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

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

Sep 1979

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979e%26psl..44..390l&link_type=abstract

Earth and Planetary Science Letters, Volume 44, Issue 3, p. 390-396.

Computer Science

28

Scientific paper

Phase transformations in baddeleyite (ZrO2) and zircon (ZrSiO4) 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 ZrO2 at room temperature and atmospheric pressure are a = 3.328 +/- 0.001, b = 5.565 +/- 0.002, and c = 6.503 +/- 0.003 Å with Z = 4, and its volume is 14.3% smaller than baddeleyite and 7.6% smaller than the fluorite-type ZrO2. It is suggested that all the polymorphic structures of ZrO2 are possible high-pressure models for the post-rutile phase of SiO2. The polyhedral coordination in these model structures varies from 7 to ``9'', compared with 6 for stishovite. If SiO2 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 ZrSiO4 was found to decompose further into a mixture of ZrO2 (cotunnite-type) plus SiO2 (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.

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