American Mineralogist, Volume 95, pages 1276–1293, 2010

Unique crystal chemistry of two polymorphs of topaz-OH: A multi-nuclear NMR andRaman study
Xianyu Xue,* Masami Kanzaki, and Hiroshi Fukui†Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori, 682-0193, JapanAbstract
A new polymorph of topaz-OH (denoted as topaz-OH II) was recently discovered at higher P-T
conditions than has been known thus far (denoted as topaz-OH I). High-resolution 1H, 29Si, and 27Al
nuclear magnetic resonance (NMR) and micro-Raman spectroscopy are applied to shed light on thecrystal chemistry of both polymorphs. Topaz-OH I, synthesized at 7 GPa and 640 °C, is stoichiometric(Si/Al = 0.5) with a largely ordered local structure. Higher P-T topaz-OH I synthesized at conditions
close to the polymorphic phase transition boundary, on the other hand, shows lower Si/Al ratios(0.44–0.45) and greater local structural disorder [including a small fraction (~3%) of octahedral Siwith a unique 29Si chemical shift near –133 ppm]. The latter may be accounted for by the developmentof defects (Si/Al in normally vacant octahedral sites and vacancies in the tetrahedral sites) at higher
P-T conditions. Topaz-OH II synthesized at 13.5~14 GPa and 1300~1400 °C similarly exhibits low
Si/Al ratios (0.41–0.46). The NMR and Raman spectra for these topaz-OH II are, in general, broader
and revealed a substantial fraction (33–37%) of octahedral Si with a range of 29Si chemical shifts(–130 to –190 ppm), a small fraction (2–3%) of tetrahedral Al, and a range of (and overall shorter)hydrogen-bonding distances than topaz-OH I. Therefore, the phase transition from topaz-OH I to II
is characterized by both a significant increase in the occupied octahedral/tetrahedral site ratio as well
as disordering of cation distribution, which is unique from the viewpoint of crystal chemistry.
Keywords: NMR spectroscopy, Raman spectroscopy, high pressure, crystal structure, phase transition, vacancy, vacant site, Al-Si disorder, hydrogen bonding

Introduction
Naturally occurring topaz [Al2SiO4(F,OH)2] are generally
limited to F-rich compositions. Topaz-OH [Al2SiO4(OH)2], theOH end-member of the F-OH solid-solution series of topaz,
was first synthesized at 5.5–10 GPa and 700–1000 °C (Wunderet al. 1993). Subsequent studies in the Al2O3-SiO2-H2O system
reported that topaz-OH can occur at pressures up to ca. 13 GPaand temperatures up to ca. 1500 °C, and breaks down to phaseegg (AlSiO3OH)-bearing assemblages at higher pressures andto kyanite (Al2SiO5) + fluid at higher temperatures (Ono 1999;Schmidt et al. 1998).
Despite the reported high-temperature stability (up to ca.
1500 °C at 13 GPa) (Ono 1999), only topaz-OH samples synthesized at relatively low P-T (up to 10 GPa and 1000 °C) havebeen well characterized. Wunder et al. (1993) first refined thecrystal structure of topaz-OH synthesized at 10 GPa and 1000°C (space group Pbnm) from single-crystal X-ray diffraction(XRD). Subsequent studies of topaz-OH synthesized at 10 GPaand 1000 °C by single-crystal XRD (Northrup et al. 1994) andtopaz-OD synthesized at 7.5 GPa and 750 °C by powder neutron
diffraction (Chen et al. 2005) located the H (D) positions, and* E-mail: xianyu@misasa.okayama-u.ac.jp† Present address: Department of Material Science, Graduate School of Material Science, University of Hyogo, 3-2-1,Kamigohri, Hyogo 678-1279, Japan.
0003-004X/10/0809–1276$05.00/DOI: 10.2138/am.2010.3471

reported that the single H position found in F-rich topaz is split
into two nonequivalent, half-occupied H (D) positions in topazOH (OD). The reported infrared and Raman spectra for topaz-OH
synthesized at 9–10 GPa and 800–1000 °C both contain two O-H
stretching bands near 3600 and 3520 cm–1 with a low-frequency
shoulder (Komatsu et al. 2005; Wunder et al. 1993). These bands
have been attributed to O-H stretching of different OH groups
from ab-initio calculations (Churakov and Wunder 2004).
We have applied a combined nuclear magnetic resonance(NMR) and micro-Raman spectroscopic study on hydrous phases(topaz-OH, phase egg, and δ-AlOOH) in the Al2O3-SiO2-H2O
system synthesized under a range of P-T conditions to place
additional constraints on their structures and potential roles aswater reservoirs in the Earth’s mantle (Kanzaki et al. 2006; Xueet al. 2006). During this study, we discovered a new polymorphof topaz-OH (denoted here as topaz-OH II) (Kanzaki et al.
2006). In-situ XRD measurements over a wide range of P-T
using synchrotron radiation at SPring-8, Japan, revealed that
topaz-OH II occurs at the highest P-T corner of the previously
reported stability field for topaz-OH, and the phase transition
between topaz-OH II and the polymorph known thus far (denotedas topaz-OH I) is reversible (Kanzaki, unpublished results). The
crystal structure of topaz-OH II has recently been refined in spacegroup Pbnm using powder XRD data obtained with synchrotron
radiation at SPring-8, and was found to bear some resemblance
to that of topaz-OH I, but with a disordered cation distribution

1276