High-pressure Silica Polymorphs Research Articles

High-pressure minerals and new lunar mineral changesite-(Y) in Chang’e-5 regolith

Forty-five years after the Apollo and Luna missions, China’s Chang’e-5 (CE-5) mission collected ∼1.73 kg of new lunar materials from one of the youngest basalt units on the Moon. The CE-5 lunar samples provide opportunities to address some key scientific questions related to the Moon, including the discovery of high-pressure silica polymorphs (seifertite and stishovite) and a new lunar mineral, changesite-(Y). Seifertite was found to be coexist with stishovite in a silica fragment from CE-5 lunar regolith. This is the first confirmed seifertite in returned lunar samples. Seifertite has two space group symmetries (Pnc2 and Pbcn) and formed from an α-cristobalite-like phase during “cold” compression during a shock event. The aftershock heating process changes some seifertite to stishovite. Thus, this silica fragment records different stages of an impact process, and the peak shock pressure is estimated to be ∼11 to 40 GPa, which is much lower than the pressure condition for coexistence of seifertite and stishovite on the phase diagram. Changesite-(Y), with ideal formula (Ca8Y)□Fe2+(PO4)7 (where □ denotes a vacancy) is the first new lunar mineral to be discovered in CE-5 regolith samples. This newly identified phosphate mineral is in the form of columnar crystals and was found in CE-5 basalt fragments. It contains high concentrations of Y and rare earth elements (REE), reaching up to ∼14 wt. % (Y,REE)2O3. The occurrence of changesite-(Y) marks the late-stage fractional crystallization processes of CE-5 basalts combined with silicate liquid immiscibility. These new findings demonstrate the significance of studies on high-pressure minerals in lunar materials and the special nature of lunar magmatic evolution.

High-pressure polymorphs of olivine and silica in Kamargaon (L6) chondrite by laser micro-Raman and XRD studies

High-pressure polymorphs of olivine and silica in Kamargaon (L6) chondrite by laser micro-Raman and XRD studies

Quartz metastability at high pressure: what new can we learn from polarized Raman spectroscopy?

The pressure dependence of the polarized Raman scattering of quartz was studied under hydrostatic conditions up to 9 GPa. We extended the available pressure calibrations, usually limited to the two most intense peaks, to a larger number of modes, providing polynomial functions that describe the relationship between pressure P and wavenumber shift $$\Delta \omega$$ for the 128-, 206-, 265-, 464-, 696-, 809-, 1080- and 1161- $$\hbox {cm}^{-1}$$ modes. For the first time, the pressure behavior of the LO-TO splitting is characterized up to 9 GPa under hydrostatic conditions. The pressure-induced wavenumber changes and derived phonon compressibilities show that longitudinal and transverse modes can be used interchangeably for the calculation of strains through the Gruneisen tensor to derive the strains in crystals, such as mineral inclusions, under non-hydrostatic conditions. A careful examination of the linewidths as a function of pressure shows that they are sensitive to the metastability of the quartz structure with respect to the high-pressure silica polymorphs. It is proposed that strong multiphonon interactions contribute to the stability of the structure of quartz at ambient conditions.

Evidence for subsolidus quartz-coesite transformation in impact ejecta from the Australasian tektite strewn field

Abstract Coesite, a high-pressure silica polymorph, is a diagnostic indicator of impact cratering in quartz-bearing target rocks. The formation mechanism of coesite during hypervelocity impacts has been debated since its discovery in impact rocks in the 1960s. Electron diffraction analysis coupled with scanning electron microscopy and Raman spectroscopy of shocked silica grains from the Australasian tektite/microtektite strewn field reveals fine-grained intergrowths of coesite plus quartz bearing planar deformation features (PDFs). Quartz and euhedral microcrystalline coesite are in direct contact, showing a recurrent pseudo iso-orientation, with the [1 1 ¯ 1] * vector of quartz near parallel to the [0 1 0]* vector of coesite. Moreover, discontinuous planar features in coesite domains are in textural continuity with PDFs in adjacent quartz relicts. These observations indicate that quartz transforms to coesite after PDF formation and through a solid-state martensitic-like process involving a relative structural shift of { 1 ¯ 0 1 1 } quartz planes, which would eventually turn into coesite (0 1 0) planes. This process further explains the structural relation observed between the characteristic (0 1 0) twinning and disorder of impact-formed coesite, and the 1 0 1 ¯ 1 PDF family in quartz. If this mechanism is the main way in which coesite forms in impacts, a re-evaluation of peak shock pressure estimates in quartz-bearing target rocks is required because coesite has been previously considered to form by rapid crystallization from silica melt or diaplectic glass during shock unloading at 30–60 GPa.

OH defects in coesite and stishovite during ultrahigh-pressure metamorphism of continental crust

The high-pressure silica polymorphs coesite and stishovite were synthesized under water-saturated conditions from a natural granitic composition doped with Li and B. Experiments were performed in a Multi-Anvil apparatus between 4 and 9.1 GPa and 900 and 950 °C, based on the conditions of a subducting continental crust as realistic for the ultrahigh-pressure metamorphic units Dora Maira and Kochetav massifs. Run products consisted of coesite/stishovite + kyanite ± phengite ± omphacite, and quench material. The synthesized silica polymorphs were successively analyzed by infrared spectroscopy, electron microprobe, and Secondary-Ion Mass Spectrometry (SIMS). No hydrous defects were observed in coesite synthesized at 4 GPa and 900 °C, whereas coesite grown at higher pressures revealed a triplet of infrared absorptions bands at 3575, 3523, and 3459 cm− 1, two minor bands at 3535 and 3502 cm− 1, and a small band at 3300 cm− 1 that was only visible at 7.7 GPa. The total amount of Al was charge-balanced by H and the other monovalent cations. However, the band triplet could not be associated with AlOH defects, while the band doublet was inferred to BOH defects and the small band probably corresponded to interstitial H. Stishovite displayed one dominant band at 3116 cm− 1 with a shoulder at 3170 cm− 1, and a minor band at 2665 cm− 1, probably all associated with AlOH defects. BOH defects were not observed in stishovite, and LiOH defects were neither observed in coesite nor stishovite, probably because of preferentially partition of Li in other phases such as omphacite. The total amount of defect protons increased with pressure and with metal impurity concentrations. The general increase in OH defects in silica polymorphs with increasing pressure (this study) contrasted the negative pressure trend of OH in quartz observed previously from the same starting material, and revealed an incorporation minimum of OH in silica polymorphs around the quartz/coesite phase transition.

Discovery of stishovite in Apollo 15299 sample

High-pressure polymorphs recovered in terrestrial craters are evidence of meteoroid impact events on the Earth’s surface. Despite countless impact craters on the Moon, high-pressure polymorphs have not been reported to date in returned Apollo samples. On the other hand, recent studies report that the high-pressure polymorphs of silica, coesite, and stishovite occur in shocked lunar meteorites. We investigated regolith breccia 15299, which was returned by the Apollo 15 mission, using the combined techniques of focused ion beam (FIB), synchrotron X‑ray diffraction (XRD), and transmission electron microscopy (TEM). The regolith breccia 15299 studied here consists of a mafic impact melt breccia with millimeter-sized, coarse-grained, low-Ti basalt clasts. The mafic melt breccia consists of fragments of minerals (olivine, pyroxene, plagioclase, silica, and ilmenite) and glass. Several quartz, tridymite, and cristobalite grains of 10-100 mm across occur in the mafic impact melt breccia. Vesicular melt veins of less than ~200 mm wide cut across the mafic melt breccia matrix and mineral fragments. Some silica grains are entrained in the melt veins. One of the silica grains entrained in the melt veins consist of stishovite [a = 4.190(1), c = 2.674(1) Å, V = 46.95 Å 3 , space group P4 2 /mnm] along with tridymite and silica glass. This is the first report of high-pressure polymorphs from returned lunar samples. TEM images show that the stishovite is needle-like in habit, and up to ~400 nm in size. Considering the lithologies and shock features of 15299, it is inferred that the stishovite possibly formed by the Imbrium impact or subsequent local impact event(s) in the Procellarum KREEP Terrane (PKT) of the nearside of the Moon.

High-pressure synthesis of mesoporous stishovite: potential applications in mineral physics

Recently, we have described a successful synthesis route to obtain mesoporous quartz and its high-pressure polymorph coesite by nanocasting at high pressure using periodic mesostructured precursors, such as SBA-16 and FDU-12/carbon composite as starting materials. Periodic mesoporous high-pressure silica polymorphs are of particular interest as they combine transport properties and physical properties such as hardness that potentially enable the industrial use of these materials. In addition, synthesis of mesoporous crystalline silica phases can allow more detailed geology-related studies such as water/mineral interaction, dissolution/crystallization rate and the surface contribution to the associated thermodynamic stability (free energy and enthalpy) of the various polymorphs and their crossover. Here, we present results of synthesis of mesoporous stishovite from cubic large-pore periodic mesoporous silica LP-FDU-12/C composite as precursor with an fcc lattice. We describe the synthesis procedure using multi-anvil apparatus at 9 GPa (about 90,000 atm) and temperature of 500 °C. The synthetic mesoporous stishovite is, then, characterized by wide and small-angle X-ray diffraction, scanning/transmission electron microscopy and gas adsorption. Results show that this new material is characterized by accessible mesopores with wide pore size distribution, surface area of ~45 m2/g and volume of pores of ~0.15 cm3/g. Results from gas adsorption indicate that both porosity and permeability are retained at the high pressures of synthesis but with weak periodic order of the pores.

Shock-induced deformation of Shergottites: Shock-pressures and perturbations of magmatic ages on Mars

Shergottites and Chassignites practiced major deformation effects whose nature, magnitude and relevance were controversially evaluated and disputatively debated. Our studies of many shocked shergottites present, contrary to numerous previous reports, ample evidence for pervasive shock-induced melting amounting of at least 23vol.% of the shergottite consisting of maskelynite and pyrrhotite, partial melting of pyroxene, titanomagnetite, ilmenite and finding of several high-pressure polymorphs and pressure-induced dissociation reactions. Our results cast considerable doubt on using the refractive index (RI) or cathodoluminescence (CL) spectra of maskelynite, in estimating the magnitudes of peak-shock pressure in both shergottites and ordinary chondrites. RI of maskelynite was set after quenching of the feldspar liquid before decompression to maskelynite glass followed by glass relaxation after decompression at the closure temperature of relaxation. The RI procedure widely practiced in the past 38years revealed unrealistic very high-pressure estimates discrepant with the high-pressure mineral inventory in shocked shergottites and ordinary chondrites and with results obtained by robust laboratory static experiments. Shergottites contain the silica high-pressure polymorphs: the scrutinyite-structured polymorph seifertite, a monoclinic ultra dense polymorph of silica with ZrO2-structure, stishovite, a dense liquidus assemblage consisting of stishovite+Na-hexa-aluminosilicate (Na-CAS) and both K-lingunite and Ca-lingunite. Applying individual high-pressure silica polymorphs alone like stishovite, to estimate the equilibrium shock pressure, is inadequate due to the considerable shift of their nominal upper pressure bounds intrinsically induced by spatially variable absorptions of minor oxides like Al2O3, Na2O, FeO, MgO and TiO2. This practice revealed variable pressure estimates even within the same shergottite subjected to the same peak-shock pressure. Occurrence of Na-CAS+stishovite, lack of the NaAlSiO4 Ca-ferrite structured polymorph or jadeite indicates that the peak-shock pressures barely exceeded 22GPa. We present convincing and ample evidence refuting the claim that the shock-induced high-pressure inventory in shergottites and ordinary chondrites are disequilibrium assemblages resulted from local pressure spikes in excess of 80GPa and during the decompression stage. Such scenario calls for a series of incomplete and quenched retrograde reactions starting with the crystallization of Mg–silicate perovskite+magnesiowüstite, if the claimed peak-shock pressure was in excess of 80GPa. This would be followed by replacement of this pair by majorite-pyropess+magnesiowüstite or akimotoite+magnesiowüstite below 23GPa and 2000°C, polycrystalline ringwoodite above 16GPa, respectively and finally replacement by polycrystalline olivine below 16GPa. Such incomplete retrograde reactions were never encountered in any shergottite, chassignite or shocked ordinary chondrite so far. Olivine–ringwoodite phase transformation in the L6 Y-791384 commences with the coherent mechanism producing ringwoodite lamellae with their (111) planes parallel to the (100) of olivine followed by the incoherent mechanism due to build up of strain in the parental olivine. This is in accord with the olivine–ringwoodite settings produced in static laboratory experiments in a multi-anvil device. Olivine–ringwoodite phase transitions were also encountered in comparable settings in the shergottite NWA 1068. Application of experimentally obtained kinetic parameters of the olivine–ringwoodite phase transitions reveals possible duration of the natural dynamic events up to few seconds thus unambiguously refuting the claimed disequilibrium decompression mechanism.The shock-induced pervasive melting of labradorite, pyrrhotite, titanomagnetite, ilmenite and partial melting of clinopyroxene strongly suggests shock-induced partial to complete resetting of the Ar–Ar, Rb–Sr, Sm–Nd, Re–Os, U–Pb and Lu–Hf radiometric systems. This also casts considerable doubt on the radiometric ages shorter than 575Ma reported in the past 38years to allegedly be the igneous crystallization ages. These short ages probably resulted from partial or total shock-induced age resetting.

Coesite and stishovite in a shocked lunar meteorite, Asuka-881757, and impact events in lunar surface

Microcrystals of coesite and stishovite were discovered as inclusions in amorphous silica grains in shocked melt pockets of a lunar meteorite Asuka-881757 by micro-Raman spectrometry, scanning electron microscopy, electron back-scatter diffraction, and transmission electron microscopy. These high-pressure polymorphs of SiO(2) in amorphous silica indicate that the meteorite experienced an equilibrium shock-pressure of at least 8-30 GPa. Secondary quartz grains are also observed in separate amorphous silica grains in the meteorite. The estimated age reported by the (39)Ar/(40)Ar chronology indicates that the source basalt of this meteorite was impacted at 3,800 Ma ago, time of lunar cataclysm; i.e., the heavy bombardment in the lunar surface. Observation of coesite and stishovite formed in the lunar breccias suggests that high-pressure impact metamorphism and formation of high-pressure minerals are common phenomena in brecciated lunar surface altered by the heavy meteoritic bombardment.

First-principles simulations of native point defects and ionic diffusion in high-pressure polymorphs of silica

Several native point defects including vacancies, interstitials, and their complexes were studied in high-pressure polymorphs of silica (stishovite, ${\text{CaCl}}_{2}$, $\ensuremath{\alpha}{\text{-PbO}}_{2}$, and pyrite types) up to 200 GPa within density-functional theory. The formation enthalpies of the individual defects strongly depend on atomic chemical potentials and the Fermi level. Their values were shown to increase by a factor of 2 over the entire pressure range studied with large differences in some cases between different phases. The Schottky defects are energetically most favorable at zero pressure whereas O-Frenkel pairs become systematically more favorable at pressures higher than 20 GPa. The activation enthalpies of ionic migrations obtained by the nudged-elastic-band method suggest that the interstitial mechanisms are favored over the vacancy hoping mechanisms. The geometric and electronic structures of defects and migrating ions vary largely among different types of defects. In particular, the O defects introduce localized electronic states. These structures remain qualitatively unchanged with compression showing similar trends among different polymorphs.

Shared and Closed-Shell O−O Interactions in Silicates

Bond paths of maximum electron density spanning O-O edges shared between equivalent or quasiequivalent MOn (n > 4) coordination polyhedra are not uncommon electron density features displayed by silicates. On the basis of the positive values for the local electronic energy density, H(rc), at the bond critical points, rc, they qualify as weak "closed-shell" interactions. As observed for M-O bonded interactions (M = first and second row metal atoms), the electron density, rho(rc), and the Laplacian of the electron density increase in a regular way as the separation between the O atoms, R(O-O), decreases. A simple model, based on R(O-O) and the distances of the Si atoms from the midpoint between adjacent pairs of O atoms, partitions the O-O bond paths in the high-pressure silica polymorph coesite into two largely disjoint domains, one with and one without bond paths. The occurrence of O-O bond paths shared in common between equivalent coordination polyhedra suggests that they may be grounded in some cases on factors other than bonded interactions, particularly since they are often displayed by inert procrystal representations of the electron density. In these cases, it can be argued that the accumulation of the electron density along the paths has its origin, at least in part, in the superposition of the peripheral electron density distributions of the metal M atoms occupying the edge-sharing polyhedra. On the other hand, the accumulation of electron density along the paths may stabilize a structure by shielding the adjacent M atoms in the edge-sharing polyhedra. For closed-shell Li-O, Na-O, and Mg-O interactions, H(rc) is positive and increases as the value of rho(rc) increases, unlike the "shared" Be-O, B-O, C-O, Al-O, Si-O, P-O, and S-O interactions, where H(rc) is negative and decreases as rho(rc) increases. The H(rc) values for the weak closed-shell O-O interactions also increase as rho(rc) increases, as observed for the closed-shell M-O interactions. On the basis of the bond critical point properties and the negative H(rc) value, the O-O interaction comprising the O2 molecule in silica III qualifies as a shared interaction.

Hydrostatic compression and high-pressure elastic constants of coesite silica

Using density-functional theory, we computed all the independent elastic constants of coesite, a high-pressure polymorph of silica, as functions of pressure up to 15 GPa. The results are in good agreement with experimental measurements under ambient conditions. Also, the predicted pressure-dependent elastic properties are consistent with x-ray data in the literature concerning lattice strains at high pressures. We find that coesite, like quartz, exhibits a gradual softening of a shear modulus B44 with increasing pressure, in contrast to the rising bulk modulus.

Classification of metal-oxide bonded interactions based on local potential- and kinetic-energy densities

A classification of the hydrogen fluoride H-F-bonded interactions comprising a large number of molecules has been proposed by Espinosa et al. [J. Chem. Phys. 117, 5529 (2002)] based on the ratio /Vr(c)/ / Gr(c) where /Vr(c)/ is the magnitude of the local potential-energy density and Gr(c) is the local kinetic-energy density, each evaluated at a bond critical point r(c). A calculation of the ratio for the M-O bonded interactions comprising a relatively large number of oxide molecules and earth materials, together with the constraints imposed by the values of inverted Delta2rho r(c) and the local electronic energy density, Hr(c) = Gr(c) + Vr(c), in the H-F study, yielded practically the same classification for the oxides. This is true despite the different trends that hold between the bond critical point and local energy density properties with the bond lengths displayed by the H-F and M-O bonded interactions. On the basis of the ratio, Li-O, Na-O, and Mg-O bonded interactions classify as closed-shell ionic bonds, Be-O, Al-O, Si-O, B-O, and P-O interactions classify as bonds of intermediate character with the covalent character increasing from Be-O to P-O. N-O interactions classify as shared covalent bonds. C-O and S-O bonded interactions classify as both intermediate and covalent bonded interactions. The C-O double- and triple-bonded interactions classify as intermediate-bonded interactions, each with a substantial component of covalent character and the C-O single-bonded interaction classifies as a covalent bond whereas their local electronic energy density values indicate that they are each covalent bonded interactions. The ratios for the Be-O, Al-O, and Si-O bonded interactions indicate that they have a substantial component of ionic character despite their classification as bonds of intermediate character. The trend between the ratio and the character of the bonded interactions is consistent with trends expected from electronegativity considerations. The ratio increases as the net charges and the coordination numbers for the atoms for several Ni-sulfides decrease. On the contrary, the ratio for the Si-O bonded interactions for the orthosilicate, forsterite, Mg2SiO4, and the high-pressure silica polymorph, stishovite, decreases as the observed net atomic charges and the coordination numbers of Si and O increase in value. The ratio for the Ni-Ni bonded interactions for the Ni-sulfides and bulk Ni metal indicate that the interactions are intermediate in character with a substantial component of ionic character.

A class of new high-pressure silica polymorphs

We synthesised a number of new metastable silica modifications in the electrically heated diamond anvil cells (DACs) at pressures over 100 GPa and temperatures over 1200 K. Our studies reveal the existence of a new class of silica phases with different degree of the ordering of silicon atoms. Silica polymorphs with octahedrally co-ordinated silicon known so far (stishovite, fully disordered phase with niccolite-type structure, α-PbO2-type, and monoclinic (P21/c space group) post-quartz phase) are members of this class. The details of the silicon distribution among free positions in the oxygen network strongly depend on the starting material and the conditions of high-pressure high-temperature treatment of silica, but a general trend is that at pressures above 40–45 GPa most of the silicon atoms are placed in octahedral positions. Our structural model provides a key for an explanation of the mysterious behaviour of silica, known for more than 30 years, when at the same pressure and temperature range different phases were synthesised.

A modeling of the structure and favorable H-docking sites and defects for the high-pressure silica polymorph stishovite

Employing first-principles methods, the docking sites for H were determined and H, Al, and vacancy defects were modeled with an infinite periodic array of super unit cells each consisting of 27 contiguous symmetry nonequivalent unit cells of the crystal structure of stishovite. A geometry optimization of the super-cell structure reproduces the observed bulk structure within the experimental error when P1 translational symmetry was assumed and an array of infinite extent was generated. A mapping of the valence electrons for the structure displays mushroom-shaped isosurfaces on the O atom, one on each side of the plane of the OSi3 triangle in the nonbonded region. An H atom, placed in a cell near the center of the super cell, was found to dock upon geometry optimization at a distance of 1.69 A from the O atom with the OH vector oriented nearly perpendicular to the plane of the triangle such that the OH vector makes a angle of 91° with respect to [001]. However, an optimization of a super cell with an Al atom replacing Si and an H atom placed nearby in a centrally located cell resulted in an OH distance of 1.02 A with the OH vector oriented perpendicular to [001] as observed in infrared studies. The geometry-optimized position of the H atom was found to be in close agreement with that (0.44, 0.12, 0.0) determined in an earlier study of the theoretical electron density distribution. The docking of the H atom at this site was found to be ∼330 kJ mol−1 more stable than a docking of the atom just off the shared OO edge of the octahedra as determined for rutile. A geometry optimization of a super cell with a missing Si generated a vacant octahedra that is 20% larger than that of the SiO6 octahedra. The valence electron density distribution displayed by the two-coordinate O atoms that coordinate the vacant octahedral site is very similar to those displayed by the bent SiOSi angles in coesite. The internal distortions induced by the defect were found to diminish rather rapidly with distance, with the structure annealing to that observed in the bulk crystal to within about three coordination spheres.

A class of new high-pressure silica polymorphs

We synthesised a number of new metastable silica modifications in the electrically heated diamond anvil cells (DACs) at pressures over 100 GPa and temperatures over 1200 K. Our studies reveal the existence of a new class of silica phases with different degree of the ordering of silicon atoms. Silica polymorphs with octahedrally co-ordinated silicon known so far (stishovite, fully disordered phase with niccolite-type structure, α-PbO 2-type, and monoclinic ( P2 1/ c space group) post-quartz phase) are members of this class. The details of the silicon distribution among free positions in the oxygen network strongly depend on the starting material and the conditions of high-pressure high-temperature treatment of silica, but a general trend is that at pressures above 40–45 GPa most of the silicon atoms are placed in octahedral positions. Our structural model provides a key for an explanation of the mysterious behaviour of silica, known for more than 30 years, when at the same pressure and temperature range different phases were synthesised.

An Exploration of Theoretical and Experimental Electron Density Distributions and SiO Bonded Interactions for the Silica Polymorph Coesite

A multipole representation of the experimental electron density distribution for the high-pressure silica polymorph coesite, using Hirshfeld-type radial functions, has been generated with single-crystal X-ray diffraction data recorded to a sin θmax/λ value of 1.21 Å-1 at 100 K. Unlike an earlier modeling of the distribution, where a more limited data set was analyzed, deformation electron density maps display banana-shaped isosurfaces in the lone-pair regions of each of the oxide anions involved in the bent SiOSi angles as well as teardrop-shaped ones along each of the SiO bond vectors. They also display a ring torus isosurface about O1, the oxide anion involved in the straight angle. Laplacian −∇2ρ maps display belt-shaped isosurfaces, centered near the apexes of the bent angles, that wrap about halfway around the oxide anions, with a ring torus-shaped isosurface surrounding O1. An analysis of −∇2ρ revealed that the (3,−3) critical point associated with the anions involved in the bent angles are associated in general with larger maxima than that associated with the straight angle, evidence that the electron density is more locally concentrated on the oxide anions involved in the bent angles. As such, these anions are asserted to be more susceptible to electrophilic attack by hydrogen, a feature that provides an experimental basis for why hydrogen in H- and Al-bearing coesite avoids O1 and is observed to dock in the vicinity of the oxide anions involved in the bent angles. The bond critical point properties of the experimental multipole representation of the electron density distribution for coesite together with those for the very high-pressure silica polymorph, stishovite, conform with those calculated for the SiO bonded interactions for a relatively large number of silicate crystals. Not only are they similar in value with the theoretical properties, but together they correlate with the observed SiO bond lengths as predicted by the calculations. The observed SiO bonds display a relatively wide range of ∇2ρ(rc) values between ∼10 e Å-5 for stishovite and ∼20 e Å-5 for coesite. The larger ∇2ρ(rc) values recorded for coesite, considered typical of first row closed-shell ionically bonded atoms, may not be typical for a closed-shell bonded interaction involving second row atoms such as the four-coordinate Si in coesite. The maxima along the bond vectors and in the lone-pair regions displayed by the experimental Δρ and −∇2ρ maps are indicative of shared covalent bonded interactions. The evidence suggests that the value of the electron density at the bond critical point for a given bonded interaction is a reliable measure of bond type: the larger the value ρ(rc), the greater the shared covalent interaction.

Ultrahigh-pressure metamorphism: tracing continental crust into the mantle

More and more evidence is being discovered in Phanerozoic collision belts of the burial of crustal rocks to previously unsuspected (and ever increasing) depths, presently on the order of 150–200 km, and of exhumation from such depths. This extends by almost one order of magnitude the depth classically ascribed to the metamorphic cycling of continental crust, and demonstrates its possible subduction. The pieces of evidence for this new, ultrahigh-pressure (UHP) metamorphism exclusively occur in the form of relics of high-pressure minerals that escaped back-transformation during decompression. The main UHP mineral indicators are the high-pressure polymorphs of silica and carbon, coesite and microdiamond, respectively; the latter often demonstrably precipitated from a metamorphic fluid and is completely unrelated to kimberlitic diamond or any shock event. Recent discoveries of pyroxene exsolutions in garnet and of coesite exsolutions in titanite suggest a precursor garnet or titanite containing six-fold coordinated silicon, therefore still higher pressures than implied by diamond stability, on the order of 6 GPa. The UHP rocks raise a formidable geological problem: that of the mechanisms responsible for their burial and, more pressingly, for their exhumation from the relevant depths. The petrological record indicates that large tracts of UHP rocks were buried to conditions of low T/P ratio, consistent with a subduction-zone context. Decompression occurred in most instances under continuous cooling, implying continuous heat loss to the footwall and hangingwall of the rising body. This rise along the subduction channel – an obvious mechanical discontinuity and weak zone – may be driven by buoyancy up to mid-crustal levels as a result of the lesser density of the acidic crustal rocks (even if completely re-equilibrated at depth) after delamination from the lower crust, in a convergent setting. Chronological studies suggest that the rates involved are typical plate velocities (1–2 cm/yr), especially during early stages of exhumation, and bear no relation to normal erosion rates. Important observations are that: (i) as a result of strain partitioning and fluid channelling, significant volumes of subducted crust may remain unreacted (i.e. metastable) even at conditions as high as 700°C and 3 GPa – with implications as to geophysical modeling; (ii) subducted continental crust shows no isotopic or geochemical evidence of interaction with mantle material. An unknown proportion of subducted continental crust must have escaped exhumation and effectively recycled into the mantle, with geochemical implications still to be explored, bearing in mind the above inefficiency of mixing. The repeated occurrence of UHP metamorphism, hence of continental subduction, through time and space since at least the late Proterozoic shows that it must be considered a common process, inherent to continental collision. Evidence of older, Precambrian UHP metamorphism is to be sought in high-pressure granulite-facies terranes.

Anion packing density: a universal quantity for both dense and porous crystalline inorganic phases.

To compare densities of inorganic high-pressure phases their molal volumes or specific gravities are usually employed, whereas for zeolites and other microporous materials the so-called framework density, FD, is applied. The definition of FD, which refers only to phases with three-dimensional tetrahedron frameworks, is extended to a 'generalized framework density' d(f), which is independent of the dimensionality of the framework and the coordination number(s) of the framework cations. In this paper the anion packing density, d(ap), is introduced as a new quantity which is not only applicable to any inorganic phase but, in contrast to FD and d(f), also allows quantitative comparisons to be made for crystalline inorganic phases of any kind. The anion packing density can readily be calculated if the volume and content of the unit cell and the radii of the anions of a phase are known. From d(ap) values calculated for high-pressure silica polymorphs studied under very high pressure, it is concluded that Shannon-Prewitt effective ionic radii do not sufficiently take into account the compressibility of the anions.

High-pressure silica polymorphs as hardest known oxides

We report the results of an experimental study of the silica phases (stishovite and SiO2 with α-PbO2 like structure). These phases were synthesized in laser- and externally electrically heated Mao–Bell type diamond anvil cells (DACs) from α-cristobalite as a starting material at pressures above 80 GPa and temperatures above 1200 K. We measured the hardness (H) of both phases and found that the value for the α-PbO2-like silica, H=31±3 GPa, is very close to that of stishovite, H=32±2 GPa, for polycrystalline material obtained in electrically heated DACs. Thus, these two polycrystalline silica phases are hardest known oxides and representatives of hardest known polycrystalline materials in general.