The chemical elements in minerals occur typically as ionic species whose electron configurations are unique and independent of whatever other ions may occur in the mineral structure. In general, a chemical bond shows some degree of ionic character and some degree of electron sharing. Aluminum, however, is exceptional in this respect, for almost all of the metal-oxygen bonds that occur in minerals are ionic. This chapter presents the Pauling’s rules as well as the anion configurations for the principal cation coordination numbers found in minerals. Differences in the stacking of Al(OH)6 octahedra result in polymorphism in the Al(OH)3 phases, bayerite, doyleite, nordstrandite, and gibbsite. Sheet structure aluminosilicates are characterized by a two-dimensional tetrahedral network of silica tetrahedra in which three oxygens from each tetrahedron are shared with other tetrahedra. Thermodynamic properties for inorganic aluminum-bearing phases are determined by a number of experimental techniques, e.g., calorimetry, solubility measurements, phase equilibria studies, and electrochemical methods.