Borates – Boracite and Colemanite Composition and Structure

Boracite
Composition, Mg₇Cl₂B₁₆O₃₀. isometric; tetrahedral. Crystals usually show cube, tetrahedron and dodecahedron in some combination. Crystals usually isolated and disseminated in other minerals. Also massive. Vitreous luster. Colorless, white, gray, green. Transparent to translucent. H. = 7. G. = 2.9-3.0. Fusible at 3 with green flame color (boron). Soluble in hydrochloric acid. Turmeric paper moistened with a solution of the mineral and then dried at 100% C. Turns reddish brown (boron). Occurs associated with halite, anhydrite, gypsum, etc., as one of the products formed by the evaporation of bodies of salt water.

Colemanite
Hidrous borate of calcium, Ca2B6O11.5H2o. Monoclinic, in short prismatic crystals, highly modified. Cleavage massive to granular and compact. Perfect pinacoidal cleavage. H. = 4-4.5. G. = 2.42. vitreous to admantine luster. Colorless to white. Transparent to translucent. Fusible at 1.5. B.B. exfoliates, crumbles and gives green flame (boron). Water in C. T. A rare mineral, but occurring in considerable quantity in the salt lake deposits, in the arid regions of southeastern California, in Death Valley, Inlo County, and in San Benardino and Los Angeles counties.