THE STRUCTURE OF METAMICT ZIRCON - A TEMPERATURE-DEPENDENT EXAFS STUDY

The thermal annealing (300-1700 K) of two metamict zircons (Ampagabe, Madagascar and Naegy, 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 z ircon are evidenced between 293 and 1700 K. The first stage (up to 600 -degrees-C) shows a decrease of the a(o)-cell parameter from 6.674 (at 300-degrees-C) to 6.610 (at 600-degrees-C) +/- 0.005 angstrom. In tha t temperature range, the average local environment around Zr (presence of (VII)Zr and d(Zr - Zr) almost-equal-to 3.3 - 3.6 angstrom) shows a weak, but significant increase of the Zr - Zr correlations located at 3.3 - 3.4 angstrom, undetectable by XRD. At temperatures up to 700-de grees-C (stage 2), the XRD-Bragg component arising from crystalline zi rcon increases in magnitude, whereas, Zr - K EXAFS analysis indicates a progressive (VII)Zr --> (VIII)Zr transition, associated with a recov ery of the crystalline zircon medium-range environment. For both techn iques, the zircon structure is fully recovered at annealing temperatur es up to 900-degrees-C. Electrostatic modelings suggest that the (VIII )Zr --> (VII)Zr transition observed in zircon with increasing alpha-de cay damage creates significantly overbonded oxygen atoms around Zr. Wi th increasing temperature, those oxygen atoms are better bonded to (VI I)Zr, due to the thermal expansion of the Zr-O bond. The congruent rec overy of the zircon structure should therefore be favoured with increa sing temperature. On the other hand, the metamict network can be also partially reorganized around 400-500-degrees-C, with the creation of Z r-rich domains, as measured by EXAFS. However, the growth of these dom ains after 3 hours annealing affects only minor portions of the aperio dic network. This model is corroborated by a similar thermal behaviour observed for a synthetic sol-gel of ZrO2 . SiO2 composition.