HR: 0830h
AN: T11C-0418    [PDF]
TI: High-Pressure Raman Spectroscopic Studies of FeS$_2$ Pyrite
AU: * Kleppe, A K
EM: annettek@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences Parks Road, Oxford, OX1 3PR United Kingdom
AU: Jephcoat, A P
EM: andrew@earth.ox.ac.uk
AF: Diamond Light Source Ltd. and University of Oxford, Department of Earth Sciences Parks Road, Oxford, OX1 3PR United Kingdom
AB: The transition metal dichalcogenide FeS$_2$, pyrite, is the most abundant of the sulphide minerals and common in a variety of geological environments. Strong geophysical interest in its physical and chemical properties under high-pressures and high-temperatures has arisen in the context of the F-S system's role in core formation, evolution and composition. It is known from X-ray diffraction studies that the equation of state of pyrite depends strongly on the degree of non-hydrostatic stress in the sample. In order to determine any possible influence of such intrinsic strength effects on the dynamical properties of FeS$_2$ we have investigated natural iron pyrite with Micro-Raman spectroscopy in the diamond-anvil cell to 55 GPa. Comparative measurements were performed under hydrostatic conditions with helium as a pressure-transmitting medium and without a pressure-medium. In fact, there have been few (if any) light scattering studies of opaque, highly reflective minerals at high-pressures. Despite the high opacity we observe (in analogy with previous work on high-pressure metallic phases) strong Raman modes at all pressures. Four out of five Raman modes are resolved with helium as a pressure-transmitting medium. The fifth mode, T$_g$(2), is $\leq$2cm$^{-1}$ apart from the strong A$_g$ mode that dominates the spectrum. We observe an increase in the separation of the E$_g$ and T$_g$(1) mode under compression. In contrast the A$_g$ (in-phase S$_2$ stretch) and T$_g$(2) (out-of-phase S$_2$ stretch) mode do not separate with pressure. All observed frequencies increase continuously under compression giving no evidence for a structural phase transition in accord with diffraction and shock wave studies. The main effect of non-hydrostatic conditions on the Raman modes is a strong pressure-induced broadening; the pressure-dependence of the frequencies is not affected within the error of the measurements. The Raman data are consistent with recent bond-length {\it vs} pressure calculations supporting a strengthening of the S-S and Fe-S bond under pressure.
DE: 3924 High-pressure behavior
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Tectonophysics [T]
MN: 2003 Fall Meeting