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Amorphous water ice comes in at least three distinct structural forms, all lacking long-range crystalline order. High-density amorphous ice (HDA) was first produced by compressing ice I to 11 kilobar at temperatures below 130 kelvin, and the process was described as thermodynamic melting1, implying that HDA is a glassy state of water. This concept, and the ability to transform HDA reversibly into low-density amorphous ice, inspired the two-liquid water model, which relates the amorphous phases to two liquid waters in the deeply supercooled regime (below 228 kelvin) to explain many of the anomalies of water2 (such as density and heat capacity anomalies). However, HDA formation has also been ascribed3 to a mechanical instability causing structural collapse and associated with kinetics too sluggish for recrystallization to occur. This interpretation is supported by simulations3, analogy with a structurally similar system4, and the observation of lattice-vibration softening as ice is compressed5,6. It also agrees with recent observations of ice compression at higher temperatures-in the 'no man's land' regime, between 145 and 200 kelvin, where kinetics are faster-resulting in crystalline phases7,8. Here we further probe the role of kinetics and show that, if carried out slowly, compression of ice I even at 100 kelvin (a region in which HDA typically forms) gives proton-ordered, but non-interpenetrating, ice IX', then proton-ordered and interpenetrating ice XV', and finally ice VIII'. By contrast, fast compression yields HDA but no ice IX, and direct transformation of ice I to ice XV' is structurally inhibited. These observations suggest that HDA formation is a consequence of a kinetically arrested transformation between low-density ice I and high-density ice XV' and challenge theories that connect amorphous ice to supercooled liquid water.

Supercritical fluids in rock–H2O systems have been proposed to be important agents of mass transfer in subduction zone environments. New experimental studies were conducted on the simple model granite system NaAlSi3O8 (Ab)–H2O in order to investigate phase relations and to develop the thermodynamic mixing properties between aqueous fluid (a.k.a. vapor, V) and silicate melt (a.k.a. liquid, L) at pressures (P) and temperatures (T) approaching those of critical mixing. We established liquidus and solvus phase relations by analyzing the quenched run products from piston–cylinder experiments over a range of $$P - T - X_{{{\text{H}}_{2} {\text{O}}}}$$ conditions from 1.0 to 1.7 GPa, 630–1060 °C and 4–92 wt% H2O. Equations for the critical curve, solidus temperatures, albite solubility at the solidus, and vapor-saturated solidus H2O content were formulated as functions of $$P - T - X_{{{\text{H}}_{2} {\text{O}}}}$$ . We constructed a subregular solution model to describe the solvus curves using P and T dependent Margules coefficients ( $$W_{{{\text{H}}_{2} {\text{O}}}}$$ and $$W_{\text{Ab}}$$ ). Activities of H2O and Ab ( $$a_{{{\text{H}}_{2} {\text{O}}}}$$ and $$a_{\text{Ab}}$$ ) could be formulated using only the input at the solidus and the critical point at each pressure because of the nearly linear dependence of the parameters on T. The solvus curves were confirmed independently by means of criteria established for classification of quenched products as L, L + V, or V and are in excellent agreement with the compositions that can be calculated using the Margules coefficients. At 1.6 GPa, the H2O content at the vapor-saturated solidus is 44.5 ± 5.5 wt% and the solubility of albite at the solidus is 42.95 ± 0.99 wt%, indicating the imminent intersection of the two curves and thus a stable critical endpoint at some slightly higher pressure. We constrain the critical endpoint at 1.63 ± 0.02 GPa, 659 ± 5 °C, and a composition of ~ 44.7 wt% H2O based on the intersections of the pressure dependent solidus curves with the critical curve, the pressure dependent albite solubility curve with the vapor-saturated solidus curve. The 1.7 GPa experiments showed no evidence for liquid–vapor immiscibility across a wide range of compositions and temperatures (4–80 wt% H2O and 630–1050 °C, and furthermore, that low albite is stable in the presence of the supercritical fluid near the breakdown of albite to jadeite and quartz. These results provide a comprehensive account of the solution properties of subcritical and supercritical fluids in this model granite system at temperatures and pressures corresponding to the deep-crust regions of granite magma generation.

The composition of Earth's core has first-order implications for understanding the thermal and chemical history of the deep Earth. The present work measures the pressure-volume equation of state of Fe5Si3 to 96 GPa in a diamond anvil cell using noble gas pressure media and demonstrates that Fe5Si3 is not stable at high temperature and pressure, but reappears during thermal quench. The isothermal equation of state at ambient temperature of Fe5Si3 is given by the bulk modulus KT,0 = 167 (8) and KT,0' = 5.1 (2)), with V0 = 56.29 cm3mol-1. At high temperatures and pressures we observed the disappearance of hexagonal Fe5Si3 diffraction peaks and the appearance of peaks corresponding to cubic FeSi and Fe3Si structures at 18 GPa and at the lowest measurable temperature (~1300 K), indicating that Fe5Si3 is not stable at high temperature. Upon temperature quench diffraction peaks corresponding to Fe5Si3 reappear, confirming its stability at ambient temperature and high pressure.

The system NaAlSi3O8 (albite, Ab)-H2O offers a simple and tractable model to study the thermodynamics of the volatile constituent H2O in felsic magmas. Although it has been studied in this context for nearly 100 years, developing a comprehensive model that adequately describes the activity of H2O ( $${a_{{H_2}O}}$$ ) in hydrous albite liquids and vapors has proven challenging. There are several problems. First, $${a_{{H_2}O}}$$ in hydrous liquids relies on melting experiments in the presence of mixed fluids with reduced H2O activity (H2O-CO2 and H2O-NaCl), but models of $${a_{{H_2}O}}$$ in these coexisting fluids have lacked sufficient accuracy. Second, the role of the solubility of albite in H2O has been assumed to be negligible; however, it is important to take solubility into account at pressure (P) above 0.5 GPa because it becomes sufficiently high that H2O activity at the wet solidus is significantly less than 1. Third, the dry melting temperatures and wet solidus temperatures are inconsistent between the datasets. We address these issues by combining previous experimental work on T– $${X_{{H_2}O}}$$ liquidus relations at 0.5–1.5 GPa with accurate activity formulations for H2O in mixed fluids (Aranovich and Newton, 1996, 1999). This yields isobaric T– $${a_{{H_2}O}}$$ sections at 0.5, 0.7, 1.0 and 1.5 GPa. Data at each isobar were fit to cubic equations, which were used to derive the following equation for liquidus T as a function of $${a_{{H_2}O}}$$ and P: $$T\left( {{a_{{H_2}O}},P} \right) = {m_0} + {m_1}{a_{{H_2}O}} + {m_2}a_{{H_2}O}^2 + {{m_3}a_{{H_2}O}^3}^\circ C$$ where T is °C, m 0 = 1119.6 + 112.3P, m 1 =–856.5–578.9P, m 2 = 1004.1 + 952.9P, and m 3 =–477.1–618.0P. The equation is valid at 0.5 < P < 1.5 GPa and T solidus < T < T dry melting. The nonzero solubility of albite in pure H2O is incorporated into the model to give the correct liquidus H2O activity when truncating the model equation in the limiting case where T → T solidus at a given pressure. This model equation reproduces both the liquidus-H2O contents and activities from the solubility measurements of Makhluf et al. (2016) in the binary system Ab-H2O at 1.0 GPa. The model equation also accurately reproduces the liquidus H2O activities from Eggler and Kadik (1979) and Bohlen et al. (1982) when the Aranovich and Newton (1999) activity formulation for CO2–H2O mixed fluids is applied to their datasets.

The role of carbon dioxide, CO2, as oxidizing agent at high pressures and temperatures is evaluated by studying its chemical reactivity with three transition metals: Au, Pt, and Re. We report systematic X-ray diffraction measurements up to 48 GPa and 2400 K using synchrotron radiation and laser-heating diamond-anvil cells. No evidence of reaction was found in Au and Pt samples in this pressure–temperature range. In the Re + CO2 system, however, a strongly–driven redox reaction occurs at P > 8 GPa and T > 1500 K, and orthorhombic β-ReO2 is formed. This rhenium oxide phase is stable at least up to 48 GPa and 2400 K and was recovered at ambient conditions. Raman spectroscopy data confirm graphite as a reaction product. Ab-initio total-energy structural and compressibility data of the β-ReO2 phase shows an excellent agreement with experiments, altogether accurately confirming CO2 reduction P–T conditions in the presence of rhenium metal and the β-ReO2 equation of state.

Amorphous water ice comes in at least three distinct structural forms, all lacking long-range crystalline order. High-density amorphous ice (HDA) was first produced by compressing ice I to 11 kilobar at temperatures below 130 kelvin, and the process was described as thermodynamic melting

The liquidus water content of a haplogranite melt at high pressure (P) and temperature (T) is important, because it is a key parameter for constraining the volume of granite that could be produced by melting of the deep crust. Previous estimates based on melting experiments at low P (≤0.5 GPa) show substantial scatter when extrapolated to deep crustal P and T (700–1000 °C, 0.6–1.5 GPa). To improve the high-P constraints on H2O concentration at the granite liquidus, we performed experiments in a piston–cylinder apparatus at 1.0 GPa using a range of haplogranite compositions in the albite (Ab: NaAlSi3O8)—orthoclase (Or: KAlSi3O8)—quartz (Qz: SiO2)—H2O system. We used equal weight fractions of the feldspar components and varied the Qz between 20 and 30 wt%. In each experiment, synthetic granitic composition glass + H2O was homogenized well above the liquidus T, and T was lowered by increments until quartz and alkali feldspar crystalized from the liquid. To establish reversed equilibrium, we crystallized the homogenized melt at the lower T and then raised T until we found that the crystalline phases were completely resorbed into the liquid. The reversed liquidus minimum temperatures at 3.0, 4.1, 5.8, 8.0, and 12.0 wt% H2O are 935–985, 875–900, 775–800, 725–775, and 650–675 °C, respectively. Quenched charges were analyzed by petrographic microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), and electron microprobe analysis (EMPA). The equation for the reversed haplogranite liquidus minimum curve for Ab36.25Or36.25Qz27.5 (wt% basis) at 1.0 GPa is $$T = - 0.0995 w_{{{\text{H}}_{ 2} {\text{O}}}}^{ 3} + 5.0242w_{{{\text{H}}_{ 2} {\text{O}}}}^{ 2} - 88.183 w_{{{\text{H}}_{ 2} {\text{O}}}} + 1171.0$$ for $$0 \le w_{{{\text{H}}_{ 2} {\text{O}}}} \le 17$$ wt% and $$T$$ is in °C. We present a revised $$P - T$$ diagram of liquidus minimum H2O isopleths which integrates data from previous determinations of vapor-saturated melting and the lower pressure vapor-undersaturated melting studies conducted by other workers on the haplogranite system. For lower H2O (

a b s t r a c t For more than half a century, thought about granite genesis and crustal evolution has been guided by the concept of partial melting in the lower crust. In this model, granitic magmas produced at depth are

Correspondence: Strongly-driven Re+CO 2 redox reaction at high-pressure and high-temperature

The system albite–H2O serves as an important model for the generation of granitic magmas, yet relatively few experimental investigations have focused on phase relations at high pressure. This study reports new experimental results, at 1.0 GPa and 690–1050 °C, on the temperature and liquid composition at vapor-saturated melting, the H2O content of undersaturated silicate liquids in equilibrium with albite, the solubility of albite in H2O–NaCl fluids immediately below the solidus, and the activity of H2O in hydrous NaAlSi3O8 liquids along the liquidus. Albite melts and dissolves congruently at all temperatures and salinities. In the NaCl-absent system, the temperature of vapor-saturated melting of low albite, confirmed by X-ray diffraction, is 695 ± 5 °C and the liquid composition is 18.14 ± 1.35 wt% H2O. The temperature dependence of the fluid-undersaturated liquidus curve in the system NaAlSi3O8–H2O varies with H2O wt% ( $$w_{{{\text{H}}_{2} {\text{O}}}}$$ ) according to $$T = - 2.0331 \times 10^{ - 3} w_{{{\text{H}}_{2} {\text{O}}}}^{3} + 1.6497w_{{{\text{H}}_{2} {\text{O}}}}^{2} {-} 58.963w_{{{\text{H}}_{2} {\text{O}}}} + 1235.5\,^{ \circ } {\text{C}}$$ indicating positive curvature in temperature-composition coordinates and a dry melting temperature of ~1235 °C. At 690 °C, immediately below the solidus, albite solubility decreases drastically with NaCl content of the fluid phase, from 8.8 ± 0.6 wt% in the NaCl-free fluid to ˂2 % at NaCl concentration of only 10 mol%. Experiments determining the activity of H2O ( $$a_{{{\text{H}}_{2} {\text{O}}}}$$ ) in liquids at vapor-saturated melting exploited low Cl solubility in liquids and low albite solubility in the presence of H2O–NaCl fluids. The maximum Cl content of quenched glasses, only 0.95 wt%, and very low albite solubility together make possible H2O activity measurement in melts equilibrated with NaCl–H2O solutions. When combined with activity data for H2O–NaCl fluids, experimentally determined $$a_{{{\text{H}}_{2} {\text{O}}}}$$ along the liquidus is described by $$T = - {469.16a_{{H_{2} O}}^{L}}^{{\frac{1}{2}}} {-} 93.382a_{{H_{2} O}}^{L} + 1235.5\;^\circ C.$$ At 1 GPa, H2O activities in hydrous albitic melts in H2O–NaCl fluids agree with those in the presence of H2O–CO2 fluids, provided appropriate fluid mixing data are used. Our results constrain an asymmetric regular solution model for the mixing of Na1/8Al1/8Si3/8O and H2O, yielding Margules parameters of W Ab = 25.56 ± 0.54 and $$W_{{{\text{H}}_{2} {\text{O}}}} = \, 10.50\; \pm \;0.74{\text{ kJ}}/{\text{mol}}$$ . The results imply critical mixing of liquid and vapor at ~1100 °C. Using existing speciation models, our results imply that H2O dissolved in hydrous albitic liquids is almost entirely in the form of molecular H2O (OH− concentration is negligible) at temperatures near vapor-saturated melting. The results provide a more complete experimental foundation for modeling the system NaAlSi3O8–H2O at high pressure.

Nanostructured silicon has recently been identified as an attractive material for a wide variety of uses from energy conversion and storage to biological applications. Here we present a new, rapid method of producing high-purity, nanostructured, unfunctionalized silicon via solid-state metathesis (SSM) in a matter of seconds. The silicon forms in a double displacement reaction between silicon tetraiodide and an alkaline earth silicide precursor. The products are characterized using powder X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive spectroscopy (EDS). Depending on the silicide precursor used, two different morphologies are obtained, either nanoparticles or dendritic nanowires. The variations in the morphologies are attributed to differences in the kinetics of the reactions.