Physical and chemical characteristics of baddeleyite (monoclinic zirconia) in natural environments: an overview and case study
1999; Elsevier BV; Volume: 274; Issue: 1-2 Linguagem: Inglês
10.1016/s0022-3115(99)00066-5
ISSN1873-4820
Autores Tópico(s)Nuclear Materials and Properties
ResumoThis report provides an overview of the natural occurrence, physical characteristics, and chemical composition of the mineral baddeleyite, ideally ZrO2. A survey of the literature shows that baddeleyite is widespread in nature as a trace mineral, but has a rather limited composition of 87–99 wt% ZrO2 with most of the remainder comprised of FeO, TiO2 and HfO2. Natural baddeleyite is invariably monoclinic due to the low concentrations of large cations such as Ca and Y. Although the concentrations of Th and U are low (generally <1500 ppm U and <100 ppm Th), the cumulative alpha-decay dose reaches 0.1–1.1 × 1016 mg−1 for samples with ages of 1–2 × 109 yr. The available data also suggest that baddeleyite is highly durable in aqueous fluids. For nuclear waste form applications, both the radiation damage effects and durability of baddeleyite need to be examined in further detail. A case study of baddeleyite from the Jacupiranga carbonatite complex of southern Brazil provides some additional information on the crystal chemistry, durability and radiation damage effects of the mineral. This work shows that baddeleyite can incorporate up to 4.1 wt% Nb2O5 and 1.2 wt% Ta2O5. Incorporation of Nb5+ and Ta5+ is partially compensated by the incorporation of up to 0.4 wt% MgO and 0.3 wt% FeO in a charge balanced substitution of the form 3Zr↔2(Nb, Ta) + (Mg, Fe). Similar substitution mechanisms may enable the incorporation of Np5+ in the ZrO2 phases of fuel and waste matrices. The mineral is highly resistant to hydrothermal alteration which affected associated uranpyrochlore crystals. Many of the baddeleyite crystals are partially enclosed within the uranpyrochlore grains which contain 19–26 wt% UO2 and 0.3–3.5 wt% ThO2. These baddeleyite crystals received maximum alpha-particle doses of 3–4.5 × 1016 mg−1 within 10 μm of the uranpyrochlore grain boundary, but the intense irradiation has not affected either the structural integrity or the aqueous durability of the mineral.
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