CaCO3 Polymorphs Research Articles

Physical and nanomechanical properties of the synthetic anhydrous crystalline CaCO3 polymorphs: vaterite, aragonite and calcite

The synthetic anhydrous crystalline CaCO3 polymorphs—vaterite, aragonite and calcite—were tested using dilatometry and nanoindentation. Microstructural changes in the samples before and after measurements were observed under scanning electron microscope and their phase composition quantified with X-ray powder diffraction with the Rietveld method. The thermal expansion coefficients of vaterite and the hardness and elastic modulus of synthetic aragonite are reported for the first time. The physical and nanomechanical properties were measured under similar conditions for each CaCO3 polymorph. Aragonite, calcite and vaterite showed volumetric thermal expansion coefficient at 303 K of 49.2(8), 48.6(2) and 44.1(3) 10−6 K−1, respectively. The elastic modulus increased from 5(4), 16(7) to 31(8) GPa for aragonite, calcite and vaterite, respectively. Average hardness was found lower than values from the literature, ranging from 0.3 to 1.3 GPa. The results are considered of interest for the design of CaCO3-based materials for applications.

Key Parameters for Size- and Shape-Controlled Synthesis of Vaterite Particles

Calcium carbonate (CaCO3) has attracted scientific attention due to its essential role in both inorganic and bioorganic chemistry. Vaterite is the least thermodynamically stable CaCO3 polymorph and has elicited great interest as an advanced biomaterial for tissue engineering, drug delivery, and a broad range of personal care products. Numerous methods of vaterite particle synthesis with different sizes and morphologies have highlighted the submicron porous particles of spherical or ellipsoidal shape as the most useful ones. In this regard, the current study is aimed at development of a reliable method for synthesis of such structures. Herein, submicron vaterite particles are synthesized by dropwise precipitation from saturated sodium carbonate and calcium chloride solutions in the presence of ethylene glycol while manipulating the concentration ratios of reagents. We demonstrate that our novel technique named “dropwise precipitation” leads to changing calcium concentrations in the reaction solution at eac...

Crystallization of Calcium Carbonate in Alginate and Xanthan Hydrogels

Calcium carbonate polymorphs were crystallized in alginate and xanthan hydrogels in which a degree of entanglement was altered by the polysaccharide concentration. Both hydrogels contain functional groups (COOH and OH) attached at diverse proportions on saccharide units. In all systems, the precipitation process was initiated simultaneously with gelation, by the fast mixing of the calcium and carbonate solutions, which contain the polysaccharide molecules at respective concentrations. The initial supersaturation was adjusted to be relatively high in order to ensure the conditions suitable for nucleation of all CaCO3 polymorphs and amorphous phase(s). In the model systems (no polysaccharide), a mixture of calcite, vaterite and amorphous calcium carbonate initially precipitated, but after short time only calcite remained. In the presence of xanthan hydrogels, precipitation of either, calcite single crystals, porous polyhedral aggregates, or calcite/vaterite mixtures were observed after five days of ageing, because of different degrees of gel entanglement. At the highest xanthan concentrations applied, the vaterite content was significantly higher. In the alginate hydrogels, calcite microcrystalline aggregates, rosette-like and/or stuck-like monocrystals and vaterite/calcite mixtures precipitated as well. Time resolved crystallization experiments performed in alginate hydrogels indicated the initial formation of a mixture of calcite, vaterite and amorphous calcium carbonate, which transformed to calcite after 24 h of ageing.

Aragonite-II and CaCO3-VII: New High-Pressure, High-Temperature Polymorphs of CaCO3

The importance for the global carbon cycle, the P–T phase diagram of CaCO3 has been under extensive investigation since the invention of the high-pressure techniques. However, this study is far from being completed. In the present work, we show the existence of two new high-pressure polymorphs of CaCO3. The crystal structure prediction performed here reveals a new polymorph corresponding to distorted aragonite structure and named aragonite-II. In situ diamond anvil cell experiments confirm the presence of aragonite-II at 35 GPa and allow identification of another high-pressure polymorph at 50 GPa, named CaCO3-VII. CaCO3-VII is a structural analogue of CaCO3-P21/c-l, predicted theoretically earlier. The P–T phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO3-VII and aragonite-II at 30–50 GPa and 0–1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO3-VI was found to be metastable in the whole pressure–temperature range.

Micro-scale geochemical and crystallographic analysis of Buccinum undatum statoliths supports an annual periodicity of growth ring deposition

The whelk Buccinum undatum is commercially important in the North Atlantic. However, monitoring the ontogenetic age and growth of populations has been problematic for fisheries scientists owing to the lack of a robust age determination method. We confirmed the annual periodicity of growth rings present in calcified statoliths located in the foot of field-collected and laboratory reared whelks using microscale measurements of trace element geochemistry. Using Secondary Ion Mass Spectrometry (SIMS), annual trace element profiles were quantified at 2μm resolution in statoliths removed from whelks collected alive from three locations spanning the length of the UK; the Shetland Isles (North), the Menai Strait, North Wales (Mid) and Jersey (South). Clear cycles in the Mg/Ca ratio were apparent with minimum values corresponding with the visible dark statolith rings and comparatively higher ratios displayed in the first year of growth. Statoliths from one and two-year-old laboratory reared whelks of known age and life history contained one and two Mg/Ca cycles respectively and demonstrated that the statolith growth ring is formed during winter (February and March). Cycles of Na/Ca were found to be anti-correlated to Mg/Ca cycles, whilst ratios of Sr/Ca were inconsistent and showed an apparent ontogenetic increase, suggesting strong physiological control. Variability in elemental data will likely limit the usefulness of these structures as environmental recorders. The results obtained using SIMS for trace element analysis of statoliths confirms the robustness of the statolith rings in estimating whelk age. μXRD at 2μm spatial resolution demonstrated the statoliths were wholly aragonitic and thus trace element variation was not the result of possible differences in CaCO3 polymorph within the statolith. Changing XRD patterns along with SEM imaging also reveal an ‘hourglass’ microstructure within each statolith. The validation of the annual periodicity of statolith growth rings now provides a robust and novel age determination technique that will lead to improved management of B. undatum stocks.

HPAM assisted controllable synthesis of peanut-like CaCO3 in fixed silicate solution

Peanut-like CaCO3 was controllable synthesized in 150mg/L silicate solutions with the help of partially hydrolyzed polyacrylamide (HPAM). The polymorph, crystals structure and composition of CaCO3 under different conditions were characterized by scanning electron microscope (SEM), powder X-ray diffraction (pXRD), energy dispersive X-ray analyser (EDX) and so on. By varying the concentrations of HPAM, initial pH and reaction temperature, peanut-like, fusiform-like and hexahedral CaCO3 particles were prepared successfully. The mechanism of stable peanut-like CaCO3 crystallization induced by silica was proposed. It was found that the adsorption of silanol groups on CaCO3 surfaces played a key role during the condensation reactions of silanol groups, which resulted in the peanut shape of CaCO3 crystals. And as a morphological modifier HPAM led to the formation of specific fibrous surface with different shapes by electrostatic interactions.

A rare mineral, vaterite, acts as a shock absorber in the eggshell of a communally nesting bird

Birds’ eggshells are primarily composed of calcite, an abundant polymorph of calcium carbonate (CaCO3). However, the eggshells of some species are coated with spherules of vaterite, a rare and thermodynamically unstable polymorph of CaCO3, the function of which remains unknown. We experimentally tested the mechanical and physiological effects of the vaterite coating on eggshells of the Greater Ani Crotophaga major, a tropical cuckoo. Vaterite removal did not affect vapour conductance rates across the eggshell, indicating that the vaterite coating does not influence gas exchange during embryonic development. However, nanoindentation revealed that the hardness and elasticity of vaterite is similar to that of calcite, and white light interferometry showed that the vaterite layer increased the total thickness of the shell cuticle by up to 10%. Furthermore, calculations of contact mechanics found that when two eggs come into contact, the depth of the surface deformation caused by the contact is far less than the thickness of the vaterite coating. These results suggest that the layer of vaterite spherules may act as a shock absorber for the underlying calcite shell, protecting it from mechanical damage caused by collision with other eggs in the nest and reducing the risk of eggshell fracture during incubation.

Impact of Citrate Ions on the Nucleation and Growth of Anhydrous CaCO3

Citrate has been shown to inhibit CaCO3 growth and it is also found in biogenic calcite, but full understanding about how citrate controls CaCO3 formation, and hence CaCO3 polymorph stability and crystal morphology, is still lacking. We investigated the effect of citrate (CIT) on CaCO3 crystallization by monitoring crystallization rates and crystal polymorph, size, and morphology as a function of increasing mol % CIT/Ca ([Ca2+] = [CO32–] = 4 mM). At CIT/Ca ≤ 50%, both vaterite and calcite formed, but nucleation and growth were retarded as CIT/Ca was increased, demonstrated by the increase in induction time and decrease in the reaction rate constant, fitted using the Johnson-Mehl-Avrami-Kolmogorov model. This is partly explained by increased CIT-Ca complexation with higher [CIT], thus reducing saturation state with respect to vaterite and calcite. Simultaneously, CIT sorption inhibited crystal growth, demonstrated by smaller vaterite spheres and more elongated calcite crystals as CIT/Ca increased. At CIT/C...

Geochemical modeling of carbonation of hydrated oil well cement exposed to CO2-saturated brine solution

Storage of CO2 into geological formations, such as deep saline aquifers and depleted oil/gas reservoirs, is being investigated to help reduce releases to the atmosphere. A critical issue is the long-term efficacy of engineered (man-made) hydrated cement seals that surround or plug boreholes. In order to investigate the impact of CO2 interactions with hydrated cement, the present study focuses on the alteration processes that were observed in a hydrated cement core (based on Type-G Portland cement) after laboratory experimental reaction with a CO2-saturated brine for 6 months. The carbonated cement block consisted of an orange outer zone, an alteration rim, and the intact cement core. The PHREEQC geochemical code was used to simulate the experimental conditions and data using 1D reactive transport modeling. The first phases to dissolve were portlandite, CS and ettringite providing calcium to solution. Three CaCO3 polymorphs were predicted to precipitate, but the less stable of these (aragonite, vaterite) were transformed to calcite over time. According to simulations the final product after 1000 years of reaction will be 68% calcite, 18% Friedel's salt and 14% Al-Fe-Si hydro/oxides. As the reaction front moved within the sample, porosity decreased due to the formation of amorphous phases and carbonates. The proposed model is a useful tool in describing longer-term hydrated cement alteration behavior during CO2 geological storage and to compare their results with in situ observations from field experiments.

Control of temperature and aqueous Mg2+/Ca2+ ratio on the (trans-)formation of ikaite

The calcium carbonate hexahydrate mineral ikaite (CaCO3⋅6 H2O) has been documented in aquatic environments at near-freezing temperatures. An increase of the prevailing temperature in the depositional environment, results in the transformation of natural ikaite into less soluble calcium carbonate phases occasionally leaving calcite pseudomorphs in the sediments, which are considered asan indicator for primary cold water temperatures. Detailed understanding on the physicochemical parameters controlling ikaite (trans-)formation however, such as temperature and reactive solution chemical composition, are still under debate. In order to study the formation of ikaite, we conducted precipitation experiments under controlled physicochemical conditions (pH = 8.3±0.1; T=6, 12, and 18±0.1°C) at defined aqueous molar Mg/Ca ratios. The transformation of ikaite into anhydrous calcium carbonate polymorphs was investigated in solution and at air exposure. The obtained results reveal the formation of ikaite at temperatures up to 12°C, whereas Mg-rich amorphous calcium carbonate precipitated at 18°C. In contact with the reactive solution ikaite transformed into aragonite at aqueous molar Mg2+/Ca2+ ratios of ≥14. In contrast, ikaite separated from the Mg-rich solution and exposed to air transformed in all cases into calcite/vaterite. The herein obtained temperature limit of ≤12 for ikaite formation is significantly higher than formerly expected and most probably caused by (i) the high saturation degree of the solution with respect to ikaite and (ii) the slow dehydration of the aqueous Ca2+ ion at low temperatures. This result questions the suitability of calcite pseudomorphs (i.e. glendonites) as a proxy for near-freezing temperatures. Moreover, our findings show that the CaCO3 polymorph formed from ikaite is strongly controlled by the physicochemical conditions, such as aqueous molar Mg2+/Ca2+ ratio of the reactive fluid and H2O availability throughout the transformation process.

Microstructural changes in alkali-activated slag mortars induced by accelerated carbonation

This study investigated microstructural changes in alkali-activated slag (AAS) mortars after carbonation using XRD, FTIR, DTG, 1H NMR and SEM examinations. The results showed that decalcification of C-S-H was the main reaction in carbonation of AAS. The C-S-H with a low Ca/Si was more vulnerable to decalcification in AAS samples activated by waterglass. Besides, AAS mortars demonstrated a lower CaCO3 formation compared to Portland cement mortars. Calcite and vaterite were the major CaCO3 polymorphs produced by carbonation of AAS precipitated mainly in gel pores and spaces in C-S-H interlayers. Meanwhile, the carbonation also caused a certain volume of contraction in AAS mortars.

Calcium Carbonate Precipitation for CO2 Storage and Utilization: A Review of the Carbonate Crystallization and Polymorphism

The transformation of CO2 into a precipitated mineral carbonate through an ex situ mineral carbonation route is considered a promising option for CO2 capture and storage (CCS) since (i) the captured CO2 can be stored permanently, and (ii) industrial wastes (i.e. coal fly ash, steel and stainless-steel slags, and cement and lime kiln dusts) can be recycled and converted into value-added carbonate materials by controlling polymorphs and properties of the mineral carbonates. The final products produced by the ex situ mineral carbonation route can be divided into two categories − low-end high-volume and high-end low-volume mineral carbonates− in terms of their market needs as well as their properties (i.e. purity). Therefore, it is expected that this can partially offset the total cost of the CCS processes. Polymorphs and physicochemical properties of CaCO3 strongly rely on the synthesis variables such as temperature, pH of the solution, reaction time, ion concentration and ratio, stirring, and the concentration of additives. Various efforts to control and fabricate polymorphs of CaCO3 have been made to date. In this review, we present a summary of current knowledge and recent investigations entailing mechanistic studies on the formation of the precipitated CaCO3 and the influences of the synthesis factors on the polymorphs.

Green technological approach to synthesis hydrophobic stable crystalline calcite particles with one-pot synthesis for oil–water separation during oil spill cleanup

The process of separating oil and water from oil/water mixtures is an attractive strategy to answer the menace caused by industrial oil spills and oily wastewater. In addition, water coproduced during hydrocarbon exploitation, which can be an economic burden and risk for freshwater resources, can become an important freshwater source after suitable water-oil separation. For oil-water separation purposes, considerable attention has been paid to the preparation of hydrophobic-oleophilic materials with modified surface roughness. However, due to issues of thermodynamic instability, costly and complex methods as well as lack of ecofriendly compounds, most of hydrophobic surface modified particles are of limited practical application. The study presents a facile procedure, to synthesize crystalline particles of calcite, which is the most stable polymorph of CaCO3 from industrial CaCO3 using oleic acid as an additive in a one-pot synthesis method. The XRD results show that the synthesized particles were a well-crystallized form of calcite. The FTIR results reflect the appearance of the alkyl groups from the oleic acid in synthesized particles which promotes the production of calcite with ‘rice shape’ (1.64 μm) (aggregated by spherical nanoparticle of 19.56 nm) morphology with concomitant changes in its surface wettability from hydrophilic to hydrophobic. The synthesized particles exhibited near to super hydrophobicity with ∼99% active ratio and a contact angle of 143.8°. The synthesized hydrophobic calcite particles had an oleophilic nature where waste diesel oil adsorption capacity of synthesized calcium carbonate (HCF) showed a very high (>99%) and fast (7 s) oil removal from oil-water mixture. The functional group of long alkyl chain including of CO bounds may play critical roles for adsorption of diesel oils. Moreover, the thermodynamically stable crystalline polymorph calcite (compared to vaterite) exhibited excellent recyclability. The isothermal study reflects the comparatively high value of correlation coefficient (R2 = 0.94) for the Langmuir isotherm compared to those of the Freundlich isotherm (R2 = 0.82) showed that the adsorption of diesel oil onto the hydrophobic CaCO3 adsorbent was much better described by the Langmuir isotherm. The kinetics study of second-order rate expression (R2 = 0.99) more fitted with the experimental data compare to first-order model (R2 = 0.92). The synthesized calcite exhibited a significant dual oleophilic and hydrophobic nature that can be applicable for oil adsorption/or removal purpose in oil contaminated areas in environment and/or industrial oily wastewater for green, simple, and inexpensive environmental cleanup.

Elastic properties of CaCO3 high pressure phases from first principles**Project supported by the National Natural Science Foundation of China (Grant Nos. 41174071, 41373060, 41374096, and 41403099) and the Seismic Fund of Institute of Earthquake Science, China Earthquake Administration (CEA) (Grant Nos. 2012IES0408, 2014IES0407, and 2016IES0101).

Elastic properties of three high pressure polymorphs of CaCO3 are investigated based on first principles calculations. The calculations are conducted at 0 GPa–40 GPa for aragonite, 40 GPa–65 GPa for post-aragonite, and 65 GPa–150 GPa for the -h-CaCO structure, respectively. By fitting the third-order Birch–Murnaghan equation of state (EOS), the values of bulk modulus and pressure derivative are 66.09 GPa and 4.64 for aragonite, 81.93 GPa and 4.49 for post-aragonite, and 56.55 GPa and 5.40 for -h-CaCO, respectively, which are in good agreement with previous experimental and theoretical data. Elastic constants, wave velocities, and wave velocity anisotropies of the three high-pressure CaCO phases are obtained. Post-aragonite exhibits 25.90%–32.10% anisotropy and 74.34%–104.30% splitting anisotropy, and -h-CaCO shows 22.30%–25.40% anisotropy and 42.81%–48.00% splitting anisotropy in the calculated pressure range. Compared with major minerals of the lower mantle, CaCO high pressure polymorphs have low isotropic wave velocity and high wave velocity anisotropies. These results are important for understanding the deep carbon cycle and seismic wave velocity structure in the lower mantle.

Effects of Experimental Parameters on the Extraction of Silica and Carbonation of Blast Furnace Slag at Atmospheric Pressure in Low-Concentration Acetic Acid

Blast furnace slag (BFS), a calcium-rich industrial byproduct, has been utilized since 2005 as a mineral carbonation feedstock for CO2 sequestration, producing calcium carbonate precipitates. In this study, the conditions for the dissolution of Ca and Si in acetic acid, and subsequent carbonation, were elaborated. For this purpose, the retardation of the polymerization of silicon was attempted by varying the concentration of acetic acid, temperature, and leaching time. An inductively coupled plasma (ICP) analysis revealed that both the Ca and Si dissolved completely within 30 min in 5% acetic acid at room temperature. This high dissolution value can be attributed to the fact that Ca was bound to O rather than to Si, as determined by X-ray photoelectron spectroscopy (XPS). The use of CO2-absorbed monoethanolamine enabled the complete carbonation of BFS at ambient conditions without the need for a pH swing. The presence of dissolved silica was found to affect the polymorphs of the precipitated CaCO3. We believe that this process offers a simple method for manipulating the composites of products obtained by mineral carbonation diminishing the leaching residues.

Polymorph and morphology of CaCO3 in relation to precipitation conditions in a bubbling system

Abstract Simulating the typical carbonation step in a mineral CO 2 sequestration, precipitated calcium carbonate (PCC) was prepared by bubbling CO 2 gas into a rich Ca solution. These carbonation reactions were conducted at three pH ranges, namely 10.0–9.0, 9.0–8.0, and 8.0–7.0, in which temperature and CO 2 flow rate are additional experimental variables. The PCC obtained in experiments was examined by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). It was found that supersaturation determined by pH value and flow rate of CO 2 has significant influence on polymorph of PCC. Vaterite was preferably formed at high supersaturation, while dissolution of metastable vaterite and crystallization of calcite occurred at low supersaturation. High temperature is a critical factor for the formation of aragonite. At 70 °C, vaterite, calcite and aragonite were observed to coexist in PCC because transformation from vaterite to aragonite via calcite occurred at this temperature. Scanning electron microscopy (SEM) technology was performed on prepared PCC, and various morphologies consistent with polymorphs were observed.

Experimental diagenesis: insights into aragonite to calcite transformation of <i>Arctica islandica</i> shells by hydrothermal treatment

Abstract. Biomineralised hard parts form the most important physical fossil record of past environmental conditions. However, living organisms are not in thermodynamic equilibrium with their environment and create local chemical compartments within their bodies where physiologic processes such as biomineralisation take place. In generating their mineralised hard parts, most marine invertebrates produce metastable aragonite rather than the stable polymorph of CaCO3, calcite. After death of the organism the physiological conditions, which were present during biomineralisation, are not sustained any further and the system moves toward inorganic equilibrium with the surrounding inorganic geological system. Thus, during diagenesis the original biogenic structure of aragonitic tissue disappears and is replaced by inorganic structural features. In order to understand the diagenetic replacement of biogenic aragonite to non-biogenic calcite, we subjected Arctica islandica mollusc shells to hydrothermal alteration experiments. Experimental conditions were between 100 and 175 °C, with the main focus on 100 and 175 °C, reaction durations between 1 and 84 days, and alteration fluids simulating meteoric and burial waters, respectively. Detailed microstructural and geochemical data were collected for samples altered at 100 °C (and at 0.1 MPa pressure) for 28 days and for samples altered at 175 °C (and at 0.9 MPa pressure) for 7 and 84 days. During hydrothermal alteration at 100 °C for 28 days most but not the entire biopolymer matrix was destroyed, while shell aragonite and its characteristic microstructure was largely preserved. In all experiments up to 174 °C, there are no signs of a replacement reaction of shell aragonite to calcite in X-ray diffraction bulk analysis. At 175 °C the replacement reaction started after a dormant time of 4 days, and the original shell microstructure was almost completely overprinted by the aragonite to calcite replacement reaction after 10 days. Newly formed calcite nucleated at locations which were in contact with the fluid, at the shell surface, in the open pore system, and along growth lines. In the experiments with fluids simulating meteoric water, calcite crystals reached sizes up to 200 µm, while in the experiments with Mg-containing fluids the calcite crystals reached sizes up to 1 mm after 7 days of alteration. Aragonite is metastable at all applied conditions. Only a small bulk thermodynamic driving force exists for the transition to calcite. We attribute the sluggish replacement reaction to the inhibition of calcite nucleation in the temperature window from ca. 50 to ca. 170 °C or, additionally, to the presence of magnesium. Correspondingly, in Mg2+-bearing solutions the newly formed calcite crystals are larger than in Mg2+-free solutions. Overall, the aragonite–calcite transition occurs via an interface-coupled dissolution–reprecipitation mechanism, which preserves morphologies down to the sub-micrometre scale and induces porosity in the newly formed phase. The absence of aragonite replacement by calcite at temperatures lower than 175 °C contributes to explaining why aragonitic or bimineralic shells and skeletons have a good potential of preservation and a complete fossil record.

Role of Brucite Dissolution in Calcium Carbonate Precipitation from Artificial and Natural Seawaters

Brucite is often associated with calcium carbonate in calcareous scales. In this study, we focus on the effects of brucite dissolution on the carbonate precipitation. Brucite-saturated solutions are elaborated by adding the same amount of brucite powder in various volumes of different natural and artificial seawaters (natural organic materials are also added, with or without Mg2+ ions). The Mg2+ and Ca2+ concentrations in seawater at the beginning and end of the experiments are determined by ICP-AES. The solid powders are ex-situ analyzed using XRD and SEM. It is observed that brucite dissolution leads to pH increase and enrichment of seawaters in magnesium ions. The Mg2+ ions, either pre-existing in the solution or added from brucite dissolution play a key role in CaCO3 polymorph selection. In order to separate the Mg2+ and OH– effects, CaCO3 precipitation is induced by NaOH addition in artificial seawater with or without Mg2+. Under both NaOH and Mg(OH)2 addition, only aragonite is precipitated from art...

Biomimetic Regulation of Microbially Induced Calcium Carbonate Precipitation Involving Immobilization of Sporasarcina pasteurii by Sodium Alginate

Many researchers in the past decade have explored the controlled synthesis of calcium carbonate with specific size, morphology, and polymorphism. This study explored the biomimetic regulation of microbially induced calcium carbonate precipitation (MICP) via employing immobilization technology. Calcium alginate gel was used to immobilize Sporosarcina pasteurii, a urea-positive microorganism. CO32– was generated driven by ureolysis and reacted directly with Ca2+ that was cross-linked in sodium alginate to produce CaCO3 precipitation. Based on SEM, TEM, XRD, HRTEM, and SAED results, amorphous calcium carbonate, vaterite, and calcite appeared in order. This evolution of CaCO3 morphology and polymorphism apparently conforms to Ostwald’s rule. Various concentrations (1–3%) of sodium alginate caused different alginate molecules to form due to the collapse of calcium alginate gel carrying negative charges and exerting a significant influence on the morphology of CaCO3 from hexagonal vaterite to capsule-shaped vat...

Carbon isotope fractionation between amorphous calcium carbonate and calcite in earthworm-produced calcium carbonate

In this study we investigate carbon isotope fractionation during the crystallization of biogenic calcium carbonate. Several species of earthworm including Lumbricus terrestris secrete CaCO3. Initially a milky fluid comprising micro-spherules of amorphous CaCO3 (ACC) is secreted into pouches of the earthworm calciferous gland. The micro-spherules coalesce and crystalize to form millimetre scale granules, largely comprising calcite. These are secreted into the earthworm intestine and from there into the soil. L. terrestris were cultured for 28 days in two different soils, moistened with three different mineral waters at 10, 16 and 20 °C. The milky fluid in the calciferous glands, granules in the pouches of the calciferous glands and granules excreted into the soil were collected and analysed by FTIR spectroscopy to determine the form of CaCO3 present and by IRMS to determine δ13C values. The milky fluid was ACC. Granules removed from the pouches and soil were largely calcite; the granules removed from the pouches contained more residual ACC than those recovered from the soil. The δ13C values of milky fluid and pouch granules became significantly more negative with increasing temperature (p ≤ 0.001). For samples from each temperature treatment, δ13C values became significantly (p ≤ 0.001) more negative from the milky fluid to the pouch granules to the soil granules (−13.77, −14.69 and −15.00 respectively at 10 °C; −14.37, −15.07 and −15.18 respectively at 16 °C and −14.89, −15.41 and −15.65 respectively at 20 °C). Fractionation of C isotopes occurred as the ACC recrystallized to form calcite with the fractionation factor εcalcite-ACC = −1.20 ± 0.52‰. This is consistent with the crystallization involving dissolution and reprecipitation rather than a solid state rearrangement. Although C isotopic fractionation has previously been described between different species of dissolved inorganic carbon and various CaCO3 polymorphs, this is the first documented evidence for C isotope fractionation between ACC and the calcite it recrystallizes to. This phenomenon may prove important for the interpretation of CaCO3-based C isotope environmental proxies.