Spectral reflectance of carbonatites and related alkalic igneous rocks: Selected samples from four North American localities

Laboratory spectral reflectance measurements were made in the 0.4- to 2.5-mu m wavelength range for samples collected from four North American carbonatite-alkalic igneous rock complexes. Alkalic rock spectra produced few absorption features; however, the carbonatites typically showed conspicuous carbonate, ferric and ferrous iron, and in some cases rare earth element absorption bands. Trivalent neodymium was the most abundant rare earth element producing absorption features and could be detected in amounts as low as 300 ppm. Ferrous iron was present in most dolomitic carbonatite samples and caused more intense ferrous absorption bands than commonly is seen in carbonate rocks of sedimentary origin. High carbonatite iron content also resulted in pervasive limonitic staining and related ferric iron absorption features. These absorption features sometimes masked other absorption bands.Because reflectance measurements can be carried out rapidly, with minimal sample preparation, reflectance spectroscopy may provide a useful field and laboratory tool for analyzing economic rare earth concentrations. In addition, the distinctive spectral characteristics of carbonatite material, in conjunction with the discordant spatial relationships between associated alkalic and carbonatite rocks, may provide a basis for remote-sensing detection of these complexes.