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(named for W. T. Coleman, 1824–93, the owner of the mine where it was discovered), a mineral of the class of borates. In terms of chemical composition it is a hydrous calcium borate, Ca[B2BO4(OH)3].H2O. Complex chains composed of B(O, OH)4 tetrahedrons and BO3 triangles bonded into a three-dimensional structure through Ca2+ ions and H2O buffer molecules are present in the crystal structure. Colemanite crystallizes in a monoclinic system, forming short, colorless crystals of columnar or dipyramidal appearance, as well as continuous fine-grained masses. Hardness on the mineralogical scale, 4–4.5; density, 2,440–2,450 kg/m3.
Colemanite is formed by precipitation, along with other borates (hydroboracite, inyoite, and borax), gypsum, and clayey deposits, from boron-containing brines of continental lakes. It is also found in the deposits of hot springs. Along with the other borates, colemanite is used as a boron ore.