Xenotime-(y)

Crystallising in the tetragonal (I41/amd) 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 (ZrSiO4), the latter having a similar crystal structure and 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.