Characterization of iron oxide minerals by second-derivative visible spectroscopy.

The second derivative of the spectral curve relating absorption and wavelength greatly enhances minor convexities and concavities of the original spectrum, and gives a much narrower band width, which provides improved resolution of subtle or overlapping bands. Synthetic goethite and hematite have distinctive second-derivative patterns. The minimum at 423 nm and the maximum at 447 nm in the second-derivative patterns were used to estimate the amount of goethite in a mixture with kaolinite. Whole soil samples can be analysed without pretreatment by this method, which means that the result of the analysis is a property of the soil itself and not possibly an artefact of the pretreatment.-D.J.M.