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The effects of pressure on the OH stretching frequencies of natural talc and two samples of synthetic 10-[Angstrom], phase have been measured using a diamond-anvil cell and a synchrotron infrared source. The 10-[Angstrom] phase was synthesized at 6.0-6.5 GPa, 600[degrees]C for 46 hours (sample 10[Angstrom],-46) and 160 hours (10[Angstrom]-160). Spectra were collected up to 9.0 GPa (talc), 9.9 GPa (10[Angstrom]-46), and 9.6 GPa (10[Angstrom]-160). The OH stretching vibration of [Mg.sub.3]OH groups in talc occurs at 3677 [cm.sup.-1] at ambient pressure, and increases linearly with pressure at 0.97(2) [cm.sup.-1] G[Pa.sup.-1]. The same vibration occurs in 10-[Angstrom] phase, but shows negligible pressure shift up to 2 GPa, above which the frequency increases linearly to the maximum pressure studied, at a rate of 0.96(3) [cm.sup.-1] G[Pa.sub.-1] (10[Angstrom]-46) and 0.87(3) [cm.sup.-1] G[Pa.sup.-1] (10[Angstrom]160). Two other prominent bands in the 10-[Angstrom], phase spectrum are suggested to be due to stretching of interlayer [H.sub.2]O, hydrogen-bonded to the nearest tetrahedral sheet. These bands also show little change over the first 2 GPa of compression, as most of the compression of the structure is taken up by closing non-hydrogen bonded gaps between interlayer [H.sub.2]O and tetrahedral sheets. Between 2 and 4 GPa, changes in band intensities suggest a rearrangement of the interlayer [H.sub.2]O. Keywords: Talc, 10-[Angstrom] phase, high-pressure studies, IR spectroscopy

An asymmetric binary Margules formulation for excess Gibbs energy, enthalpy, and entropy is adequate to describe the 1 bar halite-sylvite solvus in NaCl-KCl (Thompson and Waldbaum 1969). However, a binary Margules formulation is not adequate to describe excess volumes of single-phase chlorides in P-V-T-X from ambient P-T to ~20 kbar and 700 [degrees]C. Excess volumes across NaCl-KCl increase with temperature, decrease with pressure, and show systematic deficits in the consolute region. These patterns can be explained by the importance of a third component--vacancy defects that relieve the lattice stresses of K-Na size mismatch. New, two-phase observations in P-V-T-X allow delineation of the excess Gibbs energies to high pressures where the excess enthalpies and entropies do not depend on Tat each P, but show significant variation between 1 bar and ~20 kbar. The volume, entropy, and enthalpy of K-Na mixing become more ideal at high pressure. But the solvus expands with pressure because entropy approaches ideality faster than enthalpy and, therefore, Gibbs energy of K-Na mixing becomes less ideal with pressure. The consolute temperature rises about 80 [degrees]C in 17 kbar, with little change in consolute composition. The binary Margules equation of state provides a prediction of the rise of the solvus that is impressively convergent with the new observations. This convergence is especially impressive given the clear inadequacies of the binary excess volume formulation on which the prediction is based.

Unit-cell volumes of four single-phase intermediate halite-sylvite solid solutions have been measured to pressures and temperatures of ~28 kbar and ~700 [degrees]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 [[alpha].sub.0]*[K.sub.0] is linear (ideal) in composition between the accepted values for halite and sylvite. Taken separately, the individual values of [[alpha].sub.0] and [K.sub.0] are not linear in composition. [[alpha].sub.0] shows a maximum near the consulute composition (XNacl = 0.64) that exceeds the value for either end-member. There is a corresponding minimum in [K.sub.0]. The fact that the [[alpha].sub.0]*[K.sub.0] product 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 [[alpha].sub.0] and K0 (significantly, near the consolute composition). These extrema in [[alpha].sub.0] and [K.sub.0] provide an example of intermediate properties that do not follow simply from values for the end-members. Cell volumes across this series show small, well-behaved positive excesses, consistent with K-Na substitution causing defects through lattice mismatches. Barrett and Wallace (1954) showed maximum defect concentrations in the consolute region. Defect-riddled, weakened structures in the consolute region are more easily compressed or more easily thermally expanded, providing an explanation for our observed [[alpha].sub.0] and [K.sub.0] variations. These compliant, loosened lattices should resist diffusive transfer less than non-defective crystals and, hence, might be expected to show higher diffusivities. Tracer diffusion rates are predicted to peak across the consolute region as exchange diffusion rates drop to zero.

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. [ABSTRACT FROM AUTHOR]

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 [alpha]*[KAPPA], which is better known than either [alpha] or [KAPPA], increases from 0.0195 [+ or -] 0.0005 kbar/[degrees]C in B1 to 0.0275 [+ or -] 0.0009 kbar/[degrees]C in B2 KCl. Yagi (1978) demonstrated a similar increase for KF, also supported mainly by increases in [alpha]. 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 [alpha]-[KAPPA] across the transition. Bond tightening upon decompression reduces [alpha], 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 [degrees] 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. Kinetics data for all stages of the crystallization process were determined by analyzing the growth and decline of various diffraction peaks with time. The activation energy ([E.sub.a]) (nucleation) for Z-phase is ~39 kJ/mol while that for gyrolite is ~56 kJ/mol. [E.sub.a] (crystallization) of gyrolite is higher at ~80 kJ/mol. The reaction occurs via a two-dimensional, diffusion-controlled mechanism and is a continuous process that suggests that Z-phase is an unstable, transient phase.

KCl[O.sub.3] thermal decomposition has been studied from 29-76 kilobars using a multianvil high-pressure device and in-situ energy-dispersive X-ray diffraction and off-line quenching experiments. The rhombohedral form of KCl[O.sub.3] was found to decompose to the B2 form of KCl and [O.sub.2] via an orthorhombic KCl[O.sub.4] intermediate 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 [degrees] C. Further off-line quenching experiments using sealed gold tubes determined the equilibrium decomposition boundary to be 550 [+ or -] 15 [degrees] C over this pressure range. Unit-cell parameters and volumes were determined for the high-pressure phases of KCl[O.sub.3] and KCl from the diffraction data. The partial molar volume of [O.sub.2] was calculated from the difference in the solid volumes. Oxygen fluid volumes were then calculated along the decomposition boundary and vary from 10.6 [+ or -] 0.2 c[m.sup.3]/mol at 29 kbar to 9.6 [+ or -] 0.1 c[m.sup.3]/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 KCl[O.sub.3] decomposition experiments. The mechanical behavior of the multianvil high-pressure device was also studied and load vs. force characteristics are presented here.

Presents a study which investigated the thermal decomposition of KClO[sub 3] using a multianvil high-pressure device and in-situ energy-dispersive X-ray diffraction and off-line quenching experiments. Experimental techniques; Characterization of the multianvil device; Pressure calibration.