Mineralogical Magazine, October 2004, Vol. 68(5), pp. 801–811 Infrared and Raman spectra of ZrSiO4 experimentally shocked at high pressures 1 2, 1 2 2 2 A. GUCSIK ,M.ZHANG *, C. KOEBERL ,E.K.H.SALJE ,S.A.T.REDFERN AND J. M. PRUNEDA 1 Institute of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria 2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK ABSTRACT Zircon- and reidite-type ZrSiO4 produced by shock recovery experiments at different pressures have been studied using infrared (IR) and Raman spectroscopy. The n3 vibration of theSiO 4 group in shocked natural zircon shows a spectral change similar to that seen in radiation-damaged zircon: a decrease in frequency and increase in linewidth. The observation could imply a possible similar defective crystal structure between the damaged and shocked zircon. The shock-pressure-induced structural phasetransition from zircon ( I41/amd) to reidite (I41/a) is proven by the occurrence of additional IR and Raman bands. Although theSiO 4 groups in both zircon- and reidite-ZrSiO4 are isolated, the more condensed scheelite gives rise to SiÀO stretching bands at lower frequencies, suggesting a weakening of the bond strength. Low-temperature IR data of the reidite-type ZrSiO4 show an insignificant effect of cooling on the phonon modes, suggesting that the structural response of reidite to cooling-induced compression is weak and its thermal expansion is very small. KEYWORDS: ZrSiO4, zircon, reidite, scheelite structure, high pressure, phase transition, infrared spectroscopy, Raman spectroscopy, metamictization. Introduction lographic c axis. Under compression, zircon undergoes a structural phase transition to form a ZIRCON (ZrSiO4) is widely used in the ceramic, scheelite-structure phase (space group I41/a, a = foundry and refractory industries owing to its high 4.734 A˚ and c = 10.51 A˚ ) with an increase in thermal conductivity, chemical stability and density of 9.9% (Reid and Ringwood, 1969; Liu, ability to accommodatea numberof dopant 1979; Mashimo et al., 1983; Kusaba et al., 1985, ions. Recently, it has been the subject of extensive 1986). The scheelite-structure phase has recently research because it is commonly used in U/Pb been discovered in naturally occurring samples radiometric age dating and it is also among (Glass and Liu, 2001) and the mineral was named several crystalline phases currently under consid- reidite after Alan Reid, who first produced the eration for the immobilization and disposal of phasein thelaboratory (Glass et al., 2002). The high-actinide radioactive wastes (Anderson et al., pressure at which the transition takes place is 1993; Ewing et al., 1995; Weber et al., 1996). reported to be ~30 GPa for natural zircon in shock Zircon has a tetragonal structure with space experiments at room temperature (e.g. Kusaba et 19 ˚ group D4h or I41/amd (a = 6.607 A and c = al., 1985). A recent in situ study showed that, at 5.981 A˚ ) (e.g. Hazen and Finger, 1979). The ideal room temperature, the onset of the phase structureconsists of chains of edge-sharing, transition is at a pressure of 20 GPa in synthetic alternating SiO4 tetrahedra and ZrO8 triangular pureZrSiO 4 (van Westrenen et al., 2004). The dodecahedra extending parallel to the crystal- transition pressure is also affected by temperature as heating leads to lowering of the transition pressure (Reid and Ringwood, 1969). Due to the * E-mail: [email protected] importance of shock metamorphism in geology DOI: 10.1180/0026461046850220 and mineralogy, the effects of pressure-induced # 2004 The Mineralogical Society A. GUCSIK ET AL. shock on zircon and characterization of the extensively investigated materials in the study of scheelite-structure ZrSiO4 haveattractedconsid- metamictization, zircon has a relatively simple erable attention (e.g. Bohor et al., 1993; Knittle structure and its chemical and physical properties and Williams, 1993; Leroux et al., 1999; Glass et are well understood. We wish to investigate the al., 2002; Gucsik et al., 2002, 2004; Farnan et al., possible spectral similarities or differences 2003; Ono et al., 2004; Reimold et al., 2002; Rı´os between radiation-damaged zircon and high- and Boffa-Ballaran, 2003; Scott et al., 2002; van pressureshockedzircon for thepurposeof Westrenen et al., 2004). illuminating the metamictization process. In this study, we aim to provide a better understanding of shocked ZrSiO , especially 4 Experimental methods scheelite-type ZrSiO4 from their IR and Raman spectra. Although some previous reports have Thezircon samplesusedin this study originated shown vibrational spectra of the scheelite phase from Sri Lanka and thestarting material showed À1 (Kusaba et al., 1985; Knittleand Williams, 1993; the n3 (SiO4) Raman band at 1006.7 cm with a Gucsik et al., 2002, 2004; Scott et al., 2000; van measured width of 3.6 cmÀ1 (thevalueis Westrenen et al., 2004), these measurements were measured at the full width at half maximum – conducted in limited frequency regions, and the FWHM), which indicates that the crystal is band positions and number of bands obtained in crystallineand lacks a-decay damage. The different studies are inconsistent. Our second aim crystal was cut into thin plates ~1 mm thick, was related to understanding a-decay damage in parallel and at 45º to their crystallographic zircon. It has been documented that, due to c axes. Shock recovery experiments were radioactivedecayof naturally occurring radio- performed (shock pressures between 20 and nuclides and their daughter products in the 238U, 80 GPa) on such plates using the shock 235U and 232Th decay series, the crystal structure reverberation technique at the Ernst-Mach- of natural zircon can be heavily damaged over Institute, Germany (e.g. Deutsch and Scha¨rer, geological times, resulting in an aperiodic or 1990; Sto¨ffler and Langenhorst, 1994). The amorphous state– themetamictstate(Ewing, samples had previously been investigated by 1994; Weber et al., 1998; Salje et al., 1999). electron microscopy, and micro-Raman spectro- Extensive studies in the past decade have focused scopy (Leroux et al., 1999; Gucsik et al., 2002, on the effect of radiation on the physical and 2004). The samples were re-examined by IR, FT- chemical properties of zircon, on the structures of Raman and micro-Raman spectroscopy in the the metamict state and on the recovery of the present study to address the issues described in damaged structure at high temperatures (see earliersections and to improvethedata quality. Weber et al., 1998 and Ewing et al., 2003 for In addition to small crystals, impact-induced tiny reviews). One important scientific issue is what gains or powders from the shocked samples were happens at the atomic level during metamictiza- also measured by micro-Raman and powder tion, and what therolesof the a particleand the absorption spectroscopy. The IR powder pellet recoil in the damage process are. It is commonly techniquereportedby Zhang and Salje(2001) believed that during a-decay radiation damage, was employed in the powder absorption measure- the energetic recoil (70À100 keV) transfers most ments.Dueto thelimitation of thesample of its energy in collisions (Weber et al., 1998). powders, the IR powder absorption measure- This probably leads to an ultra-fast (in the order of ments were carried out only on the 80 GPa ps) high-temperature process in the displacement sampleand a crystallinesamplefrom Sri Lanka. cascades that could melt the material (e.g. CsI powder was used as matrix material. The Miotello and Kelly, 1997; Meldrum et al., 1998) sample/matrix ratio was 1:300. Sample powders and the melts could quench very quickly. The were thoroughly mixed with the matrix material. process might involve local high stress and/or 300 mg of the mixture were pressed into 13 mm pressure. In addition, a recent computer simula- discs under vacuum. The sample pellets were tion has proposed that radiation damage in zircon measured within 12 h of creation to avoid leads to polymerization (formation of direct possiblereactions betweenthesampleand the SiÀOÀSi linkages), shear deformation and the matrix material. Infrared reflectance and Raman formation of highly dense regions (Trachenko et measurements were also made on small crystal al., 2002), which might also beassociatedwith grains under unpolarized conditions because of local high pressures.As oneof themost thesmall sizesof thematerials. 802 EXPERIMENTALLY SHOCKED ZRSIO4 Powder IR spectra between 150 and 2500 cmÀ1 order to check any possible laser-induced were recorded using a Bruker 113v FT-IR fluorescence, FT-Raman spectra were also spectrometer (see Zhang and Salje, 2001 for recorded at room temperature using a Bruker detailed information on instrumentals). IFS 66v spectrometer adapted with a Bruker FRA Reflectance spectra between 650 and 5000 cmÀ1 106 FT-Raman accessory. A silicon-coated were recorded at an almost normal incident calcium fluoride beam-splitter and radiation of condition using an IR microscope equipped with 1064 nm from a Nd:YAG laser were used for the a mapping stage and attached to a Bruker IFS 66v excitation laser. A liquid-nitrogen-cooled, high- FT-IR spectrometer. The beam size was 80 mm. A sensitivity Ge detector was used. The FT-Raman liquid-nitrogen-cooled MCT detector, coupled spectra were recorded with a resolution of with a KBr beamsplitter and a Globar source 2cmÀ1, a laser power of ~100 mW and a back- were used. The spectra were averaged by 512 scattering geometry. scans with a spectral resolution of 2 cmÀ1. Gold mirrors were used as the reference for the reflectance measurements. For low-temperature Results and discussion measurements, a closed-cycle liquid-helium cryo- IR spectra stat(LEYBOLD),equippedwithKRS5and Unpolarized reflectance spectra (650À À1 polyethylene windows, was used. Two Si-diode 1300 cm )ofZiSiO4 shocked at different temperature sensors (LakeShore, DT-470-DI-13), pressures are shown in Fig. 1a. Thestrong well calibrated by their manufacturers, were used reflection features between 800 and 1100 cmÀ1 in thecryostat.