Clancium oxide

From Memory Alpha, the free Star Trek reference.

Clancium oxide is one of many explosive materials available to the crew of the USS Enterprise-D in 2367 when they were trapped in a Tyken's Rift.

According to the database, this material is a "Medium-yield explosive used for high-volume heat generation in emergency situations. This compound is preferred to conventional sarium krellide explosives because of superior thermal dispersion characteristics in low-pressure atmospheres." (TNG: "Night Terrors")

Atoms of this material appeared in an okudagram display of explosives listed in the Enterprise-D library computer. By the nomenclature, one could assume that clancium by itself, without the oxide component, might be a chemical element.