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. Xenotime is used chiefly as a source of yttrium and erbium and fine crystal specimens are valued by collectors. Occasionally, gemstones are also cut from the finer xenotime crystals.
The etymology of the name xenotime is uncertain: the mineral may be named so because it occurs in such small crystals that were once overlooked, or because the yttrium contained within it was first thought to be a new element.
Crystallising in the tetragonal crystal system, xenotime is typically translucent to opaque (rarely transparent) in shades of brown to brownish yellow (most common), but also reddish to greenish brown and gray. Xenotime has a variable habit: It may be prismatic (stubby or slender and elongate) with dipyramidal terminations, in radial or granular aggregates, or rosettes. A soft mineral (Mohs hardness 4.5), xenotime is—in comparison to most other translucent minerals—fairly dense, with a specific gravity between 4.4–5.1. Its lustre, which may be vitreous to resinous, together with its crystal system, may lead to a confusion with zircon with which xenotime may sometimes occur.
Xenotime has two directions of perfect prismatic cleavage and its fracture is uneven to irregular (sometimes splintery). It is considered brittle and its streak is white. The refractive index of xenotime is 1.720-1.815 with a birefringence of 0.095 (uniaxial positive). Xenotime is dichroic with pink, yellow, or yellowish brown seen in the extraordinary ray, and brownish yellow, grayish brown or greenish brown seen in the ordinary ray. There is no reaction under ultraviolet light. While xenotime may contain significant amounts of thorium or uranium, the mineral does not undergo metamictization like sphene or zircon would.
Occurring as a minor accessory mineral, xenotime is found in pegmatites and other igneous rocks, as well as gneisses rich in mica and quartz. Associated minerals include biotite and other micas, chlorite group minerals, quartz, zircon, certain feldspars, analcime, anatase, brookite, rutile, siderite, and apatite. Xenotime is also known to be diagenetic: It may form as minute grains or as extremely thin (less than 10 µ) coatings on detrital zircon grains in siliciclastic sedimentary rocks. The importance of these diagenetic xenotime deposits in the radiometric dating of sedimentary rocks is only beginning to be realised.
Discovered in 1832, xenotime's type locality is Hidra (Hitterø), Flekkefjord, Vest-Agder, Norway. Other notable localities include: Arendal and Tvedestrand, Norway; Novo Horizonte, São Paulo, Novo Horizonte, Bahia, and Minas Gerais, Brazil; Madagascar; and California, Colorado, Georgia, and North Carolina, United States. A new discovery of gemmy, colour change (brownish to yellow) xenotime has been reported from Afghanistan. North of Mount Funabuse in Gifu Prefecture, Japan, a notable basaltic rock is quarried at a hill called Maru-Yama: Crystals of xenotime and zircon arranged in a radiating, flower-like pattern are visible in polished slices of the rock, which is known as chrysanthemum stone (translated from the Japanese kiku-ishi). This stone is widely appreciated in Japan for its ornamental value.