boracite, colourless, glassy borate mineral, magnesium chloroborate (Mg3B7O13Cl). It has been found as crystals embedded in sedimentary deposits of anhydrite, gypsum, and halite. A massive variety occurs as nodules in the salt-dome deposits at Stassfurt, Ger., where it has been mined as a source of boron. For detailed physical properties, see borate mineral (table).