Ultrahydrous Stishovite from High-Pressure Hydrothermal Treatment of Sio2

Ultrahydrous stishovite from high-pressure hydrothermal treatment of SiO2

Kristina Spektora,b, Johanna Nylenb, Emil Stoyanovc, Alexandra Navrotskyc,1, Richard L. Hervigd, Kurt Leinenweberb, Gregory P. Hollande, and Ulrich Häussermanna,1

aDepartment of Materials and Environmental Chemistry, Stockholm University, S-10691 Stockholm, Sweden; bDepartment of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604; cPeter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616; dSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404; and eDepartment of Chemistry and Biochemistry, Magnetic Resonance Research Center, Arizona State University, Tempe, AZ 85287-1604

Contributed by Alexandra Navrotsky, October 19, 2011 (sent for review September 2, 2011)

Stishovite (SiO2 with the rutile structure and octahedrally coordi- A nated silicon) is an important high-pressure mineral. It has previously been considered to be essentially anhydrous. In this study, hydrothermal treatment of silica glass and coesite at 350–550 °C

near 10 GPa produces stishovite with significant amounts of H2O in its structure. A combination of methodologies (X-ray diffraction, thermal analysis, oxide melt solution calorimetry, secondary ion mass spectrometry, infrared and nuclear magnetic resonance spectroscopy) indicate the presence of 1.3 0.2wt%H2O and NMR B C suggests that the primary mechanism for the H2O uptake is a direct hydrogarnet-like substitution of 4H for Si4, with the protons clustered as hydroxyls around a silicon vacancy. This substitution is accompanied by a substantial volume decrease for the system (SiO2 H2O), although the stishovite expands slightly, and it is only slightly unfavorable in energy. Stishovite could thus be a host

for H2O at convergent plate boundaries, and in other relatively cool high-pressure environments. 1mµ 5mµ D E low temperature ∣ high-pressure synthesis ∣ hydrothermal environments ∣ multianvil technique

ilica is an archetypical system for Earth and materials science. SIts complex polymorphism remains the subject of continuing study (1–3). At ambient to moderate pressures (up to 9 GPa), all forms of silica are built up of SiO4 tetrahedra, with coesite the highest pressure polymorph of this type. At higher pressures, 1mµ 2mµ dense forms containing SiO6 octahedra occur. Stishovite with the tetragonal rutile structure is stable between 9 and 50 GPa (4). Fig. 1. Photomicrograph of a partially glass-coesite transformed particle obtained at 300 °C in plane-polarized light (A, Left) and in cross-polarized Coesite and stishovite are believed to occur in silica rich parts of ’ light (A, Right). SEM images of stishovite crystals obtained at 350 °C (B) subducted oceanic slabs and crustal fragments in the Earth s man- and 450 °C (C) using glass starting material (D) close-up of Fig. 1C.(E) SEM tle (5). They are considered nominally anhydrous minerals (6), image of stishovite crystals obtained at 450 °C from coesite. although stishovite may contain small amounts of hydroxyl in con- junction with Al incorporation (7, 8). stishovite with about 11% coesite was obtained. Products at 400– There are large kinetic barriers associated with transforma- 550 °C were coesite-free stishovite. tions among silica polymorphs (9). Silica glass can be irreversibly Fig. 1 presents optical and scanning electron microscopy densified above 15 GPa without transformation to coesite or images for selected samples. Fig. 1A shows the polarized light stishovite (10, 11). Temperatures above approximately 1,000 °C micrograph of a partially transformed larger particle consisting of were required to constrain the coesite-stishovite equilibrium a coesite rim of about 20-μm thickness and a center of strained phase boundary (12). If kinetic barriers could be lowered, new, glass. The coesite rim shows undulatory extinction, suggesting it intermediate, high-pressure forms of silica may become accessi- is composed of oriented micrometer-sized domains that grow ble (13, 14). Additionally, low temperatures can afford nanostruc- μ tured forms of high-pressure silica phases (15). To explore routes inward from the surface. Stishovite occurs as fine, 0.5 to 1- m- for lowering kinetic barriers, we investigated a high-pressure sized, euhedral tabular crystals. In the 350 °C product from glass hydrothermal environment. Experiments at 10 GPa on silica starting material, stishovite crystals are peculiarly intergrown B glass–water or coesite–water mixtures at various temperatures (Fig. 1 ). The overall texture is most likely a result of the micro- (see SI Text for details) produced stishovite containing unprecedented amounts of structural water. Author contributions: K.S., E.S., R.L.H., K.L., and G.P.H. performed research; K.S., J.N., E.S., R.L.H., K.L., G.P.H., and U.H. analyzed data; and A.N. and U.H. wrote the paper. Results and Discussion The authors declare no conflict of interest. The evolution of products during 8-h experiments using glass as 1To whom correspondence may be addressed. E-mail: [email protected] or starting material was as follows (Fig. S1). At 250 °C, the sample [email protected]. remained essentially amorphous; at 300 °C, coesite with about This article contains supporting information online at www.pnas.org/lookup/suppl/ 13% stishovite coexisted with some residual glass; and at 350 °C, doi:10.1073/pnas.1117152108/-/DCSupplemental.

20918–20922 ∣ PNAS ∣ December 27, 2011 ∣ vol. 108 ∣ no. 52 www.pnas.org/cgi/doi/10.1073/pnas.1117152108 Downloaded by guest on October 1, 2021 structure of coesite domains obtained initially from the hydro- The presence of H2O in stishovite in excess of 1 wt % is about thermal transformation of the glass. Samples prepared at higher three orders of magnitude higher than previously seen for Al-free temperatures consist of homogeneous stishovite crystals (Fig. 1C). stishovite and about one order of magnitude higher than ob- Previously intergrown crystals are now largely separated with the served for Al-bearing stishovite (7, 8). former intergrowth contacts clearly visible (Fig. 1D). In contrast, The differential scanning calorimetry (DSC) curves show when using coesite starting material, stishovite crystals appear strongly exothermic decomposition of stishovite to glass, consis- well sintered into agglomerates that have the shape and size of tent with previous thermodynamic studies (17). There are some the original coesite particles (Fig. 1E). We conclude that the differences in the shape and area of the peaks between the hydrothermal transformation of silica glass into stishovite pro- hydrous and dry samples, but the heat effects are not readily ceeds via coesite, and that the microstructure of coesite deter- quantified. A more accurate thermochemical approach utilizes mines the size and shape of stishovite crystals. high-temperature oxide melt drop solution calorimetry. Using The powder X-ray diffraction (PXRD) patterns of hydro- an appropriate thermochemical cycle (Table S2), one can calcu- thermally formed stishovite show that reflections are shifted late the enthalpy of the reaction, at ambient temperature: to lower Bragg angles (compared to dry stishovite, ref. 16), indi- ð Þþ ð Þ cating a slightly larger unit cell volume (Fig. 2A, Table 1, and SiO2 stishovite nH2O liquid c Table S1). Whereas the lattice parameter remains largely unaf- ¼ SiO2·nH2O ðhydrous stishoviteÞ: [1] fected, the a parameter increases by almost 0.5%. With increasing synthesis temperature, the a lattice parameter of anhydrous The calculated enthalpies of formation for samples 450-G and stishovite is approached. These changes, outside experimental 450-Co are 7.3 and 3.0 kJ∕mol, respectively. Assuming an error error, strongly suggest a structural role for H2O. of 0.2 wt % for the water content, the values above will change Secondary ion mass spectrometry (SIMS) was performed on by 0.4 kJ∕mol. The two materials have similar water contents the sample obtained from glass at 450 °C (i.e., sample 450-G, according to thermogravimetric analysis (TGA), but appear to see Table 1). Trace elements (B, Al, Na, Mg) did not exceed have somewhat different enthalpies. Differences between the two 60-wt-ppm and the H2O content was 1.3ð0.1Þ wt %. Selected hydrous stishovites became already apparent in the analysis of samples were subjected to thermal analysis (Fig. 2B and Table 1). their morphologies (see Fig. 1 D and E) and PXRD patterns Dry stishovite transforms exothermically into a glass at 550 °C. (see Fig. 2A). Further exploration of possible structural and The small weight loss below 300 °C in all samples represents energetic differences among differently prepared samples is the surface water. Hydrous stishovite 450-G decomposes at a lower subject of future studies. The salient point of the present findings temperature, 500 °C. Its decomposition is associated with a is that the incorporation of water into the stishovite has only a weight loss of 1.4 wt %, in agreement with the SIMS result, and small energetic penalty. Although the stishovite expands slightly we consider 1.3 0.2 wt % to be the water content of 450-G. with the incorporation of water (Table 1), the volume change for

ACB TG (wt.%) DSC (mW/mg) exo 1.2 607 557 99.8 0.4 867 1.0 840 99.6 534 dry 99.4 0.2 0.8 583 99.2 550-G 0.6 99.0 0 Absorbance 0.4 450-Co 98.8 -0.2 98.6 0.2

450-G [1] 98.4 -0.4 0 EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES 61.0 61.5 62.0 62.5 63.0 63.5 64.0 100 200 300 400 500 600 700 800 900 1200 1000 800 600 400 θ o o 2() T( C) Wavenumber (cm-1 ) F E D

dry 0.60 1418 0.50 450-G 2653 0.40 2906 3389 dry bance 450-G 0.30 1422

Absor 0.20 3322 2889 2651

0.10 450-Co CP (450-G) 0 15 10 5 0 -170 -180 -190 -200 -210 4000 3500 3000 2500 2000 1500 1H Chemical Shift (ppm) 29Si Chemical Shift (ppm) Wavenumber (cm-1 )

Fig. 2. Experimental evidence for structural water in hydrous stishovite. (A) Shape and location of the 220 reflection in the PXRD pattern for various stishovite samples, (B) TGA (solid) and DSC (dotted) traces for dry stishovite (black) and hydrous stishovite 450-G (red). (C and D) IR spectra for dry stishovite (black) and hydrous stishovites 450-G (blue) and 450-Co (red). (C) Low energy IR with Si-O vibrations. The assigned wavenumbers refer to the three Eu modes. (D) High- energy IR with O-H vibrations. (E) 29Si MAS-NMR spectra for dry stishovite and hydrous stishovite 450-G. The arrow marks the second Si site. 29Si CP-MAS spectrum for 450-G (from top to bottom). (F) 1H MAS-NMR spectra for 450-G, dry stishovite, and 450-Co (from top to bottom).

Spektor et al. PNAS ∣ December 27, 2011 ∣ vol. 108 ∣ no. 52 ∣ 20919 Downloaded by guest on October 1, 2021 Table 1. Preparation conditions, unit cell parameters, water content after TGA and SIMS and values for the enthalpies of drop solution ΔH ( ds) for various stishovite samples a c V 3 n ΔH : Sample Preparation (T, P, t, starting mat.) ,Å ,Å ,Å Water content in SiO2·nH2O ds, kJ per mol of SiO2 nH2O Dry 1,000 °C, 10 GPa, 5 h, glass 4.1779 2.6657 46.53 0.007 (TGA) 2.86 2.51 550-G 550 °C, 10 GPa, 8 h, glass, hy 4.1831 2.6647 46.63 0.030 (TGA) — 450-Co 450 °C, 10 GPa, 8 h, coesite, hy 4.1895 2.6654 46.78 0.051 (TGA) 2.40 3.02 450-G 450 °C, 10 GPa, 8 h, glass, hy 4.1953 2.6650 46.90 0.056 (TGA) −1.31 0.044 (SIMS) −1.89 Estimated standard deviations for A and C are 0.002 Å or less; hy, hydrothermal.

reaction (1) is strongly negative (−0.7 cm3∕mol at ambient con- persion. 1H∕29Si heteronuclear correlation experiments show ditions, and estimated to be −0.4 cm3∕mol at the conditions of that only the protons at 10.5 ppm are correlated with silicon synthesis). Thus the pV term (integral of ΔVdp from 1 atm to (Fig. S2). The nature of the protons at 4.7 ppm, particularly pre- high pressure) at room temperature and 10 GPa is estimated to sent in the 450-Co sample that has sintered particles, is not yet be about −7 kJ∕mol (values at higher temperature depend on clear. We hypothesize that the observed differences in the IR the equation of state of water but will be roughly similar) and spectra, PXRD patterns, and enthalpies of formation between therefore can overcome the destabilizing enthalpy at atmospheric 450-G and 450-Co samples relate to those protons. Further pressure. The contribution of entropy, TΔS, cannot be readily experiments are necessary to clarify this phenomenon. constrained, but it generally makes hydration less favorable with A 2D double quantum/single quantum (DQ/SQ) NMR cor- increasing temperature. relation spectrum was collected for sample 450-G (Fig. S3) To shed further light into the nature of the incorporated water, and shows a strong DQ signal on the diagonal for the 10.5 ppm spectroscopic investigations were performed on samples 450-G resonance, indicating a clustering of these protons. DQ spinning and 450-Co. IR spectra of hydrous and dry stishovite are pro- sideband patterns were collected for this proton environment foundly different (Fig. 2C). In the hydrous material, bands below with two different excitation periods (Fig. S4). The H–H dipolar −1 1;000 cm corresponding to Si-O vibrations are slightly red couplings thus obtained are 7.0 and 4.8 kHz, which correspond shifted (perhaps reflecting increased Si-O distances). There is an to proton–proton distances of 2.6 and 2.9 Å, respectively. We −1 intense sharp band at approximately 1;420 cm , whose origin is conclude that the protons at 10.5 ppm are clustered within the −1 unknown. Three broad bands between 2,500 and 3;500 cm may structure at a distance <3 Å. Without other metals substituting suggest O-H stretching (Fig. 2D). Whereas the location of the −1 for Si (e.g., Al), we suggest the mechanism of hydrogen incor- low-frequency band (near 2;650 cm ) is virtually identical, for poration is the hydrogarnet defect where a cluster of four hy- 2;900 −1 − 4− the 450-Co sample the intermediate band (around cm ) droxyl groups (½OH 4) replaces an entity SiO4 . Although hy- seems to split and the high-frequency band occurs at a lower −1 drogarnet defects have been established for tetrahedrally coordi- wavenumber (3,322 vs. 3;389 cm ) with considerably higher nated Si in grossular garnets (18), they are rare for minerals intensity. The high-energy region of the IR spectrum of hydrous outside the garnet group and have never been reported in struc- stishovite appears very different from that of nominally dry tures containing octahedrally coordinated Si. stishovite and Al-bearing stishovite with up to approximately Tetrahedral defects in hydrogarnets have been characterized 0.3 wt % H2O. The IR spectra of the latter two are similar and structurally for silicon-free end members [e.g., Ca3Al2ðOHÞ12, characterized by an intense, broad, and anisotropic band at Ba3In2ðOHÞ ] (24, 25). The four O atoms around a silicon 3;111–3;134 −1 12 cm (8, 18). We conclude that these pronounced vacancy are terminated as hydroxyl. The short hydroxyl O-H changes in the IR spectra of hydrous stishovite also point to a (0.91 Å) and long next nearest O…H distances (>2.5 Å) are com- unique structural role for water. 29 patible with weak hydrogen bonding. We conjecture that the un- Si-magic angle spinning (MAS) NMR experiments were ique octahedral defect in hydrous stishovite represents a more performed on dry stishovite and on the hydrous samples 450-G complex bonding situation. If, analogous to the tetrahedral de- E and 450-Co (Fig. 2 ). The latter two spectra are virtually identical. fect, four O atoms are terminated as hydroxyl, the two remaining −191 1 The octahedral Si site in stishovite has a shift of . ppm (19, ones will become formally underbonded and act as acceptors for 20), which is also the dominant resonance for the hydrous samples. However, hydrous stishovite shows a second peak at −188.6 ppm. 1 29 AB A cross polarization (CP) H → Si experiment utilizing polarization from the proton shows that the minor peak at −188.6 ppm is considerably enhanced over the resonance at −191.1 ppm, indicating that this Si site is associated with a proton environment. The 1H-MAS NMR spectrum of hydrous stishovite shows three groups of protons with chemical shifts of 10.5, 4.7, and F 1 ppm (Fig. 2 ). The resonance around 1 ppm is also observed c for dry stishovite and is known from zeolites (21) and attributed b to surface hydroxyl species (22, 23). With respect to the reso- a nances at 10.5 and 4.7 ppm, the two hydrous samples show no- ticeable differences. The peak at 4.7 ppm is much more intense for 450-Co, and when compared to the 10.5 ppm resonance shows considerable motional narrowing. The broadening of the 10.5 ppm resonance is mainly attributed to strong proton–proton Fig. 3. A simple, high-symmetry model of the octahedral hydrogarnet defect based on the established proton arrangement of the tetrahedral defect dipolar coupling which is consistent with a continuous narrowing (see SI Text for details). Si, O, and H atoms are represented as light-gray, large of this resonance when higher MAS frequencies are applied. dark-gray, and small dark-gray circles, respectively. (A) Unit cell enclosing a However, there are also heterogeneous contributions to the line defect. (B) Si atoms surrounding a defect and underbonded O atoms involved width of the 10.5 ppm resonance, indicating chemical shift dis- in strong hydrogen bonding (thick, gray lines).

20920 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1117152108 Spektor et al. Downloaded by guest on October 1, 2021 strong hydrogen bonds. This situation is illustrated schematically transport of water and its transfer among mineral assemblages is in Fig. 3 where the known arrangement of protons in the tetra- likely to be controlled by kinetics as well as thermodynamics. hedral defect has been imposed on the octahedral geometry, Furthermore, the rich variety of P–T environments inferred while observing reasonable interatomic distances for O-H, Si- for the ever-growing number of exoplanets may include rather H, and H-H (see SI Text for details). The 1H NMR and IR spec- cool high-pressure environments which, in the presence of water, tra, however, suggest multiple H environments and thus a low may result in hydrothermal conditions similar to those studied −1 symmetry. O-H stretching frequencies at 2,650 and 2;900 cm here. Finally, the octahedral hydrogarnet defect should be ex- point to strong hydrogen bonding (18). Further insight into the plored as a general hydrogen storage mechanism in other nom- structure and bonding situation of the proposed octahedral de- inally anhydrous silicates containing octahedral silicon in Earth fect may be obtained from additional spectroscopic investigation, and planetary interiors. including deuterated samples, and computational modeling. In conclusion, we have shown that high-pressure hydrothermal The unexpected discovery of hydrous stishovite shows the treatment of SiO2 catalyzes the coesite–stishovite transition and potential of hydrothermal environments at gigapascal pressures produces hydrous forms of stishovite. At pressures near 10 GPa, for creating new materials. There have been very few reports stishovite is observed at temperatures below 350 °C, with signifi- on the application of such environments at pressures approaching cant yields above 400 °C. Unique to these results is the amount 10 GPa (26), yet these conditions may be significant in Earth of water incorporated (>1 wt %), the substitution mechanism – and planetary settings. For example, conditions of 450 550 °C via unprecedented octahedral hydrogarnet defects, and the very – and 9 10 GPa can occur during the subduction of old, cold ocea- modest (<10 kJ per mole of SiO2) energetic destabilization nic crust (the Tohoku subduction zone, ref. 27), and subducted associated with the observed H2O in stishovite. There needs to mid-ocean-ridge basalts (MORB) can form free silica phases be future detailed characterization of the solubility, structural at high pressures (28). Under such low-T, high-P conditions, P state, and thermodynamics of water in stishovite as a function continuous dehydration reactions of relatively low- minerals of temperature and pressure, and investigation of the possible (e.g., serpentine) will provide H2O which could then react with occurrence of the hydrogarnet-like substitution in other minerals coesite to form hydrous stishovite (27). Rocks exhumed by containing octahedral silicon (e.g., silicate garnet, perovskite, natural processes from these environments have been found to postperovskite phases). contain coesite, and some are suggested to contain relicts of stishovite (29). If such stishovite is indeed hydrous, such a reac- Materials and Methods tion path may provide an additional mechanism for bringing In a typical synthesis, 55–65 mg of silica (glass or coesite with a particle crustal H2O into the mantle. Chung and Kagi (30) found about size distribution of about 2–200 μm) and 25–35 mg water (i.e., the molar ratio 10 times higher H2O solubility in MORB than Al-bearing stisho- was roughly 1∶1) were sealed in noble metal capsules. The capsules were vite. Although they claimed trivalent cations might be responsi- pressurized in a multianvil device (32) and subsequently heated to a tempera- ble, the discrepancy is still difficult to understand by trivalent ture between 250 and 550 °C at a rate of 20 °C∕ min. After equilibrating cations alone. The water incorporation found in this study for samples at their target temperature for 8 h, the temperature was quenched and the pressure released over a period of 11 h. Recovered capsules were pure stishovite may help to explain such large H2O solubility in MORB. Mosenfelder (31) reported water solubility in coesite to pinched open and the water removed by evaporation. The SI Text contains further details on the synthesis procedure and the analysis of products by be minor at 1,200 °C, about a factor of 50 less than that found in PXRD, optical and scanning electron microscopy, SIMS, thermal analysis stishovite at much lower temperatures in the present work. Thus (TGA/DSC), drop solution calorimetry, IR and solid-state NMR spectroscopy. it is not known whether coesite can contain and transport signifi- Additionally, parameters for the structure model presented in Fig. 3 are cant H2O under low-temperature hydrothermal conditions. In a given. relatively cold subduction regime, a number of hydrated lower pressure phases may persist, stably or metastably, into the stisho- ACKNOWLEDGMENTS. This work was supported by the Swedish Research vite stability field, with their decomposition on heating providing Council (Vetenskapsrådet) and the National Science Foundation (NSF) sources of H2O. Hydrous stishovite may provide a “bridge” at through Grants DMR-0638826, DMR-1007557, and CHE-0742006. R.L.H. acknowledges NSF EAR-0948878 supporting the Arizona State University pressures of 9–12 GPa for the transport of H2O downward into (ASU) SIMS facility. G.P.H. acknowledges NSF CHE-1011937. A.N. acknowl- EARTH, ATMOSPHERIC,

P–T AND PLANETARY SCIENCES the regime of other high-pressure phases containing signifi- edges support from The Peter A. Rock Thermochemistry Laboratory at Uni- cant H2O (e.g., wadsleyite and other high-pressure hydrous versity of California, Davis. We gratefully acknowledge the use of facilities magnesium silicates). Because of the low temperatures involved, within The LeRoy Eyring Center for Solid State Science at ASU.

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