Stability, metastability, and elastic properties of a dense silica polymorph, seifertite

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1 JOURNAL OF GEOPHYSICAL RESEARCH: SOLID EARTH, VOL. 118, 1 13, doi: /jgrb.50360, 2013 Stability, metastability, and elastic properties of a dense silica polymorph, seifertite B. Grocholski, 1 S.-H. Shim, 2 and V. B. Prakapenka 3 Received 23 July 2012; revised 20 June 2013; accepted 29 August [1] Dense silica polymorphs with sixfold coordinated Si have been found in SNC and lunar meteorites and may be important minerals for silica-rich components in the lower mantle. However, the stable crystal structure in the lower mantle and properties of dense silica remain controversial. Under stable heating and quasi-hydrostatic stress conditions, we found that the CaCl 2 type undergoes a phase transition to the -PbO 2 type (seifertite) at GPa and 2500 K. Our data suggest that this phase transition occurs at a greater depth than the perovskite! postperovskite transition in the lowermost mantle. The molar volume measured at 1 bar is the smallest among the reported silica polymorphs, therefore having the highest calculated density and in excellent agreement with recent first-principles calculations. The greater molar volume of seifertite found in the shergottite meteorite and previous experiments supports a metastable synthesis of the phase outside its stability field. Our data combined with the Hugoniots of silica polymorphs also rule out the possibility of the formation of seifertite in the meteorite within its stability field. We found very little change in bulk sound speed across the CaCl 2 -type! seifertite transition. If shear wave velocity decreases at the transition to seifertite as suggested by some computational studies, this silica transition may provide an alternative explanation for the discontinuities with a shear wave velocity decrease found at depths greater than the D 00 discontinuity. Citation: Grocholski, B., S.-H. Shim, and V. B. Prakapenka (2013), Stability, metastability, and elastic properties of a dense silica polymorph, seifertite, J. Geophys. Res. Solid Earth, 118, doi: /jgrb Introduction [2] Silica holds a special place in geophysics and planetary sciences as it is one of the most abundant components in the Earth and other terrestrial planets. The equilibrium phase diagram of silica is becoming clear from high pressure experiments, computations, and studies on AX 2 analog compounds [e.g., Haines et al., 1996; Haines and Leger, 1997; Prakapenka et al., 2003a]. The transition from fourfold coordinated (coesite) to sixfold coordinated (stishovite, rutile-type structure) Si-O occurs at 7 GPa[Zhang et al., 1996]. The stishovite structure distorts slightly to the CaCl 2 - type structure at 50 GPa [Kingma et al., 1995; Andrault et al., 1998a; Ono et al., 2002]. A few studies have suggested that the CaCl 2 type may undergo a phase transition to a mineral named seifertite with the scrutinyite ( -PbO 2 ) structure within mantle pressures [Dubrovinsky et al., 2001; 1 Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA. 2 School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA. 3 GeoSoilEnviroCARS, University of Chicago, Argonne, Illinois, USA. Corresponding author: B. Grocholski, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th & Constitution Ave., Washington DC 20560, USA. (b.grocholski@gmail.com) American Geophysical Union. All Rights Reserved /13/ /jgrb Murakami et al., 2003; Tsuchiya et al., 2004; Oganov et al., 2005; Driver et al., 2010] and persist until the pyrite structure (6 + 2 Si-O coordination) is stable above 270 GPa [Kuwayama et al., 2005]. [3] Seifertite is an important mineral for both understanding the lower mantle of the Earth [Hirose et al., 2005; Grocholski et al., 2012] and is perplexing for its appearance far outside the stability field in SNC-type and lunar meteorites [Stöffler et al., 1986; Sharp et al., 1999; Boctor et al., 2003; Aoudjehane et al., 2005; El Goresy et al., 2008; Miyahara et al., 2013]. Silica may not be a dominant mineral in the lower mantle as homogenized mantle compositions, such as pyrolite, may be silica undersaturated [Ringwood, 1982]. However, Bina [2010] suggested the possible existence of free silica even in the pyrolitic mantle as polycrystalline-armored relics with seismically detectable scale. Several recent studies have suggested the possibility of a heterogeneous lower mantle including silica oversaturated components that would provide diversity to the mineralogy of the region [Nakagawa et al., 2010; Grocholski et al., 2012]. [4] The reported transition pressure of silica to seifertite from the CaCl 2 -type structure varies over a wide range and the existence of the transition in the mantle has remained unclear. Dubrovinsky et al. [1997] reported a phase transition to a seifertite-like phase at 70 GPa, while Murakami et al. [2003] reported much higher pressure for the

2 Table 1. Experimental Conditions a Culet Pressure Temperature Total Heating Pressure Pressure Size Data Range Range Duration Phase Samples Standard Medium ( m) Used (GPa) (K) (min) Observed c Pure Silica Ar-1 Au Ar 75 0, eos sft Ar-3 Au Ar sft + CT Ne-5 Ne Ne sft Ne-7 Au Ne 150 0, eos CT Al-Bearing Silica b Ar-2Al Au Ar 75 0, eos sft + trace CT Ne-6Al Au Ne 150 0, eos CT Ar-4Al Pt Ar CT a 0 : density calculated from volume measurements at 1 bar (Table 2), eos: P-V measurements during decompression (Figure 5). b 10 mol % Al 2 O 3. c sft: seifertite, CT: CaCl 2 type. CaCl 2 -type! seifertite phase transition at 121 GPa. These earlier studies were performed without pressure transmitting medium and thermal insulation in the diamond-anvil cell, which can result in severe deviatoric stresses and thermal gradients. The free-energy differences between competing silica phases can be small [Teter et al., 1998], and hence, the formation of metastable phases under nonequilibrium conditions is particularly problematic. A recent study with quasi-hydrostatic conditions and thermal insulation [Shieh et al., 2005] failed to detect the phase transition to seifertite up to 130 GPa using the same pressure scale as Murakami et al. [2003]. However, computational studies suggest that the phase transition should occur at GPa [Karki et al., 1997a; Tsuchiya et al., 2004; Oganov et al., 2005; Driver et al., 2010]. [5] Seifertite has been found on the surface of the Earth in meteorites of Martian and lunar origin (Shergotty, Zagami, NWA 4734), both of which have high (30 80 GPa) estimated peak shock pressures [Stöffler et al., 1986; Sharp et al., 1999; Boctor et al., 2003; Aoudjehane et al., 2005; El Goresy et al., 2008; Miyahara et al., 2013]. However, the uncertainties in the stable P-T conditions for seifertite make it difficult to understand how this mineral is synthesized in these meteorites. In addition, the calculated density from the measured volume of seifertite from the Shergotty and NWA 4734 meteorites [Dera et al., 2002; Miyahara et al., 2013] and from cold decompression of cristobalite [Tsuchida and Yagi, 1990; Dubrovinsky et al., 2001] is as much as 2% lower than the recent computational calculations [Oganov et al., 2005; Driver et al., 2010]. If the density difference is converted into pressure based on equation of state of dense silica, it would be equivalent to 9 GPa. [6] We have measured the seifertite phase transition in silica at pressures up to 152 GPa and temperatures of K in the laser-heated diamond-anvil cell using insulating compressed noble gas media. We also were able to decompress seifertite synthesized at stable P-T conditions (140 GPa at 2500 K) to room pressure, allowing us to constrain the equation of state of seifertite with a 1 bar volume (V 0 ) and calculate a 1 bar density ( 0 ). 2. Experimental Method [7] A total of seven crystalline samples were synthesized at high pressure and temperature. Samples of pure (99.8%) 2 amorphous silica or 10 mol % Al 2 O 3 -bearing amorphous silica were mixed with 15 wt % Au, 10 wt % Pt, or 30 wt % Fe, compressed into platelets, and surrounded by condensed inert noble gas (Ar or Ne) for thermal insulation and quasihydrostatic stress environment (experimental setup can be found in Table 1). Argon was cryogenically loaded at the Massachusetts Institute of Technology and neon was gaspressure loaded at the Advance Photon Source [Rivers et al., 2008]. The Al-bearing starting materials were synthesized by aero-levitation [Tangeman et al., 2001]. [8] Diamond cells equipped with anvils having 75, 100, 150, or 200 m culets were used with initial sample chambers of 35, 50, 90, or 120 m in diameter, respectively. We conducted X-ray diffraction in situ at high P-T in the double-side laser-heated diamond-anvil cell at the GSECARS beamline 13IDD at the Advanced Photon Source [Prakapenka et al., 2008]. All samples were heated to T > 2000 K for min after reaching target pressure with X-ray diffraction collected using a marccd detector every 2 3 min to track crystal growth during heating (Table 1). [9] Diffraction patterns were collected during decompression at 3 5 GPa steps until catastrophic decompression of the sample occurred for the equation of state at room temperature (Table 2). Since the beam size was only 3 5 m, the sample was oscillated 1 3 m during collection of diffraction patterns to improve statistics and generate more uniformly smooth diffraction rings. This technique is used for the measurements at 300 K, but not for the measurements during laser heating. Diffraction patterns were integrated using the Fit2d image processing program [Hammersley et al., 1996] with lattice parameters and error calculated using the UnitCell program [Holland and Redfern, 1997] (Tables 3 and 4). Peak positions and background subtraction from the 1-D integrated patterns were determined with the home-written IDL fitting programs, XPEAKPO and XPEAKFIT ( Softwares.html). A total of four samples were recovered at room conditions, allowing for measurement of the room pressure volume (V 0 ). Densities were then calculated from the volumes assuming pure silica or 10 mol % Al 2 O 3 - bearing SiO 2. Room pressure sample recovery from Mbar pressure was accomplished by precompressing the rhenium gaskets to 40 GPa, well-centered circular sample chambers drilled with an EDM, and slow release of pressure using

3 Table 2. Equation of State-Fitting Results With Room Pressure Density ( 0 ) Calculated From V 0 Measured at 1 Bar and 300 K V 0 (Å 3 ) K 0 (GPa) K (g/cm 3 ) Seifertite, This Work SiO 2, all data 91.66(6) 322(2) 4 a 4.355(3) SiO 2, P >70GPa 91.66(6) 337(8) 3.5(2) Al-SiO b (8) 322(2) 4 a 4.329(4) Seifertite, Shergotty, and NWA 4734 Dera et al. [2002] Miyahara et al. [2013] Seifertite, Experiment Dubrovinsky et al. [2001] 93.52(14) 313(5) 3.43(11) 4.267(6) Seifertite, Theory Oganov et al. [2005] Driver et al. [2010] CaCl 2 Type, This Work SiO 2, all data 46.63(3) 317(3) 4 a 4.279(3) Al-SiO b (3) 298(2) 4 a 4.223(3) CaCl 2 Type, Experiment Andrault et al. [2003] 46.63(2) 334(7) 4 a 4.279(2) CaCl 2 Type, Theory Oganov et al. [2005] Driver et al. [2010] Stishovite, Experiment Andrault et al. [2003] 46.51(1) 310(1) 4.6(2) 4.290(1) Disordered Dense Silica, Experiment Dubrovinsky et al. [2004] 4.286(3) Postquartz, Experiment Haines et al. [2001] 4.29(4) Baddeleyite, Shergotty El Goresy et al. [2000] 4.30(2) a Fixed during fitting. b 10 mol % Al 2 O 3. small turns of the set screws located on the piston side of the symmetric diamond cells. [10] The metals mixed in our glass starting material were used for laser coupling, with gold and platinum also acting as internal pressure standards [Tsuchiya, 2003; Holmes et al., 1989]. The laser spot size is roughly 20 m andthe error on the temperature is 150 K, about twice the variation we obtain from the gray body fits over the heating duration. Samples did not change in optical appearance after each heating cycle or upon decompression, indicating no drastic change in visible light absorption characteristics between the densified glass, seifertite, and the CaCl 2 -type structure. We use neon as a pressure standard [Dewaele et al., 2008] in our sample mixed with iron as a laser coupler (Ne-5) since there is the possibility of carbon contamination of the metal [Prakapenka et al., 2003b] (Table 1). 3. Results 3.1. Stable Pressure-Temperature Conditions [11] A pure SiO 2 glass (Ar-1) was heated at 143 GPa and 2137 K. New diffraction peaks appeared after 10 min of heating and they persisted in subsequent heating (Table 1 and Figures 1 and 2c). Through first-principles method, 3 Teter et al. [1998] showed that four different polymorphs with sixfold coordinated Si become energetically favorable over the CaCl 2 type at P 100 GPa. We calculated the diffraction patterns of the four polymorphs based on the structural data in Teter et al. [1998] (Figure 3). The new diffraction peaks are well explained by the orthorhombic -PbO 2 -type (seifertite) structure (Figure 3). Absence of any major diffraction lines between 0.35 and 0.4 Å 1 (between 2.85 and 2.5 Å) in the measured patterns, other than nitrogen (see below), rules out the possibility of the other polymorphs, i.e., postquartz, NaTiF 4 type, and SnO 2 type (Figure 3). The weaker diffraction lines at higher angles are also explained well by the seifertite structure. [12] The other peaks in the diffraction patterns can be assigned to those from the rhenium gasket, gold internal pressure standard, argon pressure medium, and the cubicgauche phase of nitrogen (cg-n) [Eremets et al., 2004] (Figure 3a). Tails from the X-ray beam combined with the high scattering cross section of the Re gasket lead to Re peaks in the diffraction patterns for sample chambers <50 m. The cg-n phase is a rare single-bonded structure formed from small amounts of nitrogen captured during cryogenetic loading of Ar (this will be described elsewhere). The diffraction peaks which can be assigned to the cg-n phase were not observed in the Ne-loaded samples

4 Table 3. Lattice Parameters (a, b, c) and Unit Cell Volumes (V) for All Diffraction Patterns of the Seifertite Phase a Pure Silica Silica With Alumina P(GPa) ap(å) a(å) b(å) c(å) V(mol/cm 3 ) P(GPa) ap(å) a(å) b(å) c(å) V(mol/cm 3 ) b c b c b c b c b c b c a Unit cell parameter for pressure standard (gold or neon) is listed under a p. b Neon loaded. c Unit cell parameter of neon, all other are from gold. 4

5 Table 4. Lattice Parameters (a, b, c) and Unit Cell Volumes (V) for All Diffraction Patterns for the Stishovite and CaCl2-Type Phases a Pure Silica Silica With Alumina P(GPa) ap(å) a(å) b(å) c(å) V(mol/cm 3 ) P(GPa) ap(å) a(å) b(å) c(å) V(mol/cm 3 ) a Unit cell parameter of gold is listed under a p. Figure 1. Pressure-temperature conditions for the observation of the CaCl 2 type (blue symbols) and seifertite (red symbols) in pure SiO 2 (solid symbols), 10 mol % Al 2 O 3 - bearing SiO 2 (open symbols). In a pure SiO 2 sample, we found a mixture of the CaCl 2 type and seifertite (green circle), which may indicate the P-T conditions correspond to the univariant phase boundary. Laser heating during decompression (right triangles) and compression (left triangles) is conducted for the Al-bearing samples. The open squares represent the observations of the CaCl 2 type by Shieh et al. [2005]. A red solid line is the seifertite phase boundary estimated from our data. The thin dashed black line at lower P represents the seifertite formation boundary by Dubrovinsky et al. [2001]. The thin black dashed and solid lines at higher P are the CaCl 2 -type $ seifertite boundary proposed by Murakami et al. [2003] and its pressure-corrected ones for direct comparison with our data (see text for detail). Blue polygons show the pressuretemperature conditions of the postperovskite boundary in mid-ocean ridge basalt (MORB) (lower and upper bounds for the thickness of the perovskite-post perovskite mixed phase region (dark and light areas)) [Grocholski et al.,2012]. The melting line and the thick gray lines representing the Hugoniots of SiO 2 using different silica polymorphs as starting material are from Akins and Ahrens [2002]. The kinks are due to the crystalline phase changes. (e.g., Figure 2d) and disappear after unloading the Ar-loaded samples (Figure 4). [13] The synthesis of seifertite is not sensitive to laser coupler, as we used iron to synthesize the mineral at 152 GPa and 2800 K from a separate pure SiO 2 sample (Ne-5) instead of Au or Pt (Figure 2d). We found an unidentified peak in this sample at slightly higher 2 than the 002 peak from hcp iron. This peak is likely from Fe 3 C and/or Fe 3 C 7 [Sata et al., 2010; Nakajima et al., 2011] produced during the laser heating at megabar pressures and suggestive of carbon contamination. [14] A third pure SiO 2 glass (Ar-3) was heated at 136 GPa and 3100 K where we observed instantaneous crystallization of a mixture of seifertite and CaCl 2 -type structure (Figure 2b). The observation of both low- and high-pressure phases suggest that the phase boundary may exist at those 5

6 Figure 2. X-ray diffraction patterns of the silica polymorphs at high pressure-temperature. (a) The CaCl 2 type at 130 GPa and 2500 K in Al-bearing SiO 2 (Ne-6Al). (b) Mixture of seifertite and CaCl 2 type in Al-free SiO 2 synthesized at 136 GPa and 3100 K. Molecular nitrogen captured during cryogenic Ar loading (N 2?) could be partially responsible for the large peak that should be a weak seifertite reflection. (c) Seifertite at 139 GPa and 2500 K in pure SiO 2 (Ar-1). (d) Temperature quenched pure SiO 2 seifertite synthesized with Fe laser coupler (Ne-5) at 152 GPa and 2860 K. Laser heating appears to generate some Fe x C y contamination, but the diffraction peaks of seifertite are consistent with samples Ar-1 and Ar-2Al using Au as a laser absorber. We provide peak assignments (seifertite: red dots, CaCl 2 type: blue dots, unidentified: asterisks; Re: rhenium gasket, Au: gold internal pressure standard, Ar: argon pressure medium, cg-n: the cubic-gauche phase of nitrogen [Eremets et al., 2004]). Nitrogen diffraction peaks are absent in both the Ne-loaded sample (Figure 2d) and when the samples are decompressed to room pressure (Figures 4a and 4b). et al. [1998]. The peaks do not appear to be from any Al 2 O 3 polymorphs, including the Rh 2 O 3 -II-type structure identified by Lin et al. [2004]. We also examined the possibility of the CaIrO 3 -type Al 2 O 3 using the data of Ono et al. [2006]. Although the line at line at Å 1 is close to the 002 line of the CaIrO 3 -type Al 2 O 3, this reflection is expected to have very low intensity (4%). The most intense line of the CaIrO 3 -type Al 2 O 3 should exist at Å 1 (2.427 Å), which is between the most intense lines of cg-n and seifertite in Figure 3c. We conclude that these unidentified peaks are not likely from Al 2 O 3. We cannot rule out the peaks are either from some type of unknown exsolved aluminum silicate or due to a small modification of the -PbO 2 type structure from oxygen defects introduced by the incorporation of Al [Escudero and Langenhorst, 2012]. We note that we do not observe these peaks in pure SiO 2. [17] We also cannot rule out the possibility of a small amount of Al 2 O 3 exsolved from the Al-bearing SiO 2 starting materials because the detection limit of diffraction intensity at high pressure is about 5%. Hirose et al. [2005] reported Al 2 O 3 solubility of 8 mol % in seifertite, which is slightly lower than the content in our Al-bearing SiO 2 starting material (10%). If Hirose et al. [2005] is correct about Al 2 O 3 solubility, at least 8 mol % should be in our samples as well. The systematically higher molar volume of our Al-bearing seifertite compared to pure SiO 2 seifertite is consistent with P-T conditions. The fourth pure SiO 2 sample (Ne-7) was heated at 102 GPa and 2020 K, producing the diffraction peaks of the CaCl 2 -type SiO 2. [15] An Al-bearing SiO 2 glass (Ar-2Al) was heated at 152 GPa and 1960 K where diffuse but identifiable seifertite peaks appear immediately upon laser coupling, sharpening over 30 min of heating. The sample was then decompressed to 125 GPa and heated for 50 min at 3300 K, where pressure in the sample chamber increased on quenching to 130 GPa and seifertite lines sharpened. The sample was decompressed again to 125 GPa and heated for 50 min between 2100 and 3600 K, generating a small amount of CaCl 2 structured silica (a few spots from the 100% intensity 110 peak), but with no noticeable decrease in the intensity of the seifertite diffraction peaks (Figure 1 and Table 1). [16] Weak peaks from a few dots in the diffraction patterns are found at Å 1 (3.058 Å) and Å 1 (2.625 Å) (Figure 3c), but do not match with expected diffraction lines for the silica polymorphs proposed by Teter Figure 3. X-ray diffraction patterns of seifertite at high pressure and 300 K. (a) Pure SiO 2 at 140 GPa (Ar-1), which is synthesized at 152 GPa and 2440 K. (b) Pure SiO 2 at 119 GPa (Ar-1), which was decompressed from 140 GPa. (c) Al-bearing SiO 2 at 121 GPa (Ar-2Al), which is synthesized at 138 GPa and 3300 K. For comparison, we calculated diffraction patterns of the sixfold coordinated silica polymorphs using the first-principles results by Teter et al. [1998] at 120 GPa: -PbO 2 type (seifertite), postquartz (3 2 type or P2 1 /c type), NaTiF 4 type, and SnO 2 type. The symbols for the peak assignments are the same as those in Figure 2. 6

7 heating durations with cristobalite starting material and quasi-hydrostatic conditions results in diffraction peaks not consistent with the -PbO 2 phase, frequently producing a disordered -PbO 2 -type structure instead [Prakapenka et al., 2004; Dubrovinsky et al., 2004]. The lack of confirmation of the lower pressure boundary presented by Dubrovinsky et al. [2001] in experiments using high temperature (>2000 K), quasi-hydrostatic pressure media, and non-cristobalite (or -PbO 2 -type like) starting materials highlight the complex nature of the silica system and the potential to generate metastable phases [i.e., Tsuchida and Yagi, 1990; Prokopenko et al., 2001; Haines et al., 2001]. [21] The boundary reported by Murakami et al. [2003] was measured using the Pt scale [Holmes et al., 1989], which may overestimate pressure with respect to the Au scale [Tsuchiya, 2003] at 2500 K and 100 GPa by 7 GPa according to Hirose [2006] or 16 GPa according to Feietal. Figure 4. X-ray diffraction patterns of seifertite at 1 bar and 300 K. (a) Pure SiO 2 quenched from 152 GPa and 2440 K (Ar-1). (b) Al-bearing SiO 2 quenched from 128 GPa and 2590 K (Ar-2Al). For comparison, we calculated diffraction patterns of different dense silica polymorphs observed at 1 bar: seifertite [Dera et al., 2002], postquartz [Haines et al., 2001], and baddeleyite type (peak positions from El Goresy et al. [2000] and peak intensities from McCullough and Trueblood [1959]). The symbols for the peak assignments are the same as those in Figure b (Å) at least some aluminum dissolving into the structure, as shown in Figure 5 (see below for discussion). [18] A second Al-bearing SiO 2 glass (Ne-6Al) was heated at 120 GPa and 2500 K, producing the diffraction patterns of the pure CaCl 2 -type SiO 2 (Figure 2a). Sample Ar-4Al was heated at GPa and K, all resulting in CaCl 2 structured silica. [19] Figure 1 shows that our data place the CaCl 2 typeseifertite boundary very close to the core-mantle boundary (135 GPa) but still within mantle pressures. The data do not provide sufficient resolution to determine the effect of Al on the depth of the seifertite phase transition. Most data points in Figure 1 are based on the Au pressure scale [Tsuchiya, 2003], except for the data at GPa (Ar-4Al) and a single point for pure silica at 152 GPa (Ne-5) in which Pt and Ne were used as pressure scales, respectively. The Pt scale [Holmes et al., 1989] may overestimate pressure compared to the Au scale by a few GPa at 80 GPa and high temperature [Feietal., 2007]. A data point at 152 GPa in Figure 1 is from an experiment with Fe as a laser coupler, in which we found possible carbon contamination of Fe (Figure 2d). Therefore, we estimated the pressure from Ne peak positions measured after heating. The measurement after temperature quench likely underestimates pressure because of the thermal pressure effect [Heinz, 1990;Andrault et al., 1998b]. Since these data are far from the phase boundary, these uncertainties are not important for our stability diagram. [20] The boundary presented by Dubrovinsky et al. [2001] was obtained by compressing -PbO 2 -type-like material synthesized initially from cristobalite without pressure medium (Figure 1). A subsequent study using longer Pressure (GPa) Figure 5. Unit cell parameters of seifertite (pure SiO 2 (red solid symbols), Al-bearing SiO 2 (red open symbols), with an Ar medium (circles), with a Ne medium measured in this study (squares)) (Tables 3 and 4). The molar volumes of the CaCl 2 type measured in this study are also included (pure SiO 2 (blue solid squares), Al-bearing SiO 2 (blue open squares)). Solid and dashed lines are fits to the aluminafree and alumina-bearing samples, respectively (Table 2). We also plot unit cell parameters of SiO 2 seifertite reported in previous studies [Tsuchida and Yagi, 1990; Dubrovinsky et al., 1997, 2001, 2004; Murakami et al., 2003]. For the data points by Murakami et al. [2003], we present both pressures estimated from the Pt scale (gray circles) and pressures corrected for direct comparison with our data (black circles) (see the text for detail). We also include the unit cell parameters of the seifertite-like phase in MORB (green symbols) [Hirose et al., 2005; Grocholski et al., 2012] a (Å) 7

8 [2007]. When the pressure is corrected for a direct comparison with our data (solid lines in Figure 1), their boundary is at significantly lower pressures than our boundary. Unlike our measurements, Murakami et al. [2003] did not use a pressure-transmitting medium or thermal insulation, which may result in larger thermal and stress gradients. For example, studies have found that deviatoric stresses can change the high pressure behavior of quartz [Kingma et al., 1993; Haines et al., 2001]. [22] With a pressure medium in the diamond-anvil cell, Shieh et al. [2005] reported stability of the CaCl 2 type up to pressures 10 GPa higher than the boundary by Murakami et al. [2003] (130 GPa) using the same Pt scale (Figure 1). As mentioned above, because the Pt scale may overestimate the pressure [Fei et al., 2007; Hirose, 2006] with respect to the Au scale we used, the discrepancy between the Pt and Au scales may explain why Shieh et al. [2005] failed to synthesize seifertite at pressures very close to where we observed the synthesis. The low-temperature heating by Shieh et al. [2005] may also have contributed to the metastable persistence of the CaCl 2 type, which combined with the observations below provide some evidence of the sluggish nature of the transition (Figure 1). [23] The Clapeyron slope of the seifertite phase transition is poorly constrained from the previous experiments. Although Murakami et al. [2003] presented a positive slope, a negative Clapeyron slope would fit their data equally well. The slope proposed by Dubrovinsky et al. [2001] appears to be for a boundary of metastable formation of seifertite far outside the stability field. Our data are an improvement over the previous studies, although we do not have adequate coverage to quantitatively determine the slope. The kinetics of the silica system appear to be strong, evidenced by the recovery of the -PbO 2 structure at room pressure and the inability to fully convert the -PbO 2 to the lower pressure stable phase (CaCl 2 structure) near the phase boundary. Nevertheless, our data favor a positive Clapeyron slope (primarily from the Al-bearing data) which is consistent with the first-principles estimations, MPa/K [Tsuchiya et al., 2004; Oganov et al., 2005; Driver et al., 2010] Equation of State [24] Diffraction peaks at 1 bar were indexed by comparison with the seifertite found in the Shergotty meteorite [Dera et al., 2002]. Higher pressure and temperature diffraction patterns retain the same reflections but are shifted to lower d spacings as the density increases. These diffraction patterns are consistent with the -PbO 2 -type (Pbcn) silica calculated by Teter et al. [1998] (Figure 3) and allows for unambiguous indexing to determine lattice parameters and volumes. The unit-cell parameters were determined as a function of pressure during decompression (Figure 5 and Tables 3 and 4), using 8 14 reflections from Ar-1 and Ar-2Al, and 5 7 reflections for Ne-5 in which pure seifertite is observed. The use of two different pressure transmitting media and laser couplers allows us to be confident in our seifertite peak assignments. For Ar-3 in which we synthesized a mixture of seifertite and the CaCl 2 -type phase, peak overlaps between these two phases make rigorous volume determination of seifertite challenging and therefore are not included for the equation of state analysis, but the lattice parameters are still consistent with those determined from pure seifertite 8 Figure 6. Microphotograph of an Al-free seifertite sample quenched from 152 GPa and 2440 K to roomconditions. The sample chamber was collapsed to a small irregularshaped pocket due to the escape of Ar during decompression to1bar. samples within uncertainty. Volumes were also measured for stishovite and CaCl 2 -type silica in Ne-6Al and Ne-7, with V 0. [25] During decompression in experiments Ar-1, Ar-2Al, and Ne-5, the diffraction patterns remain the same without any new peaks as we cross into the CaCl 2 stability field, indicative of seifertite metastability. Our samples appear to be relatively quasi-hydrostatic during the decompression, as our Debye rings show little or no deviation from circular and different gold reflections give a similar lattice parameter. The measured volumes for the two different pure silica samples are in agreement regardless of the kind of noble gas pressure medium (Ar or Ne). The volume of Al-bearing seifertite remains slightly higher throughout the pressure range. In experiments Ar-1 and Ar-2Al, we were able to decompress seifertite to 1 bar and recovered the sample (Figure 6). The measured diffraction patterns, compared with the calculated patterns of other dense silica polymorphs found at 1 bar, i.e., baddeleyite type [El Goresy et al., 2000] and postquartz [Haines et al., 2001], show that seifertite synthesized at 140 GPa still maintains its structure without back-converting to other forms (Figure 4). This strong metastability of seifertite allowed us to calculate 1 bar density ( 0 ) and measure the P-V relations for a wide pressure range (Figure 5). [26] The density of pure SiO 2 seifertite at 1 bar calculated from V 0 measured in this study is the greatest ever recorded for a silica polymorph (Table 2). Our density of seifertite is 1.5% greater than that of stishovite [Andrault et al.,2003]. Our seifertite density is 1.4% higher than seifertite found in the Shergotty meteorite [Dera et al., 2002], and 2.1% higher than seifertite synthesized from cristobalite starting materials [Dubrovinsky et al., 2001]. Our results are virtually identical to the density obtained from quantum Monte Carlo computations, with our 0 only 0.1% higher than that from

9 Driver et al. [2010] at room pressure and remaining more dense by only 0.2% at the highest pressures of this study (150 GPa). Comparison of room pressure lattice parameters differ by only 0.22%, 0.02%, and 0.1% fora, b, and c, respectively [Driver et al., 2010]. Our high-pressure volumes (>70 GPa) are also very close to the values from the density functional theory calculation [Oganov et al., 2005], although their 0 is higher by 0.6%. Overall, the agreement with the most advanced first-principles calculations is remarkable and represents a breakthrough in the ability of computational methods to accurately predict the properties of silica at high pressure. [27] WefittheP-V data using the finite strain method [Birch, 1978] to a second-order Birch-Murnaghan equation of state (Table 2 and Figure 5) due to the trade-off between bulk modulus (K 0 ) and the pressure derivative (K 0 0 ) in the fitting [Bell et al., 1987]. We obtained K 0 = GPa for both pure SiO 2 and Al 2 O 3 -bearing seifertite, suggesting that Al 2 O 3 up to 10 mol % does not change the equation of state of seifertite. Our K 0 is in excellent agreement with computational studies [Oganov et al., 2005; Driver et al., 2010] (Table 2) but is greater than experimental results on seifertite synthesized from cristobalite, even if the difference in K 0 0 is considered (see the fitting for the data at P >70GPa in Table 2) [Dubrovinsky et al., 2001]. According to our data on the CaCl 2 -type SiO 2 using the same experimental setup, the K 0 of seifertite is higher by 1.6% than the CaCl 2 type. Our K 0 for the CaCl 2 type is lower than that of Andrault et al. [2003], with some of the discrepancy due to their use of the Pt scale [Holmes et al., 1989]. We prefer direct comparison of the seifertite K 0 with our data on the CaCl 2 type because of internal consistency between the data sets. [28] Lattice constants were fit following the modified Birch-Murnaghan equation [Xia et al., 1998]. We obtained K 0 = GPa and K 0 0 = for a 0 = Å, K 0 = GPa and K 0 = for b 0 = Å, and K 0 = GPa and K 0 0 = for c 0 = Å. The a axis is significantly more compressible than the other two axes, even considering the trade-off between K 0 and K 0 0. The a axis is perpendicular to the close packed layer of oxygen atoms. [29] Lattice parameters from previous results [Tsuchida and Yagi, 1990; Dubrovinsky et al., 1997, 2001; Murakami et al., 2003] have a lot of scatter compared to our data. For the lower pressure measurements (<100 GPa), the use of different starting materials and nonhydrostatic stress may produce seifertite in conjunction with other similar structures closely related in free energy [Teter et al., 1998; Prokopenko et al., 2001; Prakapenka et al., 2004; Dubrovinsky et al., 2004], making reliable peak identification challenging. [30] The volumes reported at P > 100 GPa by Murakami et al. [2003] are in reasonably close agreement to our data even after we correct for the difference in pressure scale [Feietal., 2007]. The agreement is coincidental, as the axial parameters are significantly different from our measurements, but have offsetting error that leads to consistent volumes (Figure 5). Murakami et al. [2003] used a small number of broad diffraction peaks (< 5) from nonhydrostatic conditions, with two to three of those peaks having overlap with the Pt laser absorber Implications 4.1. Metastability of Seifertite [31] Seifertite is remarkable in that it can be metastably preserved to 1 bar from 140 GPa over back conversion to its lower pressure forms, such as quartz, stishovite, or densified glass. This metastability hysteresis in pressure is an order of magnitude greater than diamond but lacks the dramatic change in bonding character such as the sp 2 -to-sp 3 orbital hybridization for the graphite-to-diamond transition. [32] The extreme metastability of seifertite is perhaps due to an energetically unfavorable transition pathway as has been suggested for other silica analog compounds [Haines and Leger, 1997]. Back conversion from seifertite to stishovite may require the formation of an intermediate phase such as baddeleyite from consideration of the symmetry relationships. However, because the baddeleyite-type structure has higher Si-O coordination state than seifertite, the seifertite to baddeleyite-type transition likely has a large kinetic barrier. Recovery of the -PbO 2 -type structure demonstrates that some dense high-pressure phases can be recovered to room conditions by taking advantage of transitional pathways rather than fundamental changes in bond character. [33] Conversely, although the pathway is entirely metastable, the kinetic barrier to form seifertite from cristobalite is lower than from stishovite [Donadio et al., 2008]. The significant difference in density (and perhaps bulk modulus to a lesser degree) between seifertite synthesized in its stability field (this study) and through a metastable transition from cristobalite starting material [Tsuchida and Yagi, 1990; Dubrovinsky et al., 2001; Dubrovinskaia et al., 2001] is notable (Table 2). These differences extend to the individual lattice parameters that are quite different for seifertite sythesized outside its stability field. Nonhydrostatic conditions coupled with large activation barriers for certain starting materials to convert into the stable phase (in this case stishovite or the CaCl 2 -type structure) may be the source of these differences. Furthermore, the lower density of natural seifertite compared to what we obtain in our experiments may be important in understanding the presence of the mineral in the SNC meteorites Seifertite in Meteorites [34] Seifertite has been found in nature in the Shergotty, Zagami, and NWA 4734 meteorites [Sharp et al., 1999; El Goresy et al., 2008; Miyahara et al., 2013]. Two of the meteorites are part of the achondrite SNC family of Martian origin, while NWA 4734 is of lunar origin. An important constraint on the peak shock pressure of these meteorites is the lack of evidence for whole-scale melting [Malavergne et al., 2001; El Goresy et al., 2004; Aoudjehane et al., 2005] and the appearance of small amounts of seifertite along with densified glass and stishovite [El Goresy et al., 2008]. The calculated Hugoniots for silica polymorphs [Akins and Ahrens, 2002] combined with our stability field measurements (Figure 1) would seem to rule out all precursor materials with the exception of stishovite. However, stishovite would require an unreasonably high peak shock pressures (well over 150 GPa, if kinetic issues at lower temperatures are considered) that should result in large-scale

10 melting, unless some mechanism exists for extreme pressure localization during the shock process. [35] Low-temperature compression of cristobalite is the lowest pressure pathway for generating seifertite [Tsuchida and Yagi, 1990; Dubrovinsky et al., 2001; Dubrovinskaia et al., 2001] and has been suggested as the mechanism by which it is formed in meteorites [El Goresy et al., 2008]. Previous shock wave experiments on cristobalite have been collected to 30 GPa and produce densified glass on pressure release [Gratz et al., 1993]. The production of glass over -PbO 2 structured crystals is not surprising as differences in length scales, timescales, and release path make it difficult to relate laboratory shock experiments directly to large impact events [Luo et al., 2003]. While our data do not favor any particular precursor starting material (other than ruling out glass), it does firmly establish that the seifertite in these meteorites is far outside the stability field. However, it is not inconsistent with the prevailing idea of a cristobalite origin for the mineral [El Goresy et al., 2008] Seismic Structures at the Earth s Lowermost Mantle [36] Homogenized mantle composition is likely undersaturated in silica [Ringwood, 1982], but chemically distinct materials such as oceanic crust are constantly injected into the mantle at the subduction zones. The heterogeneity may survive on a geologic timescale due to extremely sluggish solid state diffusion at mantle P-T conditions [Allegre and Turcotte, 1986; Holzapfel et al., 2005]. Oceanic crust contains silica-oversaturated sediments and basalts, transporting silica-rich materials into the mantle through subduction. Recent studies have indicated that the basaltic materials may be transported to the bottom of the mantle and persist there over long timescales [Hirose et al., 1999; Nakagawa et al., 2010]. In the lowermost mantle, basalt may contain much less Mg-silicate (perovskite or postperovskite; 30 mol %) than pyrolite (70 mol %) but contain significant amount of free silica (40 mol %) while virtually no free silica is expected in pyrolite [Xu et al., 2008]. [37] Alternatively, some free silica could exist even within a lower mantle of pyrolite composition. Bina [2010] suggested the possible existence of free silica even in the pyrolitic mantle as polycrystalline-armored relics with seismically detectable scale. Knittle and Jeanloz [1991] and Goarant et al. [1992] suggested that reaction between Mg-silicate and liquid iron may produce free silica together with iron oxide and iron silicide. The thickness of the reaction zone is unknown but should be limited to near the core-mantle boundary. Therefore, there are at least three possibilities for the existence of free silica locally in the lowermost mantle. [38] We found that the CaCl 2 type to seifertite phase boundary is consistent with a positive Clapeyron slope (Figure 1) and occurs at a pressure very close to the coremantle boundary in the Earth. Our conclusion is based on the assumption that the gold scale from Tsuchiya [2003] estimates the pressure reasonably well (see discussions in Shim et al. [2008] for the uncertainty in pressure scales). The increase in density of 1.5% across the boundary is mostly offset by a moderate increase of the bulk modulus (0.7%) (Table 2), resulting in either no change or a slight decrease in bulk sound speed by % at 130 GPa. Computational results suggest shear wave speed should decrease across the transition ( %) at 0 K [Karkietal., 1997b; Tsuchiya, 2011]. Assuming the 0 K calculations are valid at mantle-relevant temperatures, the D 00 discontinuity is associated with a shear wave increase of 2 3% [Lay et al., 1998; Wysession et al., 1998] and frequently occurs as a shallower depth, making it unlikely that the seifertite transition can be the source of the discontinuity. [39] The perovskite! postperovskite transition remains a better explanation for the D 00 discontinuity [Murakami et al., 2004; Oganov and Ono, 2004]. A decrease in bulk sound speed ( 0.8 to 2.4%) [Oganov and Ono, 2004; Iitaka et al., 2004; Shim et al., 2008] and an increase in shear velocity [Oganov and Ono, 2004; Iitaka et al., 2004; Wentzcovitch et al., 2006; Tsuchiya and Tsuchiya, 2011] across the postperovskite transition are more consistent with the D 00 discontinuity [Lay et al., 1998; Wysession et al., 1998; Hutko et al., 2008] than the seifertite transition. [40] A large boundary thickness due to a wide mixed phase region is found in pyrolitic compositions due to the partitioning behavior of Fe and Al, making a homogenized mantle composition an unlikely candidate for the discontinuity [Andrault et al., 2010; Catalli et al., 2009]. Grocholski et al. [2012] confirmed this behavior for pyrolite, but showed that the mineralogy of basaltic and harzburgitic compositions can decrease the mixed phase region. One conclusion from this study is that the postperovskite transition may only be detectable in the regions with particular types of chemical heterogeneities (such as subducted oceanic crust). This observation appears to be consistent with the fact that the D 00 discontinuity has been more readily documented in the regions which might be related to the deposition of subducted materials in the lowermost mantle [Lay et al., 1998; van der Hilst et al., 2007; He and Wen, 2011]. [41] The reason for discontinuities at greater depths than the D 00 discontinuity found in some seismic studies is even less clear. Thomas et al. [2004a] reported a laterally extending discontinuity at a greater depth than the D 00 discontinuity in the lowermost mantle beneath Caribbean. They also documented a deep discontinuity in the lowermost mantle beneath Eurasia [Thomas et al., 2004b]. Later, Hutko et al. [2008] also documented a deeper discontinuity at km depths above the core-mantle boundary. All of these discontinuities have a shear wave decrease with increasing depth and have frequently been interpreted as conversion from postperovskite to perovskite. However, the seifertite transition may have a shear wave decrease and should be considered as a candidate for discontinuities found below the D 00 discontinuity. [42] Some deep discontinuities has been explained by two separate intersections between the steep lowermost mantle geotherm and the postperovskite boundary with a positive Clapeyron slope ( double crossing ) in cold regions of the mantle [Hernlund et al., 2005]. The perovskite to postperovskite transition at a shallower depth would create a discontinuity with a shear wave speed increase, while the reverse transition at a deeper depth would result in a discontinuity with a shear wave speed decrease. This hypothesis predicts a negative correlation in the depths between the shallower and deeper discontinuities, resulting in a variation from single discontinuity to double discontinuity along a lateral decrease in temperature. Hernlund et al. [2005] 10