The degradation of monazite; implications for the mobility of rare-earth and actinide elements during low-temperature alteration

Monazite is an important economic source of thorium, the rare-earth elements and uranium. This and its chemically inert nature has led to speculation that artificial phosphate-based matrices similar in composition to monazite may prove useful as wasteforms for high-level radioactive waste. In order to assess the long-term degradation behaviour of monazite, an integrated geological, hydrogeological and mineralogical study was undertaken at the Steenkampskraal monazite mine, South Africa. Steenkampskraal is among the richest monazite ore bodies in the world comprising up to 45 weight % rare-earth oxides, 8.8% thorium oxides and 600 ppm uranium. Optical and electron-microprobe analyses of the ore reveal distinctive alteration patterns with uranium appearing to be lost preferentially. The heavy rare earths are also preferentially removed from the ore, giving rise to a marked fractionation in altered grains. Thorium, although leached, is re-concentrated together with the heavier rare-earth elements in microcrystalline silicate and oxide alteration products within the host rock. The implications for waste encapsulation are discussed from the perspective of potential groundwater transport away from the source.