Franconium is a chemical element listed on the Table of Elements. (TNG: "Rascals")
The element franconium had the symbol "Fr", which is the same atomic symbol used for francium. It is possibly a misspelling, or an homage to The Wild Wild West, which featured the fictional radioactive element franconium in two episodes: "The Night of the Glowing Corpse" (1965) and "The Night of the Cadre" (1967).