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Poly-L-glutamic (pGlu) and poly-L-aspartic (pAsp) acids, as analogues of naturally occurring soluble acidic proteins, were used to investigate specific interactions between the acidic polypeptides and calcite as a mineral surface. A basic polypeptide (poly-L-lysine, pLys) served as a negative control for testing the role of conformity between the substrate and the adsorbed polypeptides in biomineralization processes. The kinetics of calcite crystal growth was determined in a simplified precipitation model system by inoculating well defined calcite seed crystals into a moderately supersaturated solution containing one of the polypeptides. From the kinetic data, the mode and extent of the calcite/polypeptide interactions were determined. The parabolic rate law was found to be valid for the calcite crystal growth, the integration of ions into the spiral steps at the calcite crystal surface being the rate-determining mechanism. Small amounts of pGlu or pAsp caused an inhibition of calcite crystal growth, the effect being pAsp > pGlu, and the exponential dependence of the growth rate on supersaturation confirmed that surface nucleation was the growth controlling mechanism in the presence of the two acidic polypeptides. The pLys non-selective, weak, electrostatic adsorption at the crystal surface was probably responsible for increasing the calcite growth rate at low concentrations and for inhibiting it at higher concentrations. The strongest interactions between the crystal surfaces and the polypeptides were observed for the calcite/pAsp systems. They could be accounted for by coordinative interactions between the side chain carboxylic groups of the predominantly planar arrangement of the pAsp structure (β -pleated sheet) and Ca2+ ions from the calcite surface.

Many living organisms form mineral phases through biologically controlled processes, known as biomineralization. Thus created materials are composites of both, mineral and organic components. The shell of the gastropod mollusc red abalone (Haliotis rufescens) consists of calcite and aragonite layers, each of them containing characteristic biopolymers responsible for biomineralization. In this work, the effect of interstitial green sheet polypeptide, GP, extracted from the green layer of the mollusc red abalone shell, on the process of spontaneous precipitation of calcium carbonate polymorphs, was investigated. Three precipitation systems, in which the initial mineralogical composition of the precipitate was different, have been studied. Thus, in system (1) calcite appeared, in system (2) a mixture of calcite and vaterite was found, while in system (3), ASW, only aragonite precipitated. However, the precipitation kinetic measurements, X-ray diffraction, FT-IR spectroscopy, and light and scanning electron microscopy indicated that the addition of GP in the model systems caused the inhibition of precipitation and change of morphology of crystals as a consequence of GP adsorption on the crystal surfaces and its entrapment into the mineral structure. (doi: 10.5562/cca2197)

In order to find out how cadmium ions influence the formation and stability of paramagnetic centres in the vaterite phase, primary paramagnetic centres formed by γ-irradiation at 77 K were studied. Cd2+•CO33−, •Cd+• CO3− and •H atoms were detected at 77 K, and their transformation to the secondary species, •Cd+←O− and •CO2−, was followed as a function of temperature. Cd doped into vaterite caused broadening of the EPR lines due to the •CO3− hole of orthorhombic symmetry, suggesting that Cd ions are located only in the calcite structure. Thus, Cd ions may be considered as nuclei for calcite formation in the vaterite phase. The mechanism of Cd-induced calcite formation in the vaterite-precipitating system is suggested.

13 C-vaterite, the unstable calcium carbonate polymorph, doped with Cd2+ was γ-irradiated and the stable paramagnetic centres were studied using electron paramagnetic resonance (EPR) spectroscopy. The 13C isotope has nuclear spin (I=1/2) and as a consequence hyperfine interaction with the unpaired electron occurs. This revealed all paramagnetic centres of carbonate origin, i.e. CO3- and CO2- free radicals. Two paramagnetic centres induced by cadmium doped to vaterite, one characterized by ga=1.9939 and the other by gb=1.9886, were not split by the 13C-hyperfine splitting. Therefore, these paramagnetic centres are most probably due to Cdâ†�O-. The locations of these paramagnetic centres in vaterite are determined and the role of cadmium in the formation of calcite crystals in the vaterite-precipitating system is discussed.

The kinetics of transformation of vaterite to calcite in aqueous solution was studied at temperatures between 25 °C and 45 °C and ionic strengths between 15 and 415 mmol dm^-3. Both vaterite and calcite precipitated spontaneously, with vaterite being initially the only solid phase in the system. The progress of the transformation was followed by recording pH as a function of time. It was found that this transformation was solution-mediated, and that the growth of calcite was the rate-determining process. This conclusion was additionally supported by a mathematical model made to predict changes of both the solid phase and the solution during the overall process. The model is based on the rate constants and the mechanism of the processes involved in the transformation, as well as on the experimentally determined number density of the crystals formed. The seeded growth kinetics of calcite was determined in separate experiments, at temperatures between 10 °C and 55 °C, and the rate law was found to be parabolic. The high activation energy obtained for the temperature dependent rate constants on temperature, E_a=55.29 kJ mol^-1, supports the assumption that the calcite growth was controlled by processes at the crystal surface.

The calorimetric method for examination of the kinetics of solid phase dissolution is presented. The thermograms are interpreted by considering the heat evaluation due to the reaction as well as the heat transfer in the calorimeter. The kinetic interpretation is based on the change of particle surface area during the dissolution process. The experimental results confirm the applicability of the proposed method. The rate constant and the enthalpy of dissolution of solid calcium carbonate in aqueous EDTA and NaOH solutions are determined. The experiments show that the process is the first order with respect to the surface area, indicating that the detachment of EDTA complex from the surface is the responsible mechanism.

The incorporation of divalent cations (Mg^2+, Mn^2+, Cu^2+, Sr^2+, Cd^2+, Ba^2+ and Pb^2+) in vaterite and calcite was studied by atomic absorption spectroscopy (AAS), optical and scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy. The samples of vaterite and calcite were prepared by spontaneous precipitation from aqueous solutions under defined conditions. The particles of vaterite obtained were spheres, being aggregates of crystallites of 25-35 nm and the crystals of calcite were well defined rhombohedra. All ions examined, except Cu^2+, were found to be incorporated in vaterite probably forming solid solutions. Ba^2+ caused a distortion of the vaterite crystal lattice and Mg^2+ changed the shape of the particles. Mn^2+ ions were incorporated into vaterite in at least two different environments, segregated into clusters or substituted for Ca^2+, as was detected by EPR. The environment of those Mn^2+ ions substituted for Ca^2+ in vaterite was of lower symmetry than that in calcite. Characteristic EPR spectra of free radicals formed by gama-irradiation of vaterite and calcite are presented. These spectra were used to detect formation of calcite in the vaterite system, in the presence of Cd^2+ and Pb^2+ ions confirming the results obtained by EPR of Mn^2+ as a paramagnetic substitute for Ca^2+. Free radicals characteristic of Cd^2+ ions, were detected in both vaterite and calcite gama-irradiated under similar conditions.

Calcium carbonate monohydrate (CaCO3xH2O) has been synthesized and the crystals were characterized by means of optical microscopy, FT-IR spectroscopy, X-ray diffractometry and thermogravimetry. The solubility was found by recording pH during the dissolution of the monohydrate in water, preventing contact with carbon dioxide from the air. The solubility product was calculated frm the final, equilibrium, pH value taking all the relevant calcium and carbonate species into account. In the temperature range 15-50 degrees C the solubility product Kso could be expressed by -log Kso=7.050+0.000159 theta2 where temperature is expressed in degrees C. The kinetics of dissolution of calcium carbonate monohydrate crystals in aqueous solution were studied at temperatures between 15 and 45 degrees C. The progress of dissolution was followed by recording pH as a function of time. It was found that the dissolution kinetics follow a second order rate equation with the rate constant k equal to 2.205x10-4, 6.190x10-4, 1.218x10-3 and 4.369x10-3 l mol-1 s-1/mg l-1 at 15, 25, 35, 45 degrees C respectively, and that the activation energy for dissolution was 73.3 kJ mol-1, suggesting a surface-controlled process.

Biomineralization processes are the subject of numerous investigations. This article gives a review of the study on interactions between the charged polypeptides and the mineral surfaces involved in biomineralization, with an additional kinetic approach. The influence of polypeptides on two types of precipitation processes is discussed: the spontaneous precipitation from supersaturated solution and the growth kinetics of calcite seed crystals. In the first case the phenomenon of the formation and stabilization of metastable phases was found while in the second case the influence of the applied polypeptides on the kinetics and mechanisms of calcite crystal growth was investigated. Calcium carbonate polymorphs, calcite and vaterite, were used as biomineral substrates and acidic polypeptides, poly-L-aspartic (pAsp) acid and poly-L-glutamic (pGlu) acid, as simplified models of naturally occurring soluble acidic proteins. A basic polypeptide, poly-L-lysine (pLys), was also used in experiments in order to find out whether conformity between the crystal surface and the adsorbed polypeptide, or just the electrostatic interactions, have a decisive role in these processes. The addition of a particular polypeptide into the precipitation system caused a significant inhibition of nucleation and growth of vaterite, the extent of inhibition being in the order InhpAsp > InhpGlu >> InhpLys. In addition to the inhibition of precipitation, the change of the polymorphic composition and the crystal morphology of the precipitate were also achieved. The explanation of such acidic polypeptide behaviour is a consequence of kinetic constraints through the diverse efficiency of inhibition of both calcite nucleation and vaterite growth caused by adsorption of acidic polypeptides. The acidic polypeptides also caused the inhibition of calcite crystal growth, the effect being pAsp > pGlu, and changed the observed mechanism of growth controlled by the integration of ions into the spiral steps, as found for the model systems, to the surface nucleation rate-determining mechanism. Nonselective, weak and electrostatic adsorption of pLys at the crystal surface was probably responsible for increasing the calcite crystal growth rate when pLys was present at low concentrations and for inhibiting it at pLys higher concentrations. The strongest interactions between the crystal surfaces and the polypeptides, observed for the calcite/pAsp system, can account for coordinative interactions between the side chain carboxylic groups of the predominantly planar arrangement of the pAsp structure (β-pleated sheet) and Ca2+ ions from the crystal surface. (doi: 10.5562/cca1809)

Spontaneous precipitation of calcium carbonate was investigated in two precipitation systems: (1) with initial supersaturation lower than that corresponding to the solubility of amorphous calcium carbonate (ACC), at which vaterite precipitated, and (2) with initial supersaturation higher than that of ACC solubility, at which a mixture of calcite and vaterite was formed. After the addition of an acidic polypeptide, poly-L-glutamic acid (pGlu) or poly-L-aspartic acid (pAsp), into (1) a significant inhibition of nucleation, expressed as an increase in induction time, and growth of vaterite, perceived as a dead zone, was observed. Extent of inhibition decreased in the order: Inh(pAps)>Inh(pGlu)>>Inh(pLys). The addition of a polypeptide into (2) caused the inhibition of precipitation and changed the morphology and polymorphic composition of the precipitate; only vaterite appeared at approximately c(pAsp)=3 ppm, c(pGlu)=6 ppm, or c(pLys)=7 ppm. This finding is explained as a consequence of kinetic constraints through the inhibition of calcite nucleation and stronger binding of acidic polypeptide by the calcite surfaces than by the vaterite surfaces. Laboratory precipitation studies using conditions that resemble those in living organism should be run at an initial supersaturation corresponding to the solubility of ACC as a limiting condition.

Appearance of a solid phase from aqueous solution, known as precipitation, is responsible for the formation of numerous natural materials and technological products. Therefore, the knowledge on mechanisms of elementary processes involved in precipitation should be considered in the areas such as geology, oceanology, biomineralization, medicine, basic chemical and pharmaceutical industries, analytical and materials chemistry in particular. Calcium carbonates are a very suitable model system for investigations of these processes. Owing to their low solubility, a wide range of initial supersaturations can be achieved that direct to different conditions under which a particular process dominates. Calcium carbonates can precipitate in the form of six modifications (polymorphic or hydrated). In the Laboratory for Precipitation Processes, systematic investigations of the conditions for formation, crystal growth and transformation of amorphous calcium carbonate, calcium carbonate hexahydrate, calcium carbonate monohydrate, vaterite and calcite have been performed during the last nearly twenty years. An overview of these studies is presented.

The influence of different water-soluble polysaccharides, dextrans (cationic, anionic and non-ionic) and soluble starch, on the precipitation of calcium carbonate, has been investigated in the model system in which calcium hydroxide and carbonic acid were reactants. In the absence of additives, the formation of metastable phases, vaterite and amorphous calcium carbonate is observed at the early stage of the process, and as a consequence of the solution-mediated transformation process calcite appears in the system as the only solid phase in equilibrium. In the presence of starch, vaterite is found in the final precipitate, with the content increasing with the increase of starch concentration, probably as a consequence of calcite nucleation and crystal growth inhibition. Non-ionic dextran causes the inhibition of vaterite nucleation, which results in the formation of calcite as a predominant solid phase throughout the precipitation process. The crystal size of the so-formed calcite reduces by increasing the relative molecular mass ( M r ) of neutral dextran. The presence of charged dextrans, either cationic or anionic, causes inhibition of the overall precipitation process: in the case of anionic dextran the inhibition seems to be the consequence of its reaction with Ca 2+ ions (supersaturation decrease), while cationic dextran most probably adsorbs electrostatically onto the negatively charged surfaces of calcite and vaterite.

The kinetics of precipitation of vaterite (CaCO3) from aqueous solution between about 10 and 45°C and at ionic strengths from 15mM to 315mM was determined by recording pH as a function of time and analysing these data by a BASIC computer program, correcting for complexes and using activity coefficients. The particles were spherulites with diameters less than 5 μm, and the growth kinetics parabolic, with the rate constants (dr/dt)/(S-1)2 equal to 0.180, 0.468, 0.560, 1.250 and 1.860 nm/s at 11.4, 21.7, 25.0, 33.8 and 42.5°C, respectively, and 0.560, 0.588, 0.594 and 0.610 nm/s at ionic strengths 15mM, 65mM, 115mM and 315mM, respectively. The change with temperature corresponds to the activation energy, 57.1 kJ/mol. It is shown that the rate constants found are in quantitative agreement with those calculated from the theory of growth of electrolyte crystals from aqueous solution. This means that the rate- determining mechanism is the release of water molecules from the calcium ions as they “jump” into lattice positions.

Calcium carbonate was precipitated from calcium hydroxide and carbonic acid solutions at 25 degrees C, with and without addition of different magnesium (MgSO(4), Mg(NO(3))(2) and MgCl(2)) and sodium salts (Na(2)SO(4), NaNO(3) and NaCl) of identical anions, in order to study the mode of incorporation of magnesium and inorganic anions and their effect on the morphology of calcite crystals over a range of initial reactant concentrations and limited c(i)(Mg(2+))/c(i)(Ca(2+)) molar ratios. The morphology, crystal size distribution, composition, structure, and specific surface area of the precipitated crystals, as well as the mode of cation and anion incorporation into the calcite crystal lattice, were studied by a combination of optical and scanning electron microscopy (SEM), electronic counting, a multiple BET method, thermogravimetry, FT-IR spectroscopy, X-ray diffraction (XRD), and electron paramagnetic resonance (EPR) spectroscopy. In the systems of high initial relative supersaturation, precipitation of an amorphous precursor phase preceded the formation of calcite, whereas in those of lower supersaturation calcite was the first and only polymorphic modification of calcium carbonate that appeared in the system. The magnesium content in calcite increased with the magnesium concentration in solution and was correlated with the type of magnesium salt used. Mg incorporation caused the formation of crystals elongated along the calcite c axis and, in some cases, the appearance of new [011] faces. Polycrystalline aggregates were formed when the c(i)(Mg(2+))/c(i)(Ca(2+)) molar ratios in solution were increased. Addition of sulfate ions, alone, caused formation of spherical calcite polycrystalline aggregates.

Electron paramagnetic resonance (EPR) spectroscopy, along with other analytical methods and techniques (X-ray diffraction, atomic absorption spectroscopy, scanning electron microscopy, etc.) have been used to study the mode of incorporation of divalent cations (Mg2+, Sr2+, Ba2+, Cu2+, Cd2+ and Pb2+) in vaterite. Vaterite, the least stable of the calcium carbonate polymorphs, was prepared by spontaneous precipitation from aqueous solutions and the doping of vaterite was performed by adding a particular divalent cation into a calcium chloride solution before mixing it with a sodium (or potasium) carbonate solution. All vaterite samples, pure or doped with divalent cations, were subjected to γ-irradiation. By studying EPR spectra of the pure vaterite powder the following free radicals were determined: CO3- holes freely rotating at the surface of the spherullites (g=2.0113); CO2- frozen in the matrix (g‖=2.0031 and g⊥=1.9998), and freely rotating CO2- (g=2.0006) probably located between the carbonate layers. It was found that the alkaline-earth metal cations (Mg2+, Sr2+, Ba2+) did not affect the formation and stability of the above free radicals. Cu2+ and Cd2+ doped to the vaterite phase trapped electrons by forming Cu+ and Cd+, respectively, which led, as a consequence, to increased formation of freely rotating CO3- holes in the near neighbourhood. Pb2+ released electrons readily by forming Pb3+ thus causing a decrease in formation of CO3- holes. In vaterite doped with Cd2+ two additional paramagnetic centers (ga=1.9886 and gb=1.9939), imbedded in two different sites in the vaterite structure were detected.

The phase composition of mineral aragonite and synthetic vaterite samples was determined qualititavely by using IR-spectrophotometric and X-ray diffraction analyses. The transformation of aragonite, A, and vaterite, V, into the stable modification calcite, C, was followed using differential scanning calorimetry analysis. In order to determine the kinetics and mechanisms of these phase transformations, a number of experimental DSC curves were elaborated mathematically and the stationary point theory was applied. The activation energy, Ea, and the enthalpy, Delta H, were found to be, respectively, 234.5 +/- 5.6 kJ mol^-1 and 122 Jg^-1 for the phase transformation A --> C, and 252.8 +/- 48.7 kJ mol^-1 and -21.2 Jg^-1 for the V --> C transformation.

The kinetics of dissolution of vaterite (CaCO 3 ) crystals in aqueous potassium chloride solutions were studied at temperatures between 15 and 45°C and ionic strengths between 50 and 200 mol/L. The vaterite preparate used consisted of spherulites of vaterite crystals with initial radii of 2.7 to 4.6 μm. The progress of the dissolution was followed by recording pH as a function of time. From pH the concentration of the dissolved matter was calculated, and from this we found the particle size, r , and the dissolution rate, - d r /d t , where r = average particle radius and t = time. Straight lines were obtained by plotting - r (d r /d t ) as a function of undersaturation, c s - c , which may be explained by diffusional rate control. Both the values of the apparent diffusion coefficients determined and their dependence on temperature (activation energy = 24.38 kJ/mol) support the assumption that the rate-determining process for the dissolution is the diffusion of the dydrated calcium and carbonate ions away from the crystal surface into the bulk solution.

At high supersaturation in water, Ca 2+ and CO 2- 3 spontaneously form a rather unstable precursor consisting of spherical particles of calcium carbonate, 50–400 nm in diameter. X-ray diffraction indicated that the substance is amorphous. The solubility was measured at different temperatures by titration with water immediately after the precipitate was formed. The solubility product is 5.9, 4.0, 2.5, and 1.5×10 -7 M 2 at 10, 25, 40 and 55°C, respectively.