Boracite

Boracite, borate and chloride of magnesium, in which the chloride amounts to 11 per cent., is a mineral which occurs associated with gypsum and rock-salt at Stassfurt in Saxony, at Kiel in Holstein, and elsewhere. It is slightly soluble in hot water, slowly so in acid, and fuses with difficulty into a yellowish bead which becomes white, opaque and crystalline on cooling, while the flame is coloured green. Its hardness is 7, and its specific gravity nearly 3. It occurs in white, translucent, crystals of the cubic system, commonly cubes combined with the rhombic dodecahedron and the tetrahedron, and is chiefly interesting as being pyro-electric. The angles replaced by tetrahedral planes are the antilogous poles, exhibiting resinous or negative electricity when the mineral is being heated, vitreous or positive electricity when it is cooling, while the opposite unmodified angles exhibit opposite characters.