Quantification of Al-goethite from diffuse reflectance spectroscopy and magnetic methods

As one of the most abundant iron oxides in soils, the presence and nature of goethite iscontrolled by the soil conditions and burial history. The visible diffuse reflectance spectroscopy(DRS) is a useful tool for quantifying goethite. However, aluminium (Al) substitution forgoethite is very common in soils and the effects of Al content on the DRS properties ofgoethite have not been fully resolved. In this study, two series of Al substituted goethites(Al-goethite) and 20 Chinese loess/palaeosol samples were investigated using both DRS andmagnetic methods to test the feasibility of quantifying Al-goethite with the DRS method.Results show that the peak positions and amplitudes of the goethite DRS band are significantlyinfluenced by Al substitution. Specifically, the goethite concentration proxy, the amplitudeof the DRS band, is relatively stable only when Al substitution ranged between about 4 and16 mol per cent. Practically, in order to resolve the difficulty in measuring Al content innatural samples, the unblocking temperature (T_b) is proposed as the proxy for Al substitutionof goethite. When T_b of Al-goethite was above 250 K, the amplitude of DRS can be usedto reliably trace the goethite concentration variation in natural samples. For example, theDRS spectra for the Chinese loess-palaeosol samples support the idea that only haematite isenhanced via pedogenesis. In contrast, the origin of goethite seems to be mostly related to theaeolian inputs.