Why hematite is red: Correlation of optical absorption intensities and magnetic moments of Fe^(3+) minerals

Abstract

Structures with Fe^(3+) shared through oxo- or hydroxyl-groups have antiferromagnetic interactions. Such interactions result in enhanced intensity of the Fe^(3+) optical absorption bands which in some systems can be as great as a factor of 100 compared to isolated, octahedrally coordinated Fe^(3+) ions. A comparison is presented between the intensity of the lowest energy crystal-field band of Fe^(3+) 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.