Aluminium ( /ˌæljuːˈmɪniəm/ a-lew-mi-nee-əm) or aluminum (US English) ( /ˌəlˈuːmɪnəm/, ə-loo-mi-nəm) is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances. Aluminium is the third most abundant element (after oxygen and silicon), and the most abundant metal, in the Earth's crust. It makes up about 8% by weight of the Earth's solid surface. Aluminium metal is too reactive chemically to occur natively. Instead, it is found combined in over 270 different minerals. The chief ore of aluminium is bauxite.