Análise por espectroscopia vibracional, difração de raios-X e microscopia eletrônica de varredura de conchas fósseis das Formações Jandaíra e Itaituba

Abstract

In this work, we combine a set of techniques, such as Raman spectroscopy, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray fluorescence energy dispersion spectroscopy (EDS) and scanning electron microscopy (SEM) to carry out a physical-chemical characterization mineralogically characterize two specimens fossils (fossil shells from the Jandaíra Formation and brachiopod fossils from the Itaituba Formation) and infer their fossilization processes. Firstly, four fossil shells were characterized by Raman and infrared spectroscopy, XRD, EDS and SEM applied to four samples of Tylostoma sp (LP/UFC CRT 2868, LP/UFC CRT 2869, LP/UFC CRT 2870 and LP/UFC CRT 2871), which were analyzed in order to understand the different diagenetic processes involved in the preservation of the Jandaíra Formation, Potiguar Basin. In this study, fossil specimens were collected near Mossoró, state of Rio Grande do Norte. All fossil shells are preserved as internal molds, which means they have lost their original organic matter. After the decomposition of organic debris, the inner part of the shell is filled with the surrounding sediments. Our results revealed calcite as the dominant phase in LP/UFC CRT 2868, LP/UFC CRT 2869, LP/UFC CRT 2870 and LP/UFC CRT 2871. These data indicate that a similar diagenetic process was involved in the preservation of these fossil shells. the results revealed dolomite as the dominant phase in the host rock. On the contrary, the fossil samples LP/UFC CRT 2868, LP/UFC CRT 2869, LP/UFC CRT 2870 and LP/UFC CRT 2871 presented calcite as the dominant phase. These data indicate that a similar diagenetic process was involved in the preservation of these fossil shells. Later we characterized the brachiopod fossils from the Itaituba Formation, Amazon Basin. The Itaituba Formation of the Amazonas Basin, northern Brazil, is recognized for its abundant fossil content of marine invertebrates. In this study, we investigated a pavement of brachiopod fossils collected from a calcareous quarry, Caltarém quarry, Pará state. We applied Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), X-ray Fluorescence (XRF) and Raman spectroscopy techniques to characterize their chemical composition and mineralogy and discuss the possibility of polymorphs occurring through specific vibrational modes of vibration and crystallographic planes. We recorded quartz as the main phase in the brachiopod fossils and calcite as secondary minerals. Brachiopod fossils are characterized by a monotonous texture composed of an irregular surface covered by isolated patches of silicate grains forming a granular texture. This texture forms a non-pervasive fine scale replacement formed by quartz crystals associated with carbonates and other silicates. The combination of different high-resolution techniques allowed us to verify that brachiopod fossils were partially replaced by silica. Taken together, our results strongly suggest that the fossil samples studied here experienced partial silicification over geological time.