1
Jakub Haifler#, 1Radek Škoda
1 Department of Geological Sciences, Masaryk University, Brno, Czech Republic
Key words: zirconolite, hydrothermal alteration, metamict, experiment, annealing
INTRODUCTION
Zirconolite, CaZrTi2O7, is a mineral with
excellent resistance to hydrothermal alterations. It usually remains unaltered in geological environments for hundreds of millions of years or even longer periods despite of high contents of unstable elements like U and Th. For this reason synthetic analogue of zirconolite doped with components of spent nuclear fuel is suggested to be used as a form for immobilisation of high level nuclear waste. In this work we studied both the natural and experimental alterations of metamict zirconolites from Håkestad and Stålacker quarries in Norway in order to describe behaviour of this phase in contact with hydrothermal fluids – the mobility of elements and the alteration features.
CHEMICAL COMPOSITION AND XRD MEASUREMENT
Zirconolites are highly enriched in REE (Ce, Nd, Y, La), Nb, Th, U and Fe relative to ideal composition. Natural alteration features are quite rare among zirconolites, although in case of here-described samples the alteration zones are observed at some rims or along cracks. They are depleted in
Ca, Fe, Zr and Ti and enriched in Si and H2O compared to unaltered parts. The
contents of heavy elements U, Th and REE, however does not change significantly. Metamict state of zirconolite caused by high content of U and Th was confirmed by XRD-powder diffraction analysis. The annealing at temperatures between 400 and 800°C in a reaction chamber gave rise to a recrystallization of a cubic phase (Fm-3m space group with cell parameter a=5.104(3) Å). During annealing at 900°C, zirconolite-3O polytypoid was formed.
EXPERIMENTS
The fragments of metamict zirconolite were experimentally altered in three solutions (1M HCl; 1M HCl + 0.2 g of silica powder; 1M HCl + 1 g of uranium acetate) at 200°C for 30 days. Complex alteration zones rich in Zr, Nb, Ti were formed by a dissolution-reprecipitation process. Uranium, thorium and REE were not immobilised in solid products. Further experiments were performed on powdered samples - 1570 mg of powder were altered in 15 ml of 1M HCl solution and 795 mg of powder were altered in 15 ml of 1M NaOH solution at 200°C for 30 days.
49 There were 15.31 mg Ca; 0.01 mg Zr; 0.25 mg Ti; 6.39 mg Th; 2.25 mg U; 15.15 mg Ce and 5.88 mg Nd dissolved in the acid after the termination of the experiment. The relative mobility of selected elements from mineral to acidic fluid in descending order is as follows: U > Nd > Ca > Ce > Th > Ti > Zr > Nb. There were 0.04 mg Ca; 0.005 mg Zr; 0.005 mg Ti; 0.003 mg Nb; 0.056 mg U and 0.0005 mg Ce dissolved in the hydroxide. The relative mobility of the selected elements in descending order is as follows: U > Ca > Ti > Nb > Zr > Ce > Th, Nd. The complex zones of products observed within the altered fragments did not form in powders altered in HCl, but two separated products were formed. Powders altered in NaOH solution were not covered by products; they had rounded edges indicating a dissolution process only in contrast with HCl experiments, where the dissolution and reprecipitation were spatially coupled. Anatase and zircon were determined as products of alteration in HCl solutions. Zircon was determined as products of alteration in NaOH solutions and monazite overgrowth on zirconolite was observed despite the fact that phosphorus was not input deliberately. Many further unidentified phases were discovered.
CONCLUSIONS
In case of natural alterations of metamict zirconolite the heavy elements U, Th and REE are commonly preserved in solids (this work; Lumpkin 2001; Williams et al. 2001; Bulakh et al. 1998; Bulakh et al. 2006), while when the strongly corrosive solutions of HCl and NaOH were applied, these elements were released into solution. The completely metamict state of zirconolite together with hot and aggresive acidic or basic fluid disrupt the usual retention of U, Th, REE previously observed among naturally altered samples (cited above), experimentally altered crystalline samples (Pöml et al. 2011) or those altered by less corrosive fluid or at
lower temperature (Leturcq et al. 2005; Pöml et al. 2011).
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