Mineral, K₂(UO₂)₂(VO₄)₂.1–3H₂O; sp. gr. 4.5; hardness 2.0–2.5; monoclinic; normally bright yellow to greenish-yellow; pearly lustre; cleavage perfect, basal {001}; strongly radioactive; occurs in the weathered zone of sedimentary uranium ores, especially in sandstones enriched with organic matter, normally as coatings and masses in rocks. It is an important ore for uranium.