T136 Gemological Research in the Twenty-First Century: Characterization, Exploration, and Geological Significance of Diamonds and Other Gem Minerals

Monday, 26 September 2016
1:30 PM - 5:30 PM
Room 207

MICROANALYSIS OF IRIDESCENT METAMORPHIC HEMATITE FROM MINAS GERAIS, BRAZIL

Paper number: T136-11
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Abstract

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 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.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-283550

© Copyright 2016 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.