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Volatile-sensitive electrical soundings are becoming more widely adopted, with large nationwide arrays currently being acquired globally. This boom in new data is despite several key uncertainties relating to the electrical responses of a wide range of minerals that make up crustal regions. Complications include the influence of mineral chemistry, hydrous or nominally hydrous phases, and oxygen fugacity on charge-carrying ion activity within a mineral substrate. Feldspars are the most abundant mineral group in the Earth’s crust, comprising about 60% of its mineral assemblages and are particularly prevalent within subduction zones and lower crustal sequences. These areas are known locations where ore systems are commonly rooted, making them among the most widely studied regions in the Earth. To date, few studies exist that cover the electrical behavior of the feldspar mineral albite. To help address some of these issues and complications, we have undertaken electrical conductivity investigations on a single crystal of gem-quality albite (Ab49An48Or3) from Nuevo Casas Grande, Chihuahua, Mexico. Electrical conductivity measurements using impedance spectroscopy were performed at a pressure of 1 GPa over a temperature range of 373–1273 K in a multi-anvil high-pressure apparatus. Experiments were carried out using different metal electrodes: molybdenum, nickel, and rhenium to vary the oxygen fugacity during the experiments. FTIR measurements of the starting and final materials confirm that the initial samples are completely dry but absorb an average of 67 ppm H2O by mass during the experiments from the surrounding pressure medium materials. We observe no correlation in the amount of water absorbed in the feldspar to the oxygen fugacity under water-undersaturated conditions. Our investigations show that the activation enthalpy increases from ~0.77 to ~1.0 eV from the nominally hydrous to the completely dry feldspar. The activation enthalpy decreases with increasing oxygen fugacity for comparable water contents. An oxygen fugacity exponent of –0.069 is calculated at the nominal water content measured in the experiments, indicating an electrical conductivity mechanism that also involves the mobility of hydrogen.

Starting with the same sample, the electrical conductivities of quartz and coesite have been measured at pressures of 1, 6, and 8.7 GPa, respectively, over a temperature range of 373–1273 K in a multi-anvil high-pressure system. Results indicate that the electrical conductivity in quartz increases with pressure as well as when the phase change from quartz to coesite occurs, while the activation enthalpy decreases with increasing pressure. Activation enthalpies of 0.89, 0.56, and 0.46 eV, were determined at 1, 6, and 8.7 GPa, respectively, giving an activation volume of –0.052 ± 0.006 cm3/mol. FTIR and composition analysis indicate that the electrical conductivities in silica polymorphs is controlled by substitution of silicon by aluminum with hydrogen charge compensation. Comparing with electrical conductivity measurements in stishovite, reported by Yoshino et al. (2014), our results fall within the aluminum and water content extremes measured in stishovite at 12 GPa. The resulting electrical conductivity model is mapped over the magnetotelluric profile obtained through the tectonically stable Northern Australian Craton. Given their relative abundances, these results imply potentially high electrical conductivities in the crust and mantle from contributions of silica polymorphs.

The 1 bar Curie temperature, TC, at which cementite (anthropogenic form of the mineral cohenite, nominally Fe3C) abruptly loses ferromagnetism, is found to be sensitive to small deviations from the stoichiometric cementite composition. Stoichiometric Fe3C begins to lose magnetic susceptibility at 187 °C. The TCof ferromagnetic loss in cementite falls by about 13-14 °C, in either compositional direction, to the limits at either Fe-saturation or graphite-saturation. Formation of C vacancies in, or C stuffings into, Fe3C produces non-stoichiometry, disrupts and weakens the Fe magnetic ordering, and produces excess configurational entropy that is proportional to the disruption magnitude. C-excess (~0.6 at% C) at graphite-saturation is less than the C-deficiency at Fe-saturation (~2.6 at% C), so the rate at which Curie TCdrops with cementite C% variation is asymmetric about the stoichiometric composition, being steeper on the C-excess side. This asymmetry reflects the higher excess configurational entropy (and consequently greater weakening of Fe magnetic ordering) generated by C excesses than by C vacancies.

Chalcogenide glasses with tetrahedral networks can undergo significant densification under pressure owing to their open structures. The structural mechanisms of pressure-induced densification and the corresponding evolution of physical properties of glassy GeSe4alloy are studied over pressures ranging between ambient and 32.5 GPa, using X-ray scattering supplemented with 3D Monte Carlo structural modeling, Raman spectroscopy, electrical conductivity, and P–Vequation of state measurements. The results demonstrate a pressure-induced, hysteretically reversible transition between low-density semiconducting and high-density metallic amorphous phases of GeSe4near ∼10–15 GPa. These two phases are characterized by their distinct P–Vequations of state and structural mechanisms of densification. Densification in the low-density phase is dominated by large inward shifting of the second neighbors with a small amount of conversion from edge-sharing to corner-sharing GeSe4tetrahedra. On the other hand, densification in the high-density phase involves a gradual increase in the nearest-neighbor coordination numbers of Ge and Se atoms and the formation of Ge–Ge bonds between adjacent polyhedral units. These structural transformations are accompanied by a pressure-induced metallization that is reversible.

Unit-cell volumes of four single-phase intermediate halite-sylvite solid solutions have been measured to pressures and temperatures of ~28 kbar and ~700 °C. Equation-of-state fitting of the data yields thermal expansion and compressibility as a function of composition across the chloride series. The variation of the product α0·K0is linear (ideal) in composition between the accepted values for halite and sylvite. Taken separately, the individual values of α0and K0are not linear in composition. α0shows a maximum near the consolute composition (XNaCl= 0.64) that exceeds the value for either end-member. There is a corresponding minimum in K0. The fact that the α0·K0product is variable (and incidentally so well behaved as to be linear across the composition series) reinforces the significance of the complementary maxima and minima in α0and K0(significantly, near the consolute composition). These extrema in α0and K0provide an example of intermediate properties that do not follow simply from values for the end-members.

Compressibility of solids generally drops as compression proceeds. Bonds shorten and stiffen with density increase in the same structure. However for the B1-B2 phase transition in many measured alkali halides, bond lengths increase in going to the denser phase. And IR mode frequencies decrease (Hofmeister 1997), leading to the counter-intuitive expectation that compressibility should increase in going to the denser B2 phase. Past volume measurements have been equivocal at best in recognizing any such compressibility increase. New thermal equations of state for B1 and B2 KCl from in-situ X-ray diffraction measurements of phase volumes on Station 16.4 of the CLRC Daresbury Laboratory are no less equivocal about the issue of whether compressibility increases or decreases across the transition. In contrast, the new volume measurements show an easily resolved thermal expansion increase in going from B1 to B2 KCl. The product a·K, which is better known than either α or K, increases from 0.0195 ± 0.0005 kbar/°C in B1 to 0.0275 ± 0.0009 kbar/°C in B2 KCl. Yagi (1978) demonstrated a similar increase for KF, also supported mainly by increases in a. This increase can also be seen in RbCl (Walker et al. 2001). Bond weakening indicated by the thermal expansion increase is consistent with the elusive compressibility increase that is expected across the B1-B2 transition but which is not resolved from volume measurements. The thermal effects are more visible than the compressional effects on α·K across the transition. Bond tightening upon decompression reduces α, increases solid viscosity, and hence decreases the Rayleigh number. An upwelling of material undergoing a decompression phase change with decrease of coordination number may have its convective friskiness damped at such a transition

Compressibility of solids generally drops as compression proceeds. Bonds shorten and stiffen with density increase in the same structure. However for the B1-B2 phase transition in many measured alkali halides, bond lengths increase in going to the denser phase. And IR mode frequencies decrease (Hofmeister 1997), leading to the counter-intuitive expectation that compressibility should increase in going to the denser B2 phase. Past volume measurements have been equivocal at best in recognizing any such compressibility increase. New thermal equations of state for B1 and B2 KCl from in-situ X-ray diffraction measurements of phase volumes on Station 16.4 of the CLRC Daresbury Laboratory are no less equivocal about the issue of whether compressibility increases or decreases across the transition. In contrast, the new volume measurements show an easily resolved thermal expansion increase in going from B1 to B2 KCl. The product a·K, which is better known than either α or K, increases from 0.0195 ± 0.0005 kbar/°C in B1 to 0.0275 ± 0.0009 kbar/°C in B2 KCl. Yagi (1978) demonstrated a similar increase for KF, also supported mainly by increases in a. This increase can also be seen in RbCl (Walker et al. 2001). Bond weakening indicated by the thermal expansion increase is consistent with the elusive compressibility increase that is expected across the B1-B2 transition but which is not resolved from volume measurements. The thermal effects are more visible than the compressional effects on α·K across the transition. Bond tightening upon decompression reduces α, increases solid viscosity, and hence decreases the Rayleigh number. An upwelling of material undergoing a decompression phase change with decrease of coordination number may have its convective friskiness damped at such a transition

The hydrothermal crystallization of gyrolite was studied dynamically at 190-240 °C using synchrotron- based in situ Energy Dispersive Powder Diffraction (EDPD). The reaction mechanism involves the initial crystallization of a calcium silicate hydrate (C-S-H) gel, which has a sheet structure with well ordered Ca(O,OH) layers and disordered silicate layers. This is followed by the intermediate formation of Z-phase which finally transforms to gyrolite. This process involves ordering of the silicate layers and an increase in the order along c.

The hydrothermal crystallization of gyrolite was studied dynamically at 190-240 °C using synchrotron- based in situ Energy Dispersive Powder Diffraction (EDPD). The reaction mechanism involves the initial crystallization of a calcium silicate hydrate (C-S-H) gel, which has a sheet structure with well ordered Ca(O,OH) layers and disordered silicate layers. This is followed by the intermediate formation of Z-phase which finally transforms to gyrolite. This process involves ordering of the silicate layers and an increase in the order along c.

KClO3thermal decomposition has been studied from 29-76 kilobars using a multianvil highpressure device and in-situ energy-dispersive X-ray diffraction and off-line quenching experiments. The rhombohedral form of KClO3was found to decompose to the B2 form of KCl and O2via an orthorhombic KClO4intermediate over this pressure interval. The decomposition temperature was found to vary only slightly with pressure. The online experiments gave decomposition temperatures between 500 and 580 °C. Further off-line quenching experiments using sealed gold tubes determined the equilibrium decomposition boundary to be 550 ± 15 °C over this pressure range. Unit-cell parameters and volumes were determined for the high-pressure phases of KClO3and KCl from the diffraction data. The partial molar volume of O2was calculated from the difference in the solid volumes. Oxygen fluid volumes were then calculated along the decomposition boundary and vary from 10.6 ± 0.2 cm3/mol at 29 kbar to 9.6 ± 0.1 cm3/mol at 76 kbar. These volumes are 30 to 50% less than previous estimates determined from shock wave data, and imply that oxygen can be more easily stored in Earth’s mantle and core than previously believed. The thermal equation of state of the B2 form of KCl was investigated online using NaCl as an internal pressure standard. KCl was then used as an internal pressure calibrant for the online KClO3decomposition experiments. The mechanical behavior of the multianvil high-pressure device was also studied and load vs. force characteristics are presented here.

KClO3thermal decomposition has been studied from 29-76 kilobars using a multianvil highpressure device and in-situ energy-dispersive X-ray diffraction and off-line quenching experiments. The rhombohedral form of KClO3was found to decompose to the B2 form of KCl and O2via an orthorhombic KClO4intermediate over this pressure interval. The decomposition temperature was found to vary only slightly with pressure. The online experiments gave decomposition temperatures between 500 and 580 °C. Further off-line quenching experiments using sealed gold tubes determined the equilibrium decomposition boundary to be 550 ± 15 °C over this pressure range. Unit-cell parameters and volumes were determined for the high-pressure phases of KClO3and KCl from the diffraction data. The partial molar volume of O2was calculated from the difference in the solid volumes. Oxygen fluid volumes were then calculated along the decomposition boundary and vary from 10.6 ± 0.2 cm3/mol at 29 kbar to 9.6 ± 0.1 cm3/mol at 76 kbar. These volumes are 30 to 50% less than previous estimates determined from shock wave data, and imply that oxygen can be more easily stored in Earth’s mantle and core than previously believed. The thermal equation of state of the B2 form of KCl was investigated online using NaCl as an internal pressure standard. KCl was then used as an internal pressure calibrant for the online KClO3decomposition experiments. The mechanical behavior of the multianvil high-pressure device was also studied and load vs. force characteristics are presented here.

A new facility for simultaneous extended X‐ray absorption of fine structure (EXAFS), X‐ray diffraction and photoluminescence measurements under high pressures has been developed for use on station 9.3 at the Daresbury Laboratory Synchrotron Radiation Source. This high‐pressure facility can be used at any suitable beamline at a synchrotron source. Full remote operation of the rig allows simultaneous collection of optical and structural data while varying the pressure. The set‐up is very flexible and can be tailored for a particular experiment, such as time‐ or temperature‐dependent measurements. A new approach to the collection of high‐pressure EXAFS data is also presented. The approach significantly shortens the experimental times and allows a dramatic increase in the quality of EXAFS data collected. It also opens up the possibility for EXAFS data collection at any pressure which can be generated using a diamond cell. The high quality of data collected is demonstrated with a GaN case study. Particular attention will be paid to the use of energy‐dispersive EXAFS and quick‐scanning EXAFS techniques under pressure.

Potassium hydrogen fluoride (KHF2) crystallizes in the tetragonal system, space groupI4/mcm,Z= 4,a= 5.668(2)Å,c= 6.801(7)Å, under ambient conditions. Energy-dispersive X-ray diffraction was used to study the compression behavior of KHF2in a diamond anvil cell. Unit cell parameters were determined at ten different static pressures between 1 atm and 12.3 GPa, and the calculated cell volumes were found to fit a third-order Birch–Murnaghan equation of state withK0= 25.1(15) GPa andK′ = 10.0(19). The compressibility is greatest by a factor of two parallel to z despite the short K–K distances in this direction, implying that F–F repulsion controls the compressibility behavior. A reconnaissance study to much higher pressure showed that KHF2remains in this structure down to 67.6% of its ambient volume, at a pressure estimated as 50 GPa. The tetragonal structure appears to be stable at higher P than thea3 pyrite structure, which is unlikely to have a stability field for KHF2. The high-pressure stability of theI4/mcmstructure is not surprising given that it is effectively the same as that of the highly coordinated CuAl2/Fe2B group of intermetallics. In contrast,AX2compounds withoutX–Xbonding adopt Ni2In-like structures at high pressure. Electron density studies and modeling suggest that the distinction between these two groups of compounds may not be rigid.

AbstractThe thermal dehydration of naturally occurring Ca-montmorillonite has been studied by in situX-ray diffraction at temperatures between 60–120°C. The time-temperature-dependence of the position of the basal (001) reflection reveals that interlayer water loss on isothermal dehydration occurs in two stages. After an initial rapid decrease in interlayer spacing (on shock heating to an isothermal soak temperature) the reaction proceeds towards equilibrium more slowly. Furthermore, the width of the (001) reflection changes with time, reflecting transformation-dependent changes in homogeneity perpendicular to (001) with a maximum in peak width at the point where the rate of the reaction appears to change. This suggests that, as the interlayer spacing collapses, a local change is induced in the structure, affecting the means of movement of the water from the interlayer.

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