Xenotime

Xenotime
Xenotime with rutile
General
Category	Phosphate minerals
Formula	YPO4
IMA symbol	Xtm[1]
Strunz classification	8.AD.35
Crystal system	Tetragonal
Crystal class	Dipyramidal (4/mmm) H-M symbol: (4/m)
Space group	I41/a
Identification
Color	Brown, brownish yellow, gray
Crystal habit	Prismatic, radial aggregates, granular
Cleavage	Perfect [100]
Fracture	Uneven to splintery
Mohs scale hardness	4.5
Luster	Vitreous to resinous
Streak	Pale brown, yellowish or reddish, to white
Diaphaneity	Translucent to opaque
Specific gravity	4.4–5.1
Refractive index	1.720–1.815
Birefringence	δ = 0.096
Pleochroism	Dichroic
Other characteristics	Not radioactive or luminescent
References	[2][3][4][5]

Xenotime is a rare-earth phosphate mineral, the major component of which is yttrium orthophosphate (YPO₄). The phosphate ions are described by a tetrahedral shape and coordinate to the center Y³⁺ metal ion in a way that closely resembles the structure of zircon (ZrSiO₄).^[6] 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. Other iso-structural ions that undergo exchanges with PO₄ are VO₄ and NbO₄ ions, contributing to the list of possible co-occurring elements that may be in need of separation.^[7] The rare-earth elements 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 finest xenotime crystals.