The Mechanism of Precipitation of Biological Minerals. The Phosphates, Oxalates and Carbonates of Calcium

The precipitation of the phosphates, oxalates, and carbonates of calcium is complicated by the possible formation of different precursor phases involving polymorphs, hydrates, and acid salts. In order to elucidate the mechanisms of the reactions, it is necessary to study the kinetics under conditions of controlled supersaturation calculated from the activities of free ionic species. In general, the rates of formation of the salts are proportional to the (supersaturation)", where n = 1.25-2.0 suggesting a surface controlled process. However, in the case of the calcium phosphate phases, the precipitation of the thermodynamically most stable hydroxyapatite is often complicated by the formation of precursor phases which form and subsequently dissolve during the overall reactions. The sensitivity of the various solid phases to the presence of crystal growth inhibitors is markedly different. Thus in the case of calcium carbonate, it is possible to selectively inhibit calcite and aragonite by adding traces of phosphonate inhibitor, thereby encouraging the formation of vaterite, the most thermodynamically unstable phase. Such selective inhibition may explain the existence of thermodynamically unstable phases in biological systems

The Kinetics of Growth of Seed Crystals in Solution

The theory of growth of seed crystals in critically supersaturated electrolyte solutions is discussed. Techniques of studying crystal growth are reviewed, the application of which yield highly precise and reproducible data. Effects of added foreign substances on the rate of crystallization are emphasized and results, obtained for magnesium oxalate, barium and strontium sulfate crystal growth, discussed

The Crystal Growth of Sparingly Soluble Salts

The mechanism of crystal growth of sparingly soluble salts from their supersaturated solutions is discussed in the light of a number of available growth theories. It is shown that in the case of both calcium sulphate and calcium carbonate, the observed quadratic dependence of the rate of crystallization upon the relative supersaturation is consistent with a surface controlled reaction. The reaction rates are unaffected by changes in the fluid dynamics within the crystallization cell, and the activation energy for both seeded growth and the linear growth of calcium sulphate is 15.0 ± 0.5 kcal mo1e-1• Evidence is discussed for a screw dislocation mechanism for calcium sulphate crystal growth. Under conditions of relatively high supersaturation, secondary nucleation of both calcium sulphate and calcium carbonate crystals is observed upon the addition of seed crystals. The striking effect of certain additives in reducing the rate of crystal growth is discussed in terms of the important factors involved and in a number of instances it is shown that a simple adsorption isotherm of the Langmuir type satisfactory describes the growth data

The Nucleation and Growth of Calcium Phosphate Crystals

The growth of seed crystals in supersaturated solutions of calcium phosphate at 25° C is discussed. In the pH range 5-6, the crystallization of dicalcium phosphate follows a second order rate equation in a process which is surface reaction rather than· diffusion controlled. Under physiological pH conditions, the growth of hydroxyapatite seed crystals proceeds through the formation of a precursor phase which has a calcium : phosphate ratio of 1.45 ± 0.05 and which attains the HAP composition after slow interaction with the medium; the process again appears to be interface controlled

The Thermodynamics of Formation of Metal Complexes and Ion-Pairs in Solution

The problem of characterisation of species present at equilibrium in an electrolyte solution is discussed. Shortcomings of the free energy approach are shown due to inadmissible neglect of entropy changes in complex formation. It is shown that complexes formed with hard ligands are stabilised entirely by entropy changes. Soft or covalent interactions are always characterized by exothermic enthalpy changes

The Crystal Growth of Sparingly Soluble Salts

The mechanism of crystal growth of sparingly soluble salts from their supersaturated solutions is discussed in the light of a number of available growth theories. It is shown that in the case of both calcium sulphate and calcium carbonate, the observed quadratic dependence of the rate of crystallization upon the relative supersaturation is consistent with a surface controlled reaction. The reaction rates are unaffected by changes in the fluid dynamics within the crystallization cell, and the activation energy for both seeded growth and the linear growth of calcium sulphate is 15.0 ± 0.5 kcal mo1e-1• Evidence is discussed for a screw dislocation mechanism for calcium sulphate crystal growth. Under conditions of relatively high supersaturation, secondary nucleation of both calcium sulphate and calcium carbonate crystals is observed upon the addition of seed crystals. The striking effect of certain additives in reducing the rate of crystal growth is discussed in terms of the important factors involved and in a number of instances it is shown that a simple adsorption isotherm of the Langmuir type satisfactory describes the growth data

The Mechanism of Precipitation of Biological Minerals. The Phosphates, Oxalates and Carbonates of Calcium

The precipitation of the phosphates, oxalates, and carbonates of calcium is complicated by the possible formation of different precursor phases involving polymorphs, hydrates, and acid salts. In order to elucidate the mechanisms of the reactions, it is necessary to study the kinetics under conditions of controlled supersaturation calculated from the activities of free ionic species. In general, the rates of formation of the salts are proportional to the (supersaturation)", where n = 1.25-2.0 suggesting a surface controlled process. However, in the case of the calcium phosphate phases, the precipitation of the thermodynamically most stable hydroxyapatite is often complicated by the formation of precursor phases which form and subsequently dissolve during the overall reactions. The sensitivity of the various solid phases to the presence of crystal growth inhibitors is markedly different. Thus in the case of calcium carbonate, it is possible to selectively inhibit calcite and aragonite by adding traces of phosphonate inhibitor, thereby encouraging the formation of vaterite, the most thermodynamically unstable phase. Such selective inhibition may explain the existence of thermodynamically unstable phases in biological systems

The Growth of Calcium and Strontium Sulfates on Barium Sulfate Surfaces

The crystallization of calcium sulfate dihydrate (gypsum) and strontium sulfate (celestite) induced by the presence of barium sulfate (barite) surfaces has been studied using a Constant Composition kinetic method. In contrast to calcium sulfate, strontium sulfate appears to grow epitaxially on barium sulfate surfaces. However, barium sulfate induces calcium sulfate dihydrate precipitation from meta stable supersaturated solutions of the salt. Following nucleation, the rate of growth of strontium sulfate was found to be a second order with respect to supersaturation indicating a surface dislocation growth mechanism. The observed epitaxial growth is in agreement with calculations based on crystal lattice mismatch

The Kinetics of Crystallization of Calcium Fluoride. A New Constant Composition Method

A new method is described for studying reproducibly, the kinetics of crystallization of calcium fluoride under conditions of constant solution composition. The method can be used even at very low supersaturation. The rate is proportional to the square of the supersaturation, over the range of relative supersaturation, S = 0.20 - 1.66. The results point to a surface controlled crystallization. The rate is markedly influenced by the presence of phosphate

The Kinetics of Crystallization of Calcium Fluoride. A New Constant Composition Method

A new method is described for studying reproducibly, the kinetics of crystallization of calcium fluoride under conditions of constant solution composition. The method can be used even at very low supersaturation. The rate is proportional to the square of the supersaturation, over the range of relative supersaturation, S = 0.20 - 1.66. The results point to a surface controlled crystallization. The rate is markedly influenced by the presence of phosphate