FTIR spectroscopy of hydrogen in minerals: detection, structural environment, quantitative analysis and thermal behaviour

Hydrogen can be a major, a minor or a trace constituent of a large variety of minerals. It is usually bonded to oxygen to give H2O, OH-, or more rarely H3O+, H3O2- H5O2+ units, but also NH4+ or organic compounds. It can be a structural component in stoichiometric hydrates, hydroxides, and in many silicate minerals, or can be non-stoichiometrically present as a major extraframework component in microporous minerals (e.g. zeolites). Hydrogen also occur as a minor constituent in structural defects of nominally anhydrous minerals (NAMs) [1]. H exerts a strong influence on the chemical and physical properties of minerals; hence it has a controlling effect on many high-temperature geological processes (e.g. magma genesis, kinetic of phase transformations, etc.), as well as low-temperature alteration processes. Unfortunately, H is a rather elusive element, specially when it occurs in very low amounts, as in NAMs. Conventional micro-analytical techniques (EMPA) and X-ray diffraction are generally unsuitable to characterize this element; secondary ion mass spectrometry (SIMS) and neutron diffraction are not easily accessible and requires complex, time-consuming, analytical procedures. A possible alternative is offered by spectroscopy: Raman, Fourier-transform infrared (FTIR) and nuclear magnetic resonance (NMR). FTIR is particularly suitable for hydrogen, since the O-H (C-H and N-H) bond absorbs the infrared radiation very efficiently. The relatively low cost, ease of use and ease of sample preparation makes the FTIR technique an extremely powerful tool for the study of H-bearing minerals [2]. Using FTIR spectroscopy we can: a) quantify hydrogen, b) study its distribution [3], c) distinguish its speciation, d) define the structural orientation of the O-H dipole, e) study phase transitions associated with H loss, and f) study thermal processes [4]. We present here the recent developments of the FTIR activities at Roma Tre and describe the new facilities which are currently available in our laboratories (high temperature stages, Focal Plane Array detectors). We will use, as examples, the most recent results obtained by our group, including studies on beryl, cordierite, feldspathoids, phospates and hydrocarbons-bearing materials. [1] H. Keppler, J.R. Smyth (Eds.) Reviews in Mineralogy and Geochemistry 62 (2006) pp. 478. [2] E. Libowitky, A. Beran, Emu Notes in Mineralogy 6 (2004) 227-279. [3] G. Della Ventura, F. Bellatreccia, A. Marcelli, M. Cestelli Guidi, M. Piccinini, A. Cavallo, M. Piochi, Anal. Bioanal. Chem. 397 (2004) 2039-2049. [4] E. Bonaccorsi, G. Della Ventura, F. Bellatreccia, S. Merlino, Micropor. Mesopor. Mater. 99 (2006) 225-235.