GEOLOGICAL  SCIENCES

JOHN V. WALTHER
Matthews Professor

RESEARCH  PROJECTS

EXPERIMENTAL AND THEORETICAL AQUEOUS GEOCHEMISTRY
Projects involve application of computer modeling to problems in fluid-rock interaction and the analysis of the mechanisms by which progressive metamorphic reactions occur. Volatile production, transport, and fluid-rock ratios have been evaluated (Wood and Walther, 1986). The calculated rates of surface reaction (Wood and Walther, 1983; Walther and Wood, 1986) and the controls on the rate-limiting step in metamorphic reactions have been analyzed (Walther and Wood, 1984).

  1. Fluid-rock reactions during metamorphism at mid-crustal conditions (Walther, 1994).

  2. Comment: Mass transfer during Barrovian metamorphism (Walther and Holdaway, 1995).

  3. Fluid production and isograde reactions at contacts between carbonate rich and carbonate poor layers during progressive metamorphism (Walther, 1996).

  4. Should diffusion be characterized in terms of concentration or chemical potential gradients? (Walther, 1996).

Diagram showing when aqueous diffusion controls the transport of material between reactant grains and when flow controls transport for 1 mm and 1 cm average grain size for a prograde metamorphic reaction. Fluid velocity of the intergranular film is shown at the top of the diagram. The short vertical lines labeled 1, 10, 100 and 1,000 show the boundary between aqueous film diffusion and flow transport control for these values of the chemical potential gradient of the least soluble mobile component in calories per mole per cm. At very low fluid fluxes the fluid film drops below a double monomolecular absorbed film and true grain boundary diffusion control can occur. (Walther and Wood, 1984).

KINETICS OF DISSOLUTION AND FLUID-MINERAL SURFACE INTERACTIONS
Research is being carried out to study the rates and mechanism by which minerals dissolve in order to understand the controls on the composition of surface waters and the role of pH, salts and organic acids:

  1. Dissolution stoichiometry and adsorption of alkali and alkaline earth elements to the acid-reacted wollastonite surface at 25 degrees C (Xie and Walther, 1994).

  2. Dissolution kinetics of silica glass as a function of pH between 40 and 85 degrees C (Mazer and Walther, 1994).

  3. Surface charge behavior of andalusite: Implications for mixed oxide surface group contributions (Cruz and Walther, 1995).

  4. Relation between rates of aluminum silicate mineral dissolution, pH, temperature, and surface charge (Walther, 1996).

  5. Zero point of charge and surface complexation of albite (Mukhopadhyay and Walther, 1996).

  6. Comment: Feldspar dissolution at 25 degrees C and low pH (Walther, 1997).
Rate of quartz and corundum dissolution in moles per cm2 per sec times 1017 at 25 C as a function of pH plotted on the same scale. Quartz rates given by filled circles are from Wollast and Chou (1986). Rates for corundum given by open circles are from Carroll-Webb and Walther (1988). From Walther, (1998).

MINERAL SOLUBILITIES AS A FUNCTION OF TEMPERATURE, PRESSURE AND SOLUTION COMPOSITION
A continuing program in the laboratory is the measurement of solubility of minerals in aqueous solutions to determine the thermodynamic properties of species to temperatures of 650 degrees C and pressures of 3 kbar (e.g., Walther, 1986). Solubility information is necessary to help determine the way in which minerals react to equilibrium, the textures that are observed, and the extent of veining and mass transfer in geological processes. Recent and current projects including those with students:

  1. Experimental determination of the solubility of the assemblage microcline, muscovite and quartz in supercritical H2O (Walther and Woodland, 1993).

  2. Wollastonite + quartz solubility in supercritical NaCl aqueous solutions (Xie and Walther, 1993a).

  3. Quartz solubilities in NaCl solutions with and without wollastonite at elevated temperatures and pressures (Xie and Walther, 1993b).

  4. Determination of activity coefficents of neutral species in supercritical H2O solutions (Walther, 1997a).

  5. Experimental determination and interpretation of the solubility of corundum in H2O between 350 and 600 degrees C from 0.5 to 2.2 kbar (Walther, 1997c).

  6. Corundum solubilities in supercritical NaCl and HCl aqueous solutions (in progress).

Drawing after a photomicrograph of reactions zones developing between dolomite = (A) and quartz = (D) to form calcite = (B) and tremolite = (C). Shaded areas within tremolite show the location of phlogopite. Dots within quartz crystals show the orientation of fluid inclusion planes along healed micro-fractures (Walther, 1983).

Dr. John V. Walther
Department of Geological Sciences
Southern Methodist University
Dallas, Texas 75275-0395
voice: (214) 768-3174
fax: (214) 768-2701
email: walther@mail.smu.edu

Maintained by vineyard@mail.smu.edu