The study of weathering and erosion processes requires the establishment of a mass balance of minerals from the soils towards the sedimentation area. This calls for the development of quantitative tools that can be applied to mixtures of finely divided and sometimes poorly crystallized minerals found in soils and sediments. We present here the use of Fourier transform infrared (FTIR) absorbance spectroscopy in the mid-infrared as a method to calculate mineral modes for such materials. Samples were prepared using the KBr disc method. This ensures that Lambert-Beer's law is valid. A quantitative determination of the mineral content from various blends was performed by making a multicomponent analysis of the experimental spectrum using the spectra of each component in the mixture. To check the validity of the procedure, the multicomponent analysis was performed on spectra from synthetic mixtures of standard minerals (quartz, kaolinite, gibbsite, amorphous silica). Good agreement between actual and computed wt % was obtained in the range 1315-315 cm (super -1) . This application of FTIR spectroscopy was used to quantify the mineral components of a lacustrine sediment cored at Salitre, Minas Gerais, Brazil. The major phases are organic matter, kaolinite, gibbsite, quartz, anatase, and amorphous silica (phytoliths and sponge spicules). Fifty-three samples were analyzed. Quantitative results from FTIR spectroscopy were compared to chemical analyses to assess the validity of the method for the natural sediments. Phases with no sharp diagnostic spectral features, such as anatase or amorphous silica, were quantified using the multicomponent analysis. The variations of the main mineral phases along the core reflect the mechanisms of transfer from the surrounding soils to the sediment area and the role of the vegetal cover over the drainage area.