Measurement of the hematite:goethite ratio using field visible and near‐infrared reflectance spectrometry in channel iron deposits, Western Australia

Abstract

Empirical relationships and a field method have been developed for the measurement of the hematite:goethite ratio in Tertiary ooidal ironstones, locally named channel iron deposits, from the Hamersley region of Western Australia using visible to near‐infrared (400 to 1000 nm) refiectance spectrometry. The hematite:goethite ratio is important in the characterisation of these iron deposits as Al, P, water and Si are deleterious components commonly associated with goethite. The channel iron deposits typically comprise iron oxy‐hydroxides with less than 1% Fe2+ (present in maghemite or kenomagnetite), less than 8% Al3+‐substitution and with a mean crystal dimension of approximately 20 nm. The natural variations in the hematite:goethite ratio of the channel iron deposits were modelled using laboratory mixtures of pure hematite and goethite. The resultant spectral mixing trends produced consistent relationships with the hematite:goethite ratio, especially for the wavelength of the 6A1?4T1 crystal field absorption minimum. Variation in Al‐substitution and crystal size had no apparent effect on the wavelength of this feature though a change in grainsize from > 75 to < 20 μm caused an apparent 30 nm wavelength shift. However, only a 5 nm shift was apparent in spectra taken from unprepared drillchip samples probably because the finer fraction coats larger fragments. The field spectral method was found to be as accurate for measuring the hematite:goethite ratio compared with the laboratory‐based loss on ignition, XRD peaks and thermodifferential analysis techniques. Although geological samples pulverised to < 75 μm yielded the most accurate results, it was found that an additional error of only ± 5 nm was produced using unprepared drillchip samples.