Xenotime is a rare earth phosphate mineral, the major component of which is yttrium orthophosphate (YPO₄). It forms a solid solution series with chernovite-(Y) (YAsO₄) and therefore may contain trace impurities of arsenic as well as silicon dioxide and calcium. The rare earths dysprosium, erbium, terbium and ytterbium as well as metal elements such as thorium and uranium (all replacing yttrium) are the expressive secondary components of xenotime. Due to uranium and thorium impurities, some xenotime specimens may be weakly to strongly radioactive. Lithiophyllite, monazite and purpurite are sometimes grouped with xenotime in the informal "anhydrous phosphates" group. Xenotime is used chiefly as a source of yttrium and heavy lanthanide metals (dysprosium, ytterbium, erbium and gadolinium). Occasionally, gemstones are also cut from the finer xenotime crystals.