Also known as bauxite, alumina or bauxite, the main ingredient is alumina, the Department of containing impurities hydrated alumina, a mineral earthy. White or gray, iron and was yellow, or brown dignitaries. 3.9 ~ 4g/cm3 density, hardness 1 ~ 3, opaque, brittle. Melting extremely difficult. Do not dissolve in water, can be dissolved in sulfuric acid, sodium hydroxide solution. Used mainly aluminum, the system refractories.