IR  spectroscopy and OH in silicate garnet:

the long quest to document the hydrogarnet substitution



Charles A. Geiger1, George R. Rossman2
 
1 Department of Chemistry and Physics of Materials
University of Salzburg, Hellbrunnerstr. 34
A-5020 Salzburg, Austria
 2 Division of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA  91125-2500, USA

ABSTRACT

There has been much research undertaken on structural OH- in various nominally anhydrous minerals including the common silicate garnets (i.e., X3Y2Si3O12, where X = Mg, Fe2+, Mn2+ and Ca and Y = Al, Fe3+ and Cr3+). However, it is still largely not understood where small concentrations of H atoms are incorporated in the garnet crystal structure. In this work, the IR single-crystal spectra of end-member or approaching end-member composition andradite, pyrope and almandine crystals are measured. Both a natural and synthetic andradite sample show a broad, asymmetric OH-stretching mode at 3563 cm-1 that splits into two narrower modes at lower temperatures. They are located at 3575 cm-1 and 3557 cm-1 at 80 K with the higher wavenumber mode being considerably more intense compared to that at lower energy. These results are compared with published IR spectra of synthetic end-member katoite, pyrope and almandine also recorded at low temperature. These garnets show similar IR behavior with a single OH band at room temperature that splits into two at lower temperatures and with a similar intensity relationship as shown by andradite. It is argued that this behavior is indicative of the hydrogarnet substitution. The measured natural almandine- and pyrope-rich (Dora Maira, Italy, locality) crystals, on the other hand, show different spectroscopic features with several OH modes that are not consistent with the hydrogarnet mechanism. An analysis of the energy of the OH-stretching mode is made for various composition hydrogarnet clusters (i.e., X3Y2(O4H4)3, where X = Mg, Fe2+, Mn2+ and Ca and Y = Al and Fe3+) in terms of crystal-chemical properties and local atomic configurations. The OH mode, which lies between roughly between 3660 and 3550 cm-1 at RT for various end-member garnets, is a function of the mass of the X- and Y-cations due to mode coupling and/or mixing. In addition, the strength of the bonding between the X- and Y-cations and the O anion of the OH dipole plays a role in affecting the wavenumber of the OH stretching vibration. OH mode broadening in end-member garnets is primarily a result of thermal anharmonic disorder, especially with regard to the light H cation. OH mode broadening in  intermediate solid-solution composition garnets is a function of both thermal effects and variations in local atomic configurations around the OH dipole(s) in the crystal structure. Published IR spectra of certain high-pressure pyrope-rich garnets, both synthetic and natural, are analyzed and arguments made that OH can often be incorporated as the hydrogarnet substitution. IR spectra similar in appearance, having multiple relatively narrow OH modes but distinct from those indicating the hydrogarnet substitution, can be observed for certain synthetic end-member and various composition natural pyropes from Dora Maira and some natural spessartines. This indicates that other common OH substitution mechanisms, which have yet to be determined, can also occur in silicate garnet.