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European Journal of Mineralogy GSW 2008 Users' Group Meeting
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European Journal of Mineralogy; June 2001; v. 13; no. 3; p. 479-484; DOI: 10.1127/0935-1221/2001/0013-0479
© 2001 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
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Articles

Direct transition from cristobalite to post-stishovite {alpha}-PbO2-like silica phase

Natalia A. DUBROVINSKAIA1,*, Leonid S. DUBROVINSKY1, Surendra K. SAXENA1, Faramaz TUTTI1, Sandeep REKHI1 and Tristan LE BIHAN2

1 Department of Earth Sciences, Uppsala University, S-752 36 Uppsala, Sweden
2 European Synchrotron Radiation Facility, Grenoble 38043, France

* e-mail: Natalia.Dubrovinskaia{at}geo.uu.se

This paper was presented at the EMPG VIII meeting in Bergamo, Italy (April 2000)


   Abstract
Top
 Abstract
Introduction
 Experimental technique
 Results and discussion
 Conclusions
 Acknowledgements
 References
 
In situ, x-ray studies in diamond anvil cells at pressures over 80 GPa were used to determine a stable silica phase at extreme conditions. We demonstrate that so far unidentified phases obtained on compression of {alpha}-cristobalite and a new dense silica polymorph discovered in the Martian meteorite Shergotty is the {alpha}-PbO2-type silica.

Key-words: silica, {alpha}-cristobalite, phase transition, diamond anvil cell, high-pressure phase.


   Introduction
Top
 Abstract
 Introduction
Experimental technique
 Results and discussion
 Conclusions
 Acknowledgements
 References
 
Following the discovery of stishovite (the highest-pressure polymorph of silica known from natural samples so far) many attempts have been made to investigate a possible existence of denser phases at higher pressures. Several possible poststishovite phases have been suggested issuing from crystal structures observed in chemical analogues of silica, high-pressures experiments on silica and theoretical studies. However, it remains uncertain which of them is a stable silica phase at pressures and temperatures corresponding to the Earth's lower mantle. Last decade studies revealed a number of enigmatic phenomena associated with high-pressure silica polymorphs - formation of unidentified phases on compression of {alpha}-cristobalite, controversial theoretical and experimental information on possible post-stishovite phases, discovery of a new dense natural silica polymorph in the Shergotty meteorite in a mineralogical environment that is inappropriate for post-stishovite phase (Palmer & Finger, 1994, Palmer et al., 1994; Parise et al., 1994; Yahagi et al., 1994; Tsuchida & Yagi, 1990; Hemley et al., 1996; Halverson & Wolf, 1990; Yamakata & Yagi, 1997; Gratz et al., 1993; German et al., 1973; Teter et al., 1998; Sharp et al., 1999; Dubrovinsky et al., 1997). Here we present the results of in situ X-ray studies of silica in diamond anvil cells (DACs) at pressures over 85 GPa and demonstrate that the above mentioned problems can be explained from properties and behaviour of silica with {alpha}-PbO2-type structure.

On compression of {alpha}-cristobalite at pressures above 10 Gpa and ambient temperature, first Tsuchida & Yagi (1990) and later other groups (Yahagi et al., 1994; Palmer et al., 1994; Yamakata & Yagi, 1997) reported about a phase transition to one more phase (cristobalite-XI according to Tsuchida & Yagi (1990) or cristobalite-III according to Hemley et al., 1996). Under further compression at pressures above 40 GPa, cristobalite-XII (Tsuchida & Yagi, 1990) was found. On decompression of cristobalite-XII, a new polymorph - cristobalite-XIII formed (Tsuchida & Yagi, 1990). Little was known about the nature and structure of all these cristobalite-"x" phases (Hemley et al., 1996; Yamakata & Yagi, 1997). It is not clear how these phase transitions are related to amorphization of silica reported in a few papers (Palmer et al., 1994; Halverson & Wolf, 1990; Gratz et al., 1993) for the same pressure range (above 10 GPa).

Recently, the {alpha}-PbO2-type silica was experimentally obtained in a mixture with stishovite in a laser-heated diamond anvil cell at pressures above 70 GPa and temperatures above 2500 K (Dubrovinsky et al., 1997). However, there was no definite theoretical and/or experimental information on stability of the {alpha}-PbO2-type silica (Belonoshko et al., 1996; Dubrovinsky et al., 1996; Teter et al., 1998; Dubrovinsky et al., 1997; Andrault et al., 1998; Karki et al., 1997; German et al., 1973).

Intensive studies (Sharp et al., 1999) of grains of a silica material from the meteorite Shergotty led to discovery of a new natural SiO2 polymorph. On the basis of selected area electron diffraction (SAED) data, Sharp et al. (1999) suggested that the new silica polymorph was distinctly different from the ideal (with a space group Pbcn) or modified (with a space group Pnc2) {alpha}-PbO2-type phases (Teter et al., 1998; Dubrovinsky et al., 1997), but instead similar to a Pbcn structure produced in shock-waves experiments at 70 to 90 GPa (German et al., 1973). It implies that the meteorite Shergotty was subjected to unlikely high shock pressure above 80 GPa (Sharp et al., 1999).


   Experimental technique
Top
 Abstract
 Introduction
 Experimental technique
Results and discussion
 Conclusions
 Acknowledgements
 References
 
We conducted in situ X-ray high-pressure experiments at Uppsala Lab and at ESRF (beam line ID30) using Mao-Bell type DAC with constant wavelength radiation and angle sensitive detectors (Dubrovinsky et al., 1997, 2000). At Uppsala Lab we collected powder X-ray diffraction data using a Siemens X-ray system consisting of a Smart CCD Area Detector and a direct-drive rotating anode as an X-ray generator (18 kW). Mok{alpha} radiation (tube voltage 50 kV, tube current 24 mA, cathode gun 0.1 x 1 mm) was focused with a capillary X-ray optical system to {emptyset} 40 µm FWHM. More details are given in Dubrovinsky et al. (1997). At ESRF powder diffraction data were collected with a fine incident X-ray beam of approximately rectangular shape (8*9 µm2) of 0.3738 Å wavelength at the FastScan imaging plate (Dubrovinsky et al., 2000).

Pure cristobalite samples were synthesized by heating a silica gel (99.99 % purity) at 1550°C for 8 hours and then quenching it. Cristobalite with 0.5 % Na2O and 1 % Al2O3 was obtained by heating of an appropriate mixture of the silica gel, Na2CO3 and corundum at 1600°C for 12 hours and then quenching it. A pure Pt powder was used as an internal pressure standard. The experiments were conducted without any pressure medium to avoid possible chemical reactions and complication of the diffraction pattern at high pressures. Moreover, our goal was to identify the phase(s) which could appear on compression of cristobalite in non-hydrostatic conditions, while it was already found that compression in soft pressure media (H2, Ar, He) led to formation of a stishovite-like phase (Yamakata & Yagi, 1997).

In our analysis of the integrated X-ray spectra, we used the program GSAS (Larson & Von Dreele, 1994) and PeakFit 4.0.


   Results and discussion
Top
 Abstract
 Introduction
 Experimental technique
 Results and discussion
Conclusions
 Acknowledgements
 References
 
We compressed samples directly to 10–12 GPa (Fig. 1). At that pressure cristobalite-I transforms to cristobalite-XI (Fig. 1). At pressures above 37 GPa, new reflections started growing and the phase transition was completed by 45 GPa. On further compression to 89 GPa at room temperature, we did not observe any other phase transitions (Fig. 1).


Figure 1
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  		Fig. 1. Examples of diffraction patterns collected on compression of {alpha}-cristobalite at room temperature. The initial material (bottom line) is {alpha}-cristobalite (a = 4.9733(3) Å, c = 6.9234(4) Å). At pressures above 10 GPa {alpha}-cristobalite transforms to cristobalite-XI (Tsuchida & Yagi, 1990), "CrXI", second line from bottom. At pressures above 37 GPa new reflections start growing and the phase transition to {alpha}-PbO2-type structure ("S") is completed by 45 GPa. On further compression to 89 GPa at room temperature we did not observe any other phase transitions. Platinum reflections are marked as "Pt".

 
The d-values and relative intensities of reflections of the phase obtained in the present study at high pressures from {alpha}-cristobalite as a starting material, and those from the substance synthesised in a laser-heated DAC at 69 GPa (Dubrovinsky et al., 1997, 1998) are in quantitative agreement (Table 1). Such a comparison allowed us to identify the phase which we obtained from {alpha}-cristobalite at pressures above 45 GPa, as the {alpha}-PbO2-structured silica. Moreover at 52 GPa, our diffraction data and data reported by Tsuchida & Yagi (1990) for 53 GPa are close (Table 1). The lattice parameters of the {alpha}-PbO2-type silica obtained in the present study at 52(1) GPa are a = 4.326(1) Å, b = 3.939(1) Å, c = 4.805(1) Å, while by indexing reflections reported by Tsuchida & Yagi (1990) in terms of an orthorhombic lattice of the {alpha}-PbO2-type structure, we have a = 4.407(5) Å, b = 3.884(7) Å, c = 4.821(4) Å. In other words, the cristobalite-XII phase found by Tsuchida & Yagi (1990) is the {alpha}-PbO2-structured silica. On the basis of previous theoretical and experimental data (Karki et al., 1997; Sharp et al., 1999; Dubrovinsky et al., 1997), a space group of {alpha}-PbO2-type/like silica could be identified as Pbcn or Pnc2. However, these two space groups are difficult to distinguish on the basis of previous or present X-ray experimental data. The space group of {alpha}-PbO2 is Pbcn, Pnc2 is a sub-group. In this study we identify the symmetry of the high-pressure phase as Pbcn which is to be preferred as the supergroup unless we have clear experimental data to the contrary.


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  		Table 1. Comparison of d-values obtained on compresion of {alpha}-cristobalite at room temperature and 53(1) GPa and 69(2) GPa wiht earlier data on silica polymorphs at similar conditions.

 
We did not observe any phase transformations on gradual decompression from 60 GPa, and could trace all reflections of the {alpha}-PbO2-type silica down to ambient pressure obtaining the phase with lattice parameters a = 4.547(2) Å, b = 4.099(3) Å, c = 5.018(4) Å. Andrault and co-authors (1998) observed transformation from stishovite to CaCl2-type silica. Contrary, we did not observe CaCl2-structured silica in our experiments. Possible reasons for this discrepancy could be different experimental conditions. We used different initial phases and did not heat the samples.

A comparison of our X-ray data for the quenched {alpha}-PbO2-type silica phase with data published by Tsuchida & Yagi (1990) for cristobalite-XIII (Table 2), clearly shows that d-spacings, relative intensities of reflections, and lattice parameters for the two phases are close. Close agreement in X-ray data for our quenched samples and for the samples of Tsuchida & Yagi (1990) (especially taking into account the different methods of synthesis of the samples, different maximum pressures, possible differences in decompression rates etc. (Yahagi et al., 1994; Yamakata & Yagi, 1997) allows us to conclude that cristobalite-XIII (Tsuchida & Yagi, 1990) is the {alpha}-PbO2-type silica quenched from high pressure.


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  		Table 2. Diffraction data for some silica polymorphs.

 
Table 2 shows that positions of reflections and zone relations, observed for the newly discovered silica polymorph in the meteorite Shergotty (Sharp et al., 1999) by selected area electron diffraction (SAED), could be easily explained in terms of the {alpha}-PbO2-type silica structure. The ratio of lattice parameters for the silica mineral from the Shergotty meteorite (a/c = 0.890(9), b/c = 0.814(12)) are comparable to those of the {alpha}-PbO2-type silica phase obtained on decompression (a/c = 0.906(1), b/c = 0.816(1)). The reasonable small differences in the lattice parameters could be due to the different history of the synthetic and natural samples and due to a small amount of Na2O (0.4 wt. %) and Al2O3 (1.12 wt. %) in silica grains from the meteorite. Indeed, compression of synthetic cristobalite specially synthesized with 0.5 wt. % Na2O and 1 % of Al2O3 at ambient temperature to pressures above 40 GPa and further quenching led to formation of the {alpha}-PbO2-type silica phase with lattice parameters a = 4.551(1) Å, b = 4.122(1) Å, c = 5.059(2) Å fairly close to those obtained for natural SiO2 crystals from the Shergotty meteorite (Table 2). Our studies show that the silica phase discovered by Sharp et al. (1999) is actually a naturally occurring {alpha}-PbO2-structured silica.


   Conclusions
Top
 Abstract
 Introduction
 Experimental technique
 Results and discussion
 Conclusions
Acknowledgements
 References
 
In summary, X-ray in situ studies showed that on compression at ambient temperature {alpha}-cristobalite transforms to cristobalite-XI at pressures above 10 GPa. On further compression above 40 GPa at room temperature cristobalite-XI transforms to the {alpha}-PbO2-type silica phase, which is observed to 89 GPa. On the basis of our new highpressure experimental data we can propose that SiO2 in the Earth's lower mantle could have a post-stishovite {alpha}-PbO2-type structure. Careful comparison of the diffraction data of Sharp et al. (1999) and our data on {alpha}-PbO2-type silica, as well as the additional high-pressure experiments, allowed us to conclude that the silica mineral discovered in the meteorite Shergotty most probably has the {alpha}-PbO2-type structure.


   Acknowledgements
Top
 Abstract
 Introduction
 Experimental technique
 Results and discussion
 Conclusions
 Acknowledgements
References
 
We thank the Swedish Natural Science Research Council, Wallenberg and Crafoords Funds for the financial support. We thank L. Stixrude for useful comments.

Received 18 May 2000
Modified version received 28 October 2000
Accepted 2 January 2001


   References
Top
 Abstract
 Introduction
 Experimental technique
 Results and discussion
 Conclusions
 Acknowledgements
 References
 
Andrault, D., Fiquet, G., Guyot, F., Hanfland, M. (1998): Pressure-induced Landau-type transition in stishovite. Science, 282, 720–724.[CrossRef][ISI][GeoRef]

Belonoshko, A.B., Dubrovinsky, L.S., Dubrovinsky, N.A., Saxena, S.K. (1996): Phase diagram and properties of silica: lattice and molecular dynamics study. Petrology, 4, 519–535.[ISI]

Dubrovinsky, L.S., Belonoshko, A.B., Dubrovinsky, N.A., Saxena, S.K. (1996): New high-pressure silica phase obtained by computer simulation. In: "High Pressure Science and Technology", AIRAPT/EHPRG Report, 921–923.

Dubrovinsky, L.S., Saxena, S.K., Lazor, P., Ahuja, R., Eriksson, O., Wills, J.M., Johansson, B. (1997): Experimental and theoretical identification of a new high-pressure phase of silica. Nature, 388, 362–365.[CrossRef][GeoRef]

Dubrovinsky, L.S., Saxena, S.K., Tutti, F., Le Bihan, T. (2000): X-ray study of thermal expansion and phase transition of iron at multimegabar pressure. Phys. Rev. Letters, 84, 1720–1723.[CrossRef][ISI][Medline]

German, V.N., Podurets, M.A., Trunin, R.F. (1973): Shock compression of quartz to 90 GPa. Sov. Phys. JETP, 37, 107–115.

Gratz, A.J., DeLoach, L.D., Clough, T.M., Nellis, W.J. (1993): Shock amorphization of cristobalite. Science, 259, 663–666.[CrossRef][ISI][GeoRef]

Halverson, K. & Wolf, G.H. (1990): Pressure induced amorphization of cristobalite. EOS Trans. Am. Geophys. Union, 71, 1671.

Hemley, R.J., Prewitt, C.T., Kingma, K.J. (1996): High pressure behavior of silica. Rev. Miner., 29, 41–81.

Karki, B.B., Warren, M.C., Stixrude, L., Ackland, G.J., Crain, J. (1997): Ab initio studies of high-pressure structural transformations in silica. Phys. Rev. B, 55, 3465–3471.[CrossRef]

Larson, A.C. & Von Dreele, R.B. (1994): Los Alamos National Laboratory, LAUR, 86.

Palmer, D.C. & Finger, L.W. (1994): Pressure-induced phase transition in cristobalite: An X-ray powder diffraction study to 4.4 GPa. Am. Mineral., 79, 1–8.[Abstract][ISI][GeoRef]

Palmer, D.C., Hemley, L.W., Prewitt, C.T. (1994): Raman spectroscopic study of high-pressure phase transitions in cristobalite. Phys. Chem. Miner., 21, 481–488.

Parise, J.B., Yeganeh-Haeri, A., Weidner, D.J., Jorgensen, J.D., Saltzberg, M.A. (1994): Pressureinduced phase transition and pressure dependence of crystal structure in low and Ca/Al doped cristobalite. J. Appl. Phys., 75, 1361–1367.[CrossRef][ISI]

Sharp, T.G., El Goresy, A., Wopenka, B., Chen, M. (1999): A post-stishovite SiO2 polymorph in Shergotty: implication for impact events. Science, 284, 1511–1513.[CrossRef][ISI][GeoRef]

Teter, D.M., Hemley, R.J., Kresse, G., Hafner, J. (1998): High pressure polymorphism in silica. Phys. Rev. Lett., 80, 2145–2148.[CrossRef][ISI]

Tsuchida, Y. & Yagi, T. (1990): New pressure-induced transformations of silica at room temperature. Nature, 347, 267–269.[CrossRef][GeoRef]

Yahagi, Y., Yagi, T., Yamawaki, H., Aoki, K. (1994): Infrared absorption spectra of the high-pressure phases of cristobalite and their coordination numbers of silicon atoms. Solid State Comm., 89, 945–948.[CrossRef]

Yamakata, M. & Yagi, T. (1997): New stishovite-like phase of silica formed by hydrostatic compression of cristobalite. Proceedings of the Japan Academy, 73B, 85–88.[ISI]




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JOURNAL HOME HELP FEEDBACK/COMMNET SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
Copyright © 2008 by E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers