Reidite Formation From Zircon Subjected to 20, 40, and 60 GPa Shock Experiments

The deformation microstructures in shocked zircon are a key indicator of the shock conditions of hypervelocity impact events. However, calibration of these conditions using features formed in a nonequilibrium process is an extremely difficult task, resulting in large uncertainties. In the present study, we revisited products of shock experiments with zircon (20, 40, and 60 GPa), treated and studied by H. Leroux and coauthors in 1999. We applied new high‐resolution techniques such as electron backscatter diffraction (EBSD) mapping and Raman spectroscopy mapping, which are structure‐sensitive methods and allow for unambiguous phase identification. We demonstrate that the three stages of shock wave‐induced structural changes of zircon are as follows: (I) At 20 GPa, a mild degree of structural damage and cathodoluminescence (CL) dark‐ and bright‐oriented planar lamellae are developed, without the formation of reidite; (II) at 40 GPa, a moderate structural damage is observed, and crystallographically controlled reidite appears (up to 5 vol.%), being associated with CL‐bright zircon zones; and (III) at 60 GPa, CL bright moderately damaged zircon with oriented lamellar reidite coexists with band amorphous ZrSiO 4 (up to 50 vol.%), associated with patchy nonoriented reidite mixed with zircon nanophases. Bulk amorphous ZrSiO 4 does not contain reidite. The present work highlights that the formation of reidite is preceded by structural damage or shock amorphization of zircon and is induced by shock heating. The phase transition occurs by a reconstructive mechanism. Furthermore, the Raman spectral parameters of zircon were found to depend on the shock pressure experienced by zircon, offering a cost‐effective shock geobarometry tool.