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xenotime Accessory mineral, YPO and associated with monazite (Ce, La, Th) PO4; sp. gr. 4.4–5.1; hardness 4–5; tetragonal; yellowish-brown, greyish-white, or pale yellow; pale brown streak; resinous to vitreous lustre; crystals tetragonal prisms, very similar to zircon; cleavage prismatic; occurs in granitic and alkaline, igneous rocks, as well as in pegmatites and gneisses.