Apatite recrystallization

The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of material. 

Terpstra RA, Bermema P, Hartman P, Woensdregt CF, Perdok WG, Senechal ML (1986) F faces of apatite and its morphology Theory and observation. J Cryst Growth 78 468-78 Thirioux L, Baillif P, Ildefonse JP, Touray JC (1990) Surface reactions during fluorapatite dissolution-recrystallization in acid media (hydrochloric and citric acids). Geochim Cosmochim Acta 54 1969-1977... 

Experimental studies of accessory mineral solubility in aqueous fluids suggests that apatite and monazite solubility in pure H2O is low, and increases with decreasing pH (Ayers and Watson 1991). Ayers et al. (1999), in an experimental study of the coarsening kinetics of monazite in the presence of aqueous fluids, found that monazite either recrystallizes and moves as host grain boundaries migrate, or is trapped as inclusions within the host phase. Inclusion monazites, once isolated from the rock matrix, should record the time of entrapment whereas matrix monazite should record the time of final recrystallization, or, if growth zoned, a complex age spectrum. 

Figure 1. XRD traces of different bioapatite materials from modern and fossil elephants. Fossil dentin, cementum and bone all show a decrease in peak width compared to modem materials, indicating diagenetic recrystallization and coarsening of original apatite. Enamel peaks are essentially unaffected. From Ayliffe et al. (1994).

During recrystallization, trace elements are concentrated in new authigenic apatite. 

Figure 2. Schematic model of bone recrystallization by addition of authigenic apatite (note original bone crystals are on the order of 40x40x1 Onm, and are mostly poorly formed, plate-shaped crystals of dahllite).

This argument was explored by Reynard et al. (1999), using values of E and Vg obtained from the experimental partitioning data of Fujimaki (1986). Reynard et al. (1999) used Equation (1) to predict equilibrium REE-apatite partition coefficients at surface temperature and pressure, assuming that the crystal chemistry of bone apatite is broadly similar to that of HAP, and that crystal-melt partition coefficients can be used to estimate crystal-water partitioning. Reynard et al. (1999) then compared the predicted partition coefficients with measured adsorption coefficients for the REE between seawater and HAP derived by Koeppenkastrop and DeCarlo (1992), and concluded that incorporation of REE into bone via a substitution mechanism produces bell shaped REE patterns with relatively little fractionation between La and Lu. Incorporation of REE into bone via an adsorption mechanism, on the other hand, produces significant fractionation between La and Lu (La/Lu = 5). Based on REE patterns found in fossil fish teeth, they concluded that REE uptake in fossil bone was dominated by adsorption mechanisms, but that subsequent recrystallization may superimpose a degree of substitution-related fractionation over the initial, adsorption related REE pattern. It is important to note, however, that these predictions are based on crystal chemistry of hydroxyapatite and fluorapatite, and not dahllite and francolite. Variations in E and Vo will affect relative adsorption and/or partition coefficients, and may alter the predicted partition coefficient ratios (e.g., La/Lu and La/Sm). 

Background. The phosphate minerals apatite, monazite and xenotime have strongly variable retention properties for Pb under crustal conditions. The mechanisms by which the daughter product can be lost include dissolution/reprecipitation reactions, recrystallization, and diffusive loss. The latter mechanism is likely a common source of discrepancy between a mineral date and the age of the rock from which it formed. 

Apatite in living linguloid shells can rapidly recrystallize and incorporate apatite varieties with lower hydroxyl content during the post-mortem alteration processes. The study of a modern coquina of Discinisca confirms that these changes can be detected by a study of apatite lattice parameters. 

Human bones contain 60-70% (by mass) calcium phosphate. They provide mechanical support but also store the calcium and phosphate ions that are used in the body for a variety of functions. Bones are not permanent structures but instead exist in a state of dynamic equilibrium with their surrounding tissues. Biological apatite is continually dissolving and recrystallizing in the body. This equilibrium state makes it possible to maintain the necessary concentrations of calcium and phosphate ions in body fluids such as blood and saUva. 

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