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Nudged elastic band calculations of the (4H)XSi hydrogarnet type defect in Mg2SiO4forsterite
No full textFirst-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)XSi 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 ac- tivation 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 illus- trate 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
Electrical conductivities of pyrope-almandine garnets up to 19 Gpa and 1700°C
No full textElectrical conductivities of polycrystalline garnets ranging in chemical composition from almandine
(Fe3Al2Si3O12) to pyrope (Mg3Al2Si3O12) were measured at 10 GPa and 19 GPa at temperatures ranging from 300 to 1700 °C using complex impedance spectroscopy in a multianvil device. Mössbauer
spectroscopy of each sample was carried out both before and after the electrical measurements to characterize the oxidation state of Fe in the almandine bearing garnets. Similar to the behavior of other ferromagnesian silicates, the substitution of Fe for Mg along this compositional join dramatically increases electrical conductivity, but this compositional effect is reduced with increasing temperature. Conductivities increase with increasing total Fe content, as the average Fe2+-Fe3+ distance decreases. At 10 GPa, activation energies for conductivity vary smoothly with composition and increase rapidly toward the pyrope end-member composition, where it reaches a value of 2.5 eV. The results are consistent
with an electrical conductivity mechanism involving small polaron mobility in the Fe-bearing
garnets at 10 GPa. At 19 GPa, however, there is virtually no change in the activation energy as a function
of Fe-Mg substitution for the pyrope-rich garnets. These higher pressure measurements reflect a mechanism involving oxygen related point defects, as conductivities increase with pressure at constant
T for each garnet, and the effect of pressure is greater for the more Mg-rich garnets. The data also allow for a more quantitative evaluation of the effect of chemical composition, specifically Fe-Mg substitution, on the electrical conductivity profi le of the mantle, using a recently developed laboratory-
derived model. We apply the model using these data to a portion of the transition zone between 520 and 660 km, in which we vary the garnet composition from Py100 to Py85Alm15. Although only
a minor effect on bulk mantle conductivity results, we conclude that the overall garnet composition may, however, be important in characterizing the magnitude of any EC discontinuity with respect to the above-lying mantle
Electrical conductivity of hydrous wadsleite
No full textIn situ complex impedance spectroscopy of H2O-bearing wadsleyite was performed in a multianvil apparatus at 14 GPa at temperatures up to 950 °C in order to determine electrical conductivity. With increasing H2O content in wadsleyite the electrical
conductivity increases at a rate higher than observed in previous studies. The activation enthalpy in the temperature range studied
where proton conduction dominates is low (0.66 eV) suggesting an inevitable crossover to small polaron conduction at moderately
higher temperatures, depending on H2O concentration. Although the solubility of H2O in wadsleyite is significant for a silicate mineral (> 3 wt%), the presence of more than trace dissolved H2O in wadsleyite is likely to result in a conductivity too high compared to recent estimates of transition-zone conductivity. The use of complex impedance spectroscopy shows that the frequency dependence
of electrical properties is very different in the case of H2O-bearing silicate phases. At frequencies below 1000 Hz complex impedance spectra contain strong features which likely result from the sample–electrode interface such that including the low-frequency data would lead to artificially low conductivities
The Viscosities of dry and hydrous XAlSi3O8 (X=Li, Na, K, Ca0.5, Mg0.5) melts
No full textThe low-temperature viscosities of dry and hydrous X (X=Li, Na, K, Ca0.5, Mg0.5)AlSi3O8 melts have been investigated. The samples were hydrated via piston cylinder synthesis, and the water contents were subsequently determined
by Karl-Fischer titration (KFT) and IR spectroscopy. Both the anhydrous and hydrous viscosities were measured using the
micropenetration technique in the range of viscosities between 108.5 to 1011.9 Pa s, at 1 atm pressure and in the temperature ranges of 745–990 °C and 400–790°C for the dry and wet melts, respectively. The range of water content varied for all of the
samples from 0.70 to 3.13 wt.% H2O. The viscosities of dry melts vary, at fixed temperature, as a complex function of the identity of the cation in the order Li<Na<Ca<Mg<K. This trend is interpreted as due to the combined effects of cation
field strength and (Si, Al) distribution in these melts.
With the introduction of water into these melts, the viscosity decreases for all of the compositions investigated. As water
is further dissolved, the array of anhydrous viscosities converges into two distinct curves, for alkali-bearing and
alkaline-earth-bearing aluminosilicate liquids, respectively. In contrast to the insensitivity of viscosity to alkali cation
identity for hydrous melts, the alkali-aluminium ratio remains a sensitive control on viscosity. Thus, the viscosities of a
slightly peralkaline albite glass (Naexc)are lower than all of the others, both for the dry and the hydrous systems. We suggest that, in the case of alkaline-earth-bearing melts, an aluminium pair must be closely related to a doubly charged cation, to maintain electrostatic neutrality. The increase in the size of smallest rearranging species, which participates in the viscous flow process, as well as clustering of silica-rich and alumina-rich domains on an Aintermediate-rangeB scale, may be the factors resulting in the higher viscosities of Ca- and Mg-bearing compared to alkali-bearing liquids
Mixed electrical conduction in a hydrous pantellerite glass
No full textElectrical conductivity measurements were carried out on pantelleritic trachyte glasses containing up to 3.5 wt% dissolved H2O. At temperatures below about 700 K, we find evidence for
small polaron conduction due to the presence of both ferrous and ferric iron in the glass. At the higher
temperatures of the investigation (up to 973 K), a marked change in the conductivity-temperature
relation is observed, which suggests that an ionic conduction mechanism becomes the dominant means
of charge transport. In the ionic conduction regime, conductivity increases with increasing H2O
concentration. Activation energies are similar for both the anhydrous and hydrous glasses, indicating
that the conductivity is controlled by sodium diffusion even for the highest H2O concentration
examined. A slight variation in activation energy is observed depending on H2O concentration, which
is most likely associated with the depolymerising effect of dissolved water on the silicate network
structure. At low temperatures, we find a dramatic effect of fO2 on the conductivity that supports an
electronic conduction mechanism based on small polaron hopping between Fe3+ and Fe2+ sites. This
electronic pathway controls the overall electrical conductivity in these alkali-rich glasses at
temperatures exceeding 500°C if conditions remain anhydrous at an oxygen fugacity of 0.2 atm
Effect of Aluminum on Ti-coordination in Silicate Glasses: a XANES Study
No full textThe structure of glasses in the K2O-Al2O3-TiO2-SiO2 system was investigated using XANES spectroscopy. Glass samples, synthesized by quenching in air from high temperature fusions, represent
the addition of Al2O3 to a base of composition K2TiSi4O11 in amounts corresponding to 0.25,
0.50, 0.75, 1.00, 1.25, and 1.50 mol p.f.u. In the Ti-free system, this range of alkali/aluminum ratios
crosses the leucite stoichiometry at 1.0. Si K-edge and Al K-edge spectra indicate tetrahedral environments for these elements, and show no variations related to coordination change as a function of Al content. Changes in the relative intensities of peaks in the Al K-edge, however, suggest variation in the intertetrahedral (T-O-T) angle. We associate the decrease of this angle for the glasses of peraluminous composition with the presence of triclusters of tetrahedra. The pre-edge peak absorption
features in the Ti K-edge XANES spectra indicate that the average Ti coordination decreases with the addition of Al2O3. We infer depletion of fivefold-coordinated titanium (possibly as alkali
titanyl complexes), which are dominant in the Al-free glass, by the formation of fourfold coordinated
Ti and alkali aluminate complexes (up to a concentration of 40% in the most peraluminous glass). Significant amounts of [V]Ti remain present, even at peraluminous compositions, in further support of tricluster formation as a mechanism for Al incorporation
High temperature measurements of hydrous albite liquid using in situ falling sphere viscometry at 2.5 Gpa
No full textIn-situ falling-sphere viscometry using shadow radiography in a multianvil apparatus was conducted on a series of samples
along the NaAlSi3O8–H2O join up to 2.8 wt.% H2O at the Spring-8 synchrotron radiation facility (Hyogo, Japan). This allowed us
to determine viscosities normally too low to be measured at ambient pressure for hydrous silicate melts at high temperatures due to
rapid devolatilization. Pressure was fixed at 2.5 GPa for all experiments allowing us to gauge the effect of chemical composition on
viscosity. In particular, the series of samples allowed us to vary the melt's degree of polymerization while maintaining a constant Al
to Si ratio. Our results show that, for all samples, viscosity decreases as a function of pressure between 1 atm and 2.5 GPa at
1550 °C, indicating that the pressure anomaly can still be observed as depolymerization of the melt increases from nominally 0 (dry albite liquid) to NBO/T=0.8 (assuming water speciation entirely as hydroxyl groups at experimental conditions). We also find that the magnitude of the decrease in viscosity over this pressure interval does not appear to be dependent on the amount of water in the melt (i.e., NBO/T). An explanation for this behavior might be that the molar volume, at least over this limited compositional range, is
nearly constant and the effects of compression of these melts, though different in degree of polymerization, are similar
Compression mechanism in aluminosilicate melts: Raman and XANES spectroscopy of glasses quenched from pressures up to 10 Gpa
No full textRaman and XANES spectroscopy were carried out on a series of glasses of composition 44CaO–12Al2O3–44SiO2,
formed at pressures up to 10 GPa by isobaric quench from a temperature of 2200°C. The most significant changes in the Raman spectrum as a function of the synthesis pressure, or density, of the glass occur in the low-frequency region (300–700
cm-1), associated with T–O–T bending vibrations. With increasing density of the glass, the overall intensity at low frequencies decreases relative to the high-frequency portion of the spectrum. Relative intensities of bands within the low-frequency region of the Raman spectrum are also very sensitive to synthesis pressure, whereas there is little evidence that pressure influences Q-speciation as the high-frequency region of the spectrum remains virtually unchanged. With initial
compression (V/V0=1–0.96), the severe loss in intensity near 500 cm-1 indicates coordination of bridging oxygen atoms to an additional cation, which inhibits the vibrational motion that gives rise to this band normally observed for silicate
glasses formed at ambient pressure. At higher densities(V/V0<0.96), bands in the low-frequency region are shifted to higher frequencies, indicative of narrower T–O–T angles. No significant changes are observed in the Si and Ca K-edge XANES spectra with increasing densification of the glass. The Al K-edge spectra also show no significant changes among
the lower density glasses (V/V0=1–0.96), but reveal a feature near 1570 eV that dramatically increases in relative intensity
with increasing densification beyond V/V0=0.96. The observations from both Raman and XANES spectroscopy are
consistent with two different compression mechanisms operating in different pressure ranges. At lower pressures, the
spectroscopic data are characterized by features that we attribute to the presence of triclusters(OT3 units) in the quenched
melt. At higher pressures, T–O–T angle reduction and also an increase in the average coordination number of Al are likely to occur to further reduce the volume of the melt. The complex response of the structure of aluminosilicate melts to
compression suggests that their physical properties will also behave complexly as a function of pressure
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