An Exercise on Calibration: DRIFTS Study of Binary Mixtures of Calcite and Dolomite with Partially Overlapping Spectral Features

Unlike most spectroscopic calibrations that are based on the study of well-separated features ascribable to the different components, this laboratory experience is especially designed to exploit spectral features that are nearly overlapping. The investigated system consists of a binary mixture of two commonly occurring minerals: calcite and dolomite whose DRIFT (diffuse reflectance infrared Fourier transform) spectra present a peak at about 880 cm-1 due to the C-O out-of-plane bending mode separated by ~ 6 cm-1. Consequently, in the 870 - 890 cm-1 region of its medium- to low-resolution spectrum, any binary mixture made up of calcite and dolomite gives rise to a single feature due to the superposition of these two peaks. The wavenumbers of the peak center corresponding to mixtures with different composition are shown to be directly proportional to the mixture dolomiticity index ID = 100 * Wdolomite/ (Wcalcite + Wdolomite). The parameters resulting from a linear fit on the measured data for twelve mixtures with ID ranging from 0 to 100 allow the determination of the composition of any binary mixture of calcite/dolomite given the wavenumber of the 870 - 890 cm-1 peak in the corresponding DRIFT spectrum. This experience can be carried out in a single laboratory shift and allows the collection of a spectroscopic data set that can be possibly used in a discussion involving different and more conventional calibration approaches.