American Mineralogist, Volume 95, pages 1349–1352, 2010

LetterCrystal structure of a new high-pressure polymorph of topaz-OH
Masami Kanzaki*
Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori 682-0193, Japan

Abstract
A new high-pressure form of topaz-OH (denoted here topaz-OH II) was obtained at 14 GPa and1400 °C. The X-ray diffraction pattern of this phase can be indexed by an orthorhombic cell with a= 4.72318(5), b = 8.91480(9), and c = 2.77276(3) Å. The lengths of a and b are similar, but c is approximately a third of that for a previously reported topaz-OH (denoted topaz-OH I). The structural
formula of topaz-OH II can be written as (Al0.68Si0.32)(O0.66(OH)0.34)2, suggesting significant cation
disorder in the structure. The crystal structure of topaz-OH II is solved using powder synchrotron
X-ray diffraction data, and refined with constraints provided by a separate multi-nuclear NMR study.
The structure has similarities with topaz-OH I and diaspore (α-AlOOH) structures, having partiallyoccupied double edge-shared octahedral chains, and 2 × 1 tunnels with partially occupied tetrahedral
and octahedral sites.
Keywords: Crystal structure, high-pressure studies, phase transition, order-disorder

Introduction
Topaz-OH [Al2SiO4(OH)2], the OH end-member of thetopaz solid-solution series, was discovered at pressures 5.5–10
GPa and 700–1000 °C (Wunder et al. 1993). Subsequent phaseequilibrium studies revealed that topaz-OH is stable to 13 GPaand temperatures up to 1500 °C, beyond which topaz-OH breaks
down to kyanite (Al2SiO5) plus fluid at higher temperature, andto phase Egg (AlSiO3OH)-containing assemblages at higher
pressure (Schmidt et al. 1998; Ono 1999).
The crystal structure of topaz-OH has been refined in spacegroup Pbnm from single-crystal X-ray diffraction (XRD) obtained
at 10 GPa and 1000 °C by Wunder et al. (1993). Northrup et al.
(1994) also conducted single-crystal XRD of topaz-OH synthesized at the same P-T conditions, and revealed that the proton is
located in two non-equivalent, half-occupied sites. Later, Chen etal. (2005) studied a topaz-OD sample synthesized at 1023 K and
7.5 GPa by neutron powder diffraction, and confirmed the conclusions of the previous studies. The latter two studies suggested thepossibility of lower symmetry (Pbn21) by long-range order of theproton positions, although it was not confirmed. The possibilityof Pbn21 at room temperature was supported by ab-initio calculations and infrared spectroscopic study (Churakov and Wunder
2004; Watenphul and Wunder 2010). In all these structural studies, the samples were synthesized at pressures below 10 GPa andtemperatures below 1000 °C, and the cation distributions except
the proton were shown to be fully ordered. This established phaseis referred as “topaz-OH I” in this paper.
Recently, we have applied multi-nuclear ( 1H, 29Si, 27Al)
nuclear magnetic resonance (NMR) and Raman spectroscopy
to phase Egg (AlSiO3OH) and other phases in the system Al2O3SiO2-H2O (Xue et al. 2006). In the course of this study, we
discovered the existence of a new high-pressure polymorphof topaz-OH (Kanzaki et al. 2006). The stability of the phase* E-mail: mkanzaki@misasa.okayama-u.ac.jp0003-004X/10/0809–1349$05.00/DOI: 10.2138/am.2010.3555

has been studied using in situ high-pressure X-ray diffraction(Kanzaki unpublished) and found to be stable at 13–14 GPa and
1300–1500 °C, located at the high-pressure and high-temperaturecorner of the topaz-OH stability field estimated (Schmidt et al.
1998; Ono 1999). Here we report the crystal structure of thisphase (topaz-OH II hereafter) solved using powder synchrotron
X-ray diffraction data. A combined multi-nuclear NMR and Raman study of both phase I and II is reported in a separate paper(Xue et al. 2010).

Experimental methods
Sample synthesis
The topaz-OH II sample used for the present study was synthesized using aKawai-type double stage multi-anvil system with a DIA-type guide block driven
by a 1500 ton press (SPEED1500) installed at the beamline BL04B1 of SPring-8(Utsumi et al. 1998). The starting material was a mixture of dried reagent-grade
SiO2, Al2O3, and Al(OH)3 with a nominal Al2SiO4(OH)2 composition, and was put
into a Pd75Ag25 capsule (1.40/1.25 mm in o.d./i.d.). A 14 mm edge-length Cr-doped
MgO octahedral cell assembly with a tube heater (made of hBN plus TiB2) wasemployed. A thermocouple (W97Re3-W75Re25) was broken during compression, andthus temperature was estimated from applied electric power (with an uncertainty of±50 °C). The pressure cell is identical to that used for in situ high-pressure and hightemperature X-ray diffraction study of topaz-OH phase, in which gold powder wasused as a pressure standard (Kanzaki unpublished). However, the present synthesis
experiment was conducted without in situ X-ray diffraction. Therefore, to estimate
the pressure, a pressure calibration curve against press load was constructed basedon 10 in situ X-ray runs. The pressure reported here is based on that curve, andthe estimation error is <1 GPa.
The sample was kept at 14 GPa and 1400 °C for an hour, and then quenchedby cutting off the heating power. The recovered sample (K071126) was checkedby Raman spectroscopy, and was confirmed to consist of topaz-OH II with a minor
stishovite phase. This sample was further characterized in detail by NMR (Xue etal. 2010). The composition of topaz-OH II determined by an electron microprobe
is 58.54(0.86) wt% Al2O3, 31.72(0.56) wt% SiO2, and 90.26(1.17) wt% total (20points average). Details of the analyses including topaz-OH I and II synthesized
at different P-T conditions are reported in Xue et al. (2010). The Si/Al ratio is
calculated as 0.46(1), lower than that of the ideal composition (1/2), which explains
the existence of minor stishovite in the recovered sample. If all remaining Si is used
to form stishovite, 3.0(0.8) wt% of stishovite is expected from the observed Si/Alratio and nominal bulk composition. This compares well with 2.6 wt% of stishovite1349