GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 136-10
Presentation Time: 4:00 PM


LIN, Xiayang, Dept. of Geosciences, Penn State University, University Park, PA 16802, HEANEY, Peter J., Dept. of Geosciences, Penn State University, 540 Deike Bldg, University Park, PA 16802 and POST, Jeffrey E., Dept. of Mineral Sciences, Smithsonian Institution, P.O. Box 37012, Washington, DC 20013-7012,

A specular, metamorphic hematite from the Andrade Mine in João Monlevade, Minas Gerais, Brazil, is highly iridescent and thus frequently marketed as “rainbow hematite”. In an online report (, Rossman and Ma presented field-emission scanning electron micrographs (FESEM) of rainbow hematite from Brazil and other localities, and they consistently observed a “thin film” of rod-shaped nanocrystals, each measuring 5 to 35 nm in thickness and hundreds of nm in length. The nanocrystals were oriented at 120orelative to each other and enriched in Al and P. They attributed the iridescence to a thin-film effect from this coating of Al-P nanocrystals.

Using atomic force microscopy (AFM) and synchrotron X-ray diffraction (XRD) in addition to FESEM, we examined rainbow hematite from the Andrade Mine. Our study suggests that the spindle-shaped nanocrystals are not a coating of a new mineral phase. Instead, they comprise a highly porous framework of defective hematite crystals with minor concentrations of Al and P. AFM images revealed that the distances between adjacent spindle-shaped hematite crystals within a single layer are in the range of 280 – 400 nm, generating a diffraction grating for visible light. The sub-structure was apparent on all freshly fractured surfaces, indicating that it is not merely an exterior surface coating. Rietveld analysis of XRD patterns revealed non-unitary occupancies for Fe, consistent with substitution by ~12 mol% Al. We interpret the periodic nanostructure as the result of fragmentary crystal growth by the oriented aggregation of hematite nanorods.