From Halopedia, the Halo wiki
Xenotime is a type of mineral ore. Xenotime's primary use is that it yields the lanthanide elements ytterbium and erbium - critical to the manufacture of small-scale fusion reactors and ultra-efficient lasers.
The resource is extremely common on the colony of Mesra, making the planet an important strategic target for the Covenant during the Human-Covenant War. During 2526, the Covenant invaded Mesra rather than glassing the planet to take these resources; resulting in Blue Team and the 24th Marine Engineering Brigade being deployed to destroy all of the planet's mining infrastructure and deposits via the use of nuclear weapons. The Doukala Xenotime Works was the most remote of several mines on Mesra designed to extract the ore, located deep within the Karpos Mountain Range. Xenotime was not the only lanthanide ore with a very rich presence on Mesra, with many other similar resources often synonymous with a network of ancient cave systems formed long before humans colonies the planet.
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