A Re-examination of the Rare-earth-element Orthophosphate Standards in Use for Electron-microprobe Analysis

A re-examination of fourteen standards consisting of orthophosphates of the rare-earth elements, Sc and Y, grown at Oak Ridge National Laboratory in the 1980s and widely distributed by the Smithsonian Institution’s Department of Mineral Sciences, reveals that some of the material is significantly contaminated by Pb. The origin of this impurity is the Pb2P2O7 flux that is derived from the thermal decomposition of PbHPO4. The lead pyrophosphate flux is used to dissolve the oxide starting materials at elevated temperatures (~1360°C) prior to crystal synthesis. Because these rare-earth-element standards are extremely stable under the electron beam and considered homogeneous, they have been of enormous value to the technique of electron-microprobe analysis (EMPA) whenever samples are to be analyzed for the rare-earth elements, Sc, Y, and P. The monoclinic orthophosphates (monazite structure) show a higher degree of Pb incorporation than the tetragonal orthophosphates (xenotime structure). We here describe the extent of the Pb contamination and explain the degree to which the stoichiometries have been affected by the replacement of REE by Pb within the crystal structure.