Several aluminum compounds occur in bauxite ore. The mixture may include three aluminum hydroxides: gibbsite, boehmite and diaspore. Gibbsite has the chemical formula Al(OH)3. Both boehmite and diaspore have the chemical formula AlO(OH), but the two minerals differ in texture. Bauxite also contains impurities, such as silica and compounds of phosphorus and iron. The amount of impurities present varies from sample to sample.
Like all hydroxides, the three aluminum hydroxide components that occur in bauxite act as bases. Gibbsite may dissociate to form a single positively charged aluminum ion and three negatively charged hydroxyl (OH) ions. It thus acts like a base, since hydroxyl ions are characteristic of bases. However, gibbsite may also dissociate to form a negatively charged AlO2 ion and a positively charged hydrogen ion, losing a water molecule in the process. It thus acts like an acid, since hydrogen ions are an acid characteristic. Compounds that can act as either acids or bases are known as amphoteric substances.
Aluminum oxide, an intermediate compound in the extraction of aluminum from bauxite, is also an amphoteric substance. A Lewis acid accepts a pair of electrons from its reacting partner, and a Lewis base donates a pair of electrons to its reacting partner. According to an X-ray photoelectron spectroscopy study conducted by the Netherlands Institute of Metal Research and the Delft University of Technology, aluminum oxide possesses a site at which it may accept a pair of electrons and also a site at which it may donate a pair of electrons, meaning it qualifies as both a Lewis acid and a Lewis base.
Because of their amphoteric properties, the aluminum-containing minerals in bauxite dissolve in both acids and in bases. This characteristic facilitates their separation from insoluble impurities, such as silica. In addition, several acid-base reactions take place during the metal extraction process.