a mineral from the group of anhydrous borates. Chemical composition Mg₃[B₇O₁₂]OCl. It contains 62.5 percent B2O3. It crystallizes into a rhombic (pseudocubic) sys-tem. The structure is of the complex shell type and contains [BO₃]^3- and (BO₄)^5-. Part of the Mg may be replaced by Fe²⁺. Boracite forms shiny crystals of cubic, tetrahedral, or dodecahedral appearance. The color is bluish-gray; frequently, it is colorless. Its hardness on the mineralogical scale is 7.5; its density is 2,930–2,950 kg /m³. Boracite is rare. It is formed in sedimentary deposits of gypsum, anhydrite, and potassium and rock salts.