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 Si O 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 1 (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 cm 1 (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 (70 100 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 Si O Si 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 cm 1 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 cm 1 106 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 2cm 1, 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 cm 1. 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 (650 1 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 cm 1 in thecryostat. Onewas usedto control the aredueto Si O stretching bands in zircon and cryostat and the other for measuring the sample reidite. The pressure-induced phase transition temperature. The temperature stability was better from zircon (with spacegroup I41/amd)to than 1 K. reidite (with space group I41/a) is characterized Conventional Raman spectra were collected by the spectral differences between untreated with a free-sample-space LabRam micro-Raman zircon and the samples shocked at 20, 38, 40, 60 spectrometer. A 623 nm Ne-He laser was used for and 80 GPa. Thegeneralsimilaritiesin the the excitation. A CCD detector, a grating (1800 spectra of these samples shocked at and above grooves/mm), and a 506 ultra-long working- 38 GPa imply that these samples consist mainly distanceobjectiveanda 100 6 objective were of reidite, although small volumes of zircon might coupled with a free space microscope. The remain in some of the samples as a result of instrumental resolution of 3.5 cm 1 was used. In heterogeneous compression across the crystal. a 2.4 b
2.0
1.6
zircon 1.2 unshocked
20 GPa 0.8 zircon Reflectivity 38 GPa
40 GPa Absorbance (a.u.) 0.4 reidite 60 GPa
80 GPa 0.0
700 800 900 1000 1100 1200 1300 200 400 600 800 1000 1200 1400
Wavenumber (cm-1) Wavenumber (cm-1)
1 FIG.1.(a) Micro-IR reflectance spectra (650 1300 cm ) of the untreated zircon and the samples shocked at different pressures. Spectral differences between the untreated zircon and the samples shocked at and above 38 GPa are mainly due to the change of crystal symmetry during the zircon–reidite phase transition. (b) Comparison of IR powder absorption spectra (150 1500 cm 1) of zircon and the sample shocked at 80 GPa containing mainly reidite phase.
803 A. GUCSIK ET AL.
The differences between the IR features for zircon small or undetectable changes in frequency. This and reidite ZrSiO4 are attributed to the symmetry indicates that the scheelite phase of ZrSiO4 has a changesat thephasetransition as wellas very weak thermal expansion and the structural differences in bond lengths. Powder IR absorption compression caused by the cooling is weak. The spectra of reidite and zircon are shown in Fig. 1b; data from cooling measurements resolved certain the former is taken from the sample shocked to spectral features. At 20 K, the sample showed 80 GPa. Although both materials are tetragonal, clearly local maxima near 1000, 1080 and the denser phase (i.e. the scheelite-structure 1180 cm 1, which are too high in frequency to phase) is characterized by a shift of the Si O bedueto isolatedSiO 4 groups. stretching bands to lower wavenumbers. We noted the appearance of a feature near Raman spectra 1080 cm 1 in the 80 GPa sample. The wave- number of its position is >200 cm 1 higher than Room-temperature micro-Raman spectra of 1 these of the bands near 813 and 871 cm .Itis zircon and reidite-ZrSiO4 areshown in difficult to attribute the frequency difference to Fig. 3a,b. Thespectrumof zircon exhibitsa symmetry-induced band splitting of the Si O strong n3 band (anti-symmetric stretching of the 1 stretching bands of scheelite ZrSiO4, because only SiO4 group) near 1008 cm , which has been two IR-active bands of the scheelite phase are shown to have B1g symmetry (Dawson et al., expected in this frequency region. Furthermore, a 1971). Theband near974 cm 1 is assigned as a 1 frequency of 1080 cm is not consistent with the Si O stretching band (n1, symmetric stretching) Si O stretching band of zircon (Fig. 1b). Low- with A1g symmetry. Other relatively intense temperature data (see below) revealed additional Raman bands of zircon arelocatedin low- weak absorption bands near this 1080 cm 1 band. frequency regions, and they are attributed to Therefore, these features are unlikely to be bending vibrations of SiO4 groups and external associated with reidite, but probably due to a bands associated with SiO4 group motions with high-pressure-induced amorphous phase or respect to Zr atoms and motions of Zr atoms possibly dueto SiO 2 resulting from decomposi- (Dawson et al., 1971). Thestructural transition tion of ZrSiO4. from zircon (with spacegroup I41/amd) to reidite We noted that the sample shocked at 20 GPa (with spacegroup I41/a) is proven by a systematic (Fig. 1a) exhibits broad and less distinct reflec- change of Raman features (Fig. 3b). The schee- tance signals, although Raman data (see below) lite-structure ZrSiO4 (e.g. the sample shocked to indicate that the sample essentially has the zircon 80 GPa) clearly shows more Raman bands due to structure. We consider that the spectral changes its lower crystal symmetry. Just as with the IR areassociatedwith structural distortion and strain data (Fig. 1b), therelativelystrong Si O caused by high-pressure shocking. Transmission stretching bands shift to lower frequency in this electron microscopy (TEM) data (Leroux et al., structure. FT-Raman spectra of shocked samples 1999; Reimold et al., 2002) show evidence of shock-induced deformation and the formation of dislocations. Theimportanceof our finding is that line broadening seen in natural zircon could be 382 871 80 GPa 513 caused by shock-induced defects and structural 814 deformation. 1000 The IR powder absorption spectrum of the 1080 sample shocked at 80 GPa was also recorded at 868 1180 low temperatures to investigate the thermal 813 20 K 380 evolution of the phonon modes of the scheelite Absorbance (a.u.) 513 100 K phase and also to further study the observed IR 200 K 300 K features, as vibrational bands generally sharpen at low temperatures. The effect of cooling on the 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 phonon modes of the scheelite phase is weak Wavenumber (cm-1) (Fig. 2). For instance, the band at 868 cm 1 (at room temperature), which is considered to be due FIG. 2. Temperature evolution (between 20 and 300 K) to a Si O stretching vibration, shifted to of absorption spectra of the scheelite phase (sample 871 cm 1 at 20 K. The other IR bands showed shocked at 80 GPa).
804 EXPERIMENTALLY SHOCKED ZRSIO4
200 300 400 500 600 700 800 900 1000 1100
b 20 GPa
38 GPa a
B , ν (SiO ) * zircon 1g 3 4 * 1008 *
Eg 357 ν A1g, 2(SiO4) E 60 GPa g 439 B 1g225 A , ν (SiO ) B 1g 1 4 Raman intensity (a.u.) 214 1g Eg 974
Raman intensity (a.u.) 202 393
60 200 400 600 800 1000 1200 Wavenumber (cm-1) 80 GPa
200 300 400 500 600 700 800 900 1000 1100 Wavenumber (cm-1)
FIG. 3. Raman spectra of (a) zircon; (b) samples shocked at 20, 38, 60 and 80 GPa. The 20 GPa sample still has the zircon structure.
were also recorded to check laser-induced hardly be seen in the powder sample shocked to fluorescence. The FT-Raman spectra are essen- 80 GPa. We believe that they originate from tially similar to thosefrom conventionalRaman remaining untransformed zircon domains as spectroscopy, in terms of the number of bands and suggested by some previous studies (e.g. van their frequencies, although FT-Raman spectra of Westrenen et al., 2004; Gucsik et al., 2004). the shocked samples exhibit a relatively high Gucsik et al. (2004) reported that Raman spectra background at low frequencies. This confirms the of the naturally shock-deformed zircon crystals reliability of the micro-Raman data. We found from theRiesimpact structure(Germany) that theRaman bands near966 and 1000 cm 1 represent zircon-structure material, whereas a are not Raman bands of scheelite-structure sample from the shock-metamorphic (suevite) ZrSiO4. Although they were observed in some rock shows additional peaks with relatively high samples shocked at lower shock pressures, they peak intensities, which are indicative of the did not occur in all areas sampled and they could presence of the scheelite-type structure of
805 A. GUCSIK ET AL.
38 GPa * - zircon
* * * 50
40
30 m) µ ( 20
length 10
200 400 600 800 1000 1200 Wavenumber (cm-1) FIG. 4. Raman spectra from mapping measurements on the sample shocked at 38 GPa. The data show that zircon and reidite coexist in this sample. The bands marked by asterisks are due to the zircon remnants
ZrSiO4 (reidite) with zircon-structure relics. They because they are both due to Si-O stretching concluded that the naturally shock-deformed vibrations. The ratio reflects the relative contents zircons might be related to the low-shock of thetwo phases.Thedata also indicatethat the regime (<30 GPa), and do not represent the untransformed zircon remnants are of the order of same shock stages as indicated by whole-rock sub-micro or nanometers in scale, i.e. smaller than petrography. the laser beam size (~1 mm) used. Our observa- Our results suggest that zircon and reidite co- tion of coexistence of the phases in shocked exist in samples shocked at and above 38 GPa samples is consistent with the work of Leroux et (although thevolumeof zircon domains is very al. (1999), who found that at 40 GPa shock limited in the sample shocked to 80 GPa, they can pressure, the sample was partly transformed from still be detected in several regions). This is the zircon to the scheelite structure. It is also associated with the heterogeneities of the shock, consistent with XRD measurements at hydrostatic which causes some regions to remain structurally pressures >20 GPa in pure ZrSiO4 (van untransformed. The sample shocked from 38 GPa Westrenen et al., 2004). Wealso found that the was further examined using a micro-Raman mapping technique, in order to gain some 3.0
understanding of the degree of the heterogene- -1 (zircon) I 1001 cm ities. The mapping data were recorded in a grid 2.5 38 GPa I -1 (reidite) over an area of 50650 mm with a beam size of 884 cm ~1 mm and a step of 1 mm. Theresultsshow 2.0 systematic variations of the intensity of Raman bands attributed to zircon in a scale of 20 30 mm, 1.5 indicating that theamount of theuntransformed 1.0 zircon phase changes across the sample. A typical Intensity ratio spectrum evolution along one direction of the 0.5 mapped area is shown in Fig. 4. The bands
markedby asterisksaredueto thezircon 0.0 remnants. The different Raman intensity of these 0 10203040500 60 bands along the measured length reflects the Length (µm)
heterogeneity of the shock pressure. The intensity FIG. 5. Intensity ratio between the n3 band near ratio between the strong band near 884 cm 1 in 1001 cm 1 of thezircon remnantsand the884 cm 1 the scheelite phase and the n3 band in zircon is band of the scheelite phase (38 GPa sample). The lines plotted in Fig. 5. The two bands were chosen arevisual guides.
806 EXPERIMENTALLY SHOCKED ZRSIO4 shock-induced sample powders (see earlier indicatethat during thestructural transition there sections for more details) appear to have is a change of electronic structure. In fact, charge experienced more significant impact than these transfer was observed in the zircon-scheelite crystals shocked to the same pressures, because transition of other materials (Jayaraman et al., much weaker micro-Raman signals of the zircon 1987; Duclos et al., 1989). As a result, the density phase were commonly recorded in the impact- increase at the zircon-reidite structural transition is induced powders, suggesting a better transforma- expected to be associated with not only a more tion of zircon into reidite. efficient packing of the coordination polyhedra Oneof theinteresting observations in the and theelimination of a structural holein the present study is that the internal mode frequencies zircon-typestructure(Stubican and Roy, 1963; of theSiO 4 tetrahedra generally dropped to lower Reid and Ringwood, 1969), but also significant values in the scheelite phase as described earlier changes related to the SiO4 group and its local (for instance, the IR-active Si O stretching bands environment. in zircon near 980 and 880 cm 1 shift to 868 and Our Raman measurements reveal, apart from 813 cm 1 in the scheelite phase). This change is the reidite phase, an unexpected phase in some associated with the modification of local config- samples shocked to pressures >40 GPa, which can urations of thesamplethrough thetransition. In be associated with the extra feature seen in our IR particular, the frequency values for the Raman spectra.Thephaseappearedin only some Si O stretching vibrations between near 800 and sampled regions and showed bands near 212, 890 cm 1 in reidite indicate a change of the Si O 272, 383 and 586 cm 1 (their wavenumbers vary bond strength caused by the phase transition. The slightly from sampling areas to areas), which are change can be estimated by calculating the change not due to characteristic bands of either zircon or associated with force constant k, sinceunderthe reidite. These features were not seen in the harmonic approximation a phonon frequency is starting material. This implies that its occurrence given by: is not dueto potentialoriginal inclusions, but rffiffiffiffi associated with the high-pressure process. The 1 k n ¼ ð1Þ signals appeared in both FT-Raman and conven- 2pc m tional Raman spectra, suggesting that it is not associated with laser-induced fluorescence. We where k, m and c are force constant, reduced mass noted that in previous studies (Liu, 1979; Kusaba and the velocity of light, respectively. The change et al., 1985), high-pressure-induced decomposi- of force constant can be extracted from tion was reported, as proven by signals corre- 2 2 sponding to ZrO . However, the band frequencies k /k &n /n (2) 2 scheelite zircon scheelite zircon of theadditional phasematch neitherthe Thus, therelativechangein theforceconstant, characteristic bands of monoclinic, tetragonal which is directly related to the change in bond nor cubic ZrO2 (Carlone, 1992; Kim et al., strength, can be measured through the square of 1993; Hirata et al., 1994; Zhang et al., 2000a,b), their frequency ratio. This approach shows that nor thoseof SiO 2 glass and coesite (a high- there is a surprising ~11% decrease in the Si O pressure phase of SiO2), although thebands at 212 bond strength (for the above IR bands) during the and 586 cm 1 have frequencies close to those of phase transition from zircon to the scheelite two Raman bands (near 231 and 589 cm 1)of structure. This change may have several origins: stishovite, a high-pressure polymorph of SiO2.At an increase in the bond length, a change in this stageweareunableto clarify thenatureof the electron density function or a charge transfer phase, but it could be connected to the additional involving O and Si, or O and Zr or a combination phase observed in a recent electron microscopy of all these factors. Powder XRD data have shown study (Leroux et al., 1999). a small (<1%) increase in the SiO4 tetrahedral bond length when zircon is shocked-transformed Effect of shock pressure on phonon modes of zircon-type into the scheelite-type structure (Kusaba et al., ZrSiO 1986). Therefore, charge transfer might play a 4 significant rolein thechangeof phonon Oneof theimportant observationsin this study is frequencies. We noted that although the untreated the effect of high-pressure shock on the frequency zircon samples are light yellow, the shocked and width of zircon phonons, i.e. the shocked samples are commonly dark brown. This could zircon phaseshows somevariations in post-shock
807 A. GUCSIK ET AL.
four times larger than that of the starting material. zircon The frequency decrease with increasing shock ω/Γ = 1006.7 / 3.6 (unshocked) pressure shown in Fig. 6 appears opposite to the ωΓ/ = 1004.4 / 6.0 (20 GPa) pressure-induced increase of the frequency ωΓ/ = 1000.8 / 16.9 (38 GPa) reported by Knittle and Williams (1993). But thedifferenceismainly dueto different experimental methods, as the latter authors’ data zircon were obtained through in situ high-pressure Unshocked measurements, whereas our results were from shocked samples. The decompressed sample in 20 GPa theKnittleand Williams study (from in situ Raman intensity (a.u.) measurements up to 38 GPa) showed a band near reidite 1001 cm 1, which was incorrectly considered as a 38 GPa characteristic mode of scheelite-structure ZrSiO4. 800 850 900 950 1000 1050 The change of spectral parameters in the present study is related to the post-shock ‘damage’ that is -1 Wavenumber (cm ) retained in the shocked sample, whereas the
FIG. 6. The effect of high-pressure shock on the n3 increase of frequency in in situ experiments is vibration (SiO4) in zircon. The values of the frequency more closely associated with dynamic effects of (o in cm 1) and measured linewidth (g in cm 1) of the compression on the structure of zircon. The n3 vibration are given at different pressures. change in the frequency and width of the n3 band is probably due to pressure-induced peak profiles. As shown in Fig. 6, Raman bands deformation, local defects and strains. Plastic (between 970 and 1100 cm 1) of theremaining deformation was observed in a sample pressurized zircon phase in shocked samples exhibit clear to 20 GPa, and is characterized by a high density broadening upon increasing shock pressure. For of straight dislocations in glideconfiguration instance, in the sample shocked at 20 GPa the n3 (Leroux et al., 1999). According to Leroux et al. vibration of theSiO 4 group has an averaged (1999), thelargedensityof dislocations at crack frequency of ~1004 cm 1 and a mean linewidth tips shows that theplastic deformationwas (measured full width at half magnitude) of initiated by a micro-cracking process. Our data ~6 cm 1, although the measured frequency and from the Raman mapping measurements indicate width in thestarting materialare1006.7 cm 1 that the measured frequency and width of the and 3.6 cm 1, respectively. As shown by IR zircon remnants vary from area to area in the (Fig. 1a) and Raman data (Figs 3b, 6), thesample shocked sample. In order to understand the spatial shocked at 20 GPa still has the zircon structure. variation better, the data shown in Fig. 4 were Thebroadeningis moresignificant in the38 GPa curve-fitted to extract the spectral parameters. sample, which shows a bandwidth of ~17 cm 1, Figure7 a,b shows the variation in frequency and
1004 a b 22 38 GPa ν (SiO , zircon) 38 GPa ν 3 4 3 (SiO4, zircon) 1003 20 ) -1 )
-1 18
1002 16 FWHM (cm
Wavenumber (cm 14 1001
12
0 102030405060 0 1020304050 Length (µm) Length (µm) 1 FIG. 7. Spectral parameters of the n3 band near 1001 cm of the zircon remnants (38 GPa sample) as a function of length: (a) frequency, and (b) measured width. The lines are visual guides.
808 EXPERIMENTALLY SHOCKED ZRSIO4
with an increase in linewidth to 20 30 cm 1 for 20 natural metamict zircon. Data from extensive annealing experiments have also shown that the 18 band parameters can be systematically affected by ) -1 thermal annealing (Zhang et al., 2000a; Geisler et 16 al., 2001b) and by hydrothermal alteration (Geisler et al., 2001a). Thepeakprofileof this
FWHM (cm 14 Raman n3 band has been used to predict the thermal history of zircon, and hence its host rocks. Nasdala et al. (2001) used the width to estimate 12 thepossiblethermalhistory of natural zircons,
0.0 0.5 1.0 1.5 2.0 2.5 whereas Geisler et al. (2001b) proposed the use of Raman intensity ratio (zircon/reidite) both the width and frequency to trace potential thermal alterations. Although high-pressure shock 1 FIG. 8. Thewidth of the n3 band near 1001 cm of the and metamictization are two processes with zircon remnants (38 GPa sample) as a function of the distinctly different origins, the changes in Raman intensity ratio between the n3 band of zircon and frequency and linewidth are most likely to be the884 cm 1 band of the scheelite phase. The line is a associated with defects in both cases. The visual guide. similarities between the two processes, shown by the phonon modes of zircon, could reflect width of theRaman n3 along a 50 mm trajectory. similar local changes and defective crystal The regions that show a broader linewidth are structures ‘‘seen’’ by the phonons. However, the interpreted as having undergone higher local absence of the characteristic bands of scheelite- pressures, and having more defects and large typeZrSiO 4 in metamict zircon indicates that the local strains. The different width and frequency possible high pressure extremes involved in values in the mapped region further confirm the radiation damage during atomic displacements heterogeneity of the pressure impact. The simple in the cascades must be <20 GPa, because around relationship between the width and amount of the this pressure the zircon-reidite transition starts to zircon remnants in the shocked sample is shown take place at room temperature (e.g. van by plotting thewidth as a function of theRaman Westrenen et al., 2004). The real pressure intensity ratio between the n3 band of zircon and associated with the cascades could be signifi- the884 cm 1 band of scheelite phase in Fig. 8. It cantly lower than this value, as the radiation self- is clear that regions with higher values of width damage process is associated with high effective are more likely to have undergone a higher- temperatures and experiments have shown that pressure shock as fewer zircon remnants are increasing temperature lowers the zircon-reidite found. Furthermore, the correlation shown in transition pressure dramatically (Reid and Fig. 8 is consistent with the observation shown Ringwood, 1969; Liu, 1979). in Fig. 6. Oneof thepotentialapplications of our finding is that Raman measurements provide a Acknowledgements means to estimate the shock pressure through the mean phonon linewidth. But other facts could Theauthors would liketo thank Ansgar Greshake makethis correlationmorecomplex,e.g. for providing the shocked samples used in this metamictization in natural zircon. study. Financial assistancefrom thePanethTrust Theimpact of metamictizationon thephonon and British Nuclear Fuels Limited (BNFL) is frequency and width for zircon is similar to the gratefully acknowledged. The manuscript bene- impact of shock pressure in the same material. In fited from reviews by W. van Westrenen and an recent Raman studies of the effect of radiation anonymous reviewer. damage, it has been found that a-decay radiation causes a systematic variation of band frequency References and width (e.g. Nasdala et al., 1995; Wopenka et al., 1996; Zhang et al., 2000a,b; Geisler et al., Anderson, E.B., Burakov, B.E. and Vasiliev, V.G. 2001b; Palenik et al., 2003). This is clearly (1993) Creation of a crystalline matrix for actinide demonstrated by the behaviour of the n3 (SiO4) wasteat Khlopin Radium Institute. Proceedings of band near 1008 cm 1, which shifts to ~995 cm 1 Safe Waste, 93, 2,29 33.
809 A. GUCSIK ET AL.
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