HR: 0800h
AN: MR31A-0113    [Abstracts]
TI: P-V single-crystal- and P-V-T powder X-ray diffraction study of metamict zircon (ZrSiO4)
AU: * Kunz, M
EM: mkunz@lbl.gov
AF: Lawrence Berkeley Laboratory, ALS, MS 4R 0230 1 Cyclotron Rd, Berkeley, CA 94720 United States
AU: Simoncic, P
EM: psimoncic@ucdavis.edu
AF: University of California at Davis, Thermochemistry Facility 4440 Chemistry Annex 1 Shields Avenue, Davis, CA 95616 United States
AB: Zircon (ZrSiO4) is of interest due to its capabilities to retain radioactive elements such as U and Th in the crystal structure. This makes it an ideal material for U-Pb dating in geology. Zircon is also an interesting model phase to study the mechanism and properties of structural damage inflicted by self-radiation (metamictization). This is important in view of the attempts for chemical inertialization of radioactive isotopes using ceramic nuclear waste forms. While there are numerous studies on the metamictization of zircon at ambient conditions and elevated temperatures, the effect of pressure on the radiation induced structural damage is not well understood as yet. Diverging results are published on the compressibility of non-metamict zircon. Here we present a structural study of metamict zircon (~900 ppm U, ~1.5 x 10¹⁸ alpha decay events g^-1) at elevated pressure and simultaneously elevated pressure and temperature and compare it with literature data of non-metamict zircon (Hazen and Finger, 1979; Kolesov et al, 2001, Van Westrenen et al. 2004). Data were collected on a single crystal (P-V-data, structure refinement) between 0 a 7 GPa. A powder diffraction experiment was performed in a heatable diamond anvil cell at pressures up to 10 GPa and temperatures up to 573 K sample (P-V-T-data). Room temperature single-crystal compressibility data were fitted to a 3rd-order Birch Murnaghan equation of state. This gives a room-temperature compressibility of K₀ = 205.7(5) GPa, K'= 5.3(4), V₀ = 262.609(9) ų. When constraining K' to 4, the K₀ increases to 209.6(3) GPa with a V₀ of 262.585(8) ų. The a-axis is much more compressible (Ka₀ = 180.6(4) GPa) than the c-axis (Kc₀ = 304.4(18) GPa). This is in qualitative accordance with the behavior observed for non-metamict zircon, however, the axial compressibilities are slightly more isotropic in our metamict zircon compared to a non-metamict sample. The anisotropy in compressibility is reflected in the observed change in bond-length. Between 0 and 7 GPa, we observe a decrease in bond length of 1.7 % for the Zr-O(e) bond, oriented perpendicular to the c-axis, whereas the Zr-O(c), oriented sup-parallel to c, is compressed only by 0.7 %. The four Si-O bonds compress by 1 % in the same pressure range. The softer behavior of the a-b-plane relative to the c-axis is somewhat surprising in view of the well known larger swelling of the c-axis upon metamictization. The single-crystal diffraction peaks show a reversible decrease in peak width (FWHM) with increasing pressure from 0.085(5)° at ambient condition to 0.070(5)° at 7 GPa. A simultaneously loaded quartz crystal showed no variation in peak width. A decrease in peak-width with increasing pressure is unusual and may be related to a stronger relative compression of the metamict portions within the crystal compared to the intact bulk structure. The analysis of temperature dependent compressibilities is ongoing and will be presented together with the P-V data. Hazen and Finger (1979): American Mineralogist, 64, 196 - 201.
Kolesov et al. (2001): European Journal of Mineralogy, 13, 939 - 948.
Van Westrenen et al. (2004): American Mineralogist, 89, 197 - 203.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005