IR spectroscopy and OH– in silicate garnet: The long quest to document the hydrogarnet substitution

Abstract

There has been much research undertaken on structural OH^– in various nominally anhydrous minerals including the common silicate garnets (i.e., X_3Y_2Si_3O_(12), where X = Mg, Fe^(2+), Mn^(2+), and Ca and Y = Al, Fe^(3+), and Cr^(3+)). 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 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 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 analyzed together 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 broad OH^– band at room temperature that splits into two narrower bands at lower temperatures and with a similar intensity relationship as shown by andradite. This behavior is indicative of the hydrogarnet substitution. The measured IR spectra of natural almandine- and pyrope-rich (Dora Maira, Italy) 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., X_3Y_2(O_4H_4)_3, where X = Mg, Fe^(2+), Mn^(2+) and Ca and Y = Al and Fe^(3+)] in terms of crystal-chemical properties and local atomic configurations. The OH^– mode energy, which lies 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 chemical bonding between the X- and Y-cations and the O^2–anion of the OH^– dipole plays a role in affecting the wavenumber of the OH^–-stretching vibration. OH^– mode broadening, observed in the spectra of 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 cation configurations around the OH^– dipole(s). 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 or hydropyrope substitution. IR spectra similar in appearance, having multiple relatively narrow OH^– modes that are 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 different silicate garnets.