Mechanism of hydrothermal alteration of natural self-irradiated and synthetic crystalline titanate-based pyrochlore

Pyrochlore-based ceramics are considered as a potential host to immobilize high-level nuclear waste in a geological repository. For this application it is essential to investigate the fundamental mechanism of the equilibration of pyrochlore in aqueous solutions. We treated cuboids of a synthetic crystalline and a natural metamict, i.e., self-irradiation-damaged Ti-based pyrochlore in 1 ml of a 1 M HCl solution containing 43.5% O-18 at 250 degrees C for 72 h. After the experiments the colours of the samples turned from black to almost white and their surfaces were covered by micron-sized crystallites. Both samples were mainly transformed into rutile with subordinate anatase and the reaction can be traced several tens of micrometers into the cuboids. In the case of the natural pyroch lore an additional phase (aeschynite [(REE)(Ti,Nb)(2)(0,OH)(2)]) appeared as crystals on the surface and within an up to 100 mu m thick layer at the interface between TiO2 and the unreacted pyrochlore. O-18 is in both cases highly enriched in the reaction products with a sharp gradient (on a micrometer scale) to the unreacted pyrochlore. The replacement reaction retains even fine-scale morphological features typical for pseudomorphs. This observation along with the enrichment of O-18 in the product phases and the textural features clearly show that the dissolution of pyrochlore is spatially coupled with the precipitation of stable (metastable) TiO2 phases at a moving front. During such a coupled process dissolution at the proceeding front is the rate controlling step. The aeschynite phase at the interface is thought to be either an intermediate alteration product or crystallized from a fluiid which is located at the interface between the TiO2 Polymorphs and the unreacted pyrochlore due to changing transport properties between the interface and the fluid with increasing thickness of the TiO2 rim. We emphasize here that our results produced under relatively extreme batch-experimental conditions show similarities with nature as well as with results derived from experiments conducted under moderate conditions rather expected in a nuclear repository. (c) 2007 Elsevier Ltd. All rights reserved.