Hydrogarnet Defect Research Articles

Water speciation and hydrogen isotopes in hydrous stishovite: implications for the deep Earth water cycle

Stishovite is a key mineral for understanding the deep Earth water cycle because of its potential as a main carrier for water into the transition zone and lower mantle. During subduction-related metamorphism of basaltic oceanic crust, stishovite stabilizes at 8–9 GPa and comprises 10–25 vol% of the bulk mineralogy, with some experimental studies indicating that stishovite can accommodate 3.5 wt% H2O or more in the transition zone and upper lower mantle. This large water solubility has been explained by a hydrogarnet substitution mechanism (1Si4+ ↔ 4H+) and/or the incorporation of interstitial molecular water. To investigate water speciation and hydrogen isotope behavior, we synthesized partially deuterated hydrous stishovite at 9 GPa and 450 °C in a multi-anvil press (MA). The hydrous stishovite contains on average 1.69 ± 0.05 wt% water, which is consistent with earlier MA studies but is significantly lower than the 3.5 wt% reported from in situ diamond anvil cell (DAC) studies made at higher pressures and temperatures. 1H MAS NMR spinning sideband characteristics suggest a high abundance of interstitial molecular water in hydrous stishovite, while the presence of a hydrogarnet defect cannot be ruled out. Unit-cell volumes and deuterium enrichment in the quenched hydrous stishovite indicate that ~ 45% of the water is lost from the stishovite upon quenching and decompression of the experiment, consistent with a higher solubility. This implies that the pristine water contents of a P–T–fO2 equilibrated hydrous stishovite cannot be quenched to 1 atm and room temperature from classical MA experiments. We further present a capillary-based recovery method for fluid from experimental capsules, allowing direct determination of the D/H ratio of the experimental fluid and indirect determination of the hydrous stishovite. Using Rayleigh modeling to account for the quench-related water loss, we find that, at 450 °C and 9 GPa, deuterium is 3.5–4.5 times enriched in hydrous stishovite relative to coexisting aqueous fluid. This is opposite of what is commonly observed for mineral–fluid pairs above 300 °C, rendering hydrous stishovite a potential sink for deuterium and decreasing the D/H ratio of coexisting aqueous fluids. Partial decomposition (30–60%) of hydrous stishovite during mantle upwelling and production of primary basaltic melts could be accompanied by high-temperature D/H fractionation, decreasing the hydrogen isotope composition of such melts towards “mantle-like” δD values between −75 and −220‰.

Nudged elastic band calculations of the (4H)SiX hydrogarnet type defect in Mg2SiO4 forsterite

Abstract First-principles calculations based on density functional theory (DFT) using the generalized gradient approximation (GGA) were performed to assess the energetic barriers separating different topological configurations of the (4H)SiX hydrogarnet type defect in Mg2SiO4 forsterite with the climbing image nudged elastic band (CI-NEB) method. Barrier heights are low (<0.6 eV) with respect to typical activation energies observed for H-diffusion but more comparable to those for electrical conductivity of H2O-bearing nominally anhydrous minerals. As can be expected, hydrogen bonding to O atoms both within the defect and belonging to adjacent tetrahedra plays a fundamental role in the stability of each configuration. Saddle points along the minimum energy path (MEP) typically correspond to the transition of one hydrogen bond breaking to form a new hydrogen bond such that one or more OH bonds have shifted in direction without themselves breaking. MEPs show that slightly out-of-plane torsional hopping from one configuration to another can reduce the height of the barrier. We illustrate several different reaction coordinates between symmetry equivalent pairs of configurations and non-symmetry related pairs that can result in an effective means of local charge transport by shifting the center of mass of the (4H)4+ cluster within the defect site without proton transfer to an interstitial site. Especially at low temperatures in the absence of thermally activated processes that result in the breaking of stronger chemical bonds, these types of configurational transformation mechanisms are likely to be important contributors to the dielectric behavior of nominally anhydrous silicate minerals and also affect both electrical conductivity and electrical conductivity anisotropy when investigated by AC methods such as impedance spectroscopy. The NEB method can also be used to examine more effective charge and mass transport processes that involve the dissociation of the hydrogarnet defect into more complex chemical species, which might involve similar hydrogen bond breaking and forming processes observed in this study along with more significant atomic displacements.

High-Temperature Evolution of Point Defect Equilibria in Hydrous Forsterite Synthesized at 1100 °C and up to 4 GPa

Water distribution in the deep Earth represents one of the most important topics in the field of geodynamics due to its large impact on the physical and chemical properties of the Earth’s mantle, such as electrical conductivity, seismic anisotropy, diffusion, and rheology. In this study, we synthesized hydrous forsterite at 1100 °C and up to 4 GPa with either a piston-cylinder or multianvil apparatus. As a starting material, we used synthetic forsterite, unbuffered by SiO2, obtained by thermo-mechanical activation of talc and magnesium carbonate hydroxide. Hydration was carried out using liquid H2O as hydrogen source. Samples were polycrystalline in an effort to distribute H2O throughout the sample both rapidly and homogeneously. Using the Paterson calibration, we observed total water content concentrations ranging between 100 and 500 ppm wt H2O. Multiple absorption bands are found in the frequency range between 3400 and 3650 cm−1, identifying at least seven peaks in all samples. Vibrational bands were centered at 3476, 3535, 3550, 3566, 3578, 3605, and 3612 cm−1, in good agreement with experimental studies conducted on both hydrous forsterite and single crystals of olivine. The stronger OH stretching peaks can be attributed to vibrational modes associated with the hydrogarnet defect 4 H Si x in which four protons occupy a vacant tetrahedral site. None of the OH bands observed are found at frequencies associated with hydrogen occupying vacant octahedral sites. High-temperature FTIR spectroscopy was used to evaluate the evolution of IR spectra as a function of temperature, up to 500 °C. The complete reversibility of peak absorption vs. temperature in the OH stretching region confirms that no water loss occurred during heating. We observe an overall a decrease in total absorption with increasing temperature, and a prominent decrease in the relative intensities of the higher frequency bands (>3600 cm−1) with respect to lower frequency bands. We have assigned a series of equilibrium expressions based on the variation of relative peak areas with temperature and find that enthalpies of these processes range between 0.047–0.068 eV (4.5–6.5 kJ/mol), very low in comparison to activation energies observed for electrical conduction in hydrous olivine. Major changes in the vibrational spectrum are expected to be related to configurational changes of the same fully protonated hydrogarnet defect species. However, the complexity of the FTIR spectra may also be related to partially protonated defects, such as the associate defect 3 H Si ′ + H i • generated by a dissociation reaction of the hydrogarnet species.

Hydrothermal Synthesis, Crystal Structure Determination, and Magnetic Properties of a New Calcium Iron Iridium Hydrogarnet Ca3(Ir2–xFex)(FeO4)2–x(O4H4)1+x with 0 ≤ x ≤ 1

The new calcium iron iridium hydrogarnet Ca3(Ir2–xFex)(FeO4)2–x(H4O4)1+x (0 ≤ x ≤ 1) was obtained by hydrothermal synthesis under strongly oxidizing alkaline conditions. The compound adopts a garnet‐like crystal structure and crystallizes in the acentric cubic space group I43d (no. 220) with a = 12.5396(6) Å determined at T = 100 K for a crystal with a refined composition Ca3(Ir1.4Fe0.6)(FeO4)1.4(O4H4)1.6. Iridium and iron statistically occupy the octahedrally coordinated metal position, the two crystallographically independent tetrahedral sites are partially occupied by iron. Hydroxide groups are found to cluster as hydrogarnet defects, i.e. partially substituting oxide anions around the empty tetrahedral metal sites. The presence of hydroxide ions was confirmed by infrared spectroscopy and the hydrogen content was quantified by carrier gas hot extraction; the overall composition was verified by energy dispersive X‐ray spectroscopy. The structure model is supported by 57Fe‐Mössbauer spectroscopic data evidencing different Fe sites and a magnetic ordering of the octahedral iron sublattice at room temperature. The thermal decomposition proceeds via three steps of water loss and results in Ca2Fe2O5, Fe2O3 and Ir. Mössbauer and magnetization data suggest magnetic order at ambient temperature with complex magnetic interactions.

Hydrothermal synthesis and structure determination of a new calcium iron ruthenium hydrogarnet

Abstract A new calcium iron ruthenium hydrogarnet with the approximate composition Ca3(Ru2− x Fe x )(FeO4)2− y (H4O4)1+ y (x=1, y≈0.35) has been obtained by hydrothermal synthesis under oxidizing alkaline conditions. The compound crystallizes in the cubic space group Ia3̅d (No. 230) with a lattice parameter of a=12.4804(4) Å (T=100 K) and Z=8. The octahedral site of the garnet structure is equally occupied by Ru and Fe, whereas the tetrahedral site is partially occupied by Fe only. A partial substitution of the oxide anions by hydroxide ions is necessary for charge balancing, corresponding to the so-called hydrogarnet defects. The presence of hydroxide groups is proven by infrared spectroscopy. 57Fe Mössbauer spectroscopic data provide evidence for two different Fe3+ coordination environments as well as a magnetic ordering of two iron substructures with the respective ordering temperature above room temperature. The crystal composition was verified by energy-dispersive X-ray spectroscopy and the thermal behavior of the calcium iron ruthenate was studied by difference thermal analysis.

Nanoporous silica gel structures and evolution from reactive force field-based molecular dynamics simulations

Nanoporous silica-rich gel formed on silicate glass surfaces during dissolution in aqueous environment is critical in elucidating the corrosion mechanisms and the long-term residual dissolution behaviors. Silica gel models were created using two types of methods with reactive force field-based molecular dynamics simulations. The results show that the remnant silica gels created from the ISG bulk structure have a more isolated and closed pore morphology and slightly higher glass network connectivity. This contrasts with the gel structures created by hydrogarnet defect formation that exhibit more connected pore morphologies. The remnant gel structures show lower water diffusivity which was explained by the nano-confinement effect of water molecules due to frequent interactions of water molecules with adjacent silica walls and the more isolated pore morphology in the remnant gel structures. These results reveal the complexity in terms of micro and atomic structures of these silica gels, and both structure features have impact on water transport in the gel layer hence the passivating effect that controls the long-term dissolution behavior of these glasses.

Different route of hydroxide incorporation and thermal stability of new type of water clathrate: X-ray single crystal and Raman investigation

Chlormayenite Ca12Al14O32[♦4Cl2] (♦-vacancy) is partially hydrated micro porouss mineral with hydroxide groups situated at various crystallographic sites. There are few mechanisms describing its hydration. The first one assumes Cl− substitution by OH− at the center of the structural cages (W-site). The second one determines the converting a T1O4 tetrahedron to a T1O3(OH)3 octahedron due to the replacement of oxygen at the O2 site by three OH-groups according to the scheme: (O2O2− + WCl−) → 3 × O2aOH. The third mechanism, not considered so far in the case of zeolite-like minerals, includes the hydroxide incorporation in form of hydrogarnet defect due to the arrangement of tetrahedral (OH)4 in vacant cages. This yields a strong hydrated phase containing even up to 35% of water more than in any currently known mineral applicable to Portland cement. Moreover, water molecules present in different structural cages are stable up to 355 K while dehydroxylation linked to the gradual loss of only 8% of OH− groups according to 3O2aOH− → O2O2− +WOH− + gH2O occurs at temperature range from 355 K to 598 K.

Structural characterization of marine nano-quartz in chalk and flint from North Sea Tertiary chalk reservoirs for oil and gas

A new type of quartz, a nano-quartz consisting of spherical particles, is assumed to have formed by crystallization in the sea during sedimentation of the chalk in the North Sea and to have remained largely intact during burial diagenesis. The presence and nature of this nano-quartz have not been identified until recently, despite the fact that this quartz is the type present in the Upper Cretaceous-Danian chalk reservoir for oil and gas in the North Sea, both in the flint and as dispersed particles. In the present work detailed structural analysis of the nano-quartz has been carried out by X-ray diffraction, solid-state 29 Si and 27 Al MAS NMR, thermal analysis including water release, IR-absorption spectroscopy, and elemental analysis supplemented with analysis of oxygen isotope composition. It is found that IV Al substitutes for Si and that VI Al probably is bonded to hydroxyls on the particle surface of the nano-quartz structures. The charge compensation of tetrahedral Al 3+ , in addition to its conventional way of compensation by formation of the local structural arrangement [AlO 4 /M + ]° defects (M + = H + , Na, K, Li), can occur at the expense of the OH − group coordinating one of the four tetrahedral Si 4+ nearest to the Al 3+ tetrahedron. The most significant feature of the North Sea nano-quartz deduced in the present investigation is the presence of [4H] Si defects, also known as hydrogarnet defects. This defect is present in up to 5% of the tetrahedral sites, whereas Al 3+ occupies less than 1% of the tetrahedral sites. Two types of distribution of the [4H] Si defects were determined. In one of them the [4H] Si defects aggregates parallel to the (0001) plane to form platelets as cracks with hydroxylated surfaces on both sides. The second type of [4H] Si defect occurs in the form of isolated tetrahedral vacant sites.The formation of the aggregated [4H] Si platelets lying in the (0001) plane mostly increases the c parameters of the structure, whereas the isolated [4H] Si defects and K+Na impurities contribute to increasing the a parameters. The remarkable correlation of the positional distribution of the samples revealed from the relationships between a and c parameters and between amount of OH − groups responsible for formation of [4H] Si defects and a and c parameters can be considered as evidence for the validity of the structural formulas and, in general, of the main structural features of the studied samples. The unusually high content of [4H] Si defects in the nano-quartz samples may be related to their formation by precipitation in waters of the Danish North Sea.

Hydrogen mobility in transition zone silicates

We study the hydrogen mobility in ringwoodite and wadsleyite considering multiple charge-balanced defects, including Mg < = > 2H, Si < = > Mg + 2H, and the hydrogarnet defect, Si < = > 4H, using molecular dynamics simulations based on the density functional theory at transition zone pressures and temperatures between 1500 and 2500 K. We determine the diffusion coefficients and study in detail the mechanism of hydrogen mobility during lengthy simulations. Our results show that temperature, water concentration, and defect mechanism have a significant effect on mobility. We find that the fastest diffusion is for the Mg < = > 2H defect, while H is more mobile when incorporated as Si < = > Mg + 2H than as hydrogarnet defects. The computed diffusivities for ringwoodite are larger than for wadsleyite: at 2000 K, diffusivity is 1.13 × 10−09 m2/s for ringwoodite compared to 0.93 × 10−09 m2/s for wadsleyite. In general, the hydrogen atoms spend on the order of tens of picoseconds or more trapped in or around the vacancy sites with net migration between sites over timescales of tens of femtoseconds. At 2500 K, some of these hydrogen excursions take place over several angstroms, while at 2000 K, they do not always result in net diffusion. At 1500 K, most of the defects fail to make excursions from their defect sites resulting in diffusion.

Presentation of the Dana Medal of the Mineralogical Society of America for 2016 to Patrick Cordier

It is with great pleasure and with enthusiasm, I propose Patrick Cordier for MSA Dana medal. The former President and Vice-President of the Societe Francaise de Mineralogie et de Cristallographie and co-author of the book for the general public on mineralogy Ce que disent les mineraux (“What do the minerals say”), he is one of the few people that capture the essence of modern mineralogical research allied to concepts coming from materials science and physics. A physicist by training and now professor of physics at the University of Lille, he embarked on the route to become one of the world’s leading mineralogists by studying the plastic deformation of quartz and his Ph.D. in 1989. What could be simpler than to understand the plasticity of mineral with the composition of SiO2? Indeed, in reality it was a very difficult task, a subject rich in experimental folk law and dubious observations, the young researcher developed a methodology based on rigorous experimental protocol, quantitative state-of-the-art transmission electron microscopy (TEM) characterization of defect densities and their crystallography, and pertinent theoretical analysis of the results. This first adventure into the field of mineralogical research produced several well-documented publications using mechanical experiments, heat treatment, infrared spectroscopy, electron spin resonance (ESR), and, of course, TEM. The major results from this early period were the first and, I believe even today, the only determination of the equilibrium concentration of the hydro-garnet defect in quartz and the development of new vision of the importance of hydro-garnet defect (4H in Si site) (identified by ESR) in the hydrolytic weakening of quartz facilitating the easy glide of dislocations. In the following years, Patrick expanded his activities to include the characterization of shocked minerals from meteorite impacts and interstellar material. The collaborative work on Comet 81P/Wild 2, published in two …

Calculation of the energetics of water incorporation in majorite garnet

Interpretation of lateral variations in upper mantle seismic wave speeds requires constraints on the relationship between elasticity and water concentration at high pressure for all major mantle minerals, including the garnet component. We have calculated the structure and energetics of charge-balanced hydrogen substitution into tetragonal MgSiO 3 majorite up to P = 25 GPa using both classical atomistic simulations and complementary first-principles calculations. At the pressure conditions of Earth’s transition zone, hydroxyl groups are predicted to be bound to Si vacancies (□) as the hydrogarnet defect, [□ Si +4OH O ] X , at the Si2 tetrahedral site or as the [□ Mg +2OH O ] X defect at the octahedral Mg3 site. The hydrogarnet defect is more favorable than the [□ Mg +2OH O ] X defect by 0.8-1.4 eV/H at 20 GPa. The presence of 0.4 wt% Al 2 O 3 substituted into the octahedral sites further increases the likelihood of the hydrogarnet defect by 2.2-2.4 eV/H relative to the [□ Mg +2OH O ] X defect at the Mg3 site. OH defects affect the seismic ratio, R = dlnvs/dlnvp, in MgSiO 3 majorite (ΔR = 0.9-1.2 at 20 GPa for 1400 ppm wt H 2 O) differently than ringwoodite at high pressure, yet may be indistinguishable from the thermal dlnvs/dlnvp for ringwoodite. The incorporation of 3.2 wt% Al 2 O 3 also decreases R(H 2 O) by ~0.2-0.4. Therefore, to accurately estimate transition zone compositional and thermal anomalies, hydrous majorite needs to be considered when interpreting seismic body wave anomalies in the transition zone.

Mechanism of Si Island Formation in SAPO-34

With the aim of understanding the Si island formation in SAPO-34, we have carried out a computational mechanistic study. Briefly, the Si island formation in SAPO-34 is explained by three successive reactions. First, the framework Si atom is removed from the framework through the action of four water molecules. Second, the hydrogarnet defect generated by the desilication is healed by an available H3PO4 molecule. Third, the extra framework Si(OH)4 species inserts in the framework position of a phosphorus atom while, in a concerted fashion, “kicking out” the phosphorus atom as a H3PO4 extra-framework species. When these exchanges of framework and extra-framework species are repeated, the isolated Si atoms may eventually cluster into Si islands.

Ab initio study of structurally bound water at cation vacancy sites in Fe- and Al-oxyhydroxide materials

The structure and energetics of proton-compensated cation vacancies in crystalline Fe- and Al-oxide and oxyhydroxide materials are investigated using ab initio methods. In this defect model, a vacant Me3+ cation site is charge compensated by the presence of three protons, forming hydroxyls with the O atoms surrounding the vacant cation site. Proton-compensated cation vacancies are chemically equivalent to excess hydroxyl content, and are also known as hydrogarnet defects, or Ruetschi defects. These defects can be considered a particular form of structurally bound water, as the formation of the defect can be written as the product of the ideal crystalline material and water. Proton-compensated cation vacancy defects are shown to cause lattice expansion in all calculated Fe and Al materials, and are shown to destabilize all materials relative to the ideal crystalline phases. The magnitude of the destabilization due to the vacancy defects is structure dependent, thus defect content can induce shifts in the relative stability between crystalline phases.In all of the Fe-oxyhydroxide materials, proton-compensated cation vacancy defects are shown to be slightly co-stabilized in the presence of nearby Al dopant atoms, likely due to the stronger nature of Al–OH bonding (relative to Fe–OH), or from the cancellation of the opposing lattice strains introduced by the two defect types when considered in isolation. FTIR data in the literature confirms that these two defect types (proton-compensated cation vacancies and Al substitutions) have been observed to occur in tandem.Infrared vibrational frequencies are calculated for the non-stoichiometric hydroxyl groups found at the vacancy defect sites and compared with those of stoichiometric OH groups found in ideal crystalline (oxy)hydroxides. The calculated O–H stretching modes of the defect hydroxyls have higher frequencies than the modes of stoichiometric hydroxyl groups found in Fe-oxyhydroxide materials, consistent with experimental FTIR observations.

Theoretical study of OH-defects in pure enstatite

The infrared spectroscopic properties of selected defects in orthoenstatite are investigated by first-principles calculations. The considered defects include doubly protonated Mg vacancies at M1 and M2 sites, fully protonated SiA and SiB vacancies (hydrogarnet defects), and doubly protonated SiA and SiB vacancies associated with interstitial Mg2+ cations. The bands observed at 3,070 and 3,360 cm−1 in the spectrum of synthetic enstatite samples are ascribed to O2A–H and O2B–H groups, respectively, associated with M2 vacancies. The theoretical models suggest that bands observed at 3,590 and 3,690 cm−1 in the spectrum of enstatite samples synthesized under low silica-activity conditions correspond to O2H and O1H groups associated with SiB vacancies partially compensated by interstitial Mg2+ cations in fivefold coordination. The theoretical relation between the integrated absorption coefficient of OH-defects and vibrational frequencies is consistent with previous observations indicating that the absorption coefficients of OH-defects are comparatively stronger in enstatite than in the olivine polymorphs.

OH defects in quartz in the system quartz–albite–water and granite–water between 5 and 25 kbar

The incorporation of OH defects in quartz from the systems quartz–water, quartz–albite–water and granite–water at pressures between 5 and 25 kbar and temperatures between 800 and 1,000 °C was investigated by IR spectroscopy. The two most important OH absorption features can be assigned to hydrogarnet defects (absorption band at 3,585 cm−1) and coupled substitutions involving Al3+ (Al–H defects, absorption bands at 3,310, 3,378 and 3,430 cm−1). Al incorporation in quartz is controlled by mineral/melt partitioning (D l Qz/Melt = 0.01) and exhibits a negative pressure dependence. This trend is not clearly reflected by the concentration of Al–H defects, which shows positive deviations from the theoretical 1:1 correlation of Al/H for some samples. In contrast to the Al–H defects, formation of hydrogarnet defects appears to be positively correlated to pressure and water activity, and may be used a petrological indicator. The overall water concentration exhibits only minor changes with pressure and temperature, but a clear correlation of water activity (controlled by various amounts of dissolved salts) and hydrogarnet substitution could be established.

Coupled (Li+, Al3+) substitutions in hydrous forsterite

Atomistic computer simulations methods are used to examine the influence of Li and Al impurities on the uptake of hydrogen in forsterite. We find that Li′ Mg +OH • O is more stable at the Mg1 site than at the Mg2 site and that Li + increases the ability of forsterite to incorporate hydrogen associated with magnesium sites. When both Al and Li are present, then a complex comprising a bound Al • Mg2 –Li′ Mg1 defect is highly stable. When all three impurity components are mixed together, then hydrogen will strongly partition to Si vacancies forming the hydrogarnet defect. Thus the ability of forsterite to incorporate water is likely to be intimately linked to the nuances of defect chemistry, and to concentrations of impurity elements such as Li + and Al 3+ .

Theoretical infrared spectrum of OH-defects in forsterite

The infrared spectroscopic properties of selected defects in forsterite are investigated by first-principles calculations, starting from previously proposed OH-incorporation mechanisms. The considered defects include the fully protonated M1 vacancy, fully protonated Si vacancy (hydrogarnet defect), and protonated Si vacancy associated with Ti incorporation at the M1 site (''titanoclinohumite'' defect). Our findings establish that the previously proposed models for Mg and Ti defects are consistent with the observed infrared spectra. In contrast, a new model of Si defect is proposed, based on relative energies, vibrational frequencies, polarization properties and absorption coefficients. The theoretical results emphasize the critical role of the coupled vibration of OH groups in the relative intensities of OH stretching absorption bands. In contrast integrated absorption coefficients are less affected by coupling. The observed theoretical relation between the integrated absorption coefficient of OH defects and vibrational frequencies is consistent with recent experimental data.

A transmission electron microscopy study of experimentally deformed quartzite with different degrees of doping

TEM study of Heavitree quartzite deformed at high temperatures and a pressure of 1.5 GPa with three different preparations: water-added, vacuum dried, and sodium doped show a wide range of dislocation microstructure. The vacuum dried sample has a very heterogeneous dislocation distribution varying from almost no dislocations to very high densities. Dislocation nucleation occurs by cross-slip mechanisms, such as Frank-Read sources and Orowan loops around hard inclusions. Shear bands develop in the basal plane composed of two closely spaced shear planes. Dislocations have straight segments and they are aligned along directions of dense packing, suggesting strong Peierls stress control due to the crystal structure. No voids, bubbles or dislocation walls where observed in the vacuum dried sample. The sodium-doped sample had a homogeneous high dislocation density. The water-added sample has a significantly lower, relatively uniform dislocation density. Bubbles associated with dislocations are seen in every grain. Dislocations were either present in loops around bubbles or as straight dislocations with small bubbles spaced along the dislocation line. Some of the long straight dislocations are aligned parallel to the c-axis. Sub-grains where very frequent, sometimes form cells of about 2 μm in diameter with straight edges. Analysis of dislocations show 1/3<a> and [c] dislocations in sub-grain walls and free 1/3<a> dislocations on prism planes. There is clear evidence for dislocation climb with sub-grain walls, dislocation cells, dislocation junctions and dislocation debris of small loops. The major difference between vacuum dried and water-added samples are the homogeneity of the microstructure and evidence for climb in the water-added sample. Glide is clearly difficult, with a high Peierls stress in the vacuum dried sample as shown by areas of very low and very high dislocation density and the crystallographic control of dislocation line direction. The sodium-doped sample indicates that nucleation was easy by the high homogenous dislocation density, but no climb recovery has taken place. The density is high and the dislocations are strongly interacting causing tangles; no evidence for crystallographic control can be observed at these levels of strain. The activation energy for creep in Heavitree quartzite decreases with inferred water content of the specimens form 185 kJ mol −1 for vacuum dried to 151 for 0.4 wt% water-added samples. Analysis of diffusion data for oxygen under hydrothermal conditions and inferred diffusion data for the hydrogarnet defect and dislocation velocity suggests activation energies for these processes are similar to the activation energy for the dislocation creep of Heavitree quartzite and other quartz aggregates. For data from the previously published experiments with the highest stress resolution there is a correlation between the A pre-factor in the power law creep equation and the activation energy for creep. It is speculated that the correlation may be due to variable hydrogarnet defect concentrations.

A computer simulation study of OH defects in Mg2SiO4 and Mg2GeO4 spinels

Classical atomistic simulation techniques have been used to investigate the energies of hydrogen defects in Mg2SiO4 and Mg2GeO4 spinels. Ringwoodite (γ-Mg2SiO4) is considered to be the most abundant mineral in the lower part of the transition zone and can incorporate large amounts of water in the form of hydroxyls, whereas the germanate spinel (γ-Mg2GeO4) corresponds to a low-pressure structural analogue for ringwoodite. The calculated defect energies indicate that the most favourable mechanisms for hydrogen incorporation are coupled either with the reduction of ferric iron or with the creation of tetrahedral vacancies. Hydrogen will go preferentially into tetrahedral vacancies, eventually leading to the formation of the hydrogarnet defect, before associating with other negatively charged point defects. The presence of isolated hydroxyls is not expected. The same trend is observed for germanate, and thus γ-Mg2GeO4 could be used as a low-pressure analogue for ringwoodite in studies of water-related defects and their effect on physical properties.

Hydrogarnet defect in chabazite and sodalite zeolites: A periodic Hartree–Fock and B3-LYP study

The geometric, energetic, and vibrational features of the hydrogarnet defect in chabazite (CHA) and sodalite (SOD) have been studied at the Hartree-Fock and B3-LYP levels by using the periodic ab initio CRYSTAL code based on localized Gaussian basis functions. The geometry of the defective structures (CHA-HG and SOD-HG) has been fully optimized at both levels of theory by fixing the unit cell parameters to the values of the defect-free structures. The local structure of the defect is dictated by the hydrogen bond interaction among the OH groups, the strength of the shortest hydrogen bond in SOD-HG being 47 and 55 kJ/mol at the HF and B3-LYP levels, respectively. The reaction of CHA and SOD with gas phase water has been shown to be exothermic when either orthosilicic acid or α-quartz are formed as a product. On the contrary, the reactions are strongly endothermic with respect to liquid water. The vibrational spectra in the high-frequency OH stretching region for both SOD-HG and CHA-HG have been simulated using the ab initio harmonic OH stretching frequencies and compared to the experimental spectra of the mineral katoite and defective silicalite. Structure, geometry, hydrogen bond strength, defect formation energy, and OH stretching frequencies turn out to be significantly dependent on the adopted Hamiltonian for both CHA-HG and SOD-HG.