Why hematite is red: Correlation of optical absorption intensities

and magnetic moments of Fe³⁺ minerals

GEORGE R. ROSSMAN

Division of Geological and Planetary Sciences, California Institute of Technology
Pasadena, California 91125, U.S.A.

Abstract

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.

A comparison of the optical absorption spectra of hematite and andradite garnet where the spectra are normalized for density and iron concentration such that equal amounts of iron are in each sample path.

 The Geochemical Society, Special Publication No. 5, p23-27.