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

Geiger, Charles A. and Rossman, George R. (2018) IR spectroscopy and OH^– in silicate garnet: The long quest to document the hydrogarnet substitution. American Mineralogist, 103 (3). pp. 384-393. ISSN 0003-004X.

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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.

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Rossman, George R.0000-0002-4571-6884
Alternate Title:IR spectroscopy and OH– in silicate garnet: The long quest to document the hydrogarnet substitution
Additional Information:© 2018 by the Mineralogical Society of America. Manuscript received April 11, 2017; Manuscript accepted November 10, 2017; Published: March 01, 2018. C. Chopin (Paris), C. Ferraris (Paris), J. Filip (Olomouc, Czech Republic), and A.B. Woodland (Frankfurt) generously donated crystals for study. This research was supported by two grants to C.A.G. from the Austrian Science Fund (FWF: P25597-N20 and P30977-NBL) with a contribution from NSF grant EAR-1322082 to G.R.R. H. Skogby, an anonymous reviewer and the associate editor I. Kovács made constructive comments that improved the manuscript.
Funding AgencyGrant Number
FWF Der WissenschaftsfondsP25597-N20
FWF Der WissenschaftsfondsP30977-NBL
Subject Keywords:Hydrogarnet, silicate garnet, OH– in minerals, IR spectroscopy, crystal chemistry; Water in Nominally Hydrous and Anhydrous Minerals
Issue or Number:3
Record Number:CaltechAUTHORS:20180306-090925093
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Official Citation:Charles A. Geiger, George R. Rossman; IR spectroscopy and OH– in silicate garnet: The long quest to document the hydrogarnet substitution. American Mineralogist ; 103 (3): 384–393. doi:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85129
Deposited By: Tony Diaz
Deposited On:06 Mar 2018 17:32
Last Modified:03 Oct 2019 19:27

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