INSIGHT INTO THE CL-OH APATITE BINARY FROM FTIR SPECTROSCOPY

Crystal structure refinements of synthetic Cl-OH apatite show a new ClOH position that can accommodate both chlorine atoms and hydroxyl groups (Hughes et al., 2014, Am.Min). The new position gives rise to a new suite of possible column anion inter-anionic distances along the c-axis. Combination of FTIR (Fourier Transform Infrared Spectroscopy) with structural refinements allows unambiguous assignment of new absorbances in the OH stretching region.

Hydrogen bonding is governed by the distance between hydrogen donor and hydrogen acceptor, as well as the electronegativity of the acceptor. Inter-oxygen distances for O(H)-O are linearly related to wavenumber in the OH stretching domain, and compare well with previous studies. O(H)-Cl distances form a similar linear array in that space, but with a different slope as compared to O(H)-O. Libowitzky (1999, Monatshefte Für Chemie) determined, for silicates, empirical relationships between wavenumber and O(H)-O distance. This relationship is very different for phosphates and for O(H)-Cl pairs in apatite group minerals. Conclusions garnered through research into silicate minerals should be extrapolated to phosphate minerals with prudence.

Spectral fitting is improved by recognition of a low-wavenumber feature in the OH stretching region as an OH-O pair. Although previously assigned as an OH-Cl pair, this peak is found in synthetic apatites that exclude chlorine. Oxyapatite is inferred in plasma-sprayed apatite on prosthetics. These data offer a means to recognize and measure O(H)-O pairs, in synthetic and in natural apatite group minerals. The presence of oxygen as a columnar anion also provides a means to reverse anionic ordering sequences along the c-axis, and preserve hexagonal symmetry.