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Article Cite This: ACS Omega 2018, 3, 14225−14228
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Insights into the Hydrothermal Metastability of Stishovite and Coesite Nyi Myat Khine Linn,† Manik Mandal,† Baosheng Li,‡ Yingwei Fei,§ and Kai Landskron*,† †
Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States Mineral Physics Institute, Stony Brook University, Stony Brook, New York 11794, United States § Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, United States ‡
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S Supporting Information *
ABSTRACT: Hydrothermal experiments aiming at the crystal growth of stishovite near ambient pressure and temperature were performed in conventional autoclave systems using 1 M (molar) NaOH, 0.8 M Na2CO3, and pure water as a mineralizing agent. It was found that the hydrothermal metastability of stishovite and coesite is very different from the thermal metastability in all mineralizing agents and that because of this fact crystals could not be grown. While stishovite and coesite are thermally metastable up to 500 and >1000 °C, respectively, their hydrothermal metastability is below 150 and 200 °C, respectively. The thermally induced conversion of stishovite and coesite leads to amorphous products, whereas the hydrothermally induced conversion leads to crystalline quartz. Both stishovite and coesite are minerals occurring in nature where they can be exposed to hydrothermal conditions. The low hydrothermal stability of these phases may be an important factor to explain the rarity of these minerals in nature.
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INTRODUCTION Stishovite, a high-pressure polymorph of SiO2 with a rutile structure, is the hardest oxidic material with a hardness of about 33 GPa and has been the subject of intense studies.1−14 Despite this extraordinary property and its thermal metastability up to ca. 500 °C at ambient pressure, it has not found applications as an ultrahard material for abrasive, cutting, and ceramics applications. The reason is that stishovite is thermodynamically stable only above ca. 8 GPa, which poses severe limitations with regard to the scalability of the synthesis. Thus, an economic synthesis of stishovite is currently unavailable. Kinetically controlled synthesis allows for the access of materials far from chemical equilibrium. Diamond has been produced in kinetically controlled chemical vapor deposition processes from diamond seed crystals near or below ambient pressure.15−17 We set out to develop a kinetically controlled synthesis for stishovite near ambient pressure using stishovite seed crystals. The fact that stishovite is an oxide inspired us to conceive a kinetically controlled hydrothermal synthesis from the solution phase in a temperature gradient near ambient pressure. Essentially, this synthesis would work analogously to the commercial quartz growth process; however, instead of quartz seed crystals, stishovite seeds would be used. Industrially, quartz is hydrothermally grown at pressures of ca. 0.1−0.2 GPa and temperatures of ca. 300−350 °C in temperature gradients between 2 and 25 °C using a mineralizer, typically a 1 M NaOH or a 0.8 M Na2CO3 solution.18 At that temperature, stishovite is known to be thermally metastable. If stishovite was also hydrothermally metastable, conversion of the stishovite into a © 2018 American Chemical Society
lower-density phase could be avoided, and stishovite may be grown as a metastable phase from a supersaturated solution. If not, fundamental insights into the hitherto unknown hydrothermal metastability of stishovite would be obtained.
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EXPERIMENTS, RESULTS, AND DISCUSSION To test this hypothesis, we first synthesized stishovite seed crystals thermodynamically controlled at 12 GPa and 400 °C from SBA-16, an ordered mesoporous form of silica. These conditions allow for the synthesis of discrete, nonagglomerated, nanosized (ca. 300 nm) single crystals of stishovite that are described in structural detail in a previous publication.19 The use of nanosized single crystals seemed particularly attractive to us because of the high surface-to-volume ratio of the nanocrystals. In an initial experiment, 2.8 mg of the nanocrystals was pressed into a silver crucible (8 mm length and 5 mm diameter) having four 1 mm diameter holes drilled halfway around the side, and a silver lid was placed on top of the crucible. This setup ensured that the seeds were accessible by the solution, but the seeds could not be washed out by convection (it was verified in a reference experiment using quartz powder as seeds that this setup is suitable for crystal growth). The crucible was placed in the growth zone of a custom-made autoclave (High Pressure Equipment Company) made of 4340 alloy steel having a modified Bridgman-type seal. The autoclave body was 41.61 cm Received: March 14, 2018 Accepted: October 15, 2018 Published: October 26, 2018 14225
DOI: 10.1021/acsomega.8b00484 ACS Omega 2018, 3, 14225−14228
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maximum temperature gradient that could be achieved was 15 °C (Table S1). At 300 and 250 °C, well-crystalline quartz was obtained, whereas at 160 °C, the stishovite became largely noncrystalline (Figure 1b−d). The results show that under all the chosen hydrothermal conditions, stishovite was not metastable but converted into quartz. This behavior is different from the thermal behavior of stishovite near ambient pressure. Under purely thermal conditions, stishovite amorphizes at the temperature of ca. 500 °C. A 1 M sodium hydroxide solution is strongly basic. We hypothesized that the strong basicity of the solution may be the reason for the low hydrothermal stability of stishovite. Na2CO3 is also known as a good mineralizer in quartz crystal growth but is a far weaker base. We therefore decided to replace the 1 M NaOH solution by a 0.8 M Na2CO3 solution in our experiments. A dissolution temperature of 275 °C and a growth temperature of 250 °C were chosen for an initial growth experiment in 0.8 M Na2CO3 (Table S2). The XRD pattern of the material obtained after a growth time of 2 days revealed well-crystalline quartz, showing that the hydrothermal metastability of stishovite in 0.8 M Na2CO3 is also lower than the thermal metastability (Figure 2a). The mass of the recovered sample significantly increased by
(16.38 in.) tall and had an outer diameter of 6.35 cm (2.50 in.) and a volume of 100 mL. Its upper pressure limit was 0.2 GPa (30 000 psi) at 400 °C. To ensure safe operation, the autoclave was housed in a detonation-proof chamber (Fluitron). The temperatures in the dissolution and the growth zone were measured with two J-type thermocouples, which were inserted into the walls of the autoclave body. The autoclave was connected to a Viatran XL 570 pressure transmitter via a ca. 50 cm long stainless-steel high-pressure tubing having an outer diameter of 0.32 cm (1/8 in.). The autoclave was heated with two 17.78 cm (7 in.) long band heaters separated by a mica sheet as well as a bottom heater underneath the autoclave. The heaters were heated with three programmable proportional−integral− derivative temperature controllers having a maximum power of 400 W each. The setup is shown schematically in Figure S1. Quartz particles with a diameter of around 5 mm (donated by Sawyers, Inc) were used as the nutrient, and 1 M NaOH was chosen as the mineralizer. A temperature of 375 °C was chosen for the dissolution zone, and the temperature of the growth zone was 350 °C. The heating rate was 4 °C/min. The two zones were separated by a baffle with a 10% opening. The fill rate of the autoclave was 75%, the final pressure in the autoclave was 43.49 MPa (6308 psi), and the growth time was 48 h (Table S1). After the experiment was completed and the autoclave had cooled down, it was opened. The weight of the sample in the silver capsule was determined, and the phase was identified by X-ray diffraction (XRD) using a Rigaku MiniFlex II microdiffraction system with a Cu Kα radiation source (λ = 0.15405 nm). It was found that the mass of the sample had slightly decreased by 0.2 mg (7%), suggesting that crystal growth did not occur (Table S1). XRD showed reflections for quartz except a very small peak at 30.1° 2θ, which is the 100% peak for stishovite (Figure 1a). In
Figure 2. XRD patterns of products obtained from stishovite seeds in 0.8 M Na2CO3 at Tdiss = 275 °C, Tgr = 250 °C (a), Tdiss = 175 °C, Tgr = 150 °C (b), Tdiss = 137 °C, Tgr = 111 °C (c), Tdiss = 125 °C, Tgr = 105 °C (d), calculated XRD pattern of quartz (e), calculated XRD pattern of stishovite (f), calculated XRD pattern of silver (g). Q = quartz, S = stishovite, Ag = silver (from the silver capsule containing the stishovite).
139%; however, this mass increase is not due to crystal growth. It was noticed that a gel-like material, presumably amorphous silica, precipitated out of the supersaturated solution upon cooling covering the interior of the autoclave crucible and seed crystal surfaces, which explains the mass increase. To get further insights into the hydrothermal stability of stishovite in Na2CO3 solution, we carried out additional experiments at growth temperatures of 150, 111, and 105 °C and dissolution temperatures of 175, 137, and 125 °C, respectively (Table S2). At 150 °C, semicrystalline quartz was obtained together with small residual amounts of stishovite (Figure 2b). The semicrystalline nature of the product together with the different intensity ratios of the diffraction peaks indicates that the phase transformation into quartz is still incomplete at this temperature. At 111 and 105 °C, XRD showed stishovite only, showing that the hydrothermal metastability of stishovite is between 111 and 150 °C (Figure 2c). A mass increase of 45, 16, and 25% was seen for these experiments, respectively. Similar to the experiment performed at 250 °C, it was noticed that a gel-like material had
Figure 1. XRD patterns of products obtained in stishovite seeds in 1 M NaOH at Tdiss = 375 °C, Tgr = 350 °C (a), Tdiss = 325 °C, Tgr = 300 °C (b), Tdiss = 275 °C, Tgr = 250 °C (c), Tdiss = 175 °C, Tgr = 160 °C (d), calculated XRD pattern of quartz (e), calculated XRD pattern of stishovite (f), calculated XRD pattern of silver (g). These calculated XRD patterns were produced by VESTA software (http://jp-minerals. org/vesta/en/) using a cif file from Inorganic Crystal Structure Database. Q = quartz, S = stishovite, Ag = silver (because of the presence of silver from the silver capsule containing the stishovite).
view of these results, we decided to lower the growth temperature to see if the hydrothermal metastability was achieved then and if crystal growth rates were large enough to measurably grow stishovite. Additional experiments were performed at growth temperatures of 300, 250, and 160 °C. For experiments performed at growth temperatures of 300 and 250 °C, the temperature gradient was 25 °C. At 160 °C, the 14226
DOI: 10.1021/acsomega.8b00484 ACS Omega 2018, 3, 14225−14228
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temperature gradients (>50 °C) at temperatures at which stishovite is hydrothermally stable. The hypothesis was that the large temperature gradient may lead to sufficiently large crystal growth rates because of the extreme oversaturation of the solution. NaOH (1 M) was used as the mineralizer because it is known that spontaneous nucleation of crystals from such a solution does not occur, and there is no precipitation of noncrystalline silica from the supersaturated solution upon cooling. To achieve a very large temperature gradient at the growth temperature of ∼120 °C, the growth zone needed to be actively cooled. To achieve this, a 0.635 cm (1/4 in.) outer diameter copper tubing was wound around the growth zone of the autoclave and connected to a Neslab RTE 111 chilling apparatus with a cooling capacity of 375 W (BTU/h), and the apparatus was set to the lowest temperature, which is −30 °C. A shorter 8.89 cm (3.5 in.) long band heater was used to heat the dissolution zone. The distance between the cooling coil and the band heater was 5.08 cm (2 in.). The setup is schematically shown in Figure S2. The maximum dissolution temperature that was achieved at a growth temperature of 116 °C was 230 °C, corresponding to a temperature gradient of 114 °C (Table S4). However, even at this very large temperature gradient, no measurable stishovite crystal growth was achieved. Coesite is another high-pressure polymorph of silicon dioxide. In contrast to stishovite, which has octahedral SiO6 building units, it is made of interconnected SiO4 tetrahedra. The thermal metastability of coesite (>1000 °C) is known to be much higher than that of stishovite (∼500 °C).20 Our hypothesis was that because of the higher thermal metastability of coesite, the hydrothermal metastability of coesite may also be higher and that this may allow for the kinetically controlled hydrothermal growth of coesite near ambient pressure at temperatures sufficiently high to achieve reasonably large crystal growth rates. Moreover, coesite is thermodynamically stable already from ca. 2 GPa, and thus, the growth conditions would not be far from equilibrium. This is an important consideration because the free energy of the supersaturated solution must be higher than that of the seed crystals to produce a driving force for crystal growth. In an initial experiment, at a growth temperature of 200 °C and a temperature gradient of 50 °C, we found that coesite was completely converted into quartz within 24 h in a 1 M NaOH solution (Figure 5a). At 190 °C, there was the still conversion into quartz, but at 180 °C, coesite remained stable (Figure 5b,c). No crystal growth occurred, showing that crystal growth rates are too small at the temperature coesite is hydrothermally metastable (Table S5).
precipitated, which suggests that the mass increase was not due to crystal growth. To get further insights, we performed transmission electron microscopy (TEM) on the material obtained at 111 °C (Figure 3). The TEM images showed that
Figure 3. TEM images and SAED pattern of representative grains of the product obtained at Tdiss = 137 °C, Tgr = 111 °C.
the crystal sizes did not significantly change during the experiment (ca. 100−400 nm), and selected area electron diffraction (SAED) confirmed the crystallinity of the investigated particles. A very thin (∼5 nm) layer with brighter electronic contrast was noticed on the surface of the crystals, indicating that either a lower-density noncrystalline phase had precipitated on the surface of the crystals or the surface of the stishovite crystals had begun to convert into a lower-density noncrystalline phase. Overall, TEM supports that the stishovite crystals did not grow. Further experiments were carried out in pure water as the mineralizer (Table S3). At a growth temperature of 250 °C, the stishovite became completely noncrystalline (Figure 4a). A mass
Figure 4. XRD patterns of products obtained from stishovite seeds in water at Tdiss = 275 °C, Tgr = 250 °C (a), Tdiss = 175 °C, Tgr = 150 °C (b), Tdiss = 125 °C, Tgr = 100 °C (c), calculated XRD pattern of stishovite (d), calculated XRD pattern of silver (e). S = stishovite, Ag = silver (from the silver capsule containing the stishovite).
loss of 88% was measured, showing that a substantial amount of stishovite had actually dissolved. Further experiments were carried out at growth temperatures of 150 and 100 °C. In these cases, pure stishovite was obtained according to XRD (Figure 4b,c). This result shows that the hydrothermal stability of stishovite in water is slightly higher than in Na2CO3 and NaOH. For the experiment at 150 °C, a mass loss of 69% was observed, indicating stishovite dissolution. For the experiment at 100 °C, there was neither a significant mass increase nor loss, showing that stishovite remains practically inert at this temperature. Because of the limited hydrothermal metastability of stishovite, it was decided to grow stishovite in extreme
Figure 5. XRD patterns of products obtained from coesite seeds in 1 M NaOH at Tdiss = 250 °C, Tgr = 200 °C (a), Tdiss = 240 °C, Tgr = 190 °C (b), Tdiss = 230 °C, Tgr = 180 °C (c), calculated XRD pattern of quartz (d), calculated XRD pattern of coesite (e). Q = quartz, C = coesite. 14227
DOI: 10.1021/acsomega.8b00484 ACS Omega 2018, 3, 14225−14228
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CONCLUSIONS
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ASSOCIATED CONTENT
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In conclusion, our study is the first report on the hydrothermal metastability of coesite and stishovite near ambient pressure. Previously, only the thermal metastability was known. It was found that the hydrothermal metastability of these phases is significantly different from their thermal metastability. While stishovite and coesite are thermally metastable up to 500 and >1000 °C, respectively, their hydrothermal metastability is below 150 and 200 °C, respectively. The thermally induced conversion of stishovite and coesite leads to amorphous products, whereas the hydrothermally induced conversion usually leads to crystalline quartz. The difference between the thermal and the hydrothermal metastability is explained by the fact that an aqueous solution can act as a phase transformation catalyst because water (and base) can chemically attack SiO2. Both stishovite and coesite are minerals occurring in nature where they can be exposed to hydrothermal conditions. The low hydrothermal stability of these phases may be an important factor to explain the rarity of these minerals in nature. Coesite and stishovite are mostly formed because of meteorite impacts, which can lead to hydrothermal conditions. Because the high pressure produced by the shock of the impact can return to ambient conditions more quickly than the high temperature, any coesite and stishovite formed due to this shock may be exposed to hydrothermal conditions near ambient pressure. Our data indicate that minutes to hours are sufficient to convert coesite and stishovite to quartz at temperatures as low as 150 °C. At room temperature, it can be assumed that coesite and stishovite are near indefinitely metastable.
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S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00484. Cross-sectional schematic of a high-pressure experimental setup to study the hydrothermal metastability of stishovite and coesite, growth conditions of stishovite seeds in 1 M NaOH, growth conditions of stishovite seeds in 0.8 M Na2CO3, growth conditions of stishovite seeds in water, cross-sectional schematic of a high-pressure experiment designed for ultrahigh temperature gradients, growth conditions of stishovite seeds in 1 M NaOH with ultrahigh temperature gradients, and growth conditions of coesite seeds in 1 M NaOH (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 001 610 758 5788. ORCID
Kai Landskron: 0000-0001-6170-4704 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the support by DARPA award W31P4Q-12-1-0008 (10/1/2012-3/15/2014) and NSF DMR award #1463948 (7/1/2015-6/30/2018). 14228
DOI: 10.1021/acsomega.8b00484 ACS Omega 2018, 3, 14225−14228
