GEORGE R. ROSSMAN
Division of Geological and Planetary
California Institute of Technology
Pasadena, California 91125, U.S.A.
Structures with Fe3+ shared through oxo- or hydroxyl-groups have antiferromagnetic interactions. Such interactions result in enhanced intensity of the Fe3+ optical absorption bands which in some systems can be as great as a factor of 100 compared to isolated, octahedrally coordinated Fe3+ ions. A comparison is presented between the intensity of the lowest energy crystal-field band of Fe3+ minerals and their magnetic moments which demonstrates the dependence of optical absorption intensity and antiferromagnetic interactions in the host phase. Hematite, which is usually responsible for the red color of geological materials, owes its intense color to these magnetic interactions.
The Geochemical Society, Special Publication No. 5, p23-27.