pelopium

from The Century Dictionary.

• noun The name given by H. Rose to a supposed new chemical element which he believed that he had detected in the mineral columbite from Bavaria. He afterward recognized its identity with niobium.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Chem.) A supposed new metal found in columbite, afterwards shown to be identical with columbium, or niobium.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun obsolete A supposed chemical element, later found to be a mixture of niobium and tantalum.