American Mineralogist, Volume 91, pages 850–861, 2006

Cation order and hydrogen bonding of high-pressure phases in the Al2O3-SiO2-H2Osystem: An NMR and Raman study
XIANYU XUE,1,* MASAMI KANZAKI,1 HIROSHI FUKUI,1 EIJI ITO,1 AND TAKAFUMI HASHIMOTO2
1Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori, 682-0193 Japan
2
Faculty of Science, Kumamoto University, Kurokami 2-36-1, Kumamoto 860-8555 Japan

ABSTRACTTopaz-OH, phase egg, and δ-AlOOH are hydrous phases in the Al2O3-SiO2-H2O system that havebeen found to be stable at successively higher pressures up to those corresponding to the lower mantle,and thus they may be important water reservoirs in the deep mantle. We have applied 1H, 29Si, and27
Al nuclear magnetic resonance (NMR) and Raman spectroscopy to shed new light on the structureof these phases. 29Si and 27Al NMR results clearly revealed that the Si-Al distribution in phase egg ispartially disordered. The presence of structural disorder in topaz-OH was also conÞrmed. 1H NMRand Raman data are both consistent with strong, but asymmetric hydrogen bonding in δ-AlOOH andphase egg, and a range of hydrogen bonding distances in topaz-OH. The observed structural disorderand hydrogen bonding could be responsible for the high upper temperature stability limits (1500~1700°C) of phase egg and topaz-OH, and are also relevant to the incorporation mechanisms of water innominally anhydrous stishovite.
Keywords: NMR spectroscopy, topaz, phase egg, δ-AlOOH, Raman spectroscopy, crystal structure, order-disorder

INTRODUCTION
Water is among the most important volatile components thatsigniÞcantly affect phase relations, and physical and thermodynamic properties of minerals. Increasing evidence suggests thatwater is not only abundant near the Earth’s surface and crust, but
may also be present in the deep mantle as either hydrous minerals
or nominally anhydrous minerals (cf. Prewitt and Parise 2000).
In the Al2O3-SiO2-H2O system, three hydrous phases have beenfound to be stable at pressures corresponding to the upper tolower mantle and have been suggested to be potentially importantwater carriers in the subducted slabs. Among them, topaz-OH[Al2SiO4(OH)2], the OH end-member of the F-OH topaz solidsolution series, has been reported at pressures between 5.5 and13 GPa and temperatures up to 1500 °C (Wunder et al. 1993;
Schmidt 1995; Ono 1999). The initial successful synthesis of thisphase had been surprising because natural topaz minerals show
only limited substitution of OH for F, and it was once suggested
that the OH/(OH + F) content of topaz could not exceed 50%
because of the short H-H distance of 1.5 Å (Parise et al. 1980).
At pressures between about 11 and 22 GPa, and temperatures
up to about 1700 °C, phase egg (AlSiO3OH) has been reported
to become the stable phase (Eggleton et al. 1978; Irifune et al.
1995; Schmidt 1995; Schmidt et al. 1998; Ono 1999; Sano et al.
2004). The relatively high upper temperature stability limits for
topaz-OH and phase egg suggest that both may be stable not onlyin the subducted slabs, but also along normal mantle geotherm(cf. Sano et al. 2004). Above 23 GPa, phase egg breaks downto δ-AlOOH + stishovite below 1200 °C, and to corundum +stishovite + ßuid above 1200 °C (Sano et al. 2004). δ-AlOOH, a* E-mail: xianyu@misasa.okayama-u.ac.jp0003-004X/06/0506–850$05.00/DOI: 10.2138/am.2006.2064

850

high-pressure polymorph of diaspore (α-AlOOH) and boehmite(γ-AlOOH), has been found to be stable to pressures above 30
GPa at 1000~1200 °C, and thus it may serve as a water reservoir
in the cold subducted slabs, transporting water into the lower
mantle (Ohtani et al. 2001; Sano et al. 2004). In addition to thesehydrous phases, nominally anhydrous stishovite has also beensuggested to be a potentially important water reservoir, especiallybeyond the stability Þelds of hydrous phases (Pawley et al. 1993;
Chung and Kagi 2002; Panero and Stixrude 2004).
The crystal structure of δ-AlOOH was Þrst reÞned in thespace group of P21nm from powder X-ray diffraction (XRD)
by Suzuki et al. (2000), although the H position was not determined. The framework of oxygen packing is similar to the CaCl2(distorted rutile) structure, containing columns of edge-sharing
AlO6 octahedra that are joined via corners (Fig. 1a). Morerecently, Kudoh et al. (2004) reÞned the structure of a similarphase containing Mg and Si (δ-[Al0.86Mg0.07Si0.07]OOH) in spacegroup Pnn2 using single-crystal XRD, and identiÞed two H
sites from difference-Fourier maps, H1 with a short O-H1···Ohydrogen bonding distance of 2.511 Å, and H2 with two longer
O-H2···O distances of 2.743 Å. Raman spectrum of δ-AlOOH
was Þrst reported by Ohtani et al. (2001) and contains several
broad bands between 1500 and 3000 cm–1. This was interpreted
by Kudoh et al. (2004) to be related to the existence of two H
sites, although as discussed below, this is unlikely to be true. Thestrong hydrogen bonding in δ-AlOOH has also been the subjectof Þrst principles calculations because it may play a critical rolein determining the elastic properties at high pressure (Tsuchiya etal. 2002; Panero and Stixrude 2004). In general, hydrogen bonding in minerals also has great effects on many other properties
such as the strength, rheological, and electrical properties, andplays an important role in stabilizing hydrous and nominally