IS THE APATITE IN BONE A COMBINED HYDRATED-HYDROXYLATED MINERAL?

The biologically precipitated mineral component of bone is typically regarded as a nanocrystalline form of carbonated hydroxylapatite, OHAp [Ca₁₀(PO₄)₆(OH)₂]. Despite earlier evidence for structural water in nominally anhydrous apatite, that knowledge was never incorporated into accepted models of bone apatite. Our present study of carbonated OHAp and carbonated fluorapatite (FAp), formed through aqueous synthesis at 37-82°C, provides new understanding of the amounts of and the structural siting of water in bone apatite. Raman spectroscopy documents large amounts of molecular water in carbonated apatites analyzed in a dry nitrogen environment (to eliminate adsorbed water). Thermogravimetric analysis (TGA) of these samples confirms the release of several wt.% (structural, not adsorbed) water between about 200 and 500°C. Raman and IR spectroscopy of carbonated FAp synthesized in D₂O (spectroscopically distinguishable from adsorbed H₂O) show D₂O incorporation within the apatite. Our data support a model in which structural channels in OHAp partially fill with water in response to carbonate substitution. As recognized in earlier studies, some of the typical channel-filling hydroxyl ions, as well as neighboring calcium ions, are excluded from the apatite structure due to charge-balance needs as (CO₃)^2- substitutes for (PO₄)^3-. Spectroscopy and TGA suggest that water molecules fill some portion of the sites vacated by both OH and Ca. We have preliminary evidence that bone apatite is not a "flawed hydroxylapatite", but instead, a combined hydrated-hydroxylated carbonated phase, of the form Ca_10-x[(PO₄)_6-x(CO₃)_x](OH)_2-x•nH₂O. The presence of intracrystalline water in the channels of bone apatite would help explain the rapidity of the chemical response and ion exchange capability between the inorganic mineral and its biochemical environment. Carbonated apatite's unusual water-release pattern upon heating and the ability of the heated (to 400°C), partially dehydrated mineral to re-absorb water are analogous to some zeolites'. Indeed, many of bone apatite's properties are well modeled by reference to low-temperature minerals in the zeolite group, which also feature water-filled channels and the capability for rapid ion exchange.