Colemanite (Ca2B6O115H2O) is a borate mineral found in evaporite deposits. Colemanite is a secondary mineral that forms when borax is deposited, and then regularly covered with boron containing water. The boron remains after the water evaporates, and the boron and borax eventually form other minerals as for example Colemanite.

It was named after William T. Coleman (1824-1893), owner of the mine where it was first found.

Group: Borates, Hardness: 4.5, Specific gravity: 2.42, cleavage: perfect, Fracture: uneven-conchoidal, mineral may be white, yellow, or gray. Streak: white. Transparent-translucent, luster: vitreous. Forms in evaporative deposites. Soluble in hydrochloric acid. Fuses easily, breaks up, and colors a flame green. It is monoclinic. Crystals are short and prismatic.