I always wonder why "silicium" is disused in favor of silicon, given that "-ium" is used liberally today, and you made me to look it up. Apparently, "-ium" is a suffix for metal only. But when naming new/hypothetical elements that have no standard name yet, IUPAC's Systematic Element Name rules overrides it and use "-ium" for all elements, but when the name is given, the usual rules apply.