Water in Nominally Anhydrous Minerals: Hydrogen Incorporation and Defect Coupling
Nearly all of the nominally anhydrous minerals (NAMs) that compose Earth's mantle can incorporate small amounts of "water" in the form of interstitial hydrogen defects. As well as being a major repository for "water" in Earth's deep interior, this hydrogen also has a controlling influence on mineral properties and, presumably, on many mantle processes. Our understanding of how hydrogen is incorporated in mantle minerals has increased significantly over recent years, and it has been shown that hydrogen incorporation is dominantly charge-balanced by cation vacancies or lower valency cation substitutions. The techniques that we commonly use to investigate hydrogen incorporation in minerals share one major limitation. Although they can be used to give accurate information on possible sites for hydrogen incorporation in mineral structures, they often give us little or no information on how hydrogen interacts with other defects. It is widely assumed that intersititial hydrogen in NAMs is coupled with substitutional defects (lower valence cation substitutions) or cation vacancies to form stable defect complexes, even though experimental evidence to support this assertion is lacking. The degree to which hydrogen remains coupled to other defects controls not only the relative mobility of hydrogen, but also the influence that hydrogen has on mineral properties. We present results from an investigation of defect coupling in NAMs using a novel experimental technique. In situ annealing experiments have been used to determine how hydrogen interacts with other defects in neutron irradiated samples. Studies of H-bearing rutile (TiO2) demonstrate that defect coupling is more complex than previously thought. In-situ infrared (IR) analysis reveals that the main O-H stretching band in rutile spectra represents uncoupled hydrogen defects; hydrogen coupled with substitutional defects only results in minor features in IR spectra. Hydrogen in irradiated rutile remains uncoupled from other defects under high temperature conditions where there are strong driving forces towards defect coupling. Results can be used to re-interpret IR spectra from other NAMs. We present experimental data which demonstrate that only a certain proportion of the interstitial hydrogen present in minerals such as rutile and stishovite is coupled to substitutional defects. A significant proportion of the hydrogen present in these minerals always remains uncoupled from other defects. We discuss the implications that this has in terms of relative mobility of hydrogen under deep Earth conditions and the influence of hydrogen incorporation on mantle properties. We will also present new experimental data on hydrogen incorporation in spinel-type minerals which re-emphasises the importance that defect coupling has on understanding mechanisms for hydrogen incorporation in the deep Earth interior.
The Influence of Defect Associates on Diffusion in Periclase (MgO)
Interactions among point defects have a strong influence on diffusion rates in ionic crystals. In periclase, Coulombic attraction between trivalent cations (M3+) and cation vacancies leads to the formation of tightly bound pairs at conditions relevant to Earth's mantle. Binding to a vacancy enhances the diffusivity of the trivalent cation but inhibits the mobility of the vacancy, and thus reduces the ionic conductivity and the diffusivity of unbound cations. We will discuss the results of experiments designed to determine the mobility and binding energy of M3+-vacancy pairs in periclase. The binding energies determined from the diffusion experiments are in good agreement with theoretical estimates and with ionic conductivity measurements. Due to their binding to vacancies, the diffusion rates of trivalent cations in periclase are generally much faster than for divalent cations, opposite to what is observed in most silicate minerals. At the conditions of the core- mantle boundary, diffusion length scales in periclase may be up to a km or more, suggesting a possible pathway for significant chemical exchange between the core and mantle.
Cr3+-Vacancy and Ga3+-Vacancy Defect Pairs in MgO: Binding Energy, Mobility and the Influence of Electronic Structure
Trivalent impurities govern cation vacancy concentrations in most minerals, and thus play a central role in solid-state diffusion in the Earth. Although periclase is among the simplest of minerals, diffusion of trivalent cations is a complex process. Trivalent cations tend to bind to oppositely charged cation vacancies to form highly mobile pairs; the continual presence of a vacancy adjacent to the trivalent impurity allows it to move through the lattice much more rapidly than it would in the absence of binding. Diffusion experiments were performed to determine the mobility and binding energy of Cr3+- and Ga3+-vacancy pairs. Cr3+ and Ga3+ have almost identical ionic radii and polarizability, yet these experiments show that Ga3+ diffuses more than an order of magnitude faster than Cr3+, has a lower activation energy, and binds more tightly to the adjacent vacancy. All of these observations can be explained by the crystal field stabilization of Cr3+ on octahedral cation sites in MgO. Cr3+ contains only three d-orbital electrons whereas the 3d-shell of Ga3+ is full. This partial filling of the d-orbitals leads to a lowering of energy; the three d-electrons of Cr3+ occupy t2g orbitals that experience less repulsive interaction with the electrons of the six surrounding oxygen atoms. This crystal field stabilization increases the activation energy for Cr3+ migration to an adjacent vacancy. It also may explain the lower binding energy of vacancies to Cr3+. The crystal field stabilization is greatest for perfect octahedral symmetry; there is an energy cost associated with the presence of an adjacent symmetry-breaking vacancy, and consequently a reduction in the binding energy. The crystal field effect has long been known to influence the partitioning of first-row transition metals in minerals, but its influence on diffusion has not previously been considered. Our experimental results indicate that the crystal field effect may have an even stronger influence on diffusion rates than it does on partitioning.
Theoretical calculations of [AlO4/M+]0 defects in quartz and implications for the uptake of Al
Aluminum and other trace elements in quartz have attracted renewed interests in recent years, because of the development of various microbeam techniques making possible their high-precision analysis and applications as petrogenetic tools. Our knowledge about the nature and structural environments of Al in quartz comes almost exclusively from electron paramagnetic resonance (EPR) spectroscopic studies, which have identified a series of [AlO4]0 and [AlO4/M+]+ (where M = H, Li and Na) paramagnetic centers in this mineral. These paramagnetic centers are created by natural or artificial irradiations and have been interpreted to form from the diamagnetic [AlO4/M+]0 defects by acquiring an unpaired spin during irradiation, hence evidence for the coupled substitutions of Al3+ + M+ = Si4+ in quartz. However, it remains unclear how closely the geometric and electronic structural data from these paramagnetic centers reflect those of their respective diamagnetic precursors. In this study, we have performed ab initio calculations at the density functional theory (DFT) level for the [AlO4/H+]0, [AlO4/Li+]0, [AlO4/Na+]0 and [AlO4/K+]0 defects in á-quartz, by use of the CRYSTAL06 code, 72-atom supercells, and all-electron basis sets. Calculated geometries and stabilities of these [AlO4/M+]0 defects are then compared with available experimental data for their respective paramagnetic derivatives and are used to provide new insights into the uptake of Al and related trace elements in quartz.
Radiation-induced defect centers: Luminescence and optical absorption study of helium- irradiated diamond and zircon
The impact of radioactivity can generate optically active defect centers in minerals. These defects may first affect light absorption, i.e., they may cause radio-coloration or -de-coloration. Second, radio-induced defects may enhance or suppress luminescence emissions of their host minerals. The action of such centers is, for instance, seen in spotted diamond specimens showing green or brown radio-coloration (the latter is typically associated with yellowish-green photoluminescence). Another example is the yellow broad-band luminescence emission of zircon, which is commonly observed in cathodo- and photoluminescence spectra of this mineral. So study whether, and how, natural alpha radiation generates and affects such centers, flat polished diamond and zircon samples were irradiated in a tandem accelerator facility with 8.8 MeV He2+ ions, which are the analog of alpha particles generated in the 212Po α-decay (Th decay chain). Helium ions were found to penetrate 29 μm into diamond and 32 μm into zircon, respectively, which corresponds very well to ranges predicted by Monte Carlo simulations using the SRIM code. Notable pale green coloration of diamond was observed to start at 1014 to 1015 He/cm2. Spots irradiated with 1017 He/cm2 appeared dark green and were found to show initial amorphization. Green colors transformed to orange-brown through heat-treatment at about 550 °C, which is mainly due to the disappearance of the ∼16,000 cm-1 GR1 band. The latter process was found to be associated with the appearance of intense green UV-induced photoluminescence. Associated observations include strong volume expansion due to the accumulated radiation-damage, which may result in notable up-doming of radiohaloes. First studies of He-implanted zircon indicated a similar luminescence behavior, with an irradiation-induced broad-band yellow emission at 575 nm. This emission band decreases in intensity in samples that were affected by natural radiation damage prior to the He irradiation experiment. The observed depth profiles of the luminescence emission intensity in the two minerals correspond to the calculated defect distribution profiles but not to the ionization distribution profiles. This suggests that ionization alone is insufficient to create optically active centers. Additional carbon- (diamond) and oxygen-irradiation experiments (zircon) were done to generate similar structural damage with an elemental species which is already present in the respective host mineral. Observations on these samples are largely similar to those on He-irradiated samples. Consequently, optically active centers are related to structural point defects that are created by atomic knock-ons whereas our observations did not yield independent evidence which suggests that He ions themselves might be optically active species.
The Magnetic Effect of Dislocations in Small Ferromagnetic Minerals
Dislocations are known to have a major effect on magnetic hysteresis in large ferromagnetic crystals. This
effect arises from the coupling between the magnetization and the strain field around the dislocation, which
rises to a few percent at the dislocation core. The magnetization is organized in domains, and the walls
between domains move in response to a magnetic field unless they are "pinned" by a dislocations or other
Little is known about the abundance or nature of dislocations in submicron ferromagnetic minerals such as
magnetite. An extrapolation of dislocation densities from larger magnetite crystals predicts that they are rare,
but many of the likely mechanisms for crystal formation produce dislocations in crystals above a few
nanometers in size. In the few instances where anyone has looked for dislocations in submicron magnetite
crystals, they have found them.
Even less is known about how dislocations affect the magnetic properties of submicron minerals. On a
mesoscopic level, the coupling with the strain field produces a torque on the magnetization. The strains drop
off rapidly with distance from the dislocation core, but the coupling with magnetization averages to zero in
volumes centered about the core. The result is a subtle balance between magnetoelastic coupling and the
exchange coupling that gives rise to magnetic ordering. To model this, an accurate strain field must be
calculated for the dislocation. The boundary values for this calculation are very difficult because continuum
models for the strain go to infinity on the dislocation line.
A new computational approach developed at Lawrence Livermore Laboratories now solves the boundary value
problem for dislocation stress. This method can calculate the stress fields around screw dislocations in
octahedral crystals, and quantify the effect on the magnetization. Future work will apply this technique to model
dislocations within naturally occurring magnetite minerals using observations from transmission electron