Precipitation Of Amorphous Calcium Carbonate Research Articles

Revisiting geochemical controls on patterns of carbonate deposition through the lens of multiple pathways to mineralization

The carbonate sedimentary record contains diverse compositions and textures that reflect the evolution of oceans and atmospheres through geological time. Efforts to reconstruct paleoenvironmental conditions from these deposits continue to be hindered by the need for process-based models that can explain observed shifts in carbonate chemistry and form. Traditional interpretations assume minerals precipitate and grow by classical ion-by-ion addition processes but are unable to reconcile a number of unusual features contained in Proterozoic carbonates. The realization that diverse organisms produce high Mg carbonate skeletal structures by non-classical pathways involving amorphous intermediates raises the question of whether similar processes are also active in sedimentary environments. This study examines the hypothesis that non-classical pathways to mineralization are the physical basis for some of the carbonate morphologies and compositions observed in natural and laboratory settings. We designed experiments with a series of different solution Mg : Ca ratios and saturation environments to investigate the effects on carbonate phase, Mg content, and morphology. Our observations of diverse carbonate mineral compositions and textures suggest geochemical conditions bias the mineralization pathway by a systematic relationship to Mg : Ca ratio and the abundance of carbonate ions. Environments with low Mg levels produce calcite crystallites with 0–12 mol% MgCO3. In contrast, the combination of high initial Mg : Ca and rapidly increasing saturation opens a non-classical pathway that begins with extensive precipitation of an amorphous calcium carbonate (ACC). This phase slowly transforms to aggregates of very high Mg calcite nanoparticles whose structures and compositions are similar to natural disordered dolomites. The non-classical pathways are favored when the local environment contains sufficient Mg to inhibit calcite growth through increased solubility—a thermodynamic factor, and achieves saturation with respect to ACC on a timescale that is shorter than the rate of aragonite nucleation—a kinetic factor. Aragonite is produced when Mg levels are high but saturation is insufficient for ACC precipitation. The findings provide a physical basis for anecdotal claims that the interplay of kinetic and thermodynamic factors underlies patterns of carbonate precipitation and suggest the need to expand traditional interpretations of geological carbonate formation to include non-classical pathways to mineralization.

Magnesium stable isotope fractionation in marine biogenic calcite and aragonite

This survey of magnesium stable isotope compositions in marine biogenic aragonite and calcite includes samples from corals, sclerosponges, benthic porcelaneous and planktonic perforate foraminifera, coccolith oozes, red algae, and an echinoid and brachiopod test. The analyses were carried out using MC-ICP-MS with an external repeatability of ±0.22‰ (2SD for δ 26Mg; n = 37), obtained from a coral reference sample (JCp-1). Magnesium isotope fractionation in calcitic corals and sclerosponges agrees with published data for calcitic speleothems with an average Δ 26Mg calcite–seawater = −2.6 ± 0.3‰ that appears to be weakly related to temperature. With one exception ( Vaceletia spp.), aragonitic corals and sclerosponges also display uniform Mg isotope fractionations relative to seawater with Δ 26Mg biogenic aragonite–seawater = −0.9 ± 0.2. Magnesium isotopes in high-Mg calcites from red algae, echinoids and perhaps some porcelaneous foraminifera as well as in all low-Mg calcites (perforate foraminifera, coccoliths and brachiopods) display significant biological influences. For planktonic foraminifera, the Mg isotope data is consistent with the fixation of Mg by organic material under equilibrium conditions, but appears to be inconsistent with Mg removal from vacuoles. Our preferred model, however, suggests that planktonic foraminifera synthesize biomolecules that increase the energetic barrier for Mg incorporation. In this model, the need to remove large quantities of Mg from vacuole solutions is avoided. For the high-Mg calcites from echinoids, the precipitation of amorphous calcium carbonate may be responsible for their weaker Mg isotope fractionation. Disregarding superimposed biological effects, it appears that cation light isotope enrichments in CaCO 3 principally result from a chemical kinetic isotope effect, related to the incorporation of cations at kink sites. In this model, the systematics of cation isotope fractionations in CaCO 3 relate to the activation energy required for cation incorporation, which probably reflects the dehydration of the cation and the crystal surface and bond formation at the incorporation site. This kinetic incorporation model predicts (i) no intrinsic dependence on growth rate, unless significant back reaction upon slow growth reduces the isotope fractionation towards that characteristic for equilibrium isotope partitioning (this may be observed for Ca isotopes in calcites), (ii) a small decrease of isotope fractionation with increasing temperature that may be amplified if higher temperatures promote back reaction and (iii) a sensitivity to changes in the activation barrier caused by additives such as anions or biomolecules or by the initial formation of amorphous CaCO 3.

Amorphous calcium carbonate in form of spherical nanosized particles and its application as fillers for polymers

The synthesis of amorphous calcium carbonate (ACC) via a liquid precursor to give spherical particles with monodisperse distribution of diameters in the range of 0.4–1.2 μm has been optimized to the level to obtain multigram yields per batch. The synthesis was achieved by precipitation of ACC from a strongly alkaline solution of calcium chloride (CaCl 2) at ambient temperature using the hydrolysis of water soluble dimethyl carbonate (DMC) as the internal source of CO 2. As ACC produced by this novel method contains a small fraction of bound water a drying process, namely annealing at 200 °C for 6 h was developed. The water free powder was blended with conventional polyolefins (linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), polystyrene (PS)) in a melt extrusion process. Blending with poly(lactic acid) (PLA) as a model of a biomedically relevant polymer was also achieved. While a homogenous dispersion of the dried amorphous calcium carbonate (DACC) particles in the polymer composites was easily achieved, the interaction between the polymer continuous phase and particle surface seems to be rather weak. Consequently, the physical properties of the blends having volume fractions from 10 to 40% of the filler behave as expected for non-interacting materials, e.g. the melt viscosity increases as predicted from Einstein's law, the glass transition and melting temperature of the polymer matrices remain largely unaffected. The Young's modulus did increase while tensile strength and elongation at break decrease.

The mechanism of precipitation of chain-like calcite

In this work a mechanism for the precipitation of chain-like calcite agglomerates is proposed and compared to those ones leading to nano- and microcalcite in the system Ca(OH) 2–CO 2–H 2O. The experiments were carried out by semicontinuous carbonation of a slaked lime (Ca(OH) 2) suspension at 20°C, 25°C and 30°C, fixing the suspension conductivity at 5 mS/cm. The processes at 20°C and 25°C were essentially similar except in the final stage. Basically, they both occurred via surface and were characterized by the initial precipitation of amorphous calcium carbonate followed by its crystallization into chain-like aggregates of calcite. These aggregates acted as an intermediate for the final products: spheroidal nanometric calcite at 20°C and submicrometric chain-like calcite agglomerates (similar to batons) at 25°C. On the other hand, at 30°C, the precipitation took place preferentially via solution, producing micrometric calcite of scalenohedral habit as final product. The change in the mechanism of precipitation from preferentially via surface (at 20°C and 25°C) to preferentially via solution (at 30°C) seems to be directly related to the availability of Ca(OH) 2 surface in the reaction mixture during the first steps of precipitation.

Mechanisms of scale formation and carbon dioxide partial pressure influence. Part I. Elaboration of an experimental method and a scaling model

Scale formation in industrial or domestic installations is still an important economic problem. The existence of a metastable domain for calcium carbonate supersaturated solutions and its breakdown are observed under conditions rarely well defined. In most cases it is the pH rise caused by the carbon dioxide loss that involves calcium carbonate precipitation. Before studying this problem, we suggest in this first part, a new model for the evolution of the calco-carbonic system that takes into account the hydrated forms of CaCO 3 : CaCO 3 amorphous, CaCO 3·6H 2O (ikaite) and CaCO 3·H 2O (monohydrate). According to this model, the precipitation of any one of these hydrated forms could be responsible for the breakdown of the metastable state. After this first step, the solids evolve into dehydrated forms. At first, the metastable domain spread of the calcium carbonate supersaturated solutions was studied by the elaboration of computer programs in which the formation of CaCO 3 0(aq) ion pairs was taken into account. These ion pairs are supposed to evolve through dehydration to form the various calcium carbonate solid form precursors. This thermodynamic study was then compared to the experimental methods of the critical pH. Here the pH rise was caused by adding sodium hydroxide under different conditions for sodium hydroxide addition speed, agitation mode and ageing of solutions. For the highest speed of sodium hydroxide addition, the CaCO 3 ionic product reached the value of the amorphous calcium carbonate solubility product, and the reaction of the amorphous calcium carbonate precipitation was of the homogenous type. Decreasing the reagent's addition speed caused an extension of the titration time. Then, the breakdown of the metastable state was obtained with the CaCO 3·H 2O heterogeneous precipitation. This clearly illustrates the probable ageing of the precursors of the solid states that are considered in this model.

Reactive ion etching of semiconductor materials and of dielectric and metal layers : Zdenek Novotny. TESLA Electron., 3–4, 59 (1986)

In this work a mechanism for the precipitation of chain-like calcite agglomerates is proposed and compared to those ones leading to nano- and microcalcite in the system Ca(OH)2–CO2–H2O. The experiments were carried out by semicontinuous carbonation of a slaked lime (Ca(OH)2) suspension at 20°C, 25°C and 30°C, fixing the suspension conductivity at 5 mS/cm. The processes at 20°C and 25°C were essentially similar except in the final stage. Basically, they both occurred via surface and were characterized by the initial precipitation of amorphous calcium carbonate followed by its crystallization into chain-like aggregates of calcite. These aggregates acted as an intermediate for the final products: spheroidal nanometric calcite at 20°C and submicrometric chain-like calcite agglomerates (similar to batons) at 25°C. On the other hand, at 30°C, the precipitation took place preferentially via solution, producing micrometric calcite of scalenohedral habit as final product. The change in the mechanism of precipitation from preferentially via surface (at 20°C and 25°C) to preferentially via solution (at 30°C) seems to be directly related to the availability of Ca(OH)2 surface in the reaction mixture during the first steps of precipitation.