|a colourless or white glassy mineral consisting of hydrated calcium borate in monoclinic crystalline form. It occurs with and is a source of borax. Formula: Ca2B6O11.5H2O|
|[C19: named after William T. Coleman (1824--93), American pioneer, owner of the mine in which it was discovered]|
borate mineral, hydrated calcium borate (Ca2B6O115H2O) that was the principal source of borax until the 1930s. It typically occurs as colourless, brilliant crystals and masses in Tertiary sediments (those formed 66.4 to 1.6 million years ago), where it has been derived from ulexite and borax. The chief localities are the Kramer district, Death Valley, and Daggett, Calif. Colemanite is an important source of commercial borates and boric acid. For detailed physical properties, see borate mineral (table). See also borax.
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