Witherite is a barium carbonate mineral, BaCO3, in the aragonite group. Witherite crystallizes in the orthorhombic system and virtually always is twinned. The mineral is colorless, milky white, grey, pale yellow, to pale brown. The specific gravity is 4.3, which is high for a non-metallic mineral. It fluoresces light blue under both long and short-wave UV and is phosphorescent under short-wave UV light.
Witherite forms in low temperature hydrothermal environments. It is commonly associated with fluorite, celestine, galena, barite, calcite and aragonite. Witherite occurrences include: Cave-in-rock, Illinois, USA; Alston Moor, Anglezarke, Cumberland and Durham, England; Thunder Bay area, Ontario, Canada and Germany.
Witherite was named for William Withering (1741-1799) an English physician and naturalist.