Decalithium was a highly rare isotope and element which was a necessary component in making red matter.

The Romulan mining Captain Nero discovered large deposits of decalithium on an unnamed barren planet and began extraction procedures as it held great value to the Romulan Star Empire. However, he ended mining operations after a supernova from the Hobus star erupted and began threatening the Beta Quadrant, as well as Romulus itself.

In order to combat this threat, it was decided to make use of decalithium to create red matter in order to collapse the Hobus star into a singularity. However, the technology for processing the isotope was only present on Vulcan. (ST - Countdown comic: "Number One")

Quark discovered concentrations of decalithium at the pergium mine on Xoxa colony that he won from his cousin Gaila in a game of tongo. (STO novel: The Needs of the Many)

The Countdown series didn't reveal many scientific details of this element, so its position on the periodic table and specific makeup is unestablished. The formation of the name has the prefix deca- preceding lithium, indicating a tenfold increase of the basic lithium. A similar nomenclature is used for dilithium and trilithium.
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