Meisser and 4 co-authors reported in 2010- that the new mineral françoisite-(Ce), (Ce,Nd,Ca)[(UO2)3O(OH)(PO4)2]·6H2O is the Ce-analog of françoisite-(Nd). It has been discovered simultaneously at the La Creusaz uranium deposit near Les Marécottes in Valais, Switzerland, and at the Number 2 uranium Workings, Radium Ridge near Mt. Painter, Arkaroola area, Northern Flinders Ranges in South Australia.
Françoisite-(Ce) is a uranyl-bearing supergene mineral that results from the alteration under oxidative conditions of REE- and U4+-bearing hypogene minerals: allanite-(Ce), monazite-(Ce), ±uraninite at Les Marécottes; monazite-(Ce), ishikawaite-samarskite, and an unknown primary U-mineral at Radium Ridge.
The REE composition of françoisite-(Ce) results from a short aqueous transport of REE leached out of primary minerals [most likely monazite-(Ce) at Radium Ridge and allanite-(Ce) at La Creusaz], with fractionation among REE resulting mainly from aqueous transport, with only limited Ce loss due to oxidation to Ce4+ during transport.
The controls on REE composition of minerals precipitating from solutions are complex, depending under equilibrium conditions upon the crystallographic control exerted by the mineral, the REE composition of the fluid, the pressure and temperature, and the aqueous speciation of the REE (i.e. nature of the stable complexes, which itself depends upon the ligands present in solution, pH, and redox) (Lipin and McKay 1989; review of REE partitioning between minerals and fluids in Brugger et al. 2008).
In chondritic meteorites and in post-Archaean shales, Ce is the most abundant REE, followed by Nd or La (0.638, 0.474,and 0.245 ppm, respectively, in chondrites; Evensen et al. 1978; 79.6, 33.90, and 38.2 ppm in post-Archaean Australian Shales; McLennan 1989). The crystallization of minerals in which Nd or La are the dominant REE usually occurs in the weathering environment, under conditions where Ce4+ becomes stable.
Ce4+ is highly insoluble, usually coprecipitating with Fe and Mn-oxyhydroxides (e.g., Takahashi et al. 2000); hence Ce tends to be immobile where Ce4+ is present. Taunton et al. (2000) show how this behavior results in preferential leaching of trivalent REE in the top of the weathering profile in a granite, resulting in a top soil enriched in Ce relative to the fresh granite, and in soil solutions depleted in Ce relative to the other REE. This process can lead to the formation of unusual Nd- and La-rich minerals during the weathering of REE-rich ore deposits (e.g., Brugger et al. 2006a; Pring et al. 2006), and it is probably responsible for the Nd-rich and Ce-poor composition of françoisite-(Nd) at Kamoto.
Some small-scale fractionation of Ce relative to the other REE happened at both La Creusaz and Radium Ridge, although Ce remains the main REE in all analyzed samples from these localities. This hints that REE transport occurred under near-surface oxidizing conditions, but that only a small proportion of Ce was oxidized to Ce4+ along the path of REE aqueous transport.
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