Phys Chem Minerals (1999) 26: 432±436
Ó Springer-Verlag 1999
ORIGINAL PAPER
B. Reynard á M. Okuno á Y. Shimada á Y. Syono C. Willaime
A Raman spectroscopic study of shock-wave densi®cation of anorthite (CaAl2Si2O8) glass
Received: 21 July 1998 / Revised and accepted: 27 January 1999
Abstract Structural modi®cations induced by shockwave compression up to 40 GPa in anorthite glass are investigated by Raman spectroscopy. In the ®rst investigation, densi®cation increases with increasing shock pressure. A maximum densi®cation of 2.2% is obtained for a shock pressure of 24 GPa. This densi®cation is attributed to a decrease of the average ring size, favoring three-membered rings. The densi®cation is much lower than in silica glass subject to shock at similar pressures (11%), because the T-O-T bond angle decrease is impeded in anorthite glass. For higher shock pressures, the decrease of the recovered densi®cation is attributed to partial annealing of the samples due to high after-shock residual temperatures. The study of the annealing process of the most densi®ed glass by in-situ high temperature Raman spectroscopy con®rms that relaxation of the three-membered rings occurs above about 900 K. Key words anorthite glass á shock compression á densi®ed glass á Raman spectroscopy
Introduction Shock-induced structural modi®cations in glasses are of interest for the understanding of their high pressure and B. Reynard (&)1 á C. Willaime GeÂosciences Rennes CNRS UPR 4661, Universite de Rennes1, F. 35042 Rennes Cedex, France E-mail:
[email protected] M. Okuno á Y. Shimada Department of Earth Sciences, Faculty of Science, Kanazawa University, Kanazawa, 920-1192, Japan Y. Syono Institute for Material Research, Tohoku University, Sendai, 980, Japan Present address: Laboratoire de Sciences de la Terre, CNRS UMR 5570, Ecole Normale SupeÂrieure, 46 AlleÂe d'Italie, F-69364 Lyon cedex 07, France
1
high temperature behavior in geological processes (meteoritic shock, melting at high depths). So far, mostly silica glass has been studied because of its chemical simplicity (see Wolf and McMillan 1995, for a review). For earth science purposes, more complicated systems are involved, in particular feldspars during meteoritic impact shocks aecting crustal rocks. The study of such systems under dynamic compression is also of interest because these glasses are ``charge compensated'' i.e., they have a structure based on a fully polymerized tetrahedral network where alkali (Na, K) or alcaline-earth (Ca) cations act as ``charge compensators'' for Al replacing Si as a network former when compared with pure silica (Taylor and Brown 1979). As a starting point to a systematic study of their behavior at high pressure and following our study of silica glass aected by shock (Okuno et al. 1999), we have performed Raman spectroscopy on anorthite (CaAl2Si2O8) glass subject to shock in order to decipher its structural modi®cations under dynamic compression.
Experimental A synthetic transparent glass rod was cut in thin plates (10 mm in diameter, 2 mm in thickness), and was used as starting material for shock experiments. These samples were encased in a stainless steel container. Shock-wave experiments were performed by using a single stage propellant gun (25 mm bore and 4 m length; Goto and Syono 1984). The specimen container was hit by a stainless steel ¯yer, which was accelerated to a velocity of up to about 1800 m á s)1. Six shock-wave experiments were performed with shock pressures of 16.9, 19.6, 24.0, 30.7, 34.5 and 40.8 GPa. These pressures were estimated from measured projectile velocities, using the impedance matching method, with a precision of 0.1 GPa. Shock durations are of the order of 10)6 s. Shock and residual temperatures were not measured and are dicult to estimate for glasses or melts because of con®gurational changes. The refractive indices (n) of ``unshocked'' and shock-wave densi®ed anorthite glasses were measured by the immersion method and by observation of Becke line with a microscope. This method gives index values with an accuracy of about 1 ´ 10)4. Densities (q) were determined by the suspension method in mixtures of CHI2 and acetone (see Table 1).
433 Table 1 Refractive indices (n) and densities (q) of ``unshocked'' and shock-densi®ed anorthite glasses as a function of shock pressure (Ps) Ps (GPa)
q
n
0.00 16.9 19.6 24.0 30.7 34.5 40.8
2.68 2.69 2.73 2.74 2.71 2.70 2.70
1.5658 1.5662 1.5760 1.5777 1.5732 1.5712 1.5729
Raman spectra were recorded at the Universite de Rennes I, using a Dilor XY double subtractive spectrograph equipped with a premonochromator (1200 gr/mm holographic gratings) and ORTEC CCD nitrogen-cooled detector. High-temperature Raman spectra were obtained using a Leitz 1350 heating stage. Temperature was monitored with a Pt-Pt/Rh10% thermocouple calibrated against known melting point compounds. The accuracy of measured temperature is estimated as 5 K (see Okuno et al. 1999, for details). All the observed Raman spectra were corrected for the temperature and frequency dependence of the ®rst order (Stokes) Raman scattering (Long 1977; Mysen et al. 1982; Piriou and Alain 1979), using the following correction relation: Icorr Iobs x
x0 ÿ xÿ4 1 ÿ exp
ÿhx=kB T
1
where x0 is the wave number of the incident laser light (19435 cm)1 for the green Ar+ laser line), x the Raman shift, kB is Boltzmann's constant, and T the absolute temperature (K).
Fig. 1 a Density and b refractive index of anorthite glass as a function of shock pressure
Results and discussion The variations of refractive index and density with shock pressure are depicted in Fig. 1. These two variations are very similar except for the highest pressure sample. The maximum density increase (Dq/q0 = 2.2%) is obtained for a shock pressure of 24.0 GPa. This is small when compared with density increase of 11% observed for silica glass for similar shock pressures using the same apparatus and sample geometry (Okuno et al. 1999). For shock pressures above 30 GPa, only a small increase in density of less than 1% is observed. Recovery experiments on the most densi®ed glass (24.0 GPa shock pressure) show that the density decreases a little by heating to 773 K while it returns back its original value for the control or ``unshocked'' glass after quick heating at 1173 K, which is slightly above Tg (1160 K) for anorthite glass (Fig. 2). The corrected spectra of glasses subject to shock are shown in Fig. 3. The Raman spectrum of the starting material is consistent with former studies (Matson et al. 1986; McMillan et al. 1982; Mysen et al. 1981, 1980; Seifert et al. 1982; Sharma et al. 1983). It is characterized by a group of high frequency bands in the 900± 1200 cm)1 region, which are attributed to the stretching vibrations of T-O-T (T = Al, Si) linkages within the tetrahedral network. Bands in the 700±800 cm)1 have been attributed, by analogy with vitreous silica, to deformation modes involving in-cage motion of tetrahe-
Fig. 2 Variations of density of the sample subject to shock at 24.0 GPa with time after annealing at 773 and 1173 K
dral cations in the highly polymerized network and to AlO4 stretching vibrations (Galeener and Geissberger 1983; Galeener and Mikkelsen 1981; McMillan and Piriou 1983; McMillan et al. 1994, 1982; Sharma and Matson 1984; Sharma et al. 1984). Finally, bands in the 300±600 cm)1 region are attributed to the bending vibrations of T-O-T linkages and tetrahedra (Matson et al. 1986; McMillan et al. 1982; Mysen et al. 1981, 1980; Seifert et al. 1982; Sharma et al. 1983). With increasing shock pressure, the prominent evolution in the Raman spectra is the variation in relative intensity of the peak near 580 cm)1, other peaks show-
434 Fig. 3 Raman spectra of shock-densi®ed anorthite glass. The intensity of the band at 580 cm)1 is correlated with the density and refractive index increase with respect to the starting material
ing only minor broadening and frequency changes. This change is similar to that observed on statically compressed anorthite glass to 17 GPa and ambient temperature (Daniel et al. 1997). The intensity variation of the 580 cm)1 peak is proportional to that of the density increase, indicating that variations in the concentration of the species responsible for the 580 cm)1 peak is the likely densi®cation mechanism in anorthite glass subjected to shock-waves. Alternative assignments have been proposed for this peak, e. g., McMillan et al. (1982) attribute it to Al-O-Al linkages based on systematic intensity increase with decreasing silica content along the SiO2-CaAl2O4 join while Matson et al. (1986) propose that it corresponds to three-membered tetrahedral rings by analogy with the 606 cm)1 defect peak in silica glass (Galeener 1982a, b). Ab initio calculations (Kubicki and Sykes (1993) and high-temperature experiments by Daniel et al. (1995) lead to the conclusion that the latter was more plausible in anorthite glass (McMillan and Wolf 1995). We also favor that interpretation because (1) we have not observed signi®cant frequency variation of this band with increasing shock pressure or density, which is also the case for three-membered rings in densi®ed silica glass recovered from high pressures (Hemley et al. 1986; McMillan 1984; Okuno et al. 1999), indicating small geometrical variations of the corresponding species, and (2) a similar increase of the band intensity of three-membered siloxane rings is observed in densi®ed silica glasses (Hemley et al. 1986; McMillan 1984; Okuno et al. 1999). We thus attribute the observed densi®cation of anorthite glass subjected to shock waves to a change in tetrahedral ring statistics, favoring small (three-membered) rings at high pressures. It is worth noticing that this mechanism causes only a small densi®cation of anorthite glass (about 2%). In contrast, larger increases in density are observed in shock-wave altered silica glass at similar pressures (Hemley et al. 1986; McMillan 1984; Okuno et al. 1999), but with a strong decrease of the Si-O-Si bond angle marked by a signi®cant increase of the T-O-T bending mode frequency in the 400±500 cm)1 region. Such an increase is
not observed in anorthite glass, indicating that T-O-T shrinkage is either inoperative in this glass and/or that it cannot be recovered due to the presence of Ca and Al in the glass. Indeed, the frequency of the T-O-T bending mode is already high (520 cm)1) and the band narrow in anorthite glass when compared with silica glass (broad band at about 430 cm)1, e.g., Galeener and Mikkelsen 1981). This could correspond to the rather low and constrained T-O-T bond angles encountered for fourmembered rings of crystalline anorthite (bending mode frequency at 508 cm)1, e.g., Daniel et al. 1997), and suggested to be the dominant type of ring in amorphous anorthite from X-ray diraction (Taylor and Brown 1979). McMillan and Wolf (1995) have also suggested that non-bonding cation repulsion, induced by the charge compensation of tetrahedral aluminium by Ca2+ in anorthite glass, could explain the relatively narrow high-frequency band. This could also well account for the hindrance of T-O-T bond angle decrease in our shock-wave densi®ed anorthite glasses, because these non-bonding interactions are probably responsible for the constrained and high values ot T-O-T bond angle in ``unshocked'' anorthite glass, as discussed already. It is also interesting to note that the bulk moduli of melts increase along the charge compensated SiO2CaAl2O4 join (Webb and Courtial 1996) with increasing Ca content. If the change in melt compressibility and the maximum densi®cation of glasses under shock compression are related to a similar mechanism, for instance the in¯uence of non-bonding interactions, glasses along the SiO2-NaAlO2 join should have an intermediate behavior between silica and anorthite under shock compression. Further studies of shocked or compressed charge-balanced glasses such as NaAlSi3O8 or NaAlSiO4 (often considered as an analogue to anorthite glass but having dierent cationic interactions) should be useful to resolve this issue. Finally, the samples having undergone the highest shock pressures are less densi®ed than those at intermediate pressure, as already observed for shock-wave altered silica (Arndt et al. 1971; Okuno et al. 1999). We
435
attribute this eect to the increase of post-shock temperatures, which leads to a partial recovery of the highpressure structural modi®cation as demonstrated by the density decrease observed above 500 °C (Fig. 2) on the most densi®ed sample subject to shock at 24.0 GPa. High-temperature Raman spectra on this sample con®rm that this density decrease is associated with a decrease of the band intensity at 580 cm)1 (Fig. 4) which starts at about 900 K, i.e., to a relaxation of the threemembered rings quenched from dynamic compression as the temperature approaches the Tg of anorthite glass. Thus, residual temperatures were higher than this minimum value above 25 GPa. A maximum value for residual temperatures can be estimated by considering that no shock-wave altered glass is fully relaxed to its initial structure (see Figs. 1 and 3), even for a shock pressure of 40 GPa. If we assume a minimum time of 10)6 s (i.e., similar to shock duration) during which the residual temperature is maintained, a corresponding viscosity of about 105 Pa á s is required to fully relax the melt, which corresponds for anorthite to a temperature of about 1450 K (Urbain et al. 1982).
Conclusions The structural evolution of anorthite glass recovered from increasing pressure shocks can be summarized as follows. A small density increase of slightly more than 2% associated with an increase of the three-membered ring concentration is observed at intermediate pressure.
Fig. 4 High-temperature Raman spectra of anorthite glass subject to shock at 24.0 GPa (temperatures labelled in K). Notice the decrease in intensity of the 580 cm)1 band above about 900 K. It increases at 1177 K because of con®gurational changes above Tg (Daniel et al. 1995). The spectrum of the quenched sample is similar to that of the ``unshocked'' material
Higher shock pressures yield higher residual temperatures (from more than 900 K at 25 GPa to less than 1450 K at 40 GPa) resulting in partial recovery of these defects and lower density increase (less than 1%). Comparison with shock-wave altered silica glass shows that anorthite is less densi®ed at intermediate pressure due to the hindrance of the T-O-T bond angle decrease. This eect may be attributed to the metal-metal repulsion due to charge compensation of Al by Ca and/or to the starting ring statistics which favors four-membered rings in anorthite with respect to larger cavities in silica. Acknowledgements This study was partly supported by Grant-inaid for International Scienti®c Research (Joint Research, 09044069) and Foundation for Promotion of Material Science and Technology of Japan (MST Foundation), and by the French CNRS (UPR 4661 and UMR 5570).
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