Experimental study of Th-bearing LaPO ₄ (780 degrees C, 200 MPa); implications for monazite and actinide orthophosphate stability

Centre de Recherches sur la Geologie des Matieres Premieres Minerales et Energetiques, France

A complete solid solution has been hydrothermally synthesized between the two end-members LaPO ₄ and (Ca (sub 0.5) Th (sub 0.5) )PO ₄ at 780 degrees C and 200 MPa, indicating that there is no limitation in temperature and pressure conditions corresponding to those of granitic magmas for Th insertion in natural monazites. The composition limits of the (Al (super 3+) (sub 1-2chi ) B (super 2+) (sub chi ) C (super 4+) (sub chi ) )PO ₄ compounds crystallized in the monazite structure-type are determined by both r _average = (1-2chi ) (super [9]) r (sub A3+) + chi (super [9]) r (sub B2+) + chi (super [9]) r (sub C4+) and r _ratio = (1-chi ) (super [9]) r (sub A3+) + chi (super [9]) r (sub B2+) /(1-chi ) (super [9]) r (sub A3+) + chi (super [9]) r (sub C4+) parameters (where (super [9]) r _A is the ionic radius of the A element in ninefold coordination). The upper and lower values of these parameters are 1.216 Aa > or =r _average > or =1.107 Aa and 1.238> or =r _ratio > or =1. The incorporation of large amounts of trans-uranium elements in the monazite structure is deduced from this model. The limitations and geochronological inferences of this model are discussed.

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