COMPRES 2009 abstract

Synchrotron X-ray diffraction studies of metal-oxide pairs

Gregory Shofner (1)
Andrew J. Campbell (1)
Lisa Danielson (2)
Kevin Righter (2)

(1) Dept. of Geology, University of Maryland
(2) NASA Johnson Space Center

Oxygen fugacity (fO₂) controls multivalent phase equilibria and partitioning of redox-sensitive elements, and it is important to understand this thermodynamic parameter in experimental runs as well as natural systems. Constructing fO₂ buffers to pressures and temperatures corresponding to the Earth's deep interior requires precise determinations of the difference in volume (ΔV) between the buffer phases over a range of P-T conditions. Synchrotron x-ray diffraction data was collected for the present studies to measure unit-cell volumes of metals and their oxides (Co-CoO, Cr-Cr₂O₃, Mo-MoO₂, W-WO₂) under varied pressure and temperature conditions, to construct high pressure fO₂ buffer curves for these systems. Experiments were conducted in diamond anvil cells (DAC) at NSLS beamline X17C, and in the multi anvil press (MAP) at APS beamline 13-ID-GSECARS. The DAC experiments were at room temperatureand reached pressures up to 75 GPa. The MAP experiments (Co-CoO and Cr-Cr₂O₃ only) were at temperatures up to 1600 °C and at pressures up to 20 GPa. For each experiment, both the metal and its oxide were present, thus the unit-cell volume measurements were made for both phases under the same experimental conditions, minimizing the uncertainty in calculating the ΔV between the phases. Bulk moduli (K₀) and their derivatives (K₀') were determined for each phase. Room temperature values of K₀ and K₀' for Co, Cr, Mo, W, and Cr₂O₃ were within experimental uncertainty of published data for these phases. K₀ and K₀' for MoO₂ (K₀=209, K₀'=3.7) and WO₂ (K₀=302, K₀'=4) have not been previously reported. Both MoO₂ and WO₂ were determined to be monoclinic up to approximately 35 GPa, but appeared to transform to an unidentified crystal structure at higher pressures. The reversible structural transformation, from the HCP to FCC, was observed in Co at temperatures above approximately 700 °C. Using the experimentally constrained equations of state, fO₂ buffer curves were calculated for the Co-CoO and Cr-Cr₂O₃ (CRO) systems. The Co-CoO buffer ranges from approximately +4 to +2 log units above the IW buffer, whereas the CRO buffer ranges from approximately -9 to -3 log units below the IW buffer, over a temperature range of 700 to 1900 °C.