The new mineral francoisite-(Ce),
(Ce,Nd,Ca)[(UO(2))(3)O(OH)(PO(4))(2)]center dot 6H(2)O is the
Ce-analog of francoisite-(Nd). It has been discovered simultaneously at
the La Creusaz uranium deposit near Les Marecottes in Valais,
Switzerland, and at the Number 2 uranium Workings, Radium Ridge near Mt.
Painter, Arkaroola area, Northern Flinders Ranges in South Australia.
Francoisite-(Ce) is a uranyl-bearing supergene mineral that results from
the alteration under oxidative conditions of REE- and U(4+)-bearing
hypogene minerals: allanite-(Ce), monazite-(Ce), +/- uraninite at Les
Marecottes; monazite-(Ce), ishikawaite-samarskite, and an unknown
primary U-mineral at Radium Ridge. The REE composition of
francoisite-(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 Ce(4+) during transport.