High-pressure phase transformations in baddeleyite and zircon, with geophysical implications

Abstract

Phase transformations in baddeleyite (ZrO 2) and zircon (ZrSiO 4) have been investigated in the pressure range between 100 and 300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. Baddeleyite has been found to transform to an orthorhombic cotunnite-type structure at pressures greater than 100 kbar, and is the first oxide known to adopt this structure. The lattice parameters of the cotunnite-type ZrO 2 at room temperature and atmospheric pressure are a = 3.328 ± 0.001 , b = 5.565 ± 0.002 , and c = 6.503 ± 0.003A˚ with Z = 4 , and its volume is 14.3% smaller than baddeleyite and 7.6% smaller than the fluorite-type ZrO 2. It is suggested that all the polymorphic structures of ZrO 2 are possible high-pressure models for the post-rutile phase of SiO 2. The polyhedral coordination in these model structures varies from 7 to “9”, compared with 6 for stishovite. If SiO 2 were to adopt any of these structures in the deep mantle, Birch's hypothesis of a mixed-oxide lower mantle may still be viable, but the primary coordination of silicon would be greater than 6. Zircon has been found to transform to a scheelite-type structure at about 120 kbar as noted earlier. The scheelite-type ZrSiO 4 was found to decompose further into a mixture of ZrO 2 (cotunnite-type) plus SiO 2 (stishovite) in the pressure range 200–250 kbar. As implied by the transitions in zircon, the large cations of U and Th in the earth's deep mantle are most likely to occur in dioxides with structures such as the cotunnite-type, rather than to occur in silicates.