Trace Element Behavior during Shock Transformation of Zircon to Reidite

Abstract

Large single crystals of natural zircon were shock-loaded at 13.6 and 51.3 GPa in planar geometry. No structural changes were observed after loading at 13.6 GPa. Loading to 51.3 GPa resulted in zircon transformation to a denser scheelite-structured phase, reidite. The investigation of reidite samples by X-ray diffraction, Raman, photo- and cathodoluminescence spectroscopies revealed segregation of some trace cations (such as REE) on planar defects during the transformation. The segregation has occurred in a laboratory experiment without long-term annealing after the shock loading. A possible mechanism of the segregation of trivalent trace cations assumes local violation of charge balance during the zircon-reidite reconstructive transformation, which is accompanied by changes in the topology of polyhedra and second coordination spheres (Si-Zr). This results in expulsion of a fraction of the trace elements into energetically expensive interstitial positions with high diffusivity even at relatively low temperatures.