Phys Chem Minerals (1999) 26: 576±583 Ó Springer-Verlag 1999 ORIGINAL PAPER E. Huang á J.-F. Lin á J. Xu á T. Huang Y.-C. Jean á H.-S. Sheu Compression studies of gibbsite and its high-pressure polymorph Received: 28 September 1998 / Revised, accepted: 22 December 1998 Abstract Various X-ray diraction methods have been Introduction applied to study the compression behavior of gibbsite, Al(OH) , in diamond cells at room temperature. A phase 3 The water reservoir in the Earth's mantle may be do- transformation was found to take place above 3 GPa minately hosted in the hydrous minerals (e.g., Akimoto where gibbsite started to convert to its high-pressure and Akaogi 1984). Therefore, the study of hydrous polymorph. The high-pressure (HP) phase is quenchable minerals at high-pressure and temperature is crucial for and coexists with gibbsite at the ambient conditions after understanding the dynamic processes of water circula- being unloaded. This HP phase was identi®ed as nor- tion in the mantle. Examples such as triggering deep dstrandite based on the diraction patterns obtained at focus earthquakes (Kirby 1987; Meade and Jeanloz room pressure by angle dispersive and energy dispersive 1991) and regulating the water budget have been docu- methods. On the basis of this structural interpretation, mented in hydrous minerals under mantle environments. the bulk modulus of the two polymorphs, i.e., gibbsite In the past few years, some hydrous minerals with and nordstrandite, could be determined as 85 5 and trioctahedral layer structure such as brucite (Mg(OH) ) 70 5 GPa, respectively, by ®tting a Birch-Murnaghan 2 (e.g., Duy et al. 1995; Xia et al. 1998) and portlandite equation to the compression data, assuming their K0 as o (Ca(OH) ) (Desgranges et al. 1993; Pavese et al. 1997) 4. Molar volume cross-over occurs at 2 GPa, above 2 have been studied by the high-pressure experiments from which the molar volume of nordstrandite is smaller than which important geophysical implications are docu- that of gibbsite. The dierences in the molar volume and mented. These hydrous minerals show dierent behav- structure between the two polymorphs are not signi®- iors at high pressures, such as amorphization in cant, which accounts for the irreversibility of the phase Ca(OH) (Meade and Jeanloz 1990) and dehydration in transition. In gibbsite, the axial compressibility behaves 2 Mg(OH) (Johnson and Walker 1993). In our previous as c/c > a/a > b/b . This is due to the fact that the 2 o o o works, we have devoted ourselves to the studies of the dioctahedral sheets along the c-axis are held by the rel- high-pressure behaviors of hydrous and anhydrous alu- atively weak hydrogen bonding, which results in the minum oxides (Huang et al. 1995; Xu et al. 1995; Huang greater compressibility along this direction. In nord- et al. 1996). Some of these compounds, such as diaspore, strandite, the axial compressibility is b/b > c/c >a/ o o AlO(OH), do not show a signi®cant change in structure a, which can also be interpreted as resulting from the o but yield unexpected variations in Raman modes at high the existence of hydrogen bonds along the b-axis. pressures (Huang et al. 1995). On the other hand, gibbsite, Al(OH)3, shows drastic changes in both Raman and diraction patterns at high pressures (Huang et al. 1996). The results obtained from the Raman spectro- scopic studies are of interest when compared with those E. Huang (&) á J. Xu á T. Huang of other OH-bearing minerals at high pressure (Kruger Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, Taiwan, R.O.C. et al. 1989). Therefore, it is obvious that more high- pressure research on compounds in the Al O -H O J.-F. Lin 2 3 2 Geophysical Science, University of Chicago, system should be conducted in order to understand the Chicago, IL., U.S.A. behavior of the OH-bearing phases and the phase rela- tionships of minerals in the system. Y.-C. Jean á H.-S. Sheu Synchrotron Radiation Research Center, Gibbsite with a space group of P21/n has an analo- Hsinchu, Taiwan, R.O.C. gous structure as brucite but possesses dioctahedral 577 layers in contrast to those trioctahedral layers in mag- pressure of 5 GPa. The sample was then unloaded and mounted on nesian hydrous phases. Huang et al. (1996) reported that a sample holder in a Debye-Scherrer camera (with a diameter of 57.3 mm) in which a photographic ®lm was inserted. The X-ray gibbsite undergoes a polymorphic phase transition source used was CoKa radiation with running conditions of 45 KV above 3 GPa based on Raman and X-ray observations. and 30 mA. It took 24 hours for the exposure of the ®lm to collect The high-pressure phase was tentatively determined to the diraction signal. be nordstrandite (space group P1) based on the dif- In the second method, the image plate (IP) was used in coupled fraction patterns of the sample yielded from the un- with the synchrotron radiation of the Synchrotron Radiation Re- search Center (SRRC), Hsin-Chu, Taiwan. A monochromatic loading process. Unfortunately, their high-pressure beam of 14.5 KeV was used as the X-ray source. The sample was X-ray data were inadequate to justify whether this high- loaded in the hole (250 lm diameter) of a gasket made of stainless pressure phase is indeed nordstrandite. Therefore, we steel and then slightly compressed between the diamond anvil cell have carried out further in situ high-pressure study on so as to ®ll up the sample chamber. The cell was then mounted and aligned to bring the collimated X-ray beam (normally of the order gibbsite and also a more detailed structural identi®ca- of 100 lm) to fall on the sample in the diamond cell. The sample tion of the high-pressure phase. Furthermore, the com- was then compressed to approximately 6 GPa and quenched to pressibility can be determined for the two phases. room pressure. The diraction patterns of the sample at high- Energy dispersive X-ray diraction (EDXRD) has pressure and quenched condition were collected by the image plate with an exposure time of 24 hours. Optical alignment using a routinely been used as a tool in the study of materials monitor that traced the image of the sample ensured that the po- under pressures with synchrotron X-ray radiation. Re- sition of the sample did not change during the process of removing cently, much progress has been made in the detecting and remounting of the sample. system of the angle dispersive X-ray diraction (AD- XRD) method. Among all the detectors used in the EDXRD method ADXRD method, the image plate (IP) has proven to be the most ecient way for the collection of the diraction Two sets of runs were conducted in the EDXRD experiments. One signals. IP is a photosimulatable ¯uorescent plate, which was to identify the quenched phase and to correlate with the results is a digitally readable X-ray photograph. It has the obtained by the ADXRD method. The other was to determine the compressibility of the two polymorphs of Al(OH)3. The white ra- characteristics of high absorption coecient, sensitivity diation component of synchrotron radiation was used as the X-ray and greater dynamic range than the conventional ADX- source for both runs. RD detectors and it also has the capability of being In the quenching experiment, the gibbsite sample was loaded in easily read out (Shimomura et al. 1992). Hence, IP has the gasket hole of a stainless steel and compressed in a diamond cell above the phase transition pressure. Several diraction patterns proven to be a very ecient detector for the angle dis- were taken to monitor the process of the transition. The pressure persive X-ray diraction in diamond anvil cell (DAC) was then decreased to the ambient conditions and the diraction experiment (Huang 1996). Moreover, the data can be pattern of the quenched phase was taken. No pressure calibrant digitized for structural analysis, which makes this was loaded in the cell because the run was designed to take the diraction pattern of the quenched phase without interfering peaks method suitable for high-pressure diraction research from pressure calibrant. The run was carried out at the B2 station especially when synchrotron radiation is used (Chen and of Cornell High Energy Synchrotron Source, Cornell University. Takemura 1993). The application of IP to the X-ray Except for the 2h angle (taken at 14°), the experimental geometry studies and structural re®nement of samples in diamond and data acquisition time are similar to those of the compressibility anvil cells were successfully carried out by Shimomura run described below. In the compressibility run, the sample was mechanically mixed et al. (1992) and Nelmes et al. (1992). In this report, we with gold powder and loaded in a diamond anvil cell. A T301 will demonstrate the use of EDXRD and ADXRD stainless steel was used as the gasket with a diameter of 200 lm for methods in studying the pressure-induced phase transi- the sample chamber. Methanol-ethanol (4:1) solution was used as tion in Al(OH) at room temperature. pressure medium to maintain a hydrostatic condition. The experi- 3 ments were conducted at beamline X-17C, National Synchrontron Light Source, Brookhaven National Laboratory. An energy dis- persive X-ray diraction pattern was taken each time after the Experimental methods pressure increment was made. An intrinsic Ge detector was set at an angle (2h) of 7° and the acquisition time for each spectrum was about 10 minutes.