Ion Solvation, Ion Pairing and Metal Complexing in Hydrothermal Fluids Using Exafs Spectroscopy

Terry Seward (tseward@erdw.ethz.ch)¹ & Michael Henderson (chenderson@fs1.ge.man.ac.uk)²

¹ Inst. f. Mineralogie, und Petrographie, 8092 ETH-Zürich, Switzerland

² Dept. Earth Sciences, Univ. of Manchester, Manchester M13 9PL, UK

At elevated temperatures and pressures, the properties of water change quite dramatically as a result of changes in the extent and nature of hydrogen bonding. These changes are manifested by increasing ion pair and cluster molecule formation as well as by the increased stability of many simple metal complexes. We have been studying these effects in aqueous systems using X-ray absorption fine structure spectroscopy (EXAFS) over a range of temperatures and pressures of 500°C and 1500 bar pressure. Of particular interest are the changes in ion-solvent interaction which occur in response to expanding water structure due to stretching and breaking of hydrogen bonds. Such changes in the nature of ion hydration under hydrothermal conditions play a fundamental role in many geochemical processes such as the changes in the solubilities of minerals in high temperature - high pressure salt solutions resulting from changes in water activity. Our ongoing EXAFS studies using high T-P cells with both silica and diamond windows indicate that the hydration shells for both univalent and divalent cations such as Ag⁺, Sr²⁺ and Cd²⁺ contract with increasing temperature at equilibrium saturated vapour pressures. In the case of Ag⁺ and Sr²⁺, the cation-oxygen(water) bond contraction is associated with a decrease in the number of coordinated water molecules, whereas for Cd²⁺, the number of first shell waters remains unchanged at six. For the +3 cation, In³⁺, the ion-oxygen(water) distance remains constant with no change in the coordination number. Measurements on RbI solutions up to 350° indicate that the hydration shell expands as the number of first shell waters decreases from 7 to 4. These observations are consistent with our ongoing molecular dynamics simulations of high temperature electrolyte solution behaviour. Studies of metal (e.g. Cd, In, Pd) chloride complex formations are also underway in order to determine complex stoichiometry and metal-ligand bond lengths at elevated temperatures and pressures.

Th Partitioning between Monazite and Xenotime: Experimental Determination

Anne-Magali Seydoux (seydoux@gfz-potsdam.de), Richard Wirth (wirth@gfz-potsdam.de) & Wilhelm Heinrich (whsati@gfz-potsdam.de)

GeoForschungsZentrum Potsdam, Telegrafenberg - P.B. 4.1, 14473 POTSDAM, Germany

Monazite, LREEPO₄, is commonly used for U-Th-Pb age determination because of its high U and Th concentrations. Minor importance has been given to xenotime [(Y + HREE)PO₄] which also may incorporate considerable amounts of U and, to a somewhat minor extent, Th. It is known from natural assemblages that the major substitution mechanisms are:

Ca + (Th, U) = 2 REE (1)

(U, Th) + Si = REE + P (2)

Up to now quantitative relationships are unknown.

For a first step, we experimentally determined the Th distribution between the two phases using the substitution mechanism described by (2). Experiments were conducted in standard cold seal hydrothermal and internally heated pressure vessels at 200 MPa in the range of 600°C and 1100°C. Starting mixture were prepared as gels and consisted of equal amounts of CePO₄ and YPO₄ with addition of 10, 20 and 50 mole% of ThSiO₄ in the bulk composition. Grain sizes of run products were in the range of a few µm. Therefore, analytical T.E.M. methods (E.D.S., E.E.L.S.) were applied to obtain reliable chemical compositions of coexisting phases. Lattice parameters of run products were refined by Rietveld analysis of X.R.D. powder patterns.

At all temperatures, the ThSiO₄ component is partitioned almost exclusively into monazite, if the ThSiO₄ component in the bulk is 10 and 20 mole%. Xenotime is apparently ThSiO₄ - free. For ThSiO ₄ bulk of 50 mole%, between 600°C and 900°C thorite is additionally formed indicating ThSiO₄ saturation of monazite, i.e. the solvus between monazite and thorite has been intersected. At 1000°C, only monazite, a small amount of xenotime, and no thorite are present, showing that this solvus shrinks dramatically at high temperature.