Spectroscopic studies on Permian plant fossils in the Pedra de
Fogo Formation from the Parnaíba Basin, Brazil
Domingas Conceição, João da Silva, Juan Cisneros, Roberto Iannuzzi, Bartolomeu C Viana, Gilberto Saraiva, Joel Sousa, Paulo T
Freire
The “Pedra de Fogo” Formation dated from the Permian period (approximately 280 million
years ago), belongs to the sedimentary Parnaiba Basin, located in the northeastern region
of Brazil. It is recognized by their well preserved fossil contents and it is notable for having
several fossilized trunks in the growing position. Specimens from different localities were
selected in order to perform spectroscopic studies and X-ray diffraction analysis, as well,
for the purpose of identifying and characterizing compounds related to the fossilized
materials. These different techniques allowed to obtain information from the molecular
spectra in organic and inorganic substances, which are present in these above stated
fossils and in the atomic elements, as well as the crystalline phases. These aforementioned
studies have revealed promising paleoenvironmental interpretations about their
depositional strata. Regarding the techniques, these have enabled inferences with respect
to the fossil diagenetic events and allowed a better understanding of the fossilized process
and the mineralogical characteristics of the living environment, where the plants were
buried. This study presents physical and/or chemical properties of the fossilized plants
through vibrational spectroscopies, SEM/EDS spectroscopy and X-ray diffraction
techniques. Based on the results duly obtained, we were able to identify the presence of
silica and confirm that the dominant process of the fossilized specimens investigated has
occurred through quartz silicification with the contribution of persistence from amorphous
carbon.
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Spectroscopic studies on Permian plant fossils in the Pedra de Fogo
Formation from the Parnaíba Basin, Brazil
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D. M. da Conceição1, J. H. da Silva2, J. C. Cisneros3, R. Iannuzzi4, B. C. Viana5, G. D.
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Saraiva6, J. P. Sousa7, P.T.C. Freire8*
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1Programa
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Recife – PE, Brazil.
Pós-Graduação em Geociências, Universidade Federal de Pernambuco, 50670-901,
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2Campus
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– CE, Brazil.
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3Centro
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Brazil.
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4Departamento
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91509-900, Porto Alegre – RS, Brazil.
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5Departamento
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6Faculdade
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CEP 63.900-000, Quixadá – CE, Brazil.
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7Departamento
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Fortaleza – CE, Brazil
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8*Departamento
de Juazeiro do Norte – Universidade Federal do Cariri, 63000-000, Juazeiro do Norte
de Ciências da Natureza – Universidade Federal do Piauí, 64049-550, Teresina – PI,
de Paleontologia e Estratigrafia – Universidade Federal do Rio Grande do Sul,
de Física, Universidade Federal do Piauí, 64049-550, Teresina – PI, Brazil.
de Educação Ciências e Letras do Sertão Central, Universidade Estadual do Ceará,
de Geologia, Universidade Federal do Ceará Campus do Pici - Bloco 912 de Física, Universidade Federal do Ceará, 60455-970, Fortaleza – CE, Brazil.
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ABSTRACT
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PeerJ PrePrints | https://dx.doi.org/10.7287/peerj.preprints.1547v1 | CC-BY 4.0 Open Access | rec: 30 Nov 2015, publ: 30 Nov 2015
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The “Pedra de Fogo” Formation dated from the Permian period (approximately 280 million years
34
ago), belongs to the sedimentary Parnaiba Basin, located in the northeastern region of Brazil. It is
35
recognized by their well preserved fossil contents and it is notable for having several fossilized
36
trunks in the growing position. Specimens from different localities were selected in order to
37
perform spectroscopic studies and X-ray diffraction analysis, as well, for the purpose of
38
identifying and characterizing compounds related to the fossilized materials. These different
39
techniques allowed to obtain information from the molecular spectra in organic and inorganic
40
substances, which are present in these above stated fossils and in the atomic elements, as well as
41
the
42
paleoenvironmental interpretations about their depositional strata. Regarding the techniques,
43
these have enabled inferences with respect to the fossil diagenetic events and allowed a better
44
understanding of the fossilized process and the mineralogical characteristics of the living
45
environment, where the plants were buried. This study presents physical and/or chemical
46
properties of the fossilized plants through vibrational spectroscopies, SEM/EDS spectroscopy
47
and X-ray diffraction techniques. Based on the results duly obtained, we were able to identify the
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presence of silica and confirm that the dominant process of the fossilized specimens investigated
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has occurred through quartz silicification with the contribution of persistence from amorphous
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carbon.
crystalline
phases.
These
aforementioned
studies
have
revealed
promising
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Keywords: Vibrational spectroscopies; Fossil plants; Permian Period.
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1. INTRODUCTION
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The study of fossils allows understanding the complex interactions verified between living
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organisms and the physical environments in the past. Furthermore, it can provide evidence of
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the emergence, development and decline of some species. As a consequence, the fossil records
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reveal the ongoing evolution of life on Earth [1]. Fossils can help to infer the age of the rock
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strata serving for the reconstruction of the form and position of the continents, during the past
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ages, as well as to elucidate ancient environments, climate changes and extinction events. Some
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processes of fossilization can preserve biological structures or features in the organic mark
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activities during millions of years.
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Sedimentary rocks from the Parnaíba Basin contain preserved fossils from the Paleozoic Age,
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thus, recording a history of the diversification and extinction of the biota influenced by the
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movements of the tectonic plates during that geological time, as well as related to geological
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processes and the supercontinent of the Gondwana changes [2].
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The study of fossil plants is important, due to the fact that it allows the understanding in the
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mechanisms of various paleo-ecosystems in the geological history of the Earth, therefore, this
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furnishes insights about the paleo-climates, since the plants are very sensitive to the climate
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changes [3].
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The fossilization of plant remains, as well as those of other lifeforms, consists in the burial of an
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organism followed by a series of physical, chemical and even biological processes that occur in
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the sedimentary environment. Physical and chemical agents can continue after a sedimentary
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rock is formed (diagenesis). The results of all these said agents and/or the interaction of plant
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remains with the surrounded sediments along the matrix diagenesis, may lead to a fossil
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formation [3]. The study of the main aspects related to the plant fossilization process is growing
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in recent years, due to the application through physico-chemical techniques for the fossil
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characterization.
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The spectroscopic characterization of fossils [4] may contribute to understand the various
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fossilization mechanisms, conditions and processes that have enabled the preservation of animals
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and plant remains in the sedimentary Parnaiba Basin. The present study deals with the
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fossilization processes related to the plants from the Paleozoic Era (Permian Period), through
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various physico-chemical techniques [5]. Various analyses were performed aiming to determine
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the chemical and composition in the fossilized logs, therefore, inferring the sequence of chemical
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events and the main mechanisms of the fossilization process that produced the fossils.
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2. EXPERIMENTAL:
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2.1. Samples:
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The four fossils analyzed in the present studies are shown in the optical image in Figure 1. This
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picture presents four kinds of trunk fossil samples surrounded by their sediments. These samples
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are characterized as gymnosperm and pteridophyte, which came from the outcrops belonging to
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the “Pedra de Fogo” Formation in the sedimentary Parnaíba Basin. This particular basin is
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situated in the northeastern region of Brazil (in the cities of Teresina and Monsenhor Gil, located
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in the State of Piauí and in the city of Duque Bacelar, located in the State of Maranhão). The
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sample A corresponds to the fossil of a gymnosperm, collected from the Duque Bacelar
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municipality. The sample B corresponds to a pteridophyte fossil also collected from the Duque
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Bacelar area. The sample C belongs to a gymnosperm fossil from the Teresina municipality and
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finally, the sample D originated from the Monsenhor Gil municipality, corresponds to a
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pteridophyta fossil specimen.
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2.2. Characterization:
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2.2.1 X-ray diffraction
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The X-ray diffraction (XRD) patterns were obtained using a Rigaku powder diffractometer with
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the Bragg–Brentano geometry. The Co–Kα radiation was used and operated at 40 kV and 25
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mA. The XRD measurements were taken in the 2θ range of 10–100°, using the step scan
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procedures (0.02°) in counting times of 5 s. To perform the XRD measurements, we used 1 g of
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powder samples and for the data treatment, we used the Xpert High score software with the
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powder diffraction files (PDFs) included.
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2.2.2 Fourier-transform infrared spectroscopy (FTIR)
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The FTIR measurements were performed using a Bruker spectrometer, model Vertex 70. The
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spectral region analyzed spanned from 400 up to 4000 cm-1. The attenuated total reflectance
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(ATR) technique was used in the measurements of the sample.
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2.2.3 Energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) analysis
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The SEM/EDS analysis were performed using an Hitachi TM-3000 tabletop equipment,
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increasing up to 30,000 times at 15 kV with an EDS coupled Swifft ED 3000 with a solid state
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detector.
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2.2.4 Raman spectroscopy
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Fourier transform (FT)-Raman spectra, were collected with a Bruker RAM II FT-Raman module
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coupled with the VERTEX FT-IR spectrometer and with a liquid nitrogen cooled in a high-
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sensitivity Ge detector with the samples excited through the 1064-nm line of an Nd: YAG laser.
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The resolution was 4 cm-1 with an accumulation of 60 scans per spectra, as well as a nominal
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laser power of 120 mW.
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3. RESULTS AND DISCUSSION
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3.1. Analytical data
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Figure 2 shows the X-ray diffractograms for the four previously mentioned samples. According
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to the analysis of the XRD diffractograms (Figure 2), all aforementioned fossilized samples
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presented quartz as the major crystalline constituent. Such results need to be confirmed and
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expanded by the infrared and the Raman spectroscopic studies, as the amorphous phase amount
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which may be in the samples, cannot be detected by the XRD.
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Figure 3 shows the FTIR spectra in the four kinds of fossil trunks (samples A, B, C and D), duly
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collected from the “Pedra de Fogo” Formation. Regarding the profile in the FTIR spectra of
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these aforementioned fossils, we observed the presence of SiO2 vibrations associated with the
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quartz [6], confirming the previous results shown by the XRD results. The characteristics of the
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infrared bands associated with the quartz, are described on Table 2. The bands located at 462 cm-
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779, can be assigned as Si – Si stretching and the bands at 1085 and 1164 cm-1, corresponding to
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the stretching vibration of the Si-O bonds [6,7]. The doublet peaks near 800 cm-1, confirmed the
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presence of quartz in relation to other polymorphs of silica, such as the cristobalite and the
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tridymite [8]. This is an important point in certain fossilization processes, as we will discuss
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below. The quartz is the final product of a long chemical chain having cristobalite and tridymite
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as intermediate substances.
and 517 cm-1, are due to the bending vibrations of the Si-O, whereas the bands observed at 694,
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Figure 4 shows the Raman spectra of the fossils corresponding to the samples A, B, C and D.
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We also observed two intense bands around 201 cm-1 and 464 cm-1 in the spectrum of sample D,
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which were assigned, respectively as Si–O stretching/O–Si–O, bending/Si–O torsion and Si – O
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stretching/–Si–O bending from the quartz compound [9], and yet, in relation to the Raman
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spectra, three samples (A, B and C) presented the band centered at 201 cm-1 with a low intensity,
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however, the band at 464 cm-1 remained relatively intense. It is interesting to note that the peaks
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associated with the quartz have large bandwidths, which were supposed to be related with the
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distortion of the quartz crystal structures, due to the changes during the process of the
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fossilization [10]. Furthermore, two other broad bands observed around 1350 and 1600 cm-1 also
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appeared in the Raman spectra of the three said samples. These are very intense bands and can be
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associated with the carbon of carbonaceous material present in the fossil (hybridizations sp3 and
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sp2, respectively). These bands can be assigned as well, respectively as D and G bands
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corresponding to the amorphous carbon [11]. It is interesting also to note that in a previous study,
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the bands D and G were observed in the Raman spectrum of a wood fossil collected from the
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Crato Formation, belonging to the Cretaceous Period, being associated with the amorphous
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carbon, contained in the sample as well [5]. In the fossil from the “Crato” Formation, the
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presence of the amorphous carbon and the additional presence of oxygen were interpreted as one
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possible explanation, being a consequence of the natural fire. Additionally, the presence of
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carbon was interpreted as originated from the plant remain (cellulose or lignin from the life
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plant), recently [4]. It is probable that this last interpretation ought to explain the presence of
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carbon in the fossils duly analyzed in the present study.
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Figure 5(a-d) shows the scanning electron microscopy images of the four samples (A – D),
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previously analyzed by the infrared and Raman spectroscopy and the X-ray diffraction (a
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complete set of images is given in the Supplementary Materials). The analysis of the EDS
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spectroscopy for the different points on the sample surfaces, was used to give a semiquantitative
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determination of the chemical composition. The usage of this technique allowed us to understand
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that the most of the samples, are not perfectly homogeneous. For instance, in Figure 5(a) from
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the white region studied, we noted the presence of 39% weight of Si and 49% of oxygen, while
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in the black region (see the Supplementary Material), the silicon corresponded to 46% and
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oxygen, 53%. In relation to the sample B in Figure 5(b), it is possible to note a certain
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homogeneity regarding silicon and oxygen in different parts of the fossil. However, if we look at
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the concentration of iron, for example, a difference in 3% of weight in different regions of the
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sample is verified. Regarding the sample C appearing in Figure 5(c), an impressive non-
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homogeneity was observed: the quantity of silicon varies from 24% to 45% and the quantity of
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oxygen varies from 53% to 62% of weight. Similarly, the sample D in Figure 5(d) presents
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silicon varying from 20% to 26% in weight and oxygen varying from 38% to 54%. In addition to
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this, the sample D analysis showed iron with a weight of 5% in certain parts and up to 35% in
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other parts. Such non-homogeneity is not a exclusivity of the fossils studied in the present paper,
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since other studies, e.g. that performed in the plant Brachyphyllum castilhoi from the Cretaceous
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Period, showing different quantities of pyrite were recorded at different points of the sample
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[12]. With reference to the four samples of the “Pedra de Fogo” Formation, we noted that the
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SEM and the EDS techniques allowed us to realize in a clear way the non-homogeneity of the
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fossils.
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The analysis of the micromorphological patterns are characterized by the microporous on the
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surface of the samples. The images (Figure 5), showed the silica occurring primarily as masses
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corresponding to the microgranular quartz [13]. Additionally, the presence of the chemical
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elements found in smaller quantities such as aluminum, iron (except for sample D) and
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potassium, was observed.
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3.2 Fossilization process and silica source:
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In the “Pedra de Fogo” Formation, the silica supply involved in the fossilization process of the
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plant remains, could be interpreted as originated from an external source, comprised of
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terrigenous alkaline materials present in typical arid climate conditions. This could promote the
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solubility of the silica and its transference to the deposited areas [14], since the volcanic rocks
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are particularly a known source of silica [15]. In relation to the quartz found in a previous study,
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we observed large crystals formats, contained in the fine-grained matrix or glass. In these cases,
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we found combined crystalline forms, constituted of bipyramidal prisms [16].
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An important parameter to be considered during the silicification is the pH value, as this can
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present substantial changes during the process [15]. It is generally accepted that in order to occur
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silicification of wood, it must be rapidly buried in sediments and staying not exposed to the
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oxygen, therefore, avoiding in this way, a decomposition by bacteria and fungi. The environment
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where this occurs is generally a fluvial one with the pH of water varying from 6 to 9; if the
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ambient is acid, fungi can be developed and if the ambient is exaggeratedly basic, it can destroy
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the molecules of the wood itself. Ideally, dissolved silica must be found in the environment,
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which is readily available where volcanic ash was present [13, 17]. It is believed that volcanic
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ash in glass or amorphous forms absorbs water, thus, releasing silica and producing a solution
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with monosilicic acid [Si(OH)4]. According to the Ref. [15], when a concentration of the solution
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increases, the monosilicic acid polymerizes, producing siloxane bonding and releasing water.
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The pivotal point in the process is that the polymerized silicic acid leads to form hydrogen bonds
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with organic molecules in the tissue of the wood. As consequence, layers of the silicic acid are
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formed in the woody tissue. In the process of the polymerization, a coating of silica gel is
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produced on the cells (or filling voids of the woods). When the gel looses water, it solidifies in
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the amorphous opal [SiO2.nH2O]. Successive crystallizations and losses of water form the
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cristobalite, trydimite, chalcedony and finally the quartz. Therefore, an eventual silification
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process is a complex event extending over many years.
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The spectroscopic analysis used in this study showed a high prevalence of silica levels in all
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fossil plant samples duly checked. These results corroborate with the study done by Alencar et al
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[4] with the wood fossils originated from the same geological unit, e.g. “Pedra de Fogo”
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Formation. Therefore, it is possible to infer that the fossilization process that acted on the
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phytofossil records in the eastern area portion, is the same as the northern area shore of the basin.
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However, the source of silica present in these stated fossils is still a matter of debate. In the
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“Pedra de Fogo” Formation, as previously commented, the silica source ought to be external
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[14]. In addition, Matysová et al. [18] suggested a similar climatic-driven process to explain the
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silicification of plant stems recovered from the Permian Motuca Formation strata, located in
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Filadélfia municipality in the northern Tocantins State and in the southwestern Parnaíba Basin.
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For these authors, the presumed silica source for the initial stage of silicification in this case, was
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the weathering of labile minerals, mostly feldspars in the alluvium. Thus, they excluded the
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volcanic influence on the silicification mode in the plant stems from the Motuca Formation and
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interpreted as a process occurred in fluvial sediments from the basin fills, under a seasonally dry
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and warm climate, perhaps during a relatively fast tectonic basinal development.
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In the case of phytofossiliferous outcrops present in the northeastern and eastern area portions of
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the basin, some of them are associated with autochthonous assemblages, i.e., fossil woods in a
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life position. It is worth noting that in the “Pedra de Fogo” Formation, some trunks in a vertical
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position reach up to 2.3 meters of length, whereas other trunks in a horizontal position have more
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than 180 centimeters in diameter. Both instances imply that there was a sizable burial of the
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paleo flora caused by some catastrophic event. As suggested by Alencar et al. [4], possibly the
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event which enabled the permineralization of these macrophytofossils, could have been one or
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more pyroclastic depositional events in the presence of aqueous streams. However, Matysová et
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al. [18] noted, based on observations in the Australian modern environments that trees from the
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riparian vegetation, are prone to survive in semiarid climatic conditions, therefore, they are
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predisposed to be rapidly buried after extreme seasonally river discharges and later silicified. As
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a consequence, some of the trunks at these said deposits are found not too far from their sites of
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growth, so these must have also belonged to the original riparian vegetation, being preserved in
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vertical position. The aforementioned flash fluvial regimes could be easily considered as
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catastrophic events. Indeed, there is no other source of evidence, besides the abundance of silica
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over a wide geographic area, which supports the occurrence of contemporary volcanic events
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during the lifetime of plants studied herein, i.e., during the accumulation of the sedimentary
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deposits of the “Pedra de Fogo” Formation [14].
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4. CONCLUSIONS
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This study reported the characterization of four fossil trunks by X-ray diffraction measurements,
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SEM-EDS spectroscopy, as well as Raman and infrared spectroscopy. The samples in reference
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from the trunk fossils, were obtained from the sedimentary Parnaiba Basin in the Permian “Pedra
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de Fogo” Formation. These aforesaid results suggest that during the fossilization process, the
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fossils in question were affected by the environment and also suffered a process of
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permineralization with a partial substitution of its original composition by quartz. Therefore, the
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results indicated that all plant remains have undergone a similar diagenetic process, which would
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be expected if they belong to the same lithostratigraphic unit. The quartz is the most abundant
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mineral in all the analyzed samples. However, the origin of the significant amount of silica,
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available in the depositional settings that gave the origin to microcrystalline quartz available in
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the original depositional settings, is still controvertial. Furthermore, the analysis must be carried
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out in order to characterize signatures of other materials that are present in a silicified plant stems
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and also in the surrounding rocky matrixes. Nevertheless, a limited detail study of the fossils and
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the associated sedimentary deposits, may eventualy clarify the source of the silica and
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consequently, explaining the way of what initially took place with the fossilization process in the
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plant remains of the Permian strata from the Parnaíba Basin.
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Acknowledgments
G.D. Saraiva, Ph.D, acknowledges the support from the MCTI/CNPQ/Universal 14/2014
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(Grants#449471/2014-4) and the PQ – 2014 (Grants#306631/2014-8): R. Iannuzzi thanks CNPq
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for the grants (PQ 309211/2013-1) and PTCF thanks the CNPq. Acknowledgements are due to
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Francisco Carlos from the APA dos Morros Garapenses in Duque Bacelar. The authors also wish
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to thank the following Brazilian institutions, such as CAPES, FAPESPI and FUNCAP.
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do Maranhão. Dissertação de Mestrado 70f. Universidade Federal do Pará
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[15] Leo RF, Barghoon ES. 1976. Silicification of wood, Botanical Museum leaflets, Harvard
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University 25: 1-49
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[16] Tröger EW. 1979. Optical determination of rock-forming minerals. Part 1, determination
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tables. E. Content Free Trial, Stuttgart, Germany, 188 p.
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[17] Landmesser M. 1994. Versteinertes Holz. ExtraLapis 7: 1-174
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[18] Matysová P, Rössler R, Götze J, Leichmann J, Forbes G, Taylor EL, Sakala J, Grygar
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T. 2010. Alluvial and volcanic pathways to silicified plant stems (Upper Carboniferous–Triassic)
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and their taphonomic and palaeoenvironmental meaning. Palaeogeography, Palaeoclimatology,
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Palaeoecology, 292: 127-143.
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Caption for the figures:
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Figura 1: Optical images of the fossil samples. A: The gymnorsperm from the Duque Bacelar
382
municipality; B: pteridophyte from the Duque Bacelar municipality; C: The gymnosperm from
383
the city of Teresina; D: the pteridophyte from the Monsenhor Gil municipality.
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Figure 2: X-ray diffractograms of the samples A (in black), B (in red), C (in blue) and D (in
386
green) shows the crystalline phase in the samples.
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Figure 3: Infrared spectra of the fossilized logs in the spectral range from 400 – 1400 cm-1: A (in
389
black), B (in red), C (in blue) and D (in green).
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Figure 4: Raman spectra of the fossils A, B, C and D for the spectral ranges (a) from 150 to 700
392
cm-1 and (b) from 950 to 1650 cm-1. The identification of the four fossils in reference, are given
393
on Table 2.
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Figure 5: The SEM images of the following four trunk fossil samples: a) fossil A, b) fossil B, c)
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fossil C, d) fossil D (shown on the text), which demonstrated the locations where the EDS
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spectra were performed.
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Figure 1(on next page)
Figure-1
Optical images of the fossil samples. A: The gymnorsperm from the Duque Bacelar
municipality; B: pteridophyte from the Duque Bacelar municipality; C: The gymnosperm from
the city of Teresina; D: the pteridophyte from the Monsenhor Gil municipality.
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Figure 2(on next page)
Figure-2
X-ray diffractograms of the samples A (in black), B (in red), C (in blue) and D (in green) shows
the crystalline phase in the samples.
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Figure 3(on next page)
Figure-3
Infrared spectra of the fossilized logs in the spectral range from 400 – 1400 cm-1: A (in black),
B (in red), C (in blue) and D (in green).
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Figure 4(on next page)
Figure-4
Raman spectra of the fossils A, B, C and D for the spectral ranges (a) from 150 to 700 cm-1
and (b) from 950 to 1650 cm-1. The identification of the four fossils in reference, are given on
Table 2.
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Figure 5(on next page)
Figure-5
The SEM images of the following four trunk fossil samples: a) fossil A, b) fossil B, c) fossil C, d)
fossil D (shown on the text), which demonstrated the locations where the EDS spectra were
performed.
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Table 1(on next page)
Table 1
Table 1: The identification of the four log fossils with the taxonomy and the location where
these samples were collected.
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1
2
3
Table 1: The identification of the four log fossils with the taxonomy and the location where these
samples were collected.
4
Samples
5
6
7
8
9
10
11
A
Taxonomic
identification
Gymnosperm
B
Pteridophyte
C
Gymnosperm
D
Pteridophyte
Locations
APA of the Morros
Garapenses, Duque Bacelar,
MA
Geographical Coordinates:
4°10'1.28"S 42°58'52.41"O
APA of the Morros
Garapenses, Duque Bacelar,
MA
4°10'1.28"S 42°58'52.41"O
The fossil florest of the river
Poti, Teresina, PI
Geographical Coordinates:
5° 6'51.42"S 42°43'48.24"O
The Monsenhor Gil, PI
Geographical Coordinates:
5°36'56.45"S 42°30'20.48"O
Geological unity
and Period
Pedra de Fogo
Formation,
Permian
Pedra de Fogo
Formation,
Permian
Pedra de Fogo
Formation,
Permian
Pedra de Fogo
Formation,
Permian
MA = Maranhão State; PI = Piauí State
12
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Table 2(on next page)
Table 2
The infrared and Raman modes assignments of the four fossils samples.
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1
Table 2: The infrared and Raman modes assignments of the four fossils samples
IR modes
Samples
A
B
C
D
Assignment*
Wavenumber(cm-1)
Sample
Wavenumber(cm-1)
2
461
520
694
778
797
1087
1164
A
464
1347
1575
-
462
455
502
518
694
694
779
778
797
797
1087
1085
1164
1164
Raman modes
B
C
464
464
1356
1577
1595
1604
462
517
694
778
797
1087
1164
Si-O bending
Si-O bending
Si stretching
Si stretching
Si stretching
Si-O stretching
Si-O stretching
D
462
-
Assignment**
Quartzo
Carbono
Carbono
Carbono
*[6, 7] **[11-20]
3
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