Marie Curie found chalcolite to be about twice as active as uranium itself.
Chalcolite, also called torbernite, is a hydrated copper uranyl phosphate mineral. In 1898, Curie measured the ionization produced by radioactive minerals and noticed that some uranium ores were more active than their uranium content could explain. Natural chalcolite was one of those surprising samples.
The result was important because Curie had already found that radioactivity was linked to uranium itself, not simply to a mineral’s physical form. She prepared artificial chalcolite from purified materials; it showed only the activity expected from its uranium content. The stronger activity of natural specimens therefore suggested an additional, extremely radioactive substance in the ore.
That clue helped lead Marie and Pierre Curie to search pitchblende chemically for new elements. Their work resulted in the announcements of polonium in July 1898 and radium in December 1898. A common mix-up is pitchblende: some pitchblende samples were roughly four times as active as uranium, while chalcolite was described as about twice as active.