Equation of state of pyrite to 85 GPa and 2400 K

Abstract:

Pyrite (FeS₂), a Pa3 space group non-magnetic semiconductor, is the most abundant iron sulfide in nature, yet the high cosmic abundance of sulfur is not reflected in the terrestrial crust, implying it is either sequestered in the Earth’s interior or was volatilized during accretion. As it has widely been suggested that sulfur could be one of the contributing light elements leading to the density deficit of Earth’s core, a robust thermal equation of state of FeS₂ is vital for understanding the evolution and properties of Earth’s interior. We performed X-ray diffraction measurements on FeS₂ at the GSECARS sector 13-ID-D and HPCAT sector 16-ID-B beamlines at the Advanced Photon Source. Pressures from 17 to 85 GPa and temperatures up to 2400 K were achieved using laser-heated diamond anvil cells. Pressures were determined from the lattice parameters of KBr [1], which served as an insulator and pressure medium, and temperatures were determined by spectroradiometry. No phase transitions were observed in the pyrite structure over the pressure and temperature ranges investigated. By combining our new P-V-T data with previously published room temperature compression data [2], we have determined a thermal equation of state for FeS₂, with bulk modulus K=182.6(74) GPa, pressure derivative K’=3.82(25), and αK_T=0.00329(45). Our revised equation of state for pyrite is consistent with a core density deficit satisfied by 9-10 wt.% sulfur. We compare these findings to previously published ab intio equation of state parameters for pyrite under a similar range of pressures [3]. [1] Fischer et al. (2012) EPSL 357-358, 268-276. [2] Merkel et al. (2002) PCM 29, 1–9. [3] Le Page and Rodgers (2005) PCM 32, 564-567.