What is "boracite"

Boracite \Bo"ra*cite\, n. (Min.) A mineral of a white or gray color occurring massive and in isometric crystals; in composition it is a magnesium borate with magnesium chloride.

n. (context mineral English) A mixed chloride and borate of magnesium that occurs as a white to green crystalline evaporite, with the chemical formula magnesium₃boron₇oxygen₁₃chlorine.

Boracite is a magnesium borate mineral with formula: Mg B O Cl. It occurs as blue green, colorless, gray, yellow to white crystals in the orthorhombic - pyramidal crystal system. Boracite also shows pseudo-isometric cubical and octahedral forms. These are thought to be the result of transition from an unstable high temperature isometric form on cooling. Penetration twins are not unusual. It occurs as well formed crystals and dispersed grains often embedded within gypsum and anhydrite crystals. It has a Mohs hardness of 7 to 7.5 and a specific gravity of 2.9. Refractive index values are nα = 1.658 - 1.662, nβ = 1.662 - 1.667 and nγ = 1.668 - 1.673. It has a conchoidal fracture and does not show cleavage. It is insoluble in water (not to be confused with borax, which is soluble in water).

Boracite is typically found in evaporite sequences associated with gypsum, anhydrite, halite, sylvite, carnallite, kainite and hilgardite. It was first described in 1789 for specimens from its type locality of Kalkberg hill, Lüneburg, Lower Saxony, Germany. The name is derived from its boron content (19 to 20% boron by mass).