What is "guilleminite"

n. (context mineral English) An orthorhombic-pyramidal mineral containing barium, hydrogen, oxygen, selenium, and uranium.

Guilleminite (Ba(UO)(SeO)(OH)•3HO) is a uranium mineral named by R. Pierrot, J. Toussaint, and T. Verbeek in 1965 in honor of Jean Claude Guillemin (1923–1994), a chemist and mineralogist. It is a rare uranium/ selenium mineral found at the Musonoi Mine in the Katanga Province of the Democratic Republic of the Congo.

This secondary mineral also includes barium in its structure, in addition to selenium and uranium. It is bright yellow in colour and usually has an acicular crystal habit. It has a Mohs hardness of 2–3.