Crystal-liquid-vapor equilibrium experiments at high temperature (less than or equal to 1800 C) and low, controlled oxygen and hydrogen pressure (10(-1) to 10(-9) PA)

Evidence from carbonaceous chrondrites points to refractory oxides in the system CaO-MgO-Al2O3-TiO2-SiO2-Fe-O as being among the earliest phases to condense from the solar nebula. It is necessary to establish the equilibrium relationships between the relevant crystalline and amorphous phases before the chemical constraints can be meaningfully applied to models of solar system history. Preliminary experiments on earth show that such experiments are feasible. Earth-based experiments suffer from several unavoidable problems. These problems can be overcome by experimentation in the Space Station where the experiments can be conducted under near static pressure conditions and where total pressure equals the sum of controlled hydrogen and oxygen pressures and can be controlled for periods exceeding several hours

Improved modelling of liquid GeSe2_2: the impact of the exchange-correlation functional

The structural properties of liquid GeSe$_2$ are studied by using first-principles molecular dynamics in conjuncton with the Becke, Lee, Yang and Parr (BLYP) generalized gradient approximation for the exchange and correlation energy. The results on partial pair correlation functions, coordination numbers, bond angle distributions and partial structure factors are compared with available experimental data and with previous first-principle molecular dynamics results obtained within the Perdew and Wang (PW) generalized gradient approximation for the exchange and correlation energy. We found that the BLYP approach substantially improves upon the PW one in the case of the short-range properties. In particular, the Ge$-$Ge pair correlation function takes a more structured profile that includes a marked first peak due to homopolar bonds, a first maximum exhibiting a clear shoulder and a deep minimum, all these features being absent in the previous PW results. Overall, the amount of tetrahedral order is significantly increased, in spite of a larger number of Ge$-$Ge homopolar connections. Due to the smaller number of miscoordinations, diffusion coefficients obtained by the present BLYP calculation are smaller by at least one order of magnitude than in the PW case.Comment: 6 figure

Coreless Terrestrial Exoplanets

Differentiation in terrestrial planets is expected to include the formation of a metallic iron core. We predict the existence of terrestrial planets that have differentiated but have no metallic core--planets that are effectively a giant silicate mantle. We discuss two paths to forming a coreless terrestrial planet, whereby the oxidation state during planetary accretion and solidification will determine the size or existence of any metallic core. Under this hypothesis, any metallic iron in the bulk accreting material is oxidized by water, binding the iron in the form of iron oxide into the silicate minerals of the planetary mantle. The existence of such silicate planets has consequences for interpreting the compositions and interior density structures of exoplanets based on their mass and radius measurements.Comment: ApJ, in press. 22 pages, 5 figure

Effects of water and fluorine on the viscosity of albite melt at high pressure: a preliminary investigation

The viscosities of fluorine- and water-bearing melts based on albite composition have been determined at 7.5, 15 and 22.5 kbar by the falling-sphere method. All melt viscosities decrease isothermally with increasing pressure. At 1200°C the viscosity of the fluorine-bearing melt (albite + 5.8 wt.% fluorine substituted for oxygen, denoted AbF2O−1) decreases from5000 ± 750P at7.5kbar to1600 ± 240P at22.5kbar. At 1400°C the viscosity of this melt decreases from1300 ± 200P at7.5kbar to430 ± 65P at22.5kbar. At 1400°C the viscosity of albite + 2.79 wt.% water (denoted AbH2O) decreases from650 ± 100P at7.5kbar to400 ± 60P at22.5kbar. Fluorine (as F2O−1) and water strongly decrease the viscosity of albite melt over the entire range of investigated pressures. The ratio of the effects of 5.8 wt.% fluorine [F/(F + O)molar = 0.10] and 2.79 wt.% water [OH/(OH + O)molar = 0.10] on the log of melt viscosity [Δ log η(AbF2O−1)/Δ log η(AbH2O)] equals0.90 ± 0.05, 0.84 ± 0.05and0.97 ± 0.05at7.5, 15and22.5kbar, respectively. Comparison with available data on the high-pressure viscosity of albite melt indicates that both F2O−1 and H2O maintain their viscosity-reducing roles to lower crustal pressures. The difference between the viscosities of melts of albite, AbF2O−1 and AbH2O, may be explained in terms of the relatively depolymerized structures of AbF2O−1 and AbH2O melts. The depolymerization of albite melt by the addition of water results from the formation of SiOH bonds. The depolymerization of albite melt by F2O−1 substitution results from the formation of non-bridging oxygens associated with network-modifying aluminum cations that are formed upon fluorine solution. The strong viscosity-reducing effects of water and fluorine in albite melt at pressures corresponding to the mid- to lower continental crust indicate that these two components will strongly influence the dynamic behavior of anatectic melts during initial magma coalescence and restite-melt segregation

The effect of temperature and bulk composition on the solution mechanism of phosphorus in peraluminous haplogranitic magma.

Solution mechanisms of P in peraluminous glasses and melts in the system CaO-Na2O-K2O-Al2O3- SiO2-P2O5 have been examined with in-situ microRaman spectroscopy from ambient temperature to near 1200 °C. The principal aim was to examine the relative stabilities of phosphate complexes as functions of P content, peraluminosity, and temperature. Increasing peraluminosity was accomplished by increasing the proportions of Al3+ and Ca2+ of constant SiO2 content. The molar ratio Al2O3/ (CaO+Na2O+K2O) (A/CNK) ranged from ~1 to ~1.3. In all compositions, P5+ is bonded to Al3+ to form AlPO4 complexes. In addition, there is evidence for pyrophosphate complexing (P2O7). In melts with the highest (Ca+Na+K)/P, there is probably also a small fraction of orthophosphate complexes present. The relative importance of AlPO4-like complexes is correlated positively with peraluminosity (A/CNK), P2O5 content, and increasing temperature at temperatures above that of the glass transition. These structural relationships among phosphate complexes are coupled with decreasing polymerization of the aluminosilicate melts

Rigidity and intermediate phases in glasses driven by speciation

The rigid to floppy transitions and the associated intermediate phase in glasses are studied in the case where the local structure is not fully determined from the macroscopic concentration. The approach uses size increasing cluster approximations and constraint counting algorithms. It is shown that the location and the width of the intermediate phase and the corresponding structural, mechanical and energetical properties of the network depend crucially on the way local structures are selected at a given concentration. The broadening of the intermediate phase is obtained for networks combining a large amount of flexible local structural units and a high rate of medium range order.Comment: 4 pages, 4 figure

Polyhedral units and network connectivity in calcium aluminosilicate glasses from high-energy x-ray diffraction

Structure factors for Cax/2AlxSi1-xO2 glasses (x=0,0.25,0.5,0.67) extended to a wave vector of magnitude Q= 40 1/A have been obtained by high-energy x-ray diffraction. For the first time, it is possible to resolve the contributions of Si-O, Al-O and Ca-O coordination polyhedra to the experimental atomic pair distribution functions (PDF). It has been found that both Si and Al are four-fold coordinated and so participate in a continuous tetrahedral network at low values of x. The number of network breaking defects in the form of non-bridging oxygens (NBO's) increases slowly with x until x=0.5 (NBO's ~ 10% at x=0.5). By x=0.67 the network breaking defects become significant as evidenced by the significant drop in the average coordination number of Si. By contrast, Al-O tetrahedra remain free of NBO's and fully integrated in the Al/Si-O network for all values of x. Calcium maintains a rather uniform coordination sphere of approximately 5 oxygen atoms for all values of x. The results suggest that not only Si/Al-O tetrahedra but Ca-O polyhedra, too, play a role in determining the glassy structure

Using dissolved H₂O in rhyolitic glasses to estimate palaeo-ice thickness during a subglacial eruption at Bláhnúkur(Torfajökull, Iceland)

The last decade has seen the refinement of a technique for reconstructing palaeo-ice thicknesses based on using the retained H2O and CO2 content in glassy eruptive deposits to infer quenching pressures and therefore ice thicknesses. The method is here applied to Bláhnúkur, a subglacially erupted rhyolitic edifice in Iceland. A decrease in water content from ~0.7 wt.% at the base to ~0.3 wt.% at the top of the edifice suggests that the ice was 400 m thick at the time of the eruption. As Bláhnúkur rises 350 m above the surrounding terrain, this implies that the eruption occurred entirely within ice, which corroborates evidence obtained from earlier lithofacies studies. This paper presents the largest data set (40 samples) so far obtained for the retained volatile contents of deposits from a subglacial eruption. An important consequence is that it enables subtle but significant variations in water content to become evident. In particular, there are anomalous samples which are either water-rich (up to 1 wt.%) or water-poor (~0.2 wt.%), with the former being interpreted as forming intrusively within hyaloclastite and the latter representing batches of magma that were volatile-poor prior to eruption. The large data set also provides further insights into the strengths and weaknesses of using volatiles to infer palaeo-ice thicknesses and highlights many of the uncertainties involved. By using examples from Bláhnúkur, the quantitative use of this technique is evaluated. However, the relative pressure conditions which have shed light on Bláhnúkur’s eruption mechanisms and syn-eruptive glacier response show that, despite uncertainties in absolute values, the volatile approach can provide useful insight into the mechanisms of subglacial rhyolitic eruptions, which have never been observed

Thermodynamic Assessment of the CaO–Al2O3–SiO2 System

The CaO–Al2O3–SiO2 system has been assessed with the CALPHAD technique, based on recent assessments of its binary systems. A new species AlO2−1 was introduced for modeling liquid Al2O3. The ternary liquid phase was described using the ionic two-sublattice model as. The available experimental data were critically examined, and a self-consistent set of thermodynamic descriptions was obtained. Various phase diagrams and property diagrams, including isothermal sections, isoactivity lines, and a projection of the liquidus surface, are presented. Information on viscosity seems to support the use of the AlO2−1 species