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A natural intermediate member of the scapolite solid solution {Me47; chemical formula: (Na1.86Ca1.86K0.23Fe0.01)(Al4.36Si7.64)O24[Cl0.48(CO3)0.48(SO4)0.01]}, with the unusual I4/mspace group, has been studied at various temperatures and combined high-Tand high-Pby means of in situ single-crystal and powder X-ray diffraction, using both conventional and synchrotron X-ray sources. In addition, single-crystal neutron diffraction data were collected at ambient-Tand 685 °C. A fit of the experimental V-Tdata with a thermal equation of state yielded a thermal expansion coeficient at ambient conditions: αV25°C= 1/V0·(∂V/∂T)P,25°C= 1.74(3)·10–5K–1. A comparative analysis of the elastic behavior of scapolite based on this study and previous high-TXRD data suggests that a thorough re-investigation of the diferent members of the marialite-meionite solid solution is needed to fully understand the role of crystal chemistry on the thermal behavior of these complex nonbinary solid solutions. The experimental data obtained within the full temperature range of analysis at ambient pressure confirm that the investigated sample always preserves the I4/mspace group, and possible implications on the metastability of I4/mintermediate scapolite are discussed. Neutron difraction data show that no significant Si and Al rearrangement among the Tsites occurs between 25 and 685 °C. The combined high-Tand high-Pdata show that at 650 °C and between 10.30(5) and 10.71(5) GPa a phase transition toward a triclinic polymorph occurs, with a positive Clapeyron slope (i.e., dP/dT> 0). A comprehensive description of the atomic-scale structure deformation mechanisms induced by temperature and/or pressure, including those leading to structural instability, is provided based on single-crystal structure refinements.

Dense hydrous magnesium silicates (DHMSs) with large water contents and wide stability fields are a potential H2O reservoir in the deep Earth. Al-bearing superhydrous phase B (shy-B) with a wider stability field than the Al-free counterpart can play an important role in understanding H2O transport in the Earth’s transition zone and topmost lower mantle. In this study, a nominally Al-free and two different Al-bearing shy-B samples with 0.47(2) and 1.35(4) Al atoms per formula unit (pfu), were synthesized using a rotating multi-anvil press. Their single-crystal structures were investigated by X‑ray diffraction (XRD) complemented by Raman spectroscopy and Fourier-transform infrared spectroscopy (FTIR). Single-crystal XRD shows that the cell parameters decrease with increasing Al-content. By combining X‑ray diffraction and spectroscopy results, we conclude that the Al-poor shy-B crystallizes in the Pnn2 space group with hydrogen in two different general positions. Based on the results of the single-crystal X‑ray diffraction refinements combined with FTIR spectroscopy, three substitutions mechanisms are proposed: 2Al3+= Mg2++ Si4+; Mg2+= □Mg2++ 2H+(□Mg2+means vacancy in Mg site); Si4+= Al3++ H+. Thus, in addition to the two general H positions, hydrogen is incorporated into the hydrous mineral via point defects. The elastic stiffness coefficients were measured for the Al-shy-B with 1.35 pfu Al by Brillouin scattering (BS). Al-bearing shy-B shows lower C11, higher C22, and similar C33when compared to Al-free shy-B. The elastic anisotropy of Al-bearing shy-B is also higher than that of the Al-free composition. Such different elastic properties are due to the effect of lattice contraction as a whole and the specific chemical substitution mechanism that affect bonds strength. Al-bearing shy-B with lower velocity, higher anisotropy, and wider thermodynamic stability can help understand the low-velocity zone and the high-anisotropy region in the subducted slab located in Tonga.

Der Einfluss von Wasser auf die Dynamik von Planeten wie der Erde oder Neptun sind nach wie vor nur sehr rudimentär verstanden. So wird Wasser durch wasserhaltige Minerale mittels Subduktionszonen ins Erdinnere transportiert, ohne dass klar ist, ob es dort gespeichert wird oder aber durch Vulkanismus wieder an die Oberfläche gelangt. Experimente mit Hochdruckpressen erlauben es, extreme Bedingungen im Erdinneren nachzustellen und die dortigen Prozesse durch Untersuchung der physikalischen Eigenschaften mittels neuester Röntgentechnik zu entschlüsseln. Auch die innere Struktur vieler Himmelskörper ist nach wie vor ungeklärt. Die Frage, ob es unterirdische Ozeane auf Eismonden gibt oder feste Schichten von verschiedenen Eispolymorphen, ist genauso ungeklärt wie die Frage, ob das Magnetfeld der Eisriesen durch Eis bei Ultrahochdruck entsteht. Noch viel weniger wissen wir über die Prozesse im Eismantel der interstellaren Staubkörner. Auch wenn es nur kleine, ferne Körner sein mögen, liegen viele Antworten auf die Rätsel des Lebens und der Evolution in der Chemie dieser amorphen Eisschichten verborgen. Welche Rolle Wasser in der Dynamik des Inneren von Erde und Planeten sowie in vielen astrophysikalischen Prozessen spielt, ist noch weitgehend unklar. Dies liegt hauptsächlich an der Unzugänglichkeit solcher Orte. Daher kommt Laborexperimenten, die extreme Bedingungen im Inneren von Planeten und im Weltall zugänglich machen, eine hohe Bedeutung zu. In Kombination mit modernen Methoden zur Aufklärung der Struktur und Dynamik erlauben diese Methoden, Befunde aus der Ferne zu treffen. Bild: NASA/JPL‐Caltech

The synthesis of a novel type of carbonate, namely of the inorganic pyrocarbonate salt Sr[C2O5], which contains isolated [C2O5]2–-groups, significantly extends the crystal chemistry of inorganic carbonates beyond the established sp2- and sp3-carbonates. We synthesized Sr[C2O5] in a laser-heated diamond anvil cell by reacting Sr[CO3] with CO2. By single crystal synchrotron diffraction, Raman spectroscopy, and density functional theory (DFT) calculations, we show that it is a pyrocarbonate salt. Sr[C2O5] is the first member of a novel family of inorganic carbonates. We predict, based on DFT calculations, that further inorganic pyrocarbonates can be obtained and that these will be relevant to geoscience and may provide a better understanding of reactions converting CO2into useful inorganic compounds.

We have synthesized the orthocarbonate Sr3[CO4]O in a laser-heated diamond anvil cell at 20 and 30 GPa by heating to ≈3000 (300) K. Afterward, we recovered the orthocarbonate with [CO4]4–groups at ambient conditions. Single-crystal diffraction shows the presence of [CO4]4–groups, i.e., sp3-hybridized carbon tetrahedrally coordinated by covalently bound oxygen atoms. The [CO4]4–tetrahedra are located in a cage formed by corner-sharing OSr6octahedra, i.e., octahedra with oxygen as a central ion, forming an antiperovskite-type structure. At high pressures, the octahedra are nearly ideal and slightly rotated. The high-pressure phase is tetragonal (I4/mcm). Upon pressure release, there is a phase transition with a symmetry lowering to an orthorhombic phase (Pnma), where the octahedra tilt and deform slightly.

The thermal conductivity of bridgmanite, the primary constituent of the Earth's lower mantle, has been investigated using diamond anvil cells at pressures up to 85 GPa and temperatures up to 3,100 K. We report the results of time‐domain optical laser flash heating and X‐ray Free Electron Laser heating experiments from a variety of bridgmanite samples with different Al and Fe contents. The results demonstrate that Fe or Fe,Al incorporation in bridgmanite reduces thermal conductivity by about 50% in comparison to end‐member MgSiO3at the pressure‐temperature conditions of Earth's lower mantle. The effect of temperature on the thermal conductivity at 28–60 GPa is moderate, well described as k=k300(300/T)a${k={k}_{300}(300/T)}^{a}$, where ais 0.2–0.5. The results yield thermal conductivity of 7.5–15 W/(m × K) in the thermal boundary layer of the lowermost mantle composed of Fe,Al‐bearing bridgmanite. Heat transport from the Earth's core and mantle to the surface drives plate tectonics and is crucial for sustaining the magnetic field which shields the surface from the solar wind. To quantify the heat transport process across the core‐mantle boundary layer, it is important to know thermal conductivity of major constituent minerals of the lower mantle in the region. Bridgmanite, which was called silicate perovskite, is the most abundant mineral in the lower mantle. Here we measured thermal conductivity on lab‐grown bridgmanite with different Fe and Al compositions compressed at the tips of two opposing diamonds to reproduce relevant pressures in the mantle. To obtain thermal conductivity, we applied optical and X‐ray Free Electron Lasers combined with optical spectroscopy and X‐ray diffraction to heat and measure time‐dependent temperature distributions of the sample. Our study provides relevant high pressure‐temperature data sets to better constrain the heat flux across the core‐mantle boundary. We measured thermal conductivity of Fe,Al‐bearing bridgmanite, the most abundant mineral in the Earth's lower mantle, up to 85 GPa and 3,100 KFinite‐element calculations to temperatures obtained from laser flash and X‐ray Free Electron Laser heating measurements are fitted to evaluate temperature effect on conductivityWe assessed pressure, temperature, composition effects on thermal conductivity of bridgmanite at the thermal boundary layer of the lowermost mantle We measured thermal conductivity of Fe,Al‐bearing bridgmanite, the most abundant mineral in the Earth's lower mantle, up to 85 GPa and 3,100 K Finite‐element calculations to temperatures obtained from laser flash and X‐ray Free Electron Laser heating measurements are fitted to evaluate temperature effect on conductivity We assessed pressure, temperature, composition effects on thermal conductivity of bridgmanite at the thermal boundary layer of the lowermost mantle

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. The extreme pressures and temperatures of asteroid impacts on planetary surfaces result in irreversible changes in the affected rocks. The mineral quartz can then display characteristic amorphous lamellae and crystals of the high‐pressure mineral stishovite. While stishovite usually crystallizes from locally molten silica under pressure, its formation in the solid state free of silica melts has remained enigmatic to date. Here, we simulated asteroid impacts on the time scale of seconds and at ambient temperature through experiments of rapid compression and decompression of quartz. The crystallographic changes during the experiments and the microstructural observations of the recovered samples provide evidence that stishovite is indeed formed in the solid state during compression of quartz. In particular, a large number of stishovite crystals form in an identical crystallographic orientation through nucleation from the structurally similar rosiaite‐structured silica phase. The latter is formed metastably during compression. The final microstructure consists of the numerous stishovite crystals located in amorphous lamellae, because the metastable rosiaite‐structured phase transforms into amorphous silica during decompression. The discoveries describe a novel mechanism for rapid stishovite formation in the solid state, explaining very similar stishovite occurrences in meteorites and rock samples recovered from impact sites. Stishovite can form in the solid state from compressed quartz through nucleation from a transient rosiaite‐structured silica phaseThe solid state transition from quartz to finally stishovite results in a uniform orientation relationshipShock‐metamorphic features in quartz can be mimicked by compression in the diamond anvil cell Stishovite can form in the solid state from compressed quartz through nucleation from a transient rosiaite‐structured silica phase The solid state transition from quartz to finally stishovite results in a uniform orientation relationship Shock‐metamorphic features in quartz can be mimicked by compression in the diamond anvil cell

Saline water is a common fluid on the Earth‘s surface and in ice planets. Potassium chloride (KCl) is a common salt and is expected to be a ubiquitous solute in salt water in the Universe; however, few studies investigated the behavior of KCl‐H2O system at high pressures and temperatures. In this study, powder and single‐crystal X‐ray diffraction (SC‐XRD), Raman and Brillouin scattering combined with diamond anvil cells were used to investigate the phase relation in the KCl‐H2O system for different KCl concentrations at 0–4 GPa and 298–405 K. The results of powder X‐ray diffraction and Raman scattering demonstrate that a novel KCl hydrate is formed when KCl aqueous solutions transform to solid ice‐VI and ice‐VII at high pressure. Simultaneously, the single‐crystal of KCl hydrate is synthesized from a supersaturated KCl solution at 298 K and 1.8 GPa. The structure is solved by SC‐XRD, indicating a KCl monohydrate with the P21/nspace group is formed. We have verified the phase stability of KCl monohydrate by using Raman spectroscopy and density functional theory. Our results indicate that KCl monohydrate is a stable phase under pressure and temperature conditions between 1.6 and 2.4 GPa and 298–359 K. By considering the thermal profile and composition of icy moons, we hypothesize that the formation and decomposition of KCl monohydrate might induce mantle convection in these moons. More and more evidence indicates that salt ions (Na+, Ca2+, Mg2+, K+, Cl−, and SO42−${\text{SO}}_{4}^{2-}$, etc.) could be dissolved in water and ices of the Earth‐like planets and water‐rich planets. The dissolution of alkali‐ and alkaline earth chlorides and sulfates in water and the incorporation of salt ions in the solid phases of H2O ices have a significant effect on their chemical and physical properties. Potassium chloride (KCl) is a ubiquitous salt in the Universe, and we investigated the phase relationship of the KCl‐H2O system for different KCl concentrations at pressures of 0–4 GPa and temperatures of 298–405 K. We were able to produce a single crystal of KCl monohydrate by adding KCl into a KCl‐saturated solution and pressurizing the solution at 1.8 GPa and 298 K. Our findings reveal the existence of a novel KCl monohydrate phase with monoclinic structure, which remains stable within the pressure and temperature range of 1.6–2.4 GPa and 298–359 K. Considering the thermal profile and composition of icy moons, our results suggest that the stability of the KCl monohydrate phase at pressures of 1.6–2.4 GPa and temperatures of 298–359 K may drive mantle convection through its formation and decomposition. A novel KCl monohydrate is synthesized in KCl‐H2O system at high pressuresStructure, phase stability, and density of KCl monohydrate are determined at high pressures and temperaturesKCl monohydrate formation and decomposition potentially drive mantle convection in icy moons A novel KCl monohydrate is synthesized in KCl‐H2O system at high pressures Structure, phase stability, and density of KCl monohydrate are determined at high pressures and temperatures KCl monohydrate formation and decomposition potentially drive mantle convection in icy moons

The stishovite to post‐stishovite phase transition may modify the scattering of seismic waves by stishovite‐bearing rocks in the Earth's lower mantle. A series of continuous compression experiments on sintered polycrystalline stishovite was performed to study the effect of stress on the phase transition. The experimental results show that the phase transition shifts to lower pressures as the magnitude of deviatoric stress increases. Our results further show that the bulk modulus of sintered polycrystalline stishovite differs from that derived from single crystal measurements and decreases at the phase transition. In cold regions, such as subducted slabs, stresses may accumulate and shift the phase transition to a shallower depth. In hot regions with less stress, such as rising plumes, the phase transition is shifted to a greater depth. In addition, the phase transition may have varying seismic signatures depending on the behavior of the grain boundaries in mantle rocks and the micro‐stresses present in neighboring grains. Stishovite and post‐stishovite are high‐pressure polymorphs of SiO2that are stable in the Earth's lower mantle. The stishovite to post‐stishovite phase transition may affect the way seismic waves travel through mantle rocks. To detect SiO2phases in the deep mantle, it is essential to investigate the depth of the phase transition and its effects on seismic wave propagation. Experiments were conducted to simulate the high‐pressure environment of the lower mantle and to study the phase transition of sintered polycrystalline stishovite aggregates, in which crystals are firmly connected to each other. Our study finds that the phase transition is sensitive to deviatoric stress, which is a measure of the directional stress difference. An increase in deviatoric stresses shifts the phase transition to shallower depths in the mantle. We further find that at pressures below the phase transition, sintered polycrystalline stishovite aggregates exhibit lower compressibility compared to stishovite powder and single crystals, but the compressibility increases abruptly at the phase transition, probably caused by grain‐grain interactions in the sintered samples. Therefore, the behavior of seismic waves will be different when passing through stishovite‐bearing rocks with different degree of grain‐grain interactions, particularly under conditions close to the phase transition boundary. Continuous compression experiments were performed to study the post‐stishovite phase transition under different stress conditionsDeviatoric stresses shift the phase transition to a shallower depth in the lower mantleThe bulk modulus of sintered polycrystalline stishovite differs from that of stishovite powder and drops at the phase transition Continuous compression experiments were performed to study the post‐stishovite phase transition under different stress conditions Deviatoric stresses shift the phase transition to a shallower depth in the lower mantle The bulk modulus of sintered polycrystalline stishovite differs from that of stishovite powder and drops at the phase transition

In this work, the hydrogen absorption and desorption properties of the system K2[Zn(NH2)4]-8LiH are investigated in detail via in situ synchrotron radiation powder X-ray diffraction (SR-PXD), Fourier transform infrared spectroscopy (FT-IR), and volumetric methods. Upon milling, K2[Zn(NH2)4] and 8LiH react to form 4LiNH2-4LiH-K2ZnH4, and then 4LiNH2-4LiH-K2ZnH4releases H2in multiple steps. The final products of the desorption reaction are KH, LiZn13, and Li2NH. During rehydrogenation, KH reacts with LiZn13under 50 bar of hydrogen producing K3ZnH5. This phase appears to enhance the hydrogenation process which after its formation at ca. 220 °C takes place in only 30 s. The system 4LiNH2-4LiH-K2ZnH4is shown to be reversible under the applied conditions of vacuum at 400 °C for desorption and 50 bar of H2at 300 °C for absorption.

Skiagite-Fe-majorite garnets were synthesized using a multianvil apparatus at 7.5–9.5 GPa and 1400–1600 K. Single-crystal X-ray diffraction at ambient conditions revealed that synthesized garnets contain 23 to 76% of an Fe-majorite component. We found that the substitution of Fe2+and Si4+for Fe3+in the octahedral site decreases the unit-cell volume of garnet at ambient conditions. Analysis of single-crystal X-ray diffraction data collected on compression up to 90 GPa of garnets with different compositions reveals that with increasing majorite component the bulk modulus increases from 159(1) to 172(1) GPa. Our results and literature data unambiguously demonstrate that the total iron content and the Fe3+/Fe2+ratio in (Mg, Fe)-majorites have a large influence on their elasticity. At pressures between 50 and 60 GPa we observed a significant deviation from a monotonic dependence of the molar volumes of skiagite-Fe-majorite garnet with pressure, and over a small pressure interval the volume dropped by about 3%. By combining results from single-crystal X-ray diffraction and high-pressure synchrotron Mössbauer source spectroscopy we demonstrate that these changes in the compressional behavior are associated with changes of the electronic state of Fe in the octahedral site.

The 3.65 Å phase [MgSi(OH)6] is likely to be formed by decomposition of the hydrous 10 Å phase [Mg3Si4O10(OH)2·H2O] at pressures of 9-10 GPa. In this study, we use a combination of X-ray diffraction and first-principles simulations to constrain the equation of state and elasticity of the 3.65 Å phase. We find that the equation of state results for the 3.65 Å phase, from X-ray diffraction data are well represented by a third-order Birch-Murnaghan formulation, with K0= 83.0 (±1.0) GPa, K0′ = 4.9 (±0.1), and V0= 194.52 (±0.02) Å3. Based on the first-principles simulations, the full single-crystal elastic constant tensor with monoclinic symmetry shows significant anisotropy with the compressional c11= 156.2 GPa, c22= 169.4 GPa, c33= 189.3 GPa, the shear components c44= 55.9 GPa, c55= 58.5 GPa, c66= 74.8 GPa, and c46= 1.6 GPa; the off-diagonal components c12= 38.0 GPa, c13= 26.5 GPa, c23= 22.9 GPa, c15= 1.5 GPa, c25= 1.5 GPa, and c35= -1.9 GPa at zero pressure.

The high-pressure behavior of a synthetic mordenite-Na (space group: Cmcmor Cmc21) was studied by in situ single-crystal synchrotron X-ray diffraction with a diamond anvil cell up to 9.22(7) GPa. A phase transition, likely displacive in character, occurred between 1.68(7) and 2.70(8) GPa, from a C-centered to a primitive space group: possibly Pbnm, Pbnnor Pbn21. Fitting of the experimental data with III-BM equations of state allowed to describe the elastic behavior of the high-pressure polymorph with a primitive lattice. A very high volume compressibility [KV0= 25(2) GPa, βV0= 1/KV0= 0.040(3) GPa–1; KV′= (?KV/?P)T= 2.0(3)], coupled with a remarkable elastic anisotropy (βb>>βc>βa), was found. Interestingly, the low-Pand high-Ppolymorphs show the same anisotropic compressional scheme. A structure collapse was not observed up to 9.22(7) GPa, even though a strong decrease of the number of observed reflections at the highest pressures suggests an impending amorphization. The structure refinements performed at room-P, 0.98(2) and 1.68(7) GPa allowed to describe, at a first approximation, the mechanisms that govern the framework deformation in the low-Pregime: the bulk compression is strongly accommodated by the increase of the ellipticity of the large 12-membered ring channels running along [001].

AbstractThe high-pressure crystal chemistry of coesite was studied by means of single crystal X-ray diffraction in the pressure interval ∼2–34 GPa and at ambient temperature. We compressed the samples using diamond-anvil cells loaded with neon as pressure-transmitting medium and collected X-ray diffraction data using synchrotron radiation. The thermodynamically stable coesite – coesite-I – was observed up to ∼20 GPa, with the following unit-cell parameters: a = 6.6533(12) Å, b = 11.9018(10) Å, c = 6.9336(10) Å, β = 121.250(20)° and V = 469.38(15) Å3. The volume-pressure data of coesite-I are described by means of a third-order Birch-Murnhagan EoS with parameters V0= 547.26(66) Å3, KT0= 96(4) GPa, K′To${K'_{To}}$= 4.1(4). Above such pressure we witness the formation of a well crystallized coesite-II, previously observed only by spectroscopic studies. The structure of the novel high-Ppolymorph was determined and refined at ∼28 and ∼31 GPa with final Rindices of 8% and 12%, respectively. Coesite-II has P21/n symmetry and a unit cell that is “doubled” along the b-axis with respect to that of the initial coesite-I: a = 6.5591(10) Å, b = 23.2276(14) Å, c = 6.7953(9) Å, β = 121.062(19)° and V = 886.84(19) Å3at ∼28 GPa. All Si atoms are in tetrahedral coordination. The displacive phase transition I->II is likely driven by the extreme shortening (0.05 Å or 3.2%) of the shortest and the most compressible Si1-O1 bond, related to the stiff 180° Si1-O1-Si1 angle. Under compression the linear angle bends, resulting in two independent angles, one of which, however, retains almost linear geometry (∼178°). The requirement of this angle to be close to linear likely causes further Si-O compression down to an extremely short distance of ∼1.52 Å which prompts subsequent structural changes, with the formation of a triclinic phase at ∼31 GPa, coesite-III.

High-pressure behavior of coesite was studied on single crystals using diamond-anvil cells with neon as the pressure-transmitting medium by means of in situ Raman spectroscopy up to pressures of ~51 GPa. The experimental observations were complemented with theoretical computations of the Raman spectra under similar pressure conditions. We find that coesite undergoes two phase transitions and does not become amorphous at least up to ~51 GPa. The first phase transition (coesite I to coesite II) is reversible and occurs around 23 GPa. The second transition (coesite II to coesite III) at about 35 GPa is also reversible but involves a large hysteresis. Samples recovered from the highest pressure achieved, ~51 GPa, show Raman spectra of the initial coesite. The ab initio calculations gave insight into the initiation mechanism of the first phase transition, implying, from the analysis of unstable phonon modes, that it is probably a displacive phase transition due to shearing of the four-membered rings of SiO4 tetrahedra upon compression. The transition to the lowest-symmetry phase, coesite III, is possibly a first-order phase transition that leads to a very distinct structure. None of the metastable high-pressure phases of coesite has been previously studied and it was widely accepted that coesite undergoes pressure-induced amorphization at significantly lower pressures (30 GPa). The study of the high-pressure behavior of coesite is important to better constrain the metastable phase diagram of silica. Further crystallographic investigations are necessary for characterizing the structures of these metastable coesite forms. Crystalline or amorphous metastable phases derived from coesite under high-pressure conditions are of particular interest because they can be used as potential tracers of peak transient pressures (stress) reached in processes such as impacts or faulting.