ZIRCON: CRYSTAL CHEMISTRY, STRUCTURAL VARIATIONS, AND RADIOLOGICAL EFFECTS

This study investigates the variations of structural parameters and chemistry of a partially metamict and seven detrital zircons from different localities using single-crystal X-ray diffraction (SCXRD), synchrotron high-resolution powder X-ray diffraction (HRPXRD), and electron microprobe techniques (EMPA). The unit-cell parameters for the eight zircon samples vary linearly with increasing unit-cell volume, V. The detrital zircon sample 7 from the Canadian Arctic region has the lowest unit-cell parameters and bond distances, ideal stoichiometric composition, and is unaffected by α-radiation damage. Therefore, sample 7 is chemically and structurally pure zircon. The sample 8 from Jemaa, Nigeria shows the significant change throughout the synchrotron HRPXRD trace and reveals the largest structural parameters after the Rietveld refinement. The increase of unit-cell parameters, average <Zr-O>, and Si-O distances for sample 8 are the effect of α-radiation doses received over long geologic time (2384 Ma) and are not related to the substitutions in Zr and Si sites as both sites are fully occupied by Zr and Si, respectively. The short Zr-O^I distance for the sample 8 is not affected by the radiation accumulation and annealing processes. Except for sample 8, all the other zircon samples show very good correlations between the V and Zr and Si (apfu) contents. They received α-radiation doses which are lower than ~3.5 ´ 10¹⁵ α-decay events/mg. Substitutions of other cations at the Zr and Si sites control the variations of structural parameters for samples 1 to 7. The sample 5 shows relatively long unit-cell parameters and bond distances because the Zr site accommodates other cations that have higher ionic radii. Geological age increases the radiation doses in zircon and it is also related to the V.