X-RAY DIFFRACTION STUDY OF BRABANTITE-MONAZITE SOLID SOLUTIONS

Abstract

We synthesized compounds with stoichiometry of natural brabantite M2+Th(PO4)2 with M2+=Ca, Cd, Sr, Pb, Ba at 1 bar, 1200 °C and 2.5 kbar, 700 °C. Those compounds were studied by powder X-ray diffraction and electron microprobe. For Ca, Sr, and Pb, we obtained always crystals with the monazite structure. For Cd, the monazite structure is obtained at 1 bar, but not at 2.5 kbar. For Ba, we obtained the monazite structure only at 2.5 kbar. The unit-cell parameters of the compounds with monazite structure vary regularly with the size of the M2+ ion. We studied also the solid-solution LaPO4-M2+Th(PO4)2, at 1200 °C, 1 bar. The solid solutions for Cd, Ca, and Sr are continuous, and the unit-cell parameters vary linearly with the degree of substitution. For Pb, the solid solution is continuous, but the unit-cell parameters do not vary linearly. For Ba, there is a miscibility gap, with a maximum of about 50 mol% of BaTh(PO4)2 in LaPO4. These results have several consequences for U-Th-Pb geochronology of monazite, and for utilization of monazite as a nuclear-waste ceramic.