A New Mechanism for Stishovite Formation During Rapid Compression of Quartz and Implications for Asteroid Impacts

Shock‐induced transformations of quartz to high‐pressure polymorphs and diaplectic glass are decisive in identifying impact cratering events. Under shock compression, quartz can melt in local hot spots and crystallization of these silica melts under pressure can yield the high‐pressure mineral stishovite. A solid‐state transition to stishovite in relation to the formation of amorphous lamellae was already suggested in the late 1960s, but this idea was never comprehensively proven. Therefore, the mechanism responsible for such an intracrystalline stishovite formation is unknown to date. Herein, crystallographically oriented single crystals of quartz were compressed and decompressed in a membrane‐driven diamond anvil cell. These experiments aim at simulating the pressure paths of natural impacts on the timescale of seconds using compression rates between 0.2 and 0.6 GPa/s and peak pressures between 20 and 37 GPa. During the compression of quartz, the time‐resolved synchrotron X‐ray diffraction patterns reveal the almost simultaneous formation of two high‐pressure polymorphs, the recently identified rosiaite‐structured silica and stishovite. Transmission electron microscopic observations of recovered samples show that stishovite occurs as arrays of uniformly oriented nanometer‐sized crystals in amorphous intracrystalline lamellae. These observations indicate that the numerous stishovite crystals likely nucleated from the structurally similar rosiaite phase and thus inherited their uniform orientation during compression. During decompression, the metastable and non‐quenchable rosiaite‐structured phase collapsed to the amorphous stishovite‐containing lamellae. These findings attest to a novel mechanism of the formation of stishovite in the solid state and provide an explanation for similar microstructural occurrences of stishovite in impact‐metamorphic rocks and shocked meteorites.


10.1029/2023JE008126
2 of 14 Evidence of impacts are recorded through specific modifications in the crystal structures of rock-forming minerals, which are known as shock-metamorphic effects (Langenhorst & Deutsch, 2012).Because quartz is one of the major constituents of the Earth's continental crust and can show a large variety of different shock-metamorphic effects, it represents the best indicator mineral of past impacts and can even serve as a shock barometer (Stöffler & Langenhorst, 1994).For instance, at relatively low pressures, quartz forms mechanical Brazil twins through a well-known mechanism of plastic shear deformation (McLaren et al., 1967).Shock experiments have shown that such mechanical Brazil twins are indicative of a low-pressure regime <17.5 GPa (Fazio et al., 2018).At peak pressures between 10 and 35 GPa shocked quartz displays the formation of planar deformation features (PDFs), which can be described as sets of amorphous lamellae with specific crystallographic orientations (Stöffler & Langenhorst, 1994).Their abundance and crystallographic orientations are widely used to determine the peak pressures of past impacts.
The formation of PDFs through the passage of a shock wave has previously been suggested through different models (Fazio et al., 2018;Goltrant et al., 1992;Grady, 1980;Langenhorst, 1994;v. Engelhardt & Bertsch, 1969).However, quartz also amorphizes through the application of dynamic and static loads.This pressure-induced amorphization of quartz was first attributed to the compression above 25 GPa (Hazen et al., 1989;Hemley et al., 1988) and found to occur in a lamellar morphology (Kingma, Meade, et al., 1993).It was then recognized that crystalline high-pressure phases can form prior to or during amorphization (Kingma et al., 1996;Kingma, Hemley, et al., 1993).To date, numerous different metastable high-pressure phases have been suggested (e.g., Badro & Teter, 1997;Choudhury & Chaplot, 2006;Dubrovinsky et al., 1997;Haines et al., 2001;Liu et al., 1978;Prakapenka et al., 2004;Teter et al., 1998;Tsuchiya & Nakagawa, 2022;Wentzcovitch et al., 1998).For instance, recent shock experiments have indicated the formation of silica with a defective niccolite (NiAs)-structure, in which silicon is disordered in a hexagonally closest packed arrangement of oxygen (Tracy et al., 2020).However, rapid compression experiments in the membrane-driven diamond anvil cell (mDAC) revealed that quartz transforms into a metastable high-pressure polymorphic phase displaying a rosiaite-like structure during compression (Otzen et al., 2023a).This crystal structure provides an explanation why it is not quenchable under ambient conditions and collapses to amorphous silica during decompression.The resulting amorphous lamellae are very similar to PDFs observed in shocked quartz.
Although typical pressures during impacts fall in the stability fields of the high-pressure silica polymorphs coesite and stishovite, these minerals are found only in trace quantities in impact sites (Stöffler, 1971) and meteorites (Miyahara et al., 2014).This can be explained with the substantial atomic rearrangements required for the solid-state formation of coesite and stishovite from quartz.Such reconstructive solid-state phase transformations usually require too much time to occur within the short duration of impacts.The transformations are, however, possible if the temperatures are locally higher than in the bulk of a shocked rock.For example, Kieffer (1971) demonstrated that coesite and stishovite occur as randomly oriented nanocrystalline aggregates in the interstitial pore glass of the Coconino sandstone collected from the Meteor crater.This observation has been interpreted as the formation of the high-pressure polymorphs through crystallization from rapidly cooled silica melt in localized hot spots.The liquid crystallization hypothesis was corroborated by shock experiments on sandstone (Mansfeld et al., 2017) and granite (Hamann et al., 2023).However, early studies (v.Engelhardt & Bertsch, 1969) have argued that stishovite is located in PDFs and is formed from quartz by means of a solid-solid transformation process.The assumption could initially not be substantiated by microstructural studies with the transmission electron microscope (TEM) (Leroux, 2005), but was later confirmed in a set of PDFs through Raman spectroscopy (Stähle et al., 2008).The observations indicate that stishovite might form by an alternative, so far unknown solid-state mechanism in contrast to the proposed melt crystallization upon pressure release during impacts.
The compression of silica in the DAC to pressures above 8 GPa provides another possibility to form stishovite in the laboratory, but this usually requires external heating (Stishov & Popova, 1961) to overcome the energy barrier of the transition and accelerate the transformation kinetics (Haines et al., 2001).Therefore, the identification of nanometer-sized stishovite crystals embedded in amorphous silica (Carl et al., 2017) after DAC compression without external heating corroborates the possibility of an alternative transition mechanism.In both shock and DAC compression of quartz, the occurrence of stishovite always seems to be related to the amorphization of quartz.
Moreover, rapid compression experiments in the mDAC were shown to provide means to simulate the non-hydrostatic and heterogeneous stress conditions of impacts (Carl et al., 2017;Otzen et al., 2023a).Although 10.1029/2023JE008126 3 of 14 the timescales of compression experiments performed in the mDAC are much slower than those of natural impacts, they are still closer to the time scales of large meteorite impacts than laboratory shock experiments (Otzen et al., 2023a).Furthermore, shock temperatures are below 500°C for shock pressures up to approximately 35 GPa (Wackerle, 1962), which are low compared to the melting temperature of silica above 1,700°C (Jackson, 1976;Shen & Lazor, 1995).Shock temperatures can thus be argued to play only a minor role for the transitions of quartz in this pressure regime.Rapid compression experiments in the mDAC may thus represent a reasonable technique for the simulation of impact conditions.
The overall goal of this study is to evaluate the possibility of a direct solid-state transformation of quartz to stishovite during an impact and to understand the required pressures and underlying mechanisms.In our previous study, we compressed single crystals of quartz parallel to the crystallographic c direction (Otzen et al., 2023a).In this direction of highest symmetry in quartz, we identified the rosiaite-structured silica phase forming above 15 GPa, but did not observe the formation of stishovite.The compression direction might, however, be unrepresentative for impacts on the surface of the Earth, because natural targets consist of quartz grains with random crystallographic orientations, where the c axis is inclined to the compression direction.Here, we describe experiments where we choose a compression direction perpendicular to the 1011 plane of quartz, along which there is no symmetry element of the structure present.

Sample Preparation
Similar to the sample preparation described in our previous publication (Otzen et al., 2023a), thin sections of single crystalline quartz (thickness of approx.30 μm) were prepared parallel to the 1011 plane from a natural quartz specimen.A thin layer of gold (thickness of approx.100 nm) was sputtered onto the surface and used as an internal pressure standard.Using a focused ion beam (FIB) dual beam instrument, a total of five disc-shaped samples (diameter of approx.130 μm) were prepared from the coated thin sections.The samples were then loaded into mDACs for the experiments.

Experiments
Single crystal X-ray diffraction with simultaneous uniaxial compression and decompression of the quartz samples was carried out at the Extreme Conditions Beamline P02.2 at the third generation light source PETRA III, DESY, Germany.A monochromatic X-ray beam was tuned to an energy of 25.6 keV (exact wavelengths in Table 1) and focused to a spot size of 8 × 3 μm 2 (Full-Width-Half-Maximum) employing a compound refractive lens system.The images were collected on a Perkin Elmer flat panel detector (PE XRD1621).The sample detector distance and tilt of the detector were calibrated using a CeO 2 standard (NIST 674b) and the DIOPTAS software (Prescher & Prakapenka, 2015).
At the beginning of the compression-decompression experiments, pressure was first increased in small steps with the membrane until slight shifts or broadening of diffraction peaks indicated the start of sample compression.Thereafter, continuous compression at rates between 0.1 and 0.6 GPa/s was used to reach various peak pressures between 20 and 37 GPa (see Table 1  subjected to the peak pressure for approximately 2 min and decompressed thereafter.Due to the strong friction between the piston and cylinder of the DACs, decompression was incomplete for samples #1, #2 and #4.Employing the double-sided mDAC (Sinogeikin et al., 2015), complete decompression was attained for samples #3 and #5.During the entire course of compression, constant peak pressure and decompression, diffraction images were collected using acquisition times of 1 or 2 s.

Preparation of the Recovered Samples for TEM
The samples were recovered after the experiments and prepared using the FIB technique employing the gallium ion gun at the FEI Quanta 3D FEG work station at the Friedrich Schiller University Jena.A section was cut perpendicular to the surface right at the sample center and thinned using ion beam currents between 30 and 0.1 nA and an acceleration voltage of 30 keV.The section was carefully cleaned using a beam current of 48 pA at an acceleration voltage of 5 keV.The final thin sections had sizes of approximately 10 × 20 μm 2 and thicknesses between 100 and 200 nm.Immediately after finishing the preparation, the sections were investigated with the FEI Tecnai G2 FEG TEM operated at an acceleration voltage of 200 kV.

Analysis of the X-Ray Diffraction Patterns
The crystalline phases appearing and disappearing during compression and decompression of the quartz samples were determined using the interplanar spacings and angles of reflections in the two-dimensional diffraction images.Additionally, these images were integrated into diffractograms in order to determine the course of interplanar spacings and peak intensities, and analyze the diffuse scattering of amorphous silica.
The X-ray diffractograms show a high background intensity mainly due to the Compton scattering originating from the diamonds.To determine the background, reference points were set at local minima of each diffractogram and fitted by tenth order polynomial functions using a least squares method.Care was taken to use equal fit parameters and positions of reference points for each data set to reduce changes in the shape of the backgrounds induced by the fitting procedure.Some of the positions of the reference points, however, had to be adapted during compression and decompression due to the shift and broadening as well as appearance and disappearance of diffraction peaks as a function of pressure.
The background-free diffractograms were obtained by subtracting the fitted backgrounds from the measured diffractograms.Using the software package LMFIT (Newville et al., 2014), peak positions and integrated intensities were obtained by fitting the Voigt distribution function to each peak of the background-free diffractograms by minimizing the deviation of the sum curve to the background-free diffractograms with the least squares method.Pressures were calculated from diffraction peaks of the ( 111), ( 200), ( 220), (311), and (222) planes of gold employing the third order Birch-Murnaghan equation of state parameterized by Fei et al. (2007).

In Situ High-Pressure X-Ray Diffraction Experiments
We performed five experiments on single crystals of quartz (Table 1 and Figure S1 in Supporting Information S1) employing rapid and uniaxial compression perpendicular to the 1011 plane in the mDAC.Synchrotron X-ray diffraction images were collected simultaneously during the experiments in order to observe the time-resolved crystallographic changes.A representative diffraction image in Figure 1a displays the single crystal-like reflections of the sample and the diffraction rings of the pressure calibrant gold compressed to 25 GPa.The single crystal-like reflections of the sample are outlined schematically in Figure 1b.For simplification, only the most important reflections with low Miller indices are depicted, providing information on the orientation relationships between phases (Figure 1b).
Only two reflections of the 1011 planes of quartz are present at the beginning of the experiments.When the pressure builts up, we observed large intensity fluctuations as well as the appearance of additional quartz reflections.Both phenomena can be attributed to the non-hydrostaticity of the compression causing strong and varying lattice distortions in the quartz samples (see Otzen et al., 2023a).The evolution of elastic anisotropy of quartz as a function of pressure may also be another factor that influences the stress field in such types of experiments.In the course of compression, the intensities of quartz reflections stabilize until, at the pressure of 18 GPa, new single crystal-like reflections appear at positions that correspond to interplanar spacings of 3.7, 2.7, and 1.7 Å.These reflections can be ascribed to the 1010 , 1011 , and 1012 planes of rosiaite-structured silica, respectively (Otzen et al., 2023a).At slightly higher pressures of 21 GPa, diffraction peaks with interplanar spacings of 2.9 and 1.8 Å appear.The enclosing angles are 90° and 37°/53° between the reciprocal directions.These spacings and angles can be assigned to those of the  {110} and  {120} planes of stishovite viewed along the c axis.All reflections possess similar positions relative to the remaining quartz reflections in all compression experiments performed, indicating specific crystallographic orientation relationships between the observed phases.Besides quartz, rosiaite-structured silica, and stishovite, no reflections could be identified that would indicate the formation of other high-pressure silica polymorphs such as seifertite, which is known to form when the starting phase is α-cristobalite (Černok et al., 2017).The corresponding intensities of the most intense diffraction peaks during compression and decompression are shown in Figure 1c.After the appearance of diffraction peaks from the 1010 and 1012 planes of rosiaite-structured silica, the intensities of the quartz peaks decrease sharply until the peak pressure is reached, indicating that quartz breaks down during further compression.Simultaneously, the intensities of the diffraction peaks of stishovite and rosiaite-structured silica increase sharply.Only the 1011 reflections of rosiaite-structured silica remain weak during the entire experiments.Depending on the reflection, the intensity increase terminates at different pressures or when the peak pressure (between 20 and 37 GPa for the samples #1 to #5) is reached at the latest.At peak pressure, the intensities of all the diffraction peaks remain constant.During decompression of the sample, an apparent partial reappearance of the 1011 reflections of quartz can be observed, whereas the intensities of the stishovite peaks decrease slightly and the peaks associated to rosiaite-structured silica vanish.At the same time, the intensity of the fitted backgrounds increases in the region between 1.5 and 2.0 Å −1 (Figure S2 and S3 in Supporting Information S1).
The interplanar spacings of quartz, rosiaite-structured silica and stishovite calculated from the integrated diffraction patterns decrease during compression and increase during decompression.The only exception is the interplanar spacings of reflections of the 10 10 planes of rosiaite-structured silica that increase in the pressure range between 22 and 29 GPa (Figure 1d), indicating a kind of a relaxation of the rosiaite-structured high-pressure phase.

Transmission Electron Microscopy of Microstructures in Recovered Quartz
In order to investigate the microstructure of the samples after the experiments, cross sections of recovered samples were prepared employing a FIB instrument and immediately thereafter examined with the TEM.In the experiment with the peak pressure of 20 GPa (sample #1), many sets of diffraction contrast fringes containing partial dislocations are observed in quartz (Figure 2) using the bright field (BF) and dark field (DF) modes.In the experiment with the peak pressure of 23 GPa (sample #2), partial amorphization is observed as evident through the presence of sets of planar, amorphous lamellae (Figure 3).The individual lamellae are lenticular, that is, they broaden around the center and taper off toward both ends.Lens widths range from 50 to 150 nm.Differently oriented lamellae terminate at one another.The traces of the most frequently occurring lamellae form angles of 31°, 46°, and 61° with respect to the trace of the 1011 surface of the single crystal sample.In the experiment with the peak pressure of 30 GPa (sample #4), amorphization is more advanced affecting most of the sample volume while islands of quartz remain (Figure 4).Lenticular lamellae are observed resembling in their shape, size and number those described for the experiment with the peak pressure of 23 GPa (sample #2).They form angles of 19°, 26°, 35°, and 60° with the sample surface.In addition, thin amorphous lamellae with a few nanometers in width appear in the interstices between lenticular amorphous lamellae.They occur with a higher abundance and form various angles to the sample surface while dominantly displaying an angle of 25°.
In addition to amorphous lamellae, numerous nanometer-sized objects can be observed in the sample compressed to the peak pressure of 30 GPa (sample #4), appearing as dark patches in the BF images (Figures 4a-4c).The corresponding selected area electron diffraction (SAED) pattern shows the single crystal reflection of quartz and, additionally, single crystal-like reflections compatible with those of stishovite (Figure 4d).Using the reflections of the  (110) planes of stishovite and the 1101 planes of quartz, the DF images reveal that the majority of the dark patches in the BF images can be identified as stishovite.
Large lenticular amorphous lamellae are free of stishovite.Only in the interstices between these stishovite-free amorphous lamellae, stishovite crystals occur within thin amorphous lamellae.Additionally, the stishovite crystals seem to align parallel to the orientation of the thin amorphous lamellae.They display facets of various forms, which gives them the overall appearance of subspherical to ellipsoidal grains with sizes between mostly 10-20 nm.Moreover, the absence of streaks in the X-ray diffraction and SAED images suggests that the stishovite crystals are free of planar defects.

Microstructural Occurrence of Stishovite
The main response of quartz compressed dynamically to high pressures are the transformations to metastable rosiaite-structured silica and stishovite, followed by the formation of amorphous lamellae during decompression.Quartz can transform directly to rosiaite-structured silica (Otzen et al., 2023a), which occurs when the compression exceeds 18 GPa.During decompression, the rosiaite-structured silica collapses to amorphous silica due to its instability at low pressures.The amorphous lamellae (Figure 3a) resemble PDFs observed in natural shock metamorphic quartz (Otzen et al., 2023a).The formation of amorphous silica is corroborated by the observation that during decompression, x-ray diffraction peaks associated with rosiaite-structured silica disappear and a distinct broad peak appears simultaneously in the backgrounds of integrated diffractograms (Figures S2 and S3 in Supporting Information S1).The broad peak is associated with a first sharp diffraction peak (FSDP) typical of diffuse X-ray scattering of amorphous materials (Otzen et al., 2023a;Prescher et al., 2017), because the positions of the FSDP in the Q-space between 1.5 and 2 Å −1 agree well with the positions measured for vitreous silica in the same pressure range (Prescher et al., 2017).
The transformation mechanism to stishovite during the compression of quartz has previously been regarded as too sluggish to occur at room temperature without external heating (Haines et al., 2001).However, this assumption has been questioned by the discovery of stishovite crystals after an mDAC experiment using powder samples of quartz (Carl et al., 2017) and the observations of the present study.The use of single crystalline quartz samples with predefined crystallographic orientations in the present investigation may provide insights into the previously unknown transition mechanisms.The most significant observation is the appearance of stishovite as numerous nanometer-sized and defect-free crystals, which is indicative of nucleation in a strongly undercooled parent phase (Langenhorst & Poirier, 2000).Subsequent free growth is indicated by the development of multiple crystal faces of the stishovite grains.In contrast, previously proposed shear mechanisms for the solid-state stishovite transformation (Martoňák et al., 2007;Stolper & Ahrens, 1987) would lead to planar defects known for the isostructural mineral rutile (Langenhorst & Deutsch, 2012).However, structural defects are not visible in the TEM images of stishovite (Figure 4) and the proposed shear mechanisms (Martoňák et al., 2007;Stolper & Ahrens, 1987) are thus unlikely.
The stishovite crystals are located inside thin amorphous lamellae and completely surrounded by the amorphous phase.This indicates that, during compression, the nucleation occurred inside these lamellae of rosiaite-structured silica because, according to our previous work, the amorphous lamellae represent the breakdown product of rosiaite-structured silica.Furthermore, the absence of palisade structures separated by a central ridge (Langenhorst & Poirier, 2002) rules out nucleation at the interfaces between rosiaite-structured silica and quartz.
These findings imply that increased temperatures must be induced internally during or prior to the formation of stishovite.The heat is required to promote the diffusion of silicon atoms and thus accelerate the reconstructive transition to the ordered stishovite structure.The small, nanometer-sized stishovite crystals indicate then that the grain growth is limited, which might be caused by rapid cooling of the narrow lamellae.This can explain why stishovite crystals are located in the centers of the lamellae, where the cooling is slowest and the time for stishovite growth by silicon diffusion is longest.The most plausible heat source might be the involvement of shear during the formation of stishovite within lamellae through which high temperatures can be generated via friction.Shear processes also occurred in the quartz crystals as demonstrated by the presence of mechanical Brazil twins (Figure 2).This can also explain why stishovite occurs only in specific amorphous lamellae oriented at angles of approximately 45° to the compression direction, corresponding to the directions of maximum resolved shear stress.Another factor, which may also play a role in the development of maximum shear stresses, might be the elastic anisotropy of quartz (Healy et al., 2020).
Finally, the described formation mechanism can explain the evolution of the diffraction peak intensities measured during the experiments (Figure 1c).As stishovite nucleates inside the lamellae of rosiaite-structured silica, the latter phase must appear first, that is, at the pressure of 18 GPa, whereas stishovite appears only thereafter at the slightly increased pressure of 21 GPa.During further compression, the abundances of both phases increase as seen by the increasing intensities.The intensity maximum shown by the rosiaite-structured phase at approximately 27 GPa can be interpreted with a change in transformation rates.The transformation rate from quartz to rosiaite-structured silica declines at around 27 GPa, because a large fraction of the initial quartz is already transformed into rosiaite-structured silica at this pressure.On the contrary, stishovite can continue to nucleate inside the broadly present lamellae of rosiaite-structured silica.In total, this leads effectively to a decrease in the phase fraction of rosiaite-structured silica, as seen through the decreasing intensities above approximately 27 GPa.The intensities stabilize when the formation of stishovite ceases.While the rosiaite-structured phase transforms into amorphous silica during decompression, the stishovite crystals remain intact, as evident through the preserved growth faces (Figure 2b).

Orientation Relationships Between Quartz and High-Pressure Phases and the Origin of Stishovite
Quartz, rosiaite-structured silica and stishovite form in particular crystallographic orientation relationships to each other, as evident in the single crystal-like diffraction images.This observation indicates that high-pressure phases likely formed in the solid state.If the nucleation of stishovite occurred from a high-pressure melt, diffraction rings of random crystallographic orientations would be expected (Ashworth & Schneider, 1985;Mansfeld et al., 2017).
Orientation relationships between quartz and the rosiaite-structured phase have previously been determined from the compression of quartz along the c axis corresponding to the direction of highest symmetry in quartz (Otzen et al., 2023a).The compression direction perpendicular to 1011 , however, does not coincide with a symmetry element.The reduced symmetry seen in the single crystal-like diffraction images thus prevents us from determining the exact crystallographic orientation relationships of the rosiaite-structured phase with quartz.On the contrary, the diffraction patterns indicate a uniform crystallographic orientation of stishovite crystals with the c axis of stishovite parallel to the compression direction and the  {110} planes subparallel to the 1011 and 0111 planes of quartz and the 1010 planes of rosiaite-structured silica.
10.1029/2023JE008126 10 of 14 Because the microstructural observations suggest that stishovite crystals form within lamellae of the rosiaite-structured phase, their identical crystallographic orientation is likely to be inherited from the crystallographic orientations of rosiaite-structured silica.The possibility for an inheritance of the crystallographic orientation from quartz can be tested with the metrical fit between both structures.BF TEM images suggest that one of the major orientations of stishovite-containing lamellae are parallel to the 1010 plane of quartz (Figure 4).Stishovite and quartz must be intergrown at this plane if the crystallographic orientation of stishovite is inherited from quartz.The planes oriented perpendicular to the plane of intergrowth, which are the  (0001) and 2110 of quartz and the 201 and  (100) planes of stishovite, must then exhibit similar interplanar spacings and the arrangement of atoms at the interface must display a good match between the two structures.At the pressure of 26 GPa, the positions of the quartz reflections indicate unit cell parameters of approximately a = 4.6 Å and c = 4.9 Å, from which interplanar distances of 4.9 and 2.3 Å can be calculated for the  (0001) and 2110 planes, respectively.While the stishovite reflections indicate an a parameter of approximately 4.1 Å, the c parameter can be approximated as 2.6 Å if quasi-hydrostatic conditions are considered (Andrault et al., 2003).These values yield interplanar spacings of 1.6 and 4.1 Å for the 201 and  (100) planes, respectively.The disagreement of interplanar spacings of parallel planes between quartz and stishovite thus suggest that the structures of stishovite and quartz match poorly at the suggested 1010 intergrowth plane.This corroborates that the identical crystallographic orientations of the stishovite crystals are probably inherited from the rosiaite-structured phase, agreeing with the observation that stishovite grains occur inside amorphous lamellae without contact with the quartz interface.
In summary, the rapid compression experiments performed in the mDAC on single crystals of quartz suggest a new transformation mechanism to stishovite, which might similarly occur during natural impacts.The transformation processes can be depicted in a chronological order (Figure 5).Lamellae of rosiaite-structured silica are first formed during compression (Figure 5a) above approximately 18 GPa.At slightly increased pressures, the lamellae of rosiaite-structured silica are sheared if the lamellae orientations are close to the angle of maximum resolved shear, that is, at 45°, with respect to the compression direction (Figure 5b).The heat induced through friction initiates the nucleation of stishovite crystals inside the lamellae of rosiaite-structured silica.A high undercooling is then responsible for a high nucleation rate while rapid cooling of the lamellae limits crystal growth, leading to the presence of a large number of nanometer-sized crystals.During decompression, the rosiaite-structured silica collapses to amorphous silica, while the stishovite crystals are preserved metastably down to ambient pressures (Figure 5c).The final microstructure consists of untransformed quartz that is crosscut by amorphous lamellae with different crystallographic orientations, while some amorphous lamellae contain fine-grained stishovite.The abundance and crystallographic orientations of amorphous lamellae are dependent on the peak pressure attained.Above approximately 30 GPa, the quartz samples are completely transformed into the high-pressure rosiaite-structured silica.This model of interplanar formation of stishovite confirms early ideas that PDFs in impact metamorphic quartz may carry stishovite (v.Engelhardt & Bertsch, 1969).
In the context of impact events, the inheritance of an orientation relationship between precursor and high-pressure phases has been observed in a number of systems such as for zircon, monazite and rutile (Erickson et al., 2019;Langenhorst & Deutsch, 2012;Timms et al., 2017).These transformations operate by martensitic shear transformations, which is neither the case for the displacive transition from quartz to rosiaite-structured silica nor for the final formation of stishovite by nucleation and growth from rosiaite-structured silica.Another difference is that no coordination changes occur during the transformation of zircon, monazite, and rutile, while in our system silicon undergoes a coordination change from four-fold to six-fold.To the best of our knowledge, we thus present here a unique example of an inherited orientation relationship with an associated coordination change.Another example could be the cristobalite to seifertite transition, in which cristobalite X-I forms as an intermediate polymorph (Černok et al., 2017).In this case, an orientation relationship is not yet proven.

Relevance of Experimental Observations for Naturally Shocked Quartz
The presented mechanism and chronology of phase transitions can explain the microstructures observed in natural shock metamorphic rock samples from the Ries impact crater (Stähle et al., 2008;Stöffler, 1971;v. Engelhardt & Bertsch, 1969) and meteorites (Miyahara et al., 2014), which are very similar to those obtained in the present compression experiments.In suevites from the Ries crater, for example, PDFs oriented parallel to the 1013 rhombohedral planes of quartz were found to contain fine-grained stishovite ("filled lamellae") (Stöffler, 1971;v. Engelhardt & Bertsch, 1969), whereas differently oriented PDFs contain only amorphous silica.This supports 10.1029/2023JE008126 11 of 14 the observations that stishovite formation is preferred by particular lamellae orientations, and that the crystallographic orientation of quartz with respect to the compression direction determines the possibility for stishovite formation.Finally, it shows that the presented mechanism of stishovite formation might also be applicable to shock compression of quartz.
The BF TEM images also suggest that the amount of stishovite is low compared to the amount of rosiaite-structured silica formed during the compression of quartz.Furthermore, the density of stishovite is only slightly higher than the density of rosiaite-structured silica.Consequently, the density increase through the phase transitions from quartz is mainly driven by the formation of the rosiaite-structured phase.This means that the of stishovite to the density increase in shocked quartz within the mixed phase regime of the Hugoniot curve is negligible.

Figure 1 .
Figure 1.X-ray diffraction data of a quartz single crystal (sample #5) during compression and decompression perpendicular to the 1011 plane.From a (a) representative diffraction pattern measured at 30 GPa, the (b) relative positions of the most important low-index single crystal-like reflections are schematically depicted and assigned to the corresponding phases, that is, quartz (Qtz), stishovite (Stv) and the metastable rosiaite-structured high-pressures phase (ros-SiO 2 ).These reflections provide information on the orientation relationships between the phases.Additional reflections are visible in (a), which can be indexed as 1013 of quartz and 1121 and 2021 of rosiaite-structured silica.As a function of compression and decompression, the (c) intensities of the prominent reflections and the (d) interplanar spacings of the crucial reflections of ros-SiO 2 are shown.The pressure range of substantial stishovite formation and the attained peak pressure are indicated.

Figure 2 .
Figure 2. Mechanical Brazil twins in the quartz sample #1 (peak pressure of 20 GPa) viewed in (a) bright field mode and (b) dark field mode in the transmission electron microscope.The orientations of the sample surface (i.e., 1011 quartz) and the compression direction perpendicular to the 10 11 plane are indicated by line and arrow, respectively.

Figure 3 .
Figure 3. Amorphous lamellae of the quartz sample #2 (peak pressure of 23 GPa).(a) The bright field image shows lenticular amorphous lamellae with different crystallographic orientations, while the (b) corresponding selected area electron diffraction image indicates the crystallographic orientation of the quartz specimen.The orientation of the sample surface (i.e., 1011 in quartz) and the compression direction perpendicular to the 1011 plane are indicated by line and arrow, respectively.

Figure 4 .
Figure 4. Microstructure of the quartz sample #4 (peak pressure of 30 GPa).(a-c)The bright field images show differently oriented amorphous lamellae (bright) in quartz (large dark patches).The vertically oriented, thin amorphous lamellae contain stishovite (nanometer-sized dark patches).The traces of the 1011 sample surface and the compression direction perpendicular to the 1011 plane are indicated by line and arrow, respectively.(d) A selected area electron diffraction image indicates the crystallographic orientations of the quartz specimen and the amorphous lamellae.

Figure 5 .
Figure 5. Sequence of phase transitions involving the formation of rosiaite-structured silica (ros-SiO 2 ), stishovite and glass (amorphous silica) in quartz compressed dynamically in the membrane-driven diamond anvil cell perpendicular to the 1011 plane.(a) Lamellae of rosiaite-structured silica form above 18 GPa.(b) Above 21 GPa, lamellae of rosiaite-structured silica are sheared if their orientations are close to the angle of maximum resolved shear at 45°.The generated heat initiates the nucleation of stishovite inside these lamellae.(c) The rosiaite-structured silica phase collapses to a glass during decompression.The final microstructure consists of untransformed quartz that is crosscut by amorphous lamellae with specific crystallographic orientations.Amorphous lamellae with an orientation prone to shear during compression contain nanometer-sized stishovite crystals.

Table 1 Summary
for compression conditions of individual samples).The samples were of Experimental Parameters Employed for the Compression and Synchrotron X-Ray Diffraction of Quartz Samples in the Membrane-Driven Diamond Anvil Cell and Microstructural Observations of the Recovered Samples