The elements erbium and terbium were discovered by Carl Gustaf Mosander, a Swedish chemist, in 1843. He had discovered lanthanum four years earlier. His former supervisor, Jacob Berzelius, had discovered the new element cerium in cerite, so Mosander continued to study cerite and discovered lanthanum. Later, in 1843, he began studying the mineral gadolinite. Johan Gadolin had discovered yttrium from gadolinite in 1794.
Mosander began studying gadolinite in the hope of finding another element. He made studies on a sample of yttria (that is, yttrium oxide) obtained from this mineral, and found that it actually contained two other, previously unknown, metal oxides. He called them erbia and terbia. They contained the two new rare earth metals called erbium and terbium. What is interesting is that Mosander had now discovered three new elements, (lanthanum, erbium, and terbium) all rare earths, in minerals that other scientists had discarded after making their discoveries.
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