Apatite equilibrium

The main assumption in this model is that matrix carbonate in solution is the principle agent for alteration of carbonates in biological apatites. Although the liquid and solid carbonate phases differ isotopically (the latter is enriched by about 8%o relative to former, under equilibrium conditions), the isotopic relationship between phases is well-defined (Emrich et al. 1970) hence values obtained from solid are at least representative of the dissolved phase. As shown in the model in Fig. 5.3, isotopic contamination of... 

It is assumed that in this experiment (58), stable or metastable equilibrium had been reached between the aqueous solution and a surface layer of the apatite particles. 

Once calcium deficient hydroxyapatite DOHA (between pH 6.8 and 8.2) is formed, a metastable equilibrium is created with the aqueous solution which may last indefinitely at room or body temperature.If carbonate ions are present in addition the apatite preferably formed is... 

The behavior of apatite during crustal anatexis equilibrium and kinetic... 

To a good first approximation, the Great Lakes fit a model involving the equilibrium of calcite, dolomite, apatite, kao-Unite, gibbsite, Na- and K-feldspars at 5°C., 1 atm. total pressure with air of PCo2 = 3.5 X 10" atm. and water. Dynamic models, considering carbon dioxide pressure and temperature as variables (but gross concentrations fixed), show that cold waters contain excess carbon dioxide and are unsaturated with respect to calcite, dolomite, and apatite, whereas warm waters are nearly at equilibrium with the atmosphere but somewhat supersaturated with respect to calcite, dolomite, and apatite. 

One of the first studies to show this was performed on Kilboume Hole spinel Uierzolites (Jagoutz et al, 1980). Equihbrated neodymium isotopes in orthopyroxene and diopside defined essentially zero age isochrons, consistent with the very recent eruption age of the host volcanic rocks, while strontium isotopes were un-equilibrated. Stolz and Davies (1988) found varying degrees of equihbration between amphibole, clinopyroxene and apatite in peridotite xenoliths from S.E. Australia. Several samples contained coexisting amphibole and clinopyroxene and had almost reached isotopic equilibrium for strontium but displayed disequilibrium relations for lead and neodymium isotopes. This was taken to indicate more rapid diffusion of strontium than lead and neodymium. Some peridotite and eclogite... 

Sedimentary phosphate ores, such as those found in Florida and Morocco, tend to have high concentrations of uranium, whereas the opposite occurs with magmatic ores, such as apatite from Kola. Typical activity concentrations of U are 1500 Bq kg in sedimentary phosphate deposits and 70 Bq kg in apatite. U is generally found in radioactive equilibrium with its decay products. The activity concentrations of Th... 

The equilibrium constant for equation i is of course at least as uncertain as the solubility product of apatite. With a value of for Caio(P04)6(OH)2(s) of... 

However, the reverse situation — that is, replacement of apatitic substance by quartz — has been described (Horowitz, 1967) as a dij enetic process in sediments, so it becomes difficult to understand the conditions of Eh and pH which govern reactions of this sort without a knowledge of the entire history of events although Cook (1970, p. 2115) has concluded that phosphatiza-tion and calcitization of silica, and the reverse reaction, may be explained by pH fluctuation, probably within the range of pH 7 to 10 . Furthermore, laboratory study of the system CaO-PaOs-HjO (Skinner, 1973) indicates that equilibrium obtains slowly even at temperatures about 300°C. While most minerals can be assumed to have formed under equilibrium conditions — in environments that existed at the time — it must be remembered that amorphous mineral substances [such as, bolivarite and evansite] have not attained a status of minimum energy during thousands to millions of years. 

Metal ions in biological systems tend to be distributed between four different states in vivo. First, there is the inert form of the metal complex whereby it is laid down into a solid matrix, just like the calcium in the apatite of bones and teeth (Scheme 1). Secondly, there are the three states of metal-protein having reversibly bound metal ions, known as high molar mass species, that are in turn in equilibrium with low molecular mass complexes and, for purely thermodynamic reasons, there must be the occasional aquated metal ion, although this is highly unlikely with a pH of 7.4 because of the threat of hydrolysis. 

As in the case of calcite, the activities of various dissolved species have been calculated at different pH values for other minerals using the relevant chemical equilibria and stoichiometric restrictions. The distribution of the activities of dissolved species of hydroxy apatite is given in Figs. 3.5a and 3.5b for open and closed systems, respectively. If the system is brought to equilibrium with atmospheric carbon dioxide, the distribution will follow that shown in Fig. 3.5a. For the closed system, Ca species are the most predominant... 

On the other hand, most interestingly, the calcite surface can also be converted to apatite under appropriate conditions. Fig. 3.5 shows the amount of phosphate required to convert calcite to apatite (along with the total phosphate in equilibrium with apatite) far exceeds the amount required for this if the system is open to atmospheric CO2. If the system is closed, it is seen that the conversion is possible in the entire pH range. It is interesting to note the amount of phosphate required for this process at each pH value. For example, for the closed system, only about 10 kmol/m of phosphate is required to produce the conversion effect around pH 13. 

Surface conversion due to reactions of the dissolved species with the mineral surface can be predicted using thermodynamic stability diagrams for heterogeneous mineral systems based on relevant mineral dissociation equilibria. This is illustrated in Fig. 3.10 for the calcite/apatite/dolomite system. The activities of Ca + species in equilibrium with various solid phases show that the singular point for calcite and apatite is 9.3. Above this pH, apatite is less stable than calcite and hence conversion of apatite surface to that of calcite can be expected in calcite-apatite system. Similarly, the calcite-dolomite and apatite-dolomite singular points occur at pH 8.2 and 8.8, respectively. 

The caseins exist in milk as polydisperse aggregates ranging in size from ca. 40 to 220nm (3), but the size distribution of micelles depends upon the method of measurement. These casein micelles scatter light and are responsible for the whitish, opaque nature of skim milk. The casein micelles are also associated with a colloidal apatite comprised of calcium-phosphate-citrate (CPC) which has a stabilizing influence on the micelle structure. The colloidal CPC is in equilibrium with soluble CPC in the milk serum phase and is solubilized as the pH is reduced. Thus, as the pH is reduced to the isoelectric point of the caseins (4.6), the colloidal CPC solubilizes, and the caseins precipitate (143). This phenomenon should be kept in mind during some of the following discussions. 

Solubility, equilibrium, and precursor phases in apatite growth... 

Equations (2) and (3) were used to generate Figure 6. The calculations were performed using constant Cl/OH in apatite of 4, 1.5, 0.67 and 0.25 (as mole fractions) at temperatures of 800 and 1000°C, at a pressure of 1000 bars. These calculations are independent of melt composition. The ratio ci/ 20 is valid whether or not the melt is saturated with an aqueous phase. The /whci represents the HCl molality in an aqueous phase that would be in equilibrium with both the analyzed apatite as well as the melt. 

---