FTIR spectral signatures of amazon inorganic phosphates: Igneous, weathering, and biogenetic origin

The characterization of phosphates is generally hampered by the variability of their sources, the complexity of the mineralogical assemblies and/or the thermochemical transformations undergone. Fourier transform infrared (FTIR) spectroscopy can characterize and differentiate phosphates in a practical and efficient way. In this sense, in order to differentiate phosphates from different Amazonian deposits and establish a spectral database, initially small because it is starting, six samples of phosphate rocks were analyzed by FTIR spectroscopy in the near-IR and middle-IR regions using the transmittance, attenuated reflectance, and diffuse reflectance methods. X-ray diffraction and X-ray fluorescence spectroscopy were also used as complementary analyses. The IR results revealed that the transmittance and diffuse reflectance methods are the most suitable for the analysis of phosphate materials, and they should be used together whenever possible. The identification of the PO₄ bands, as well as of the (CO₃)^2-, Al₂OH, and NH₄ bands, allowed the differentiation of the phosphate materials according to their geological source and the establishment of a database of the studied materials by both the transmittance and diffuse reflectance methods.