Silicate melts at magmatic temperatures: in-situ structure determination to 1651°C and effect of temperature and bulk composition on the mixing behavior of structural units

The abundance of coexisting structural units in K-, Na-, and Li-silicate melts and glasses from 25° to 1654°C has been determined with in-situ micro-Raman spectroscopy. From these data an equilibrium constant, K_x, for the disproportionation reaction among the structural units coexisting in the melts, Si₂O₅(2Q³)SiO₃(Q²)+SiO₂(Q⁴), was calculated (K_x is the equilibrium constant derived by using mol fractions rather than activities of the structural units). From ln K_x vs l/T relationships the enthalpy (H_x) for the disproportionation reaction is in the range of-30 to 30 kJ/mol with systematic compositional dependence. In the potassium and sodium systems, where the disproportionation reaction shifts to the right with increasing temperature, the H_x increases with silica content (M/Si decreases, M=Na, K). For melts and supercooled liquids of composition Li₂O·2SiO₂ (Li/Si=1), the H_x is indistinguishable from 0. By decreasing the Li/Si to 0.667 (composition LS3) and beyond (e.g., LS4), the disproportionation reaction shifts to the left as the temperature is increased. For a given ratio of M/Si (M=K, Na, Li), there is a positive, near linear correlation between the H_x and the Z/r² of the metal cation. The slope of the H_x vs Z/r² regression lines increases as the system becomes more silica rich (i.e., M/Si is decreased). Activity coefficients for the individual structural units, _i, were calculated from the structural data combined with liquidus phase relations. These coefficients are linear functions of their mol fraction of the form _i=a lnX _i+b, where a is between 0.6 and 0.87, and X _i is the mol fraction of the unit. The value of the intercept, b, is near 0. The relationship between activity coefficients and abundance of individual structural units is not affected by temperature or the electronic properties of the alkali metal. The activity of the structural units, however, depend on their concentration, type of metal cation, and on temperature.     

Mineral-solution equilibria—III. The system Na2OAl2O3SiO2H2OHCl

Chemical equilibrium between sodium-aluminum silicate minerals and chloride bearing fluid has been experimentally determined in the range 500–700°C at 1 kbar, using rapid-quench hydrothermal methods and two modifications of the Ag + AgCl acid buffer technique. The temperature dependence of the thermodynamic equilibrium constant (K) for the reaction $\text{NaAlSi}{}_3\text{O}{}_8\text{+ HCl}{}^{\mathrm{o}}\text{= NaCl}{}^{\mathrm{o}}\text{+}\text{1}\text{2Al}{}_2\text{SiO}{}_5\text{, +}\text{5}\text{2SiO}{}_2\text{+}\text{1}\text{2H}{}_2\text{O}$ Albite Andalusite Qtz. can be described by the following equation: log k = ?4.437 + 5205.6/T(K) The data from this study are consistent with experimental results reported by Montoya and Hemley (1975) for lower temperature equilibria defined by the assemblages albite + paragonite + quartz + fluid and paragonite + andalusite + quartz + fluid. Values of the equilibrium constants for the above reactions were used to estimate the difference in Gibbs free energy of formation between NaCl^o and HCl^o in the range 400–700°C and 1–2 kbar. Similar calculations using data from phase equilibrium studies reported in the literature were made to determine the difference in Gibbs free energy of formation between KCl^o and HCl^o. These data permit modelling of the chemical interaction between muscovite + kspar + paragonite + albite + quartz assemblages and chloride-bearing hydrothermal fluids.     

Solving 3D boundary element problems using constrained iterative approach

Some major challenges for geophysicists and structural geologists using three-dimensional boundary element method codes (3D-BEM) are: (1) reducing the amount of memory required to solve large and dense systems and (2) incorporation of inequality constraints such as traction inequality constraints (TIC) and displacement inequality constraints (DIC). The latter serves two purposes. First, for example, inequality constraints can be used to simulate frictional slip (using TIC). Second, these constraints can prevent element interpenetration while allowing opening mode (using DIC). We have developed a method that simultaneously incorporates both types of functionality of the inequality constraints. We show that the use of an appropriate iterative solver not only avoids the allocation of significant memory for solving the system (allowing very large model computation and simplifying parallelization on multi-core processors), but also admits interesting features such as natural incorporation of TICs and DICs. Compared to other techniques of contact management (e.g., Lagrange multipliers, penalty method, or complementarity problem), this new simple methodology, which does not use any incremental trial-and-error procedures, brings more flexibility, while making the system more stable and less subject to round-off errors without any computational overhead. We provide validations and comparisons of the inequality constraints implementation using 2D analytical and numerical solutions.     

The earth is our home: systemic metaphors to redefine our relationship with nature

Climate change is one of the most compelling challenges for science communication today. Societal reforms are necessary to reduce the risks posed by a changing climate, yet many people fail to recognize climate change as a serious issue. Unfortunately, the accumulation of scientific data, in itself, has failed to compel the general public on the urgent need for pro-environmental policy action. We argue that certain metaphors for the human-environment relationship can lead people to adopt a more nuanced and responsible conception of their place in the natural world. In two studies, we tested properties of multiple metaphors with the general public (study 1) and experts on climate change (study 2). The metaphor “the earth is our home” resonated with climate experts as well as diverse subpopulations of the general public, including conservatives and climate-change deniers.     

The ionization constants of aqueous MgCl2 at elevated temperatures and pressures—a revision

The ionization constants of aqueous MgCl₂ have been revised, in the light of recent measurements of electrical conductance, from the calculations reported previously by Frantz and Popp (1979).     

Mineral-solution equilibria—V. Solubilities of rock-forming minerals in supercritical fluids

The solubility constants of sixty-nine rock-forming minerals have been computed for temperatures between 400 and 600°C at 1000 and 2000 bar pressure using the free-energy data for aqueous solutes presented in Parts I through IV of this series combined with the thermodynamic properties of minerals from Helgesonet al. (1978). An example describing solution compositions in equilibrium with a spilite is discussed. A computer program for calculating solution compositions in equilibrium with mineral assemblages is included as an appendix.     

Mineral-solution equilibria—I. An experimental study of complexing and thermodynamic properties of aqueous MgCl2 in the system MgO-SiO2-H2O-HCl

Speciation of aqueous magnesium in the system MgO-SiO₂-H₂O-HCl in supercritical aqueous fluids has been investigated using standard rapid-quench hydrothermal techniques and a modification of the Ag + AgCl buffer method (Frantz and Eugster, 1973. Am. J. Sci.267, 268–286). A concentric double-capsule charge was utilized. The outer gold capsule contained the assemblage talc + quartz + Ag + AgCl + H₂O-MgCl₂ fluid; the inner platinum capsule, Ag + AgCl + H₂O-HCl fluid. During the experiments, and thus $\text{?}{}_{\mathrm{HCl}}$ equilibrated between the two capsules. After quenching, measurement of the chloride concentration in the fluid in the inner capsule and total magnesium in the fluid in the outer capsule defines the concentrations of HCl and Mg that coexist with talc + quartz in the outer capsule. Changes in the measured molality of HCl as a function of the total magnesium concentration at constant P and T were used to identify the predominant species of magnesium in the hydrothermal fluid. Experimental results showed that at 2000 bar, MgCl°₂ is the predominant species above 550°C and Mg²⁺, below 400°C. Data at intermediate temperatures when combined with the dissociation constant for HCl were used to obtain the dissociation constant for MgCl°₂. The results of these experiments were combined with results from experiments using Ag + AgCl in conjunction with the oxygen buffer, hematite-magnetite, to obtain the equilibrium constant for the reaction $\text{1}\text{3}\text{Talc + 2HC1° H}{}_2\text{O MgCl°}{}_2\text{+}\text{4}\text{3}\text{Quartz +}\text{4}\text{3}\text{H}{}_2\text{O}$ from which the difference in Gibbs free energy of MgCl°₂ and HC1° was obtained as a function of temperature at 1000, 1500 and 2000 bar pressure, Solubility constants for brucite. forsterite, chrysotile, and talc were calculated.     

The Salt River Indian Reservation: land use conflicts and aspects of socioeconomic change on the outskirts of Metro-Phoenix, Arizona

Several North American metropolitan areas including Albuquerque, El Paso and Seattle in the U.S., and the Canadian urban areas of Montreal and Vancouver, are characterized by Indian reservations situated either within or on the outskirts of these cities. This type of Indian reservation or reserve within metropolitan areas has been the subject of limited geographical research, and is nowhere as manifest as in Metro-Phoenix. Three reservations obstruct the otherwise unlimited growth of Arizona??s capital. This phenomenon is most striking in the eastern regions of Phoenix where, until the end of the 1980s, the almost exclusively rural, agrarian Salt River Indian Reservation formed a physical and cultural barrier to urban development, preventing metropolitan sprawl beyond a clearly marked boundary. Continued urban pressures have led to changing land uses, effectively hollowing out this embankment or barrier, which has caused a profound change in the reservation. Both the Salt River and Gila River Indian reservations maintain large tracts of irrigated agricultural land, but the transformation that converted farms to suburbs off the reservation is now changing the reservation itself. In fact, this change actually began more than a hundred years ago with the Anglo-American colonization of what is today Arizona??s central region.