Reflectance spectroscopy of ilmenites and related Ti and Ti[sbnd]Fe oxides (200 to 2500 nm): Spectral–compositional–structural relationships

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.