This occurs because there is enough overlap between the outer orbitals of Fe 2+ and Ti 4+ to allow an electron to pass from one ion to another. When Fe 2+ and Ti 4+ ions occupy two adjacent sites in a corundum crystal, the transfer of an electron from the iron cation (positively charged) to the titanium cation (positively charged) can now change the valence state of both. If Fe 2+ and Ti 4+ ions are substituted for Al 3+, localized areas of charge imbalance are created. Iron can take the form Fe 2+ or Fe 3+, while titanium generally takes the form Ti 4+. The process at work is called intervalence charge transfer or cooperative charge transfer, which is the transfer of an electron from one transition-metal ion to another. If both titanium and iron impurities are present together, however, the result is a magnificent deep-blue color.

If a similar amount of iron is present, a very pale yellow color may be seen. Corundum contains a repeating unit of aluminum oxide and is colorless.Ĭorundum that contains a few hundredths of one percent of titanium is colorless.

Ruby and blue sapphire together