Xenotime (from the Greek words xenos, "foreign", and time, "honour") is a rare yttrium phosphate mineral whose chemical formula is 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, thorium, uranium, ytterbium; and zirconium (all replacing yttrium): 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 (waterfree) phosphates" group.