Class : Carbonates, nitrates, borates
    Subclass : Borates
    Crystal System : Monoclinic
    Chemistry : Ca₂B₆O₁₁ H₂O
    Rarity : Fairly rare

Colemanite is like the vast majority of borates, a mineral formed by evaporation from salt lakes and lagoons in a desert environment. The waters of these lakes contain large amounts of borated brines due to volcanic vapors. It owes its name to William Tell Coleman, founder of the Californian boron industry and owner of the mine where it was discovered (Furnace Creek, California). In its salt lake deposits, colemanite is associated with many salts : borates (ulexite, borax, kernite...), halite, glauberite, thenardite, gypsum... More rarely, it is present in hot spring deposits (Chile). It appears to originate from the hydration of primary borates (borax and ulexite). The crystals occupy large dissolution cavities in these primary borates, which allows them to develop various automorphic facies : short prismatic to isometric (pseudo-octahedral), or lanceolate with sharp edges (pseudorhombohedral). Colemanite also occurs in compact masses, sometimes in spherolitic aggregates. It has a vivid vitreous to adamantine luster, a high density for a borate (2.4) ; it is colorless to milky white, sometimes yellowish-white, gray or brown. In the past, it was the world's main source of boron before being replaced by kernite.

No twin report for this mineral species.