FTIR of phosphate minerals with complex H-bonds network

Phosphates are among the most complex and variegated compounds in the entire mineral kingdom. Currently the total number of distinctive phosphate species is about 300 and most of them contains hydrogen as OH, H2O or in HPO4 2- group. Hydrogen bond has a central role in stabilizing the hydroxy-hydrated phosphate (HHPh) structures because it supplies the additional bond-valence (0.1 - 0.3 vu) contribution to the anions. Hence the (PO4) groups can link easily to all other interstitial cations (Huminicki & Hawthorne, 2002). For this reason, most HHPhs are characterized by the presence of complex tridimensional networks of O-H…O hydrogen bonds, which connect the polyhedral units making up a three dimensional framework. Consequently, the dimensionality of the structural unit is controlled primarily by the amount and role of hydrogen in the structure (Hawthorne, 1998). This could explain the observed correlation between the position in the paragenetic sequence of pegmatitic phosphates, and the amount of H2O in their formula (Fisher, 1958). FTIR spectroscopy is a powerful tool for the study of hydrogen in minerals, but HHPhs are rather challenging to study because of their complex structures. Moreover, due the high OH/H2O contents, these minerals show extremely intense IR absorptions in the OH region. In this work, we describe the results obtained by IR spectroscopy in different spectral regions of selected phosphates (veszelyite, whiteite, vauxite, paravauxite, metavauxite, augelite, wardite, wavellite, lazulite, arrojadite). O-H and hydrogen bonds orientation were studied by single crystal polarized light m-IR-spectroscopy, and experiments at HT- and LT were performed to study phase transitions and dehydration mechanisms with the aim of defining the thermal stability of these minerals. Finally, some applications done by using the novel FPA-FTIR imaging methods are presented. Fisher D.J. 1958. Pegmatite phosphates and their problems. Am. Mineral., 43, 181-207. Hawthorne F.C. 1998. Structure and chemistry of phosphate minerals. Mineral. Mag., 62, 141-164. Huminicki D.M.C. & Hawthorne F.C. 2002. The Crystal Chemistry of the Phosphate Minerals. In: Kohn M.L., Rakovan J. & Hughes J.M. Eds., Phosphates Geochemical, Geobiological, and Materials Importance. Rev. Mineral. Geochem., 48, 123-254.