Shock-induced growth and metastability of stishovite and coesite in lithic clasts from suevite of the Ries impact crater (Germany)

The microtextures of stishovite and coesite in shocked non-porous lithic clasts from suevite of the Ries impact structure were studied in transmitted light and under the scanning electron microscope. Both high-pressure silica phases were identified in situ by laser-Raman spectroscopy. They formed from silica melt as well as by solid-state transformation. In weakly shocked rocks (stage I), fine-grained stishovite (≤1.8 μm) occurs in thin pseudotachylite veins of quartz-rich rocks, where it obviously nucleated from high-pressure frictional melts. Generally no stishovite was found in planar deformation features (PDFs) within grains of rock-forming quartz. The single exception is a highly shocked quartz grain, trapped between a pseudotachylite vein and a large ilmenite grain, in which stishovite occurs within two sets of lamellae. It is assumed that in this case the small stishovite grains formed by the interplay of conductive heating and shock reverberation. In strongly shocked rocks (stages Ib–III, above ∼30 GPa), grains of former quartz typically contain abundant and variably sized stishovite (<6 μm) embedded within a dense amorphous silica phase in the interstices between PDFs. The formation of transparent diaplectic glass in adjacent domains results from the breakdown of stishovite and the transformation of the dense amorphous phase and PDFs to diaplectic glass in the solid state. Coesite formed during unloading occurs in two textural varieties. Granular micrometre-sized coesite occurs embedded in silica melt glass along former fractures and grain boundaries. These former high-pressure melt pockets are surrounded by diaplectic glass or by domains consisting of microcrystalline coesite and earlier formed stishovite. The latter is mostly replaced by amorphous silica.     

Quartz paramorphs after former stishovite in UHP eclogite from the South Altyn Tagh,Western China and its Significance

The long prism/needle‐shaped polycrystalline quartz aggregates and square/parallelogram‐shaped singlephase quartz inclusions in omphacite and garnet of ultrahigh pressure eclogite were first discovered from the Jiangalesayi area, South Altyn UHP belt. Based on their morphology, these quartz inclusions are quartz paramorphs after stishovite. The minimum peak pressure of the eclogite is estimated to be >8–9 GPa at 800– 1000 °C based on the stability field of stishovite. This new evidence, together with previous stishovite exsolution microstructure in the gneiss from the same region, suggests an ultra‐deep subduction and exhumation of the South Altyn continental rocks to/from mantle depths in stishovite stability field. Evidence of ultra‐deep subduction of continental materials might be more common and diverse than previous thought. Exhumation of subducted continental rocks from≥300 km has been considered impossible because they are denser than mantle at these depths. How did the stishovite bearing continental rocks of the South Altyn exhumated? As we all know, the densities of stishovite (4.3 g/cm³) are much higher than coesite (2.9 g/cm³), and stishovite transforms into coesite with temperature increases. Density calculations were performed for subducted continental rocks along phase transition of stishovite to coesite, using the third‐order Birch‐Murnaghan equation of state based on mineral fractions obtained from experiments and Perple_X. The results show that the density of Siliceous rocks decrease remarkably, lower than the surrounding mantle in coesite stability field, whereas the density of Oligosiliceous and Silicon unsaturated rocks is higher than surrounding mantle. Thus, we propose that the thermal induced transformation could provide an initial driven force for the exhumation of ultra‐deep subducted silica‐enriched felsic continental rocks. Temperature increase could be derived from an increased geothermal gradient from convective mantle or mantle plume. Mafic to ultra‐mafic rocks and silica‐deficient rocks may be captured by the upwelling subducted continental rocks and exhumated together.     

蛇绿岩地幔岩中自由SiO2的发现及其地质意义

自由SiO_2系指石英及其同质多型物(polymorphs)柯石英、斯石英等。石英广泛分布于地壳中的各种岩石中,柯石英和斯石英只存在于超高压岩石和陨石坑中。由于石英和非饱和SiO_2的橄榄石不能共生,因此在地幔橄榄岩和超镁铁岩中不存在原生石英。最近笔者在西藏罗布莎蛇绿岩的地幔岩(方辉橄榄岩)的豆荚状铬铁矿中发现了自由SiO_2和柯石英相。根据高温高压相平衡实验资料,橄榄石、辉石这样的硅酸盐矿物在地幔深部的压力条件下可以分解成简单氧化物,如FeO(方铁矿)、MgO(方镁石)以及SiO_2(斯石英)等。由此推测,西藏蛇绿岩地幔岩中自由SiO_2可能是来自于下地幔的矿物,是地幔柱作用将其搬运到上地幔浅部。     

Calorimetric study of the coesite-stishovite transformation and calculation of the phase boundary

High temperature drop-solution calorimetry in molten 2 PbO · B₂O₃ at 1044 K for coesite and stishovite polymorphs of silica was carried out to determine the enthalpy of the coesite-stishovite transition. These experiments were performed on high-purity, single-phase samples of coesite and stishovite. Our new value for the enthalpy of the coesitestishovite transition (ΔH ₂₉₈ ⁰ ) is 29.85 ± 0.78 kJ/mol, which is about 35% lower than previously reported by Akaogi and Navrotsky (1984) and Holm et al. (1967), but which compares well with new measurements by Akaogi et al. (1994b). Using these new data, we have calculated the equilibrium phase boundary between coesite and stishovite and obtained a slope, dP/dT=0.0031 (2) GPa/K. This calculated slope is in good agreement with that determined [0.0026 (2) GPa/K] from the in-situ X-ray diffraction study of Zhang et al. (1996).     

Fe2N-type SiO2 from shocked quartz

The modified niccolite structure (Fe₂N-type) of SiO₂, previously found in diamond anvil experiments at 35 to 40 GPa, was formed in a porous mixture of crystalline α-quartz and copper powder at shock pressures estimated at 12 to 27 GPa. It is suggested that quartz can invert during shock compression not only to coesite, stishovite and an amorphous or glass phase of silica, but also to Fe₂N-type SiO₂, depending upon the shock history.     

Raman spectra of high-pressure polymorphs of SiO2 at various temperatures

Raman spectra of the two high-pressure polymorphs of SiO₂ (coesite and stishovite) were investigated in the temperature range 105–875 K at atmospheric pressure. Coesite remained intact after the highest temperature run, but stishovite became amorphous at temperatures above about 842～872 K. Most Raman modes exhibit a negative frequency shift with temperature for these polymorphs, but positive trends were also observed for some modes. Except for some weak modes, nonlinear temperature variation were established for these polymorphs within the experimental uncertainty and temperature range spanned. The slopes of the variation (δv_i/δT)_P for these polymorphs were compared with the published values. When compared with quartz and stishovite, the four-membered rings of SiO₄-tetrahedra in coesite exhibit very little change with both temperature and pressure. It is also suggested that temperature and pressure should have opposite effects on the Raman shift of each vibrational mode.     

石英高压相变研究进展

文中总结了前人有关石英高温高压相变的实验结果。根据以前的实验,在静水压条件下,石英-柯石英-斯石英-CaCl2结构超斯石英相-α-PbO2结构超斯石英相之间的相变方程分别是:p(GPa)=(2.11±0.03)+(9.8×10-4±1.2×10-4)×T(℃),p(GPa)=(8.0±0.2)+(1.1×10-3±3×10-4)×T(℃),p(GPa)=(51±2)+(0.012±0.005)×T(K),p(GPa)=98+(0.0095±0.0016)×T(K)。文中还初步探讨了非静水压状态对石英相变的影响。实验结果表明,差应力的出现降低了石英相变所需要的围压,即相变边界向低压方向偏移,在周永胜等人实验数据的基础上,笔者尝试将二维的相图扩展到三维相图以考虑差应力的影响。最后讨论了石英相变在地学研究中的作用,对比不同的观点分析了前人对超高压变质作用过程的解释,希望可以为以后解释地质资料提供较为广泛的可能性,促进我们对地球内部动力学过程的了解。     

A mapping of the electron localization function for the silica polymorphs: evidence for domains of electron pairs and sites of potential electrophilic attack

The electron localization function, η, evaluated for first-principles geometry optimized model structures generated for quartz and coesite, reveals that the oxide anions are coordinated by two hemispherically shaped η-isosurfaces located along each of the SiO bond vectors comprising the SiOSi angles. With one exception, they are also coordinated by larger banana-shaped isosurfaces oriented perpendicular to the plane centered in the vicinity of the apex of each angle. The hemispherical isosurfaces, ascribed to domains of localized bond-pair electrons, are centered ～0.70 Å along the bond vectors from the oxide anions and the banana-shaped isosurfaces, ascribed to domains of localized nonbonding lone-pair electrons, are centered ～0.60 Å from the apex of the angle. The oxide anion comprising the straight SiOSi angle in coesite is the one exception in that the banana-shaped isosurface is missing; however, it is coordinated by two hemispherically shaped isosurfaces that lie along the bond vectors. In the case of a first-principles model structure generated for stishovite, the oxide anion is coordinated by five hemispherically shaped η-isosurfaces, one located along each of the three SiO bond vectors (ascribed to domains of bonding-electron pairs) that are linked to the anion with the remaining two (ascribed to domains of nonbonding-electron pairs) located on opposite sides of the plane defined by three vectors, each isosurface at a distance of ～0.5 Å from the anion. The distribution of the five isosurfaces is in a one-to-one correspondence with the distribution of the maxima displayed by experimental Δρ and theoretical ??²ρ maps. Isosurface η maps calculated for quartz and the (HO) ₃ SiOSi(OH) ₃ molecule also exhibit maxima that correspond with the (3,?3) maxima displayed by distributions of ??²ρ. Deformation maps observed for the SiOSi bridges for the silica polymorphs and a number of silicates are similar to that calculated for the molecule but, for the majority, the maxima ascribed to lone-pair features are absent. The domains of localized nonbonding-electron pair coordinating the oxide anions of quartz and coesite provide a basis for explaining the flexibility and the wide range of the SiOSi angles exhibited by the silica polymorphs with four-coordinate Si. They also provide a basis for explaining why the SiO bond length in coesite decreases with increasing angle. As found in studies of the interactions of solute molecules with a solvent, a mapping of η-isosurfaces for geometry-optimized silicates is expected to become a powerful tool for deducing potential sites of electrophilic attack and reactivity for Earth materials. The positions of the features ascribed to the lone pairs in coesite correspond with the positions of the H atoms recently reported for an H-doped coesite crystal.     

Calculated thermodynamic properties of silica polymorphs

Accurate interatomic potentials have been employed to compute the phonon density of states of αquartz, stishovite and coesite polymorphs of silica. The temperature variation of several thermodynamic properties is calculated by using the phonon density of states to describe the vibrational entropy contribution to the free energy. Results for these polymorphs are in surprisingly good agreement with available experimental data. Moreover, the microscopic origin of quantitative differences in the heat capacity behavior of low and high density polymorphs is established.     

Prograde evolution of Sulu UHP metamorphic rock in Yangzhuang,Junan region,deduced by combined Raman and petrological studies

The prograde metamorphic history of the Sulu ultrahigh‐pressure metamorphic terrane has been revealed using Raman‐based barometry of the SiO₂ phases and other mineral inclusions in garnet porphyroblasts of a coesite eclogite from Yangzhuang, Junan region, eastern China. Garnet porphyroblasts have inner and outer segments with the boundary being marked by discontinuous changes in the grossular content. In the inner segment, the SiO₂ phase inclusions are α‐quartz with no coesite or relict features such as radial cracks. The residual pressures retained by the quartz inclusions systematically increase from the crystal centre to the margin of the inner segment. The metamorphic conditions estimated by calculation from the residual pressure and conventional thermodynamic calculation range from 500 to 630 °C and 1.3 to 2.3 GPa for the stage of the inner segment. Coesite and its pseudomorph occur as inclusions in the outer segment of the garnet and matrix omphacite. This occurrence of coesite is consistent with the pressure and temperature conditions of 660–725 °C and 3.1 GPa estimated by conventional geothermobarometry. Our results suggest that the quartz inclusions in the inner segment were trapped by garnet under α‐quartz‐stable conditions and survived phase transition to coesite at the peak metamorphic stage. The SiO₂ phases and other inclusions in the garnet have retained evidence of the pre‐eclogite prograde stage even during exhumation stage. The combined Raman spectroscopic and petrological approaches used here offers a powerful means for obtaining more robust constraints prograde stages involving garnet growth where different SiO₂ phases are present as inclusions.     

In situ X-ray observations of the coesite-stishovite transition: reversed phase boundary and kinetics

Using a DIA-type, cubic-anvil, high-pressure apparatus (SAM-85) in conjunction with in situ X-ray diffraction, we have investigated phase relations between coesite and stishovite up to 12 GPa and 1530 °C using synthetic powders of the two phases as the starting materials. The phase transition between coesite and stishovite was identified by observing the first appearance of a phase that did not already exist or by a change in the relative intensity of the two patterns. In most experiments, the diffraction patterns on samples were collected within 10 minutes after reaching a pressure and temperature condition. On this time scale, two phase boundaries associated with the coesite-stishovite transition have been determined: (1) for the stishovite-to-coesite transition, observations were made in the temperature range of 950–1530 °C, and (2) for the coesite-to-stishovite transition from 500 to 1300 °C. These observations reveal that there exists a critical temperature of about 1000 °C to constrain the coesite-stishovite equilibrium phase boundary. Above this temperature, both boundaries are linear, have positive dP/dT slopes, and lie within a pressure interval of 0.4 GPa. Below this temperature, the dP/dT slope for the stishovite-to-coesite phase boundary becomes significantly larger and that for the coesite-tostishovite phase boundary changes from positive to negative. As a result, an equilibrium phase boundary can only be determined from the results above 1000 °C and is described by a linear equation P (GPa)=6.1 (4)+ 0.0026 (2) T (°C). This dP/dT slope is in good agreement with that of Zhang et al. (1993) but more than twice that of Yagi and Akimoto (1976). For the kinetics of the phase transition, preliminary rate data were obtained for the stishovite-to-coesite transition at 1160 and 1430 °C and are in agreement with the simple geometric transformation model of Avrami and Cahn.     

X-ray Raman scattering for structural investigation of silica/silicate minerals

We have performed X-ray Raman scattering (XRS) measurements on the oxygen K and silicon L absorption edges of four silica minerals: α-quartz, α-cristobalite, coesite, and stishovite. We have also calculated the partial electron densities of states (DOSs) and compared these with the XRS spectra. This study demonstrates that the short-range structure around the atom of interest strongly influences the XRS spectral features. Importantly, the oxygen K-edge XRS spectra are found to reflect the p-orbital DOS while the silicon L-edge spectra reflect the s- and d-orbital DOSs, even when a product of a momentum transfer and a mean radius of a electron orbit (1s for oxygen and 2p for silicon), Qr, is close to or larger than unity. Building on this, calculations of the partial DOSs for other silica phases are presented, including ultra-high-pressure phases, which provide a good reference for further XRS study of silica and silicate minerals. XRS measurements should be performed on not only either of oxygen or silicon but also on many kinds of constituent elements to reveal the structural change of glasses/melts of silicates under extreme conditions.     

Pressure-induced incipient amorphization of α-quartz and transition to coesite in an eclogite from Antarctica: a first record and some consequences

Monocrystalline quartz inclusions in garnet and omphacite from various eclogite samples from the Lanterman Range (Northern Victoria Land, Antarctica) have been investigated by cathodoluminescence (CL), Raman spectroscopy and imaging, and in situ X‐ray (XR) microdiffraction using the synchrotron. A few inclusions, with a clear‐to‐opalescent lustre, show ‘anomalous’ Raman spectra characterized by weak α‐quartz modes, the broadening of the main α‐quartz peak at 465 cm^?1, and additional vibrations at 480–485, 520–523 and 608 cm^?1. CL and Raman imaging indicate that this ‘anomalous’α‐quartz occurs as relicts within ordinary α‐quartz, and that it was preserved in the internal parts of small quartz inclusions. XR diffraction circular patterns display irregular and broad α‐quartz spots, some of which show an anomalous d‐spacing tightening of ～2%. They also show some very weak, hazy clouds that have d‐spacing compatible with coesite but not with α‐quartz. Raman spectrometry and XR microdiffraction characterize the anomalies with respect to α‐quartz as (i) a pressure‐induced disordering and incipient amorphization, mainly revealed by the 480–485 and 608‐cm^?1 Raman bands, together with (ii) a lattice densification, evidenced by d‐spacing tightening; (iii) the cryptic development of coesite, 520–523 cm^?1 being the main Raman peak of coesite and (iv) Brazil micro‐twinning. This ‘anomalous’α‐quartz represents the first example of pressure‐induced incipient amorphization of a metastable phase in a crustal rock. This issue is really surprising because pressure‐induced amorphization of metastable α‐quartz, observed in impactites and known to occur between 15 and 32 GPa during ultrahigh‐pressure (UHP) experiments at room temperature, is in principle irrelevant under normal geological P–T conditions. A shock (due to a seism?) or a local overpressure at the inclusion scale (due to expansion mismatch between quartz and its host mineral) seem the only geological mechanisms that can produce such incipient amorphization in crustal rocks. This discovery throws new light on the modality of the quartz‐coesite transition and on the pressure regimes (non‐lithostatic v. lithostatic) during high‐pressure/UHP metamorphism. In particular, incipient amorphization of quartz could favour the quartz‐coesite transition, or allow the growth of metastable coesite, as already experimentally observed.     

Melting relations of basalt-andesile-rhyolite-H2O and a pelagic red clay at 30 kb

An olivine basalt, a tonalite (andesite), a granite (rhyolite), and a red clay (pelagic sediment) were reacted, with known quantities of water in sealed noble metal capsules, in a piston-cylinder apparatus at 30 kb pressure. For the pelagic sediment, with H₂O+=7.8% and no additional water, the liquidus temperature is 1240°C, the primary phases are garnet and kyanite. The subsolidus phase assemblage is phengite mica+garnet+clinopyroxene+coesite+kyanite. With 5 wt.% water added, the liquidus temperatures and primary phases for the calc-alkaline rocks are 1280°-1180°-1080°, garnet+clinopyroxene, garnet, and quartz respectively. Garnet and clinopyroxene occur throughout the melting interval of the olivine tholeiite for all water contents. Garnet is joined by clinopyroxene 80° below the andesite plus 5% H₂O liquidus, quartz is joined by clinopyroxene 180° below the rhyolite plus 5% H₂O liquidus. The subsolidus phase assemblage is garnet+clinopyroxene+coesite+minor kyanite for all the calc-alkaline compositions. We conclude that calc-alkaline andesites and rhyolites are not equilibrium partial melting pruducts of subducted oceanic crust consisting of olivine tholeiite basalt and siliceous sediments. Partial melting in subduction zones produces broadly acid and intermediate liquids, but these liquids lie off the calc-alkaline basalt-andesite-rhyolite join and must undergo modification at lower pressures to produce calcalkaline magmas erupted in overlying island arcs.     

Raman microprobe (RMP) determinations of natural and synthetic coesite

Raman microprobe (RMP) spectra of synthetic coesiste and three natural coesites from eclogite — facies rocks are provided and evaluated for characterisation purposes. The main coesite line lies at 521 cm^?1 and the other characteristic lines attributed to coesite occur at 117, 177 and 271 cm^?1. Two petrologically useful applications were (a) the confirmation of the coesite structure in very small natural crystals deduced to be coesite from petrographic observations only, and (b) the recognition of sub-microscopic crystallites of quartz in incipiently — transformed coesite in all the natural samples.     

Raman spectroscopy of natural silica in Chicxulub impactite,MexicoSpectroscopie Raman de silices naturelles dans l'impactite de Chicxulub,Mexique

A series of natural silica impactite samples from Chicxulub (Mexico) was investigated by Raman microprobe (RMP) analysis. The data yield evidence for high-pressure shock metamorphism in the rock. The impactite contains three polymorphs of silica: the original α-quartz, and two high-pressure varieties – coesite and disordered quartz representing various degrees of crystallinity. We found systematic changes in frequencies and half-widths of the Raman bands, caused by increasing irregularities of bond-lengths and bond-angles and a general breaking-up of the structure as a result of impact events. Therefore, RMP is an adequate tool for measuring the crystallinity of disordered quartz. The half-width Γ and the frequency ω of the symmetric SiOSi stretching vibrational band (A₁ mode) of the SiO₄ tetrahedra are the most amenable parameters for estimating the degree of crystallinity. In well-crystallized quartz, Γ=5 cm^?1 and ω=464 cm^?1, while in highly disordered quartz this line shifts up to ω=455 cm^?1 and broadens up to Γ=30 cm^?1. The Raman lineshapes appear to depend strongly on the degree of lattice disorder subsequent to impact events. To cite this article: M. Ostroumov et al., C. R. Geoscience 334 (2002) 21–26     

Factors affecting preservation of coesite in ultrahigh‐pressure metamorphic rocks: Insights from TEM observations of dislocations within kyanite

To understand the preservation of coesite inclusions in ultrahigh‐pressure (UHP) metamorphic rocks, an integrated petrological, Raman spectroscopic and focussed ion beam (FIB) system–transmission electron microscope (TEM) study was performed on a UHP kyanite eclogite from the Sulu belt in eastern China. Coesite grains have been observed only as rare inclusions in kyanite from the outer segment of garnet and in the matrix. Raman mapping analysis shows that a coesite inclusion in kyanite from the garnet rim records an anisotropic residual stress and retains a maximum residual pressure of ~0.35 GPa. TEM observations show quartz is absent from the coesite inclusion–host kyanite grain boundaries. Numerous dislocations and sub‐grain boundaries are present in the kyanite, but dislocations are not confirmed in the coesite. In particular, dislocations concentrate in the kyanite adjacent to the boundary with the coesite inclusion, and they form a dislocation concentration zone with a dislocation density of ~10⁹ cm^?2. A high‐resolution TEM image and a fast Fourier transform‐filtered image reveal that a tiny dislocation in the dislocation concentration zone is composed of multiple edge dislocations. The estimated dislocation density in most of the kyanite away from the coesite inclusion–host kyanite grain boundaries is ~10⁸ cm^?2, being lower than that in kyanite adjacent to the coesite. In the case of a coesite inclusion in a matrix kyanite, using Raman and TEM analyses, we could not identify any quartz at the grain boundaries. Dislocations are not observed in the coesite, but numerous dislocations and stacking faults are developed in the kyanite. The estimated overall dislocation density in the coesite‐bearing matrix kyanite is ~10⁸ cm^?2, but a high dislocation density region of ~10⁹ cm^?2 is also present near the coesite inclusion–host kyanite grain boundaries. Inclusion and matrix kyanite grains with no coesite have dislocation densities of ≤10⁸ cm^?2. Dislocation density is generally reduced during an annealing process, but our results show that not all dislocations in the kyanite have recovered uniformly during exhumation of the UHP rocks. Hence, one of the key factors acting as a buffer to inhibit the coesite to quartz transformation is the mechanical interaction between the host and the inclusion that lead to the formation of dislocations in the kyanite. The kyanite acts as an excellent pressure container that can preserve coesite during the decompression of rocks from UHP conditions. The search for and study of inclusions in kyanite may be a more suitable approach for tracing the spatial distribution of UHP metamorphic rocks.     

TEM investigation of the crystal microstructures in a quartz-coesite assemblage of the western alps

Atransmission electron microscope (TEM) study of quartz-coesite inclusions in garnet in crustal rocks from the Western Alps is presented. Coesite shows a low dislocation density (<10⁷ cm^?2), and quartz a higher density of defects, Brasil twins (10⁴ cm^?1) and dislocations (10⁸ cm^?2). It is concluded that coesite has been not or only slightly plastically deformed and that the yield strength of coesite is higher than that of quartz. The large scale deformation implications are briefly discussed. TEM observations show no systematic topotactic relationship between the two polymorphs and their boundaries have a scalloped morphology which suggests that growth of quartz from coesite was controlled by a diffusion process.