MS15-03 - The True Structural Relationship between Zircon and Scheelite Structure Types, and a New Polymorph of Zircon

Ross Angel (University of Pavia, Italy)

Zircon (ZrSiO4, I41/amd) is a very common accessory mineral and important ceramic component. Its high-pressure polymorph with a scheelite-type structure, reidite (I41/a), has been detected in impact structures where it is diagnostic of shock impacts. Because I41/a is a sub-group of  I41/amd many authors have assumed that reidite (and scheelite structures in general) can be obtained by a displacive-type distortion of the zircon structure. Careful analysis of the atomic linkages and symmetry elements  in the two structure types shows that this is incorrect, and that the transformation between them must be reconstructive and first order. This is consistent with survival of reidite in meteorite craters, and confirmed by our new ab-initio DFT simulations [1].

The DFT simulations also revealed that zircon has a soft phonon mode that triggers a displacive transition  near 20 GPa to a previously-unreported ZrSiO4 phase with symmetry I-42d. We have confirmed the occurrence of a soft-mode driven second-order phase transition by in-situ high-pressure Raman spectroscopy [2]. This also showed that reidite starts to form in the sample just after the transition to the new phase. Thus, at room temperature, the first-order activation barrier between zircon and reidite can be overcome via the formation of the new high-pressure phase as an intermediate bridging state.

Supported by the European Research Council (Grant 714936).

[1] Stangarone et al. (2019) American Mineralogist, doi: 10.2138/am-2019-6827

[2] Mihailova et al. (2019) Physics & Chemistry of Minerals, submitted