MICROANALYSIS OF IRIDESCENT METAMORPHIC HEMATITE FROM MINAS GERAIS, BRAZIL

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 (http://minerals.gps.caltech.edu/), 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 120^orelative 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.