The structure of metamict zircon: A temperature-dependent EXAFS study

Abstract

The thermal annealing (300–1700 K) of two metamict zircons (Ampagabe, Madagascar and Näegy, Japan) has been studied using X-Ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure spectroscopy (EXAFS) at Zr K-edge. Two stages of thermal annealing within the aperiodic zircon are evidenced between 293 and 1700 K. The first stage (up to 600° C) shows a decrease of the a ₀-cell parameter from 6.674 (at 300° C) to 6.610 (at 600° C)± 0.005 Å. In that temperature range, the average local environment around Zr (presence of ^VIIZr and d(Zr-Zr) ≈ 3.3–3.6 Å) shows a weak, but significant increase of the Zr-Zr correlations located at 3.3–3.4 Å, undetectable by XRD. At temperatures up to 700° C (stage 2), the XRD-Bragg component arising from crystalline zircon increases in magnitude, whereas, Zr-K EXAFS analysis indicates a progressive ^VIIZr→^VIIIZr transition, associated with a recovery of the crystalline zircon medium-range environment. For both techniques, the zircon structure is fully recovered at annealing temperatures up to 900° C.

Electrostatic modelings suggest that the ^VIIIZr→^VIIZr transition observed in zircon with increasing alpha-decay damage creates significantly overbonded oxygen atoms around Zr. With increasing temperature, those oxygen atoms are better bonded to ^VIIZr, due to the thermal expansion of the Zr-O bond. The congruent recovery of the zircon structure should therefore be favoured with increasing temperature. On the other hand, the metamict network can be also partially reorganized around 400–500° C, with the creation of Zr-rich domains, as measured by EXAFS. However, the growth of these domains after 3 hours annealing affects only minor portions of the aperiodic network. This model is corroborated by a similar thermal behaviour observed for a synthetic sol-gel of ZrO₂ · SiO₂ composition.