Charge Transfer Transitions

Charge transfer transitions - also called intervalence charge transfers or molecular orbital transitions - produce mineral colour when a valence electron is transferred from one ion to an adjacent ion rather than simply promoted to a higher orbital on the same ion. The transfer requires absorbing a photon whose energy equals the energy difference between the donor ion’s occupied orbital and the acceptor ion’s vacant one. Charge transfers may occur between two cations, between an anion and a cation, or between two adjacent anions. ^[1]

The Transfer Mechanism

In a charge transfer event, two different ions occupy adjacent structural sites X and Y. A photon is absorbed and promotes a lower-energy electron from ion X to a vacant higher-energy orbital on ion Y. The charge on each ion changes temporarily - for example, Fe²⁺ at site X ejects an electron and becomes Fe³⁺, while Fe³⁺ or Ti⁴⁺ at site Y receives the electron and its charge decreases by one. When the electron returns to its original position, both ions revert to their original charges. The process is therefore cyclic - no permanent change occurs - but the photon energy is consumed in driving the transfer. ^[1]

In both the Fe²⁺→Fe³⁺ and Fe²⁺→Ti⁴⁺ transfers, the energy difference is about 1.8 eV. This corresponds to light at the red end of the visible spectrum. Red light is therefore absorbed, and the transmitted or reflected light is perceived as blue. This single energy value explains why two otherwise unrelated mineral systems - sapphire and magnetite - are both controlled by charge transfer. ^[1]

Notable Examples

Sapphire (Fe²⁺-Ti⁴⁺ Transfer)

Sapphire is a blue gem variety of corundum (Al₂O₃). Small amounts of Fe²⁺ and Ti³⁺ substitute for Al³⁺ in the structure. Intervalence charge transfer between Fe²⁺ and Ti⁴⁺ in adjacent sites absorbs red-end photons and produces the characteristic blue colour. ^[1]

Magnetite and Mafic Silicates (Fe²⁺-Fe³⁺ Transfer)

The black colour of magnetite (Fe₃O₄) arises from Fe²⁺-Fe³⁺ charge transfer. The same process is responsible for the dark colour of many common mafic silicate minerals - hornblende, augite, and biotite all contain both Fe²⁺ and Fe³⁺ in adjacent sites, and their characteristic dark colours follow from the same mechanism. ^[1]

Hematite (O²⁻-Fe³⁺ Transfer)

Hematite (Fe₂O₃) is deep red through a different type of charge transfer - an anion-to-cation transfer. An electron from O^2- is transferred to an adjacent Fe³⁺. The energy difference absorbed in this transfer falls in the blue-to-green end of the spectrum, so those wavelengths are removed from the transmitted light, leaving the red colour that gives hematite and iron-stained rocks their characteristic appearance. ^[1]

Lazurite (S-S Anion-to-Anion Transfer)

Lazurite (Na₆Ca₂[Al₆Si₆O₂₄]S₂) is the lapis lazuli mineral. Its blue colour arises from a charge transfer between adjacent sulfur atoms in different structural sites whose electron energy levels differ by a couple of electron volts. Light at the red end of the spectrum is absorbed when an electron from one sulfur atom is promoted to its neighbour. This is an anion-to-anion transfer - neither cation nor oxygen is involved. ^[1]

References

1. Nesse, W. D. (2018). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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