APATITE CRYSTAL GROWTH FROM SOLUTION

The driving force for nucleation and crystal growth from solution can be expressed as the positive difference between the chemical potential of a species in a supersaturated solution and that of a saturated solution. Once a solution becomes supersaturated with respect to a particular mineral, 3-D nucleation may occur. For the spontaneous formation of an apatite nucleus (for the sake of discussion we will refer to fluorapatite) in solution the change in the Gibbs free energy (AG) of the reaction... 

Valsami-Jones, E., Ragnarsdottir, K.V., Putnis, A., Bosbach, D., Kemp, A.J., andCressey, G. (1998) The dissolution of apatite in the presence of aqueous metal cations at pH 2-7. Chem Geol 151 215-233 Van Cappellen P, Berner RA (1991) Fluorapatite crystal-growth from modified seawater solutions. Geochim Cosmochim Acta 55 1219-1234... 

However, methane-diphosphonate could not prevent the growth of apatite crystals in vitro on prepared sinews of rats tail out of a metastable solution with calcium and phosphate ions. On the contrary, the precipitated crystalline particles were bigger and better crystallized than those from control solutions. This is in surprising contrast to most of the information from the literature. No other calcium phosphate minerals besides apatite have been found by X-ray diffraction, whereas under comparable conditions brushite and octacalcium phosphate grow on collagenous sinews549. ... 

Moreno. E. C. and Vanighese. K. (1981). Crystal growth of calcium apatite from dilute solutions. J. Crystal Growth. 53 20-30. 

Boudreau AE, McCallum IS (1989) Investigations of the Stillwater Complex Part 5, Apatites as indicators of evolving fluid composition. Contrib Mineral Petrol 102 138-153 Brecevic LJ, Furedi-Milhofer H (1972) Precipitation of calcium phosphates from electrolyte solutions II The formation and transformation of the precipitates. Calc Tissue Res 10 82-90 Brown WE (1966) Crystal Growth of Bone Minerd. Clin Orthopaed 44 205-220... 

Moreno EC, Varaghese K (1981) Crystal growth of calcium apatites from dilute solutions. J Cryst Growth... 

Mg-inhibition. Even if sediments or interstitial waters are at or near saturation with phosphate, there are still impediments to CFA crystallization. One of the most significant is the role of Mg-inhibition. Martens and Harris (1970) demonstrated in the laboratory that when phosphate and fluorine are added to a solution with a composition similar to that of ocean water in Mg VCa ratio, apatite does not crystallize. Instead, a Ca-phosphate gel precipitates with a Ca/P ratio of 1.35 compared to the ratio of 1.67 found in apatite. In their experiments, the gel was kept in the proxy seawater solution for 8 months with no appearance of apatite. However, when the gel was placed in a Mg-free solution, apatite quickly formed. Mg-inhibition results from the smaller Mg ion replacing Ca in the apatite structure, the resultant lattice distortion prevents effective crystal growth (Martens and Harris 1970). Later work by Gulbrandsen et al. (1984) showed that given enough time, in their experimentation seven years, apatite would indeed crystallize from the gel despite the Mg in solution. 

A solution containing Ca and HPO4 " ions and having the same ionic product as serum (18 (mg per 100 ml) ) in a clean container can be kept indefinitely without the separation of crystals. If, however, apatite crystals are introduced into the solution, they grow in size at the expense of ions from solution and the ionic product of the solution falls until it is equal to the solubility product for hydroxyapatite, at which point crystal growth stops. 

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