate calcite. Four distributions of solvent exchanges are found with residence times of 8.9 ps for water bound to calcium sites, 14 ps for that bound to carbonate sites and 16.7 and 85.1 ps for two bound waters in a shared calcium-carbonate conformation. By comparing rates and activation energies, it is found that solvent exchange limits reaction rates neither for growth nor dissolution, likely due to the necessity to form intermediate states during ion sorption. However, solvent exchange forms the ceiling for reaction rates and yields insight into more complex reaction pathways.

Synthesis, scale and corrosion inhibition evaluation and mechanism of 2-aminobenzimidazole modified polyaspartic acid

With the growing public concern for environmental preservation, this work employs environmentally friendly polyaspartic acid (PASP) as the main chain, to develop a green and efficient dual-function inhibitor with scale and corrosion inhibition. The new inhibitor PASP-2-A was synthesized by grafting 2-amino-benzimidazole (2-A) containing nitrogen heterocyclic into polysuccinimide under alkaline conditions. The corrosion inhibition of carbon steel and brass was determined by static test, polarization curve and electrochemical impedance spectroscopy (EIS). It can be found that PASP-2-A had better scale inhibition performance than PASP under different conditions. The addition of 5 mg·L−1 PASP-2-A can almost 100% prevent the formation of CaCO3 scale. Molecular dynamics simulation results showed that PASP-2-A significantly destroys the crystal structure of CaCO3 scale due to the coordination between PASP-2-A and Ca2+ groups. The modified PASP scale inhibitor can prevent the growth of calcite (110) and (104) crystal faces, regulate the transformation of CaCO3 from stable calcite to more soluble spherical vaterite, making it less deposit. The electrochemical test results showed that the corrosion inhibition of PASP-2-A is greatly improved compared with PASP, and the corrosion inhibition of Q235 carbon steel increased about 40% at the dosage of 200 ppm. The corrosion inhibition efficiency of H62 brass in 3.5 wt% NaCl was 97.97% with 300 ppm PASP-2-A. Notably, due to its degradable nature, low toxicity, and affordability, PASP-2-A serves as an excellent inhibitor, exhibiting remarkable performance in both scale inhibition and corrosion inhibition. Consequently, it presents a promising avenue for further advancements in modified PASP.

Electrochemically Assisted Growth of Hopper and Tabular Calcite under Confinement

Electrochemically Assisted Growth of Hopper and Tabular Calcite under Confinement

Guinea fowl eggshell structural analysis at different scales reveals how organic matrix induces microstructural shifts that enhance its mechanical properties

Guinea fowl eggshells have an unusual structural arrangement that is different from that of most birds, consisting of two distinct layers with different microstructures. This bilayered organization, and distinct microstructural characteristics, provides it with exceptional mechanical properties. The inner layer, constituting about one third of the eggshell thickness, contains columnar calcite crystal units arranged vertically as in most bird shells. However, the thicker outer layer has a more complex microstructural arrangement formed by a switch to smaller calcite domains with diffuse/interlocking boundaries, partly resembling the interfaces seen in mollusk shell nacre. The switching process that leads to this remarkable second-layer microstructure is unknown. Our results indicate that the microstructural switching is triggered by changes in the inter- and intracrystalline organic matrix. During production of the outer microcrystalline layer in the later stages of eggshell formation, the interactions of organic matter with mineral induce an accumulation of defects that increase crystal mosaicity, instill anisotropic lattice distortions in the calcite structure, interrupt epitaxial growth, reduce crystallite size, and induce nucleation events which increase crystal misorientation. These structural changes, together with the transition between the layers and each layer having different microstructures, enhance the overall mechanical strength of the Guinea fowl eggshell. Additionally, our findings provide new insights into how biogenic calcite growth may be regulated to impart unique functional properties. Statement of significanceAvian eggshells are mineralized to protect the embryo and to provide calcium for embryonic chick skeletal development. Their thickness, structure and mechanical properties have evolved to resist external forces throughout brooding, yet ultimately allow them to crack open during chick hatching. One particular eggshell, that of the Guinea fowl, has structural features very different from other galliform birds – it is bilayered, with an inner columnar mineral structure (like in most birds), but it also has an outer layer with a complex microstructure which contributes to its superior mechanical properties. This work provides novel and new fundamental information about the processes and mechanisms that control and change crystal growth during the switch to microcrystalline domains when the second outer layer forms.

Reactions in UHT calc-silicate granulites from the In Ouzzal terrane (Western Hoggar, South Algeria): Implication for pressure-temperature-fluid evolution

Calc-silicate granulites constitute a relatively small part of the whole granulitic material outcrops characterizing the In Ouzzal terrane (NW Hoggar, South Algeria). However, these rocks preserve a number of spectacular reaction textures that could be effectively used to infer their pressure-temperature-fluid history. These textures are interpreted using P-T and T-XCO2 grids in the simplified CaO-Al2O3-SiO2-Vapor system. In this process, sequences of reactions have been subdivided into two distinct stages: (i) the early prograde stage that was accompanied by significant rise of temperature from about 800 °C up to 1050 °C at around10 kbar followed by (ii) the decompression stage from about 9 to 6 kbar. During the prograde stage, coarse grained wollastonites were produced according to the reaction calcite + quartz → wollastonite + CO2. Furthermore, in the peak pressure temperature stage, the reaction producing wollastonite + scapolite from coarse primary garnet consumes CO2 with temperature increasing from 850 °C to 1000 °C according to the reaction 3grossular + 3CO2 → 3wollastonite + 2calcite + scapolite. The latest reactions have been occurred during the decompression stage from about 10 kbar to 5 kbar and cooling from 1000 °C to 800 °C. The growth of calcite + quartz around wollastonite besides to garnet coronas between wollastonite, calcite and scapolite are explained by the reaction: calcite + quartz → wollastonite + CO2 and 3wollastonite + scapolite +2calcite → 3grossular + 3CO2. The appearance of anorthite around scapolite occurs following a decrease of temperature independently to the fluids according to the reaction scapolite → 3anorthite + calcite. All reactions took place at CO2 low pressure which was estimated between 0.04 and 0.55.

Bioinspired controllable CaCO3 synthesis from solid waste by an “all in one” amino acid-in strategy: Implication for CO2 mineralization

CO2 mineralization suffers from extensive consumption of acidic/alkaline reagents due to the limited kinetics, which necessitates the development of an atom/energy-efficient process. An “all in one” reagent amino acid-based Ca2+ leaching-CO2 mineralization process inspired by biomineralization was proposed. This study explored the interactions between amino acids and Ca leaching, CaCO3 pre-nucleation, nucleation, and growth by investigating the evolution of Ca concentration, solution turbidity and CaCO3 morphology in complex CFA. Promising Ca2+ leaching efficiencies were observed, being 51.1 %, 34.2 %, and 21.0 % for glycine (Gly), L-alanine (Ala), and L-arginine (Arg), respectively, negatively correlated with their pI but positively correlated with their bonding energy with Ca2+. These amino acids ultimately led to a CaCO3 production of Gly (114.8 g/kg) > Ala (78.2 g/kg) > Arg (54.5 g/kg) from CFA. Gly leachate remained transparent (1.23 NTU) in the first 3 min of CO2 bubbling even though the solution was supersaturated and then got turbid suddenly (6508.75 NTU in 10 min) as continued CO2 bubbling, indicating the inhibition of Gly on CaCO3 nucleation. Observations using Cryo-SEM confirm the presence of abundant CaCO3 nanoparticles in the early stages of mineralization, which later assemble into vaterite crystal particles with a sub-structure and coalesce into larger particles in Gly and Ala. Meanwhile, Arg inhibited calcite growth along the C-axis, leading to the formation of thin, blocky-like particles. Furthermore, 0.010 ∼ 0.027 mmol/g amino acids are occluded into the CaCO3, stabilizing the metastable vaterites and imparting thermal stability, which was believed to be associated with this multistep pathway involving a transient prenucleation cluster, nanoparticle precursor formation, and particle–particle coalescence.

A Study on Ikaite Growth in the Presence of Phosphate

Phosphate is a common component in natural growth solutions of ikaite. Although phosphate often occurs as a minor constituent, its presence may promote the formation of ikaite as it significantly inhibits the precipitation of calcite. The interactions of phosphate with ikaite and the role of a potential uptake of phosphate by ikaite, however, are poorly understood. In this study, the influence of phosphate on ikaite growth at 1 °C was investigated. Ikaite- and calcite-seeded growth experiments were conducted in cryo-mixed-flow reactors at saturation ratios 1.5 ≤ Ωikaite ≤ 2.9 (Ω = ionic activity product/solubility product). From these growth experiments, the rate constant k = 0.10 ± 0.03 µmol/m2/s and the reaction order n = 0.8 ± 0.3 were derived for ikaite. The reaction order implies a transport- or adsorption-controlled growth mechanism which supports a low energy pathway of ikaite growth via an attachment of hydrous CaCO30 complexes without any extensive dehydration of aqueous species as, for instance, required for calcite growth. A potential depletion of aqueous phosphate due to an uptake by ikaite growth was not detectable. Furthermore, growth retardation by phosphate, as known for calcite growth, was not evident. Thus, a significant incorporation of phosphate into growing ikaite could be precluded for the conditions applied in this study. The observed lack of incorporation of phosphate agrees with the previously suggested growth mechanism via the attachment of hydrous CaCO30 complexes which likely does not facilitate substantial substitution of carbonate by phosphate ions.

Study on the growth mechanism of porous spherical calcium carbonate synthesized by carbonization controlled by amino acids

The formation of vaterite is well known to be influenced by many low molecular weight additives. In this study, Density functional theory calculations was employed to study the effect of biological material lysine(Lys), aspartic acid(Asp), threonine(Thr) and glycine(Gly) on the formation of vaterite CaCO3. The interaction energies between Gly and CaCO3(110) and (104) surfaces were −2.618 and −1.784 eV, respectively. With the molar ratio of Ca2+ to Gly at 1.5, the reaction time of 11.5 min, the pH value of 7.5, the crystalline product of the reaction was almost monodisperse spherical vaterite in Ca(OH)2-Gly-CO2 reaction system. The obtained products were characterized by the XRD, RAM, SEM, TEM, and FT-IR. The results showed that the increase of Gly concentration inhibited the nucleation and the growth of calcite. Lys, Asp, Thr and Gly were employed as stabilizers for controlling particles, the crystalline product of the reaction was almost monodisperse vaterite CaCO3. Its surface chemistry can be explored using this mechanism analysis. The methods are expected to be a reference for industrial production process.

Unravelling calcite-to-aragonite evolution from a subsurface fluid - Formation pathway, interfacial reactions and nucleation effects

Calcite-aragonite alternations are documented in sedimentary deposits worldwide, but their formation is still poorly understood and individual CaCO3 precipitation pathways are rarely confirmed experimentally. Therefore, (sub)recent CaCO3 sinter formation in a historic subsurface adit at Erzberg (Austria) was used as a natural laboratory to monitor and assess the calcite-to-aragonite evolution pathway and intergrowth mechanism in terms of solid-liquid-atmosphere dynamics and their relevance for solid-liquid interface reactions and nucleation effects. Our results indicate an initial homogeneous nucleation of low-Mg calcite (LMC: ∼3 ± 1 mol% MgCO3), induced by CO2 degassing from the percolating geogenic fluid, which is originated from seepage of local meteoric water and incongruent dissolution of Mg-Ca-Fe-bearing minerals from the host rock. Progressive LMC growth leads to an increase in the aqueous molar ratio of Mg/Ca, causing a Mg/Ca zonation pattern with transitions to high-Mg calcite (HMC: up to 7 mol% MgCO3). At a critical Mg concentration, the available Mg calcite crystal surfaces are acting as a nucleation site for heterogenous aragonite formation. In this way, fast growing acicular aragonite crystals are initiated, which impede further calcite growth. The calcite-to-aragonite transition is thus controlled by the reaction kinetics and mechanisms of Mg-calcite formation and the chemical evolution of the precipitating solution at the nano- to micro-spatial scale, creating Mg-enriched HMC surface sites for aragonite to be nucleated and preferentially grown. In the present case, the dynamics of the formation of calcite-aragonite sequences are triggered by distinct local environmental changes, in particular seasonal variations in seepage fluid flow behavior and progress in CO2 degassing. These considerations are relevant for a better understanding of proxy signal development and preservation in calcareous sedimentary sequences forming under highly dynamic environmental conditions.

Revealing the Secrets behind the Color and Sea-Wave Patterns of Larimar

In the last century, a blue–green colored gemstone known as Larimar with a special sea-wave pattern was discovered in the Dominican Republic. Larimar is composed of the mineral pectolite, which has a chemical composition of NaCa2Si3O8(OH) and is usually white in color. Cu2+ has always been considered to be the primary genesis of the blue color shown in Larimar, because native copper often grows together with Larimar. To clarify whether copper is the main reason for the origin of blue–green pectolite, we utilized laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) techniques to analyze trace elements in the pectolite samples and compared the relationship between elements and colors. The results show that vanadium and iron are the main origins of the sky-blue and green color of Larimar. We also discovered that it is not only the chemical elements that affect the color shades of the mineral, but the orientation of the radial fiber crystals also plays a critical role. The sea-wave pattern and the changes in the color saturation of radial pectolite are due to the transmittance of visible light through different viewed angles under changing crystal orientations. Our results reveal the chemical and physical factors behind the color and sea-wave pattern of Larimar. In addition, to our knowledge, this is the first time that the formation age of Larimar has been proven to be approximately equal to or younger than 40 ka, using the U-Th dating of calcite growth together with pectolite.

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.

Influence of impurity metal doping on calcite growth: A first-principles study

To better understand the effect of impurity metal doping on the initial stage of calcite growth, the attachment of the CaCO3 molecule on anhydrous and hydrated pristine and doped (Mg-, Cu-, Zn-, Sr- and Ba-doped) calcite (101¯4) surfaces and the analysis of the interactions of the water molecule with various dopants were studied using the first-principles methods. Simulation results suggest that the adsorption ability of anhydrous calcite surfaces toward the CaCO3 molecule is determined by the size of the dopant. The adsorption behavior for water molecules is dominated by chemical bonding. For the adsorption of the CaCO3 molecule on hydrated calcite surfaces, both the dopant size and the chemical bonding factors influence its stability, resulting in the decrease of the CaCO3 adsorption on all doped surfaces. By considering synergetic effects of impurity metal doping on the water and CaCO3 adsorption, we provide new insights into the understanding of the inhibiting mechanism of Mg, Cu and Zn impurities on calcite growth. Moreover, the predicted facilitation of Sr and Ba dopants on calcite growth highlights the feasibility of applying the calcite precipitation technique in mineralizing toxic Sr and Ba from contaminated environments.

On hydrogen-cement reaction: Investigation on well integrity during underground hydrogen storage

Underground hydrogen storage (UHS) is becoming a promising technology to help achieve full hydrogen economy and support the energy transition to carbon net-zero. One of the important parameters for securing UHS for example in depleted gas reservoirs is related to the wellbore integrity and cement seal, which is rarely addressed. When hydrogen is injected, it can react with the cement around the wellbore, and cause various alterations in cement petro-physical properties. Therefore, it is crucial to understand the associated reactions between hydrogen and cement and their implications for cement degradation. In this study, hydrogen injection experiments were performed on two cement core plugs (class G) under 1400 psi and 75 °C for 125 days, and the impact on cement integrity was evaluated. The results show low reactivity of hydrogen with cement and minor alteration in the petro-physical properties of the cement samples. The XRD results show a minor increase in Alite contents after hydrogen injection, while the observed calcite growth was attributed to the cement hydration process. Despite the presence of high mobility elements such as Ca, Mg, K and Al in the cement plugs, the minor redox reactions between cement minerals and hydrogen inhibit the precipitation of high-density minerals. However, we indeed observed for the first time an increase in cement weight and density with hydrogen injection, which suggest that hydrogen/brine injection may promote the precipitation of high density products. This caused a reduction in the CT porosity by 8.86% and 8.43% in samples C#1 and C#2 respectively. However, the total reduction in porosity is considered minor, suggesting a limited impact on cement integrity -which is favorable for UHS-. Furthermore, the alteration in the statistical distributions of P- and S-waves caused a minor increase in Poisson's ratio and dynamic elastic modulus (E), suggesting a low risk of wellbore cement degradation during UHS. The obtained results provide clear analyses of the reactivity of cement to hydrogen injection, and suggest that no significant impact on wellbore integrity or cement seal would be expected during UHS.

Surface roughness and overgrowth dynamics: The effect of substrate micro-topography on calcite growth and Sr uptake

Substrate roughness influences epitaxial nucleation, growth, and trace element incorporation. We qualitatively and quantitatively examine the role of calcite surface roughness on overgrowth morphology and Sr concentration. Surfaces were made by cleaving optically clear Iceland spar, then abrading calcite cleavage surfaces with sandpaper (physical weathering analogues), or by attacking the cleavage surface with dilute HCl (chemical weathering analogues). Calcite precipitated on “weathered” surfaces have morphologies that vary systematically from one substrate type to the next. Growth on the abraded substrate was shingled, didn't form a continuous growth layer, and was poorly adhered to the seed crystal. Growth on the acid etched substrate mostly formed a continuous growth layer in optical continuity with the seed crystal. Differences in overgrowth morphology are interpreted to be due to differences in island coalescence dynamics on the different substrates. Rough substrates have a high density of island nuclei per unit surface area, which inhibits island coalescence due to interfacial misfit between individual islands. In contrast, overgrowths on smooth substrates coalesced into a single crystalline overgrowth layer. There is a weak positive correlation between the initial fractal dimension of the substrate and average Sr concentration in the overgrowth.

In Situ Rb/Sr Geochronology and Stable Isotope Geochemistry Evidence for Neoproterozoic and Paleozoic Fracture‐Hosted Fluid Flow and Microbial Activity in Paleoproterozoic Basement, SW Sweden

AbstractRecent studies have shown that biosignatures of ancient microbial life exist in mineral coatings in deep bedrock fractures of Precambrian cratons, but such surveys have been few and far between. Here, we report results from southwestern Sweden in an area of 1.6–1.5 Ga Paleoproterozoic rocks heavily reworked by the 1.14–0.96 Ga Sveconorwegian orogeny, a terrane previously scarcely explored for ancient microbial biosignatures. Calcite‐pyrite‐adularia‐illite‐coated fractures were analyzed for stable isotopes via Secondary Ion Mass Spectrometry (δ13C, δ18O, δ34S) and in situ Rb/Sr geochronology via Laser‐ablation inductively coupled plasma mass spectrometry. The Rb/Sr ages for calcite‐adularia and calcite‐illite show that several fluid flow events can be discerned (797 ± 18–769 ± 7, 391 ± 5–387 ± 6, 356 ± 5–347 ± 4, and 301 ± 7 Ma). The δ13C, δ18O and 87Sr/86Sr values of different calcite growth zones further confirmed episodic fluid flow. Pyrite δ34S values down to −49.9‰V‐CDT, together with systematically increased δ34S from crystal core to rim, suggest formation following microbial sulfate reduction under semi‐closed conditions. Assemblages involving MSR‐related pyrite generally have Devonian to Permian Rb/Sr ages, indicating an association to extension‐related fracturing and fluid mixing during foreland‐basin formation linked to Caledonian orogeny in the northwest. An assemblage with an age of 301 ± 7 Ma is potentially related to Oslo Rift extension, whereas the Neo‐Proterozoic ages relate to post‐Sveconorwegian extensional tectonics. Remnants of short‐chained fatty acids in the youngest calcite coatings further indicate a biogenic origin, while the absence of organic molecules in older calcite is in line with thermal degradation, potentially related to heating during Caledonian foreland basin burial.