A TEM investigation of natural metamict zircons: structure and recovery ofamorphous domains

The nature of the amorphous regions and their recovery processes in two nat ural metamict zircon samples from Sri Lanka have been studied by high rt so lution and analytical transmission electron microscopy. Samples untreated a nd annealed at different temperatures were investigated. Nanoprobe analyses on untreated samples and samples annealed at 1000 K show that within exper imental uncertainties, no chemical segregation occurred. In samples anneale d at higher temperatures (greater than or equal to 1100 It) recovery occurs in a two-stage process and leads to different microstructures, which depen ded on the initial amount of metamictization. In highly amorphized samples, recrystallization starts at 1200 K. Randomly oriented ZrO2 grains embedded in a silica-rich matrix are detected. At higher temperature (16 h at 1600 K), the assemblage transforms into a polygonal texture of small zircon grai ns. Some untransformed zirconia grains and Dockets of silica-rich glass are still present, however. In partially metamict samples, recovery starts at 1100 K. The small surviving oriented zircon domains grow at the expense of the surrounding amorphous material. At 1200 K, new zirconia grains nucleate with random orientations. After 1 h annealing at 1400 K, the zircon struct ure is restored and the microstructure coarse-grained. The proportion of cr ystalline zirconia and silica-rich glass has dramatically decreased.