Geoffrey D. Bromiley

Hydrogen solubility and speciation in natural, gem-quality chromian diopside

Abstract A new technique for performing long duration (up to 300 hours) high-pressure annealing experiments under water-saturated conditions has been developed. This technique has been used to investigate watersolubility and speciation in natural, gem-quality chromian diopside. Capsule design for the technique is a variant of the double-capsule technique, and relies on the use of a semi-permeable Pt membrane, which permits free hydrogen diffusion into samples, but protects samples from reacting with buffer mixtures. The investigation of a natural single crystal of chromian diopside revealed a very unusual annealing behavior: water contents increase sharply after a short annealing period and then decrease slowly to some metastable equilibrium value. The main process that takes place during the annealing experiments is hydrogen diffusion coupled with Fe3+ reduction. This essentially reverses the main mechanism for hydrogen loss from mantle samples during exhumation, and the technique therefore provides sample-specific information on original water contents. Absorption bands at 3646 and 3434 cm-1 in IR spectra from annealed samples suggest two main mechanisms for hydrogen incorporation in the diopside sample: (1) incorporation of hydrogen onto the O2 site, with vibration of the OH dipole in the direction of a nearby O3 site (along the edge of an M2 site), and (2) incorporation of hydrogen onto the O2 site with vibration of the OH dipole toward a nearby O1 site (along a shared M1-M2 edge) or O2 site (along the edge of an M1 site). The ratio of peak heights between the absorption bands at 3646 and 3434 cm-1 is independent of water fugacity but dependent on oxygen fugacity, and appears to provide a measure of the redox state “frozen” into the sample. This ratio could be used to determine whether pyroxenes from upper-mantle xenoliths had experienced concurrent hydrogen-loss and oxidation during exhumation.

Lithium/nickel mixing in the transition metal layers of lithium nickelate: high-pressure synthesis of layered Li[LixNi1−x]O2 oxides as cathode materials for lithium-ion batteries

Novel compositions Li[LixNi1−x]O2, 0<x<0.2, with a layered structure, have been prepared under high-pressure in an oxygen-rich atmosphere using a piston-cylinder type apparatus. A new structural feature of these compositions as compared to well-known Li[Ni]O2 is the development of mixed [LixNi1−x]O2-layers in addition to nearly pure Li-layers. The replacement of Ni3+ by Li+ in nickel-rich layers is compensated for by the appearance of Ni4+ ions. The use of Li[LixNi1−x]O2 as a cathode material in a lithium ion cell shows a different electrochemical behavior as compared to Li[Ni]O2. Below 4.6 V the cell capacity is always below 115 mA g−1. On increasing the voltage upper limit, an irreversible transformation into a solid with a stoichiometry to LixNiO2 takes place. On further cycling Li[LixNi1−x]O2 displays a reversible lithium insertion, reaching a capacity of about 190 mA h g−1 between 5.0 and 2.7 V after several cycles.

Hydrogen and minor element incorporation in synthetic rutile

Abstract The solubility and incorporation mechanisms of H and various trivalent and divalent cations in synthetic rutile have been investigated. Experiments performed using different bulk Fe2O3 contents demonstrate that Fe3+ substitutes onto the main Ti site, charge-balanced by oxygen vacancies. Under more reducing conditions in Fe-poor systems, the concentration of Ti interstitials in rutile is increased, resulting in a decrease in H solubility. Variation in the solubility of different oxides in rutile as a function of ionic radius implies substitution onto the main Ti site, probably charge-balanced by oxygen vacancies. To a lesser degree, substitution of trivalent and divalent cations is locally charge-balanced by H incorporation. Variation in OH-stretching frequencies in infrared spectra as a function of composition implies that octahedral defects and structurally-incorporated H are coupled. However, in all samples, some of the H is also decoupled from substitutional impurities, as is evident from an OH-absorption band at 3279 cm-1. This band corresponds to the main OH band seen in spectra of many natural rutiles, implying that in most rutiles, H defects are decoupled from substitutional defects.

The real topological configuration of the extra-framework content in alkali-poor beryl: A multi-methodological study

Abstract The crystal structure of alkali/water-poor beryl (H2O + Na2O + Cs2O < 1.2 wt%) was reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, thermogravimetric analysis, neutron diffraction, and polarized infrared spectroscopy to determine the real topological configuration of the extra-framework content in the six-membered ring channels. Analysis of the nuclear density Fourier map suggests that the (water) oxygen is located along the sixfold axis at the 2a site (0,0,1/4), whereas the (water) protons are at -0.028(7), -0.071(3), 0.332(1). The hydrogen atoms are distributed in 6 × 2 equivalent positions, above and below the oxygen site. Geometrical configuration of the water molecule is well defined: the O-H bond distance is 0.949(18) Å and the H-O-H bond angle is 106.9(2.2)°. The H···H vector is oriented at -4° from [001]. This configuration is completely different from that found in alkali-rich beryl, where the H···H vector is perpendicular to [001]. Na is probably located, with the H2O oxygen, at the 2a site. According to the chemical analysis, which shows that the amounts of other alkali and earth-alkali cations are negligible (Rb, K, Mg, Mn ≤ 110 ppm, Ca ≤ 225 ppm, Cs ≤ 430 ppm), no effect of other cations on the extra-framework population was observed in the structural refinement. The final agreement index (R1) of the structural refinement was 0.037 for 34 refined parameters and 160 unique reflections with Fo > 4σ(Fo). The topological configuration of the H2O molecule into the channel is confirmed by the spectroscopic investigation. Polarized single-crystal IR spectra show that the H2O molecule is oriented with the molecular symmetry axis perpendicular to the hexagonal axis and H···H vector parallel (or quasi-parallel) to [001].

The structure of (Ca,Co)CoSi2O6 pyroxenes and the Ca-M2+ substitution in (Ca,M2+)M2+Si2O6 pyroxenes (M2+ = Co, Fe, Mg)

Abstract The crystal structure of three C2/c clinopyroxenes with composition (Ca0.8Co0.2)CoSi2O6, (Ca0.6Co0.4) CoSi2O6 and (Ca0.4Co0.6)CoSi2O6 was refined down to R4σ = 2.6% by single-crystal X-ray diffraction. The crystals were synthesized at P = 3 GPa by cooling from 1500 to 1200 °C in a piston-cylinder apparatus. At the end of the refinement cycles, electron density residuals (up to 2.1 e-) were observed close to the M2 site and related to the site splitting of Ca and Co in the M2 polyhedron in the two subsites M2 and M2′. Split refinement significantly improved the agreement factor and decreased the uncertainty in the atomic coordinates. Similar features were found in (Ca,Mg)MgSi2O6 and (Ca,Fe) FeSi2O6 intermediate pyroxenes. The average structural changes related to the cation substitution at the M2 site in (Ca,Co)CoSi2O6, (Ca,Mg)MgSi2O6, and (Ca,Fe)FeSi2O6 pyroxenes are similar: the T tetrahedron becomes more regular, the difference between M1-O bond lengths increases, and the M2-O3 bond lengths with the furthermost O3 oxygen atoms become longer. The changes in the M2-O bond distances are, however, not linear, and they are higher for more increased substitution. The largest structural deformation occurs on the (010) plane, with higher deformation at about 60° from the c axis for any composition. The orientation of the deformation ellipsoid is most related to a shift in tetrahedral chains. The scalar deformation for the cation substitution, εs, is linearly related to the cation radius of the average M2 site (IRM2), i.e., the deformation is higher as the cation size decreases, following the equation: εs = -0.0072(12)IRM2 + 0.0082(13), R2 = 0.75. Increasing deformation with cation substitution is supported as the major limiting factor for solid solution. The displacement parameters for unsplit M2, O2, and O3 atoms increase up to the intermediate composition, indicating a local configuration for the M2 polyhedron centered by Ca and Co. However no significant change in Ueq of the O3 atom is observed up to 20% substitution of the smaller cation in the M2 site. Comparison with Raman spectral data suggests that local chain structural configurations occur only for the substitution of the smaller cation in the M2 site higher than 20%, and that the substitution mechanism is different for C2/c clinopyroxenes with lower and higher Ca content

New insight into crystal chemistry of topaz: A multi-methodological study

Abstract The crystal chemistry of a natural topaz [with OH/(OH + F) < 0.5] was reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, single-crystal X-ray diffraction (at 298 K) and neutron diffraction (at 298 and 10 K), and polarized infrared spectroscopy to define unambiguously the real symmetry of topaz, the location of the proton and its thermal displacement parameters at room and low temperatures, the hydrogen-bonding and the vibration modes (stretching and bending) of the OH dipole. X-ray and neutron structural refinements allow us to infer that the crystal structure of natural topaz with OH/(OH + F) < 0.5 can be described with the Pbnm space group. Violating reflections, found in the previous investigations and in this study, are likely due to Renninger effect (double diffraction phenomenon). The nuclear density Fourier map shows that the proton is located at Wyckoff 8d position and the refined coordinates are: x = 0.495(2), y = 0.252(1), z = 0.1629(7). The O-H bond lies on the (010)-plane and forms an angle of about 28.9° with the c-axis. Neutron structural refinements at 298 and 10 K show that the displacement ellipsoid of the proton is highly anisotropic. The H-bonding arrangement appears to be complex, with at least four potential H···O/F interactions (distances < 2.38 Å). The topological configuration of the O-H group described by the neutron structural refinements is confirmed by the infrared investigation: the OH stretching mode (at 3640 cm-1) has no component of vibration parallel to the b axis (i.e., the O-H direction is perpendicular to [010]). The OH bending mode (at 1161 cm-1) shows components along the three crystallographic axes, which appear to be more prominent along the a and b-axes. The possible distribution into the crystal structure of topaz of the minor/trace elements found (Na, Ca, Fe Cr, V, Ti, B), and the implied topological effects, is discussed.

High-pressure phase transitions and hydrogen incorporation into MgSiO3 enstatite

Abstract Hydrogen incorporation into orthoenstatite (Pbca), low-clinoenstatite (P21/c), and high-pressure clinoenstatite (C2/c) has been investigated using polarized and unpolarized infrared spectroscopy. Using shifts in OH stretching frequencies between the spectra and data from different crystal models, we test various models for hydrogen incorporation. The only significant differences between orthoenstatite and low-clinoenstatite spectra relate to anisotropy of the higher wavenumber bands, which implies a change in orientation of longer OH dipoles between the two structures. High-pressure clinoenstatite reverts to low-clinoestatite during depressurization, but subtle differences are noted between IR spectra of samples synthesized in the high-pressure clinoenstatite and low-clinoenstatite stability fields. Differences probably relate to the splitting of oxygen sites into two sets of non-equivalent sites during transformation of high-pressure clinoenstatite. The most realistic models for hydrogen incorporation into all three polymorphs involve association of hydrogen with the underbonded O2a and O2b sites. However, changes in OH dipole orientation between the different polymorphs and the effects of phase transitions on water solubility in the system MgSiO3 mean the effects and implications of hydrogen incorporation into the three polymorphs may differ considerably.

The origin of Earth’s first continents and the onset of plate tectonics

The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth’s mantle, hydrosphere, atmosphere, and biosphere for ~4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P-T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6–4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6–2.2 GPa and 900–950 °C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of ~30–45-km-thick Eoarchean oceanic crust, modified Earth’s mantle and Eoarchean environments and ecosystems.

Solid solutions and phase transitions in (Ca,M2+)M2+Si2O6 pyroxenes (M2+ = Co, Fe, Mg)

Abstract The effect of the substitution of Ca with Co, on the phase transition and on the extension of the miscibility gap, was studied to model the general mechanism of phase transitions and solid solutions in (Ca,M2+)M2+Si2O6 pyroxenes. Eleven pyroxenes with composition Ca1-xCo1+xSi2O6, (0 ≤ x ≤ 1) were therefore synthesized by piston cylinder at P = 3 GPa, and T between 1100 and 1350 °C. The samples were characterized by SEM-EDS, XRD powder diffraction, and TEM. The results were compared with those of Ca-Fe and Ca-Mg pyroxenes. The phase diagram of Ca-Co pyroxenes is similar to that of Ca-Fe and Ca-Mg ones, with a wide asymmetric miscibility gap, and higher solubility in the Ca-rich side of the gap. The solubility on the Ca-rich side of the gap is related to the radius of the cation substituting. The cell parameters of the Ca-Co pyroxenes undergo a sudden change at the composition of about 0.4 Ca apfu, due to the C2/c-P21/c phase transition. The change in volume with composition follows an ideal trend, in the C2/c phase, dictated by the ionic size of the substituting cation. Deviation from the C2/c behavior are instead observed in the P21/c field and ascribed to volume strain. The same turnover was found in Ca-Mg, Ca-Fe, and Ca-Mn pyroxenes. The C2/c-P21/c transition occurs with decreasing the M2 average cation radius, down to a critical value between 0.85 and 0.88 Å, depending on the series. A stabilization of the C2/c phase related to crystal field in Ca-Fe and Ca-Co pyroxenes is suggested by the analysis of the volume strain in the P21/c field. A key finding is that a miscibility gap may develop either by lattice strain related to cation substitution, within a series where all endmembers have the same structure, or for the combined effect of lattice strain and a phase transition, as is the case for pyroxenes.

A portable high-pressure stress cell based on the V7 Paris–Edinburgh apparatus

We describe a new device, based on a V7 Paris–Edinburgh press, for torsional testing of material at pressures up to 7 GPa (extendable to 15 GPa). Samples are deformed using a simple shear geometry between opposed anvils by rotating the lower anvil, via a rotational actuator, with respect to an upper, stationary, anvil. Use of conical anvil profiles greatly increases sample dimensions more than other high-pressure torsional apparatus did. Samples of polycrystalline Zr (2 mm thick, 3.5 mm diameter) have been sheared at strains exceeding γ ∼1.5 at constant strain rate and at pressures from 1.8 to 5 GPa, and textural development has been studied by electron microscopy. Use of amorphous-boron-epoxy gaskets means that nearly simple shear of samples can be routinely achieved. This apparatus allows study of the plastic and anelastic behaviour of materials under high pressure, and is particularly suited for performing in situ investigations using synchrotron or neutron radiation.