Trevium was a substance that reacted with tri-magnesite to produce ultraviolet radiation.

In 2267, ultraviolet satellites containing trevium and tri-magnesite were placed in orbit over the planet Deneva to destroy flying parasites that had taken over the bodies of the inhabitants. (TOS: "Operation -- Annihilate!")

According to the Star Fleet Medical Reference Manual (p. 19), the element trivium, symbol Tv, atomic weight 328, atomic number 127, was discovered by the Federation from the asteroid belt of the Denevan system in 2153.
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