TY - JOUR
T1 - Reflectance spectroscopy of ilmenites and related Ti and Ti[sbnd]Fe oxides (200 to 2500 nm)
T2 - Spectral–compositional–structural relationships
AU - Izawa, Matthew R.M.
AU - Applin, Daniel M.
AU - Morison, Matthew Q.
AU - Cloutis, Edward A.
AU - Mann, Paul
AU - Mertzman, Stanley A.
N1 - Funding Information:
This research was funded the Natural Sciences and Engineering Research Council of Canada , the Canadian Space Agency , and the University of Winnipeg . The authors also wish to thank the Canada Foundation for Innovation, the Manitoba Research Innovations Fund, the Canadian Space Agency, and the University of Winnipeg for supporting the establishment of the Centre for Terrestrial And Planetary Exploration (C-TAPE) where this work was conducted. Mertzman thanks National Science Foundation for XRF lab up-grade support through MRI-0923224 . We also wish to thank Caroline Morisset for providing a number of the ilmenite samples and accompanying compositional data. We thank two anonymous reviewers for very insightful and constructive critiques of the manuscript, and Will Grundy for editorial handling.
Publisher Copyright:
© 2021 Elsevier Inc.
PY - 2021/7/1
Y1 - 2021/7/1
N2 - Ilmenite is an important mineral for understanding lunar evolution. Ilmenite is also a primary ore of titanium on the Earth. Here we present a comprehensive examination of the spectral-compositional-structural relationships of ilmenites and related Fe[sbnd]Ti oxides. Ilmenite spectral features of interest include maxima near ~250 nm (due Ti4+-O and Fe2+-O charge transfers), ~335 nm (due to Fe2+-Ti4+ charge transfer), and 950 nm (interband maximum), and absorption features near ~540 nm (due to Fe2+-Ti4+ charge transfers), ~630 nm (due to Ti3+-Ti4+ charge transfers), and a ~ 1300/1600 nm absorption doublet (due to Fe2+ crystal field transitions). Absorption features transition from Fresnel peaks to valley around 400 nm, as absorption coefficients decrease toward longer wavelengths. Ilmenite powders are generally darkest and show the greatest spectral contrast and reflectance rise beyond ~1300 nm for the smallest grain sizes. Other Ti and Fe[sbnd]Ti oxides share some spectral properties with ilmenites but also exhibit many differences. The most common feature they share is a rise in reflectance toward shorter wavelengths below ~500 nm (i.e., blue spectral slope). Ti ± Fe oxide spectra can also exhibit absorption features attributable to Ti, Fe, and Ti ± Fe, and these features vary in intensity, shape and wavelength position due to factors such as Ti and Fe oxidation states, coordination environment, and nearest neighbor cation types. Ilmenite differs most from silicates in the region below ~500 nm: it shows a reflectance increase versus decrease toward shorter wavelengths for silicates, as well as diagnostic Ti/Ti-Fe maxima or minima. Thus, detection of ilmenite in mixtures is best accomplished by including the UV region in spectral analysis. With increasing ilmenite in mixtures, its diagnostic spectral features become increasingly apparent.
AB - Ilmenite is an important mineral for understanding lunar evolution. Ilmenite is also a primary ore of titanium on the Earth. Here we present a comprehensive examination of the spectral-compositional-structural relationships of ilmenites and related Fe[sbnd]Ti oxides. Ilmenite spectral features of interest include maxima near ~250 nm (due Ti4+-O and Fe2+-O charge transfers), ~335 nm (due to Fe2+-Ti4+ charge transfer), and 950 nm (interband maximum), and absorption features near ~540 nm (due to Fe2+-Ti4+ charge transfers), ~630 nm (due to Ti3+-Ti4+ charge transfers), and a ~ 1300/1600 nm absorption doublet (due to Fe2+ crystal field transitions). Absorption features transition from Fresnel peaks to valley around 400 nm, as absorption coefficients decrease toward longer wavelengths. Ilmenite powders are generally darkest and show the greatest spectral contrast and reflectance rise beyond ~1300 nm for the smallest grain sizes. Other Ti and Fe[sbnd]Ti oxides share some spectral properties with ilmenites but also exhibit many differences. The most common feature they share is a rise in reflectance toward shorter wavelengths below ~500 nm (i.e., blue spectral slope). Ti ± Fe oxide spectra can also exhibit absorption features attributable to Ti, Fe, and Ti ± Fe, and these features vary in intensity, shape and wavelength position due to factors such as Ti and Fe oxidation states, coordination environment, and nearest neighbor cation types. Ilmenite differs most from silicates in the region below ~500 nm: it shows a reflectance increase versus decrease toward shorter wavelengths for silicates, as well as diagnostic Ti/Ti-Fe maxima or minima. Thus, detection of ilmenite in mixtures is best accomplished by including the UV region in spectral analysis. With increasing ilmenite in mixtures, its diagnostic spectral features become increasingly apparent.
KW - Ilmenite
KW - Iron‑titanium oxides
KW - Reflectance spectroscopy
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U2 - 10.1016/j.icarus.2021.114423
DO - 10.1016/j.icarus.2021.114423
M3 - Article
AN - SCOPUS:85103081960
VL - 362
JO - Icarus
JF - Icarus
SN - 0019-1035
M1 - 114423
ER -