Estimation of surface iron oxide abundance with suppression of grain size and topography effects

Abstract

Mineral forms of iron oxide, such as hematite, goethite and jarosite, are important because they are widely distributed at the Earth’s surface and because they are used as indicators for mineral exploration. Iron oxide abundance in rocks containing these minerals can be estimated from the absorption depth at wavelengths of around 900nm in a reflectance spectrum, but this depth is also affected by extraneous factors such as grain size and topography. This paper investigated the effect of grain size on reflectance spectra and proposed a method for estimating iron oxide abundance in surface rocks by using remotely sensed data with suppression of the effects of grain size and topography. Reflectance spectra were measured in a laboratory from rock powder samples of different grain sizes containing iron oxide minerals. While the reflectance increased with decreasing grain size, the presence of ferric iron caused the absorption depth to be almost constant at around 900nm, irrespective of the chemical composition of the sample. In addition, the difference between the reflectance at 550nm and 760nm (Slope) was a function of grain size. Iron oxide abundance can be estimated accurately by MCR-900D, which is the maximum absorption depth at the absorption center after the effect of grain size and topography was suppressed by Slope and the continuum-removal method, which takes the ratio between the original spectrum and its continuum, respectively. Correlation of MCR-900D results with datasets of actual spectral and chemical iron oxide laboratory measurements revealed that the mineral forms also need to be considered. MCR-900D results were significantly correlated with rock samples classified as containing different forms of iron oxide minerals (hematite, goethite and jarosite). Finally, MCR-900D was applied to an AVIRIS dataset for the Cuprite site in Nevada, USA. The results represented the enrichment zones of iron oxide within hydrothermally altered areas.