Characteristic of Fe in tektite observed from XANES and UV-Vis spectroscopy - IOPscience
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Journal of Physics: Conference Series
PAPER • OPEN ACCESS
Characteristic of Fe in tektite observed from XANES and UV-Vis
spectroscopy
To cite this article: S Paisarnsombat et al 2021 J. Phys.: Conf. Ser. 1719 012002
View the article online for updates and enhancements.
This content was downloaded from IP address 46.4.80.155 on 29/03/2021 at 13:49
Siam Physics Congress (SPC) 2020 IOP Publishing
Journal of Physics: Conference Series 1719 (2021) 012002 doi:10.1088/1742-6596/1719/1/012002
Characteristic of Fe in tektite observed from XANES and UV-
Vis spectroscopy
S Paisarnsombat*, N Monarumit and S Aimploysri
Department of Earth Sciences, Faculty of Science, Kasetsart University, Bangkok,
Thailand
*E-mail: fscisnpa@ku.ac.th
Abstract. Tektite is a geological sample formed as a result of a collision of the meteorite on the
surface. An energy from the collision results in an excavation of the melted material into the
Earth’s atmosphere and landed back on the surface further away from the impact site having
glassy or crystalline textures. Tektite was deposited in Thailand and was identified as part of the
Australasian strewn field. Chemical compositions of tektites have previously been studied.
However, there is no study has established a relationship between composition and color of
tektites. Thus, this study aims to relate the characteristic of color-center element, Fe, to the color
of tektites using X-ray Absorption Near Edge Structure (XANES) and UV-Vis spectroscopy.
Nine tektite samples were collected from Korat plateau and purchased from Thailand and
Vietnam gem markets. XANES spectra of all (nine) samples show similar Fe K-edge pattern
with pre-edge and E0 at 7080-7089 and 7118 eV, respectively. The spectrum is 100 percent
matched with XANES structure of FeO standard suggesting Fe component of Fe2+ oxidation state
in the sample. Seven, out of nine, samples establish similar UV-Vis broad absorption peak at
492-539 nm, which corresponds to absorption peak of Fe2+. The UV-Vis absorption spectrum
cannot be obtained from two samples possibly due to high Fe-content in the samples. The
calculated energy band gaps (Eg) of the samples are in between 1.35 eV to 1.76 eV with
maximum absorption in between 1.88 eV to 1.95 eV. The result is in consistent with a black
color of the tektite. It indicates that cause of color in tektites is related to Fe-content and can be
explained by the energy band. This study can be furthered compare with moldavite (green variety
of tektite) in order to identify cause of color in impact-related materials like tektites, as well as
to provide information for identification of synthetic and genuine moldavites and also for
characterization of tektites sources using the component of various Fe oxidation states.
1. Introduction
Tektites are small glassy objects, one or more centimeters in diameter, formed as a product of impact
event. They are found distributed in certain areas of the Earth’s surface, which is known as strewn fields
[1]. There are four strewn fields around the world including North American, Central European, Ivory
Coast and Australasian strew fields [2]. The latter is directly related to tektites deposited in Thailand
and other Southeast Asian countries such as Vietnam, Laos and Philippines. The Australasian strewn
field was believed to be quenched from molten ejecta during an impact event collided 0.79 Ma ago [3].
Tektite is interesting because it is a geological material that is not formed by a typical geological
process, and some of its origin is mostly unknown. Not only be an important geological material, tektite
has been in gemstone market for a long time. The most famous tektite gemstone is called moldavite,
which is a green variety of tektite. Tektites have previously been studied in many aspects such as
composition, spectral data [4], and its origin. However, there is no study suggesting the cause of color
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd 1
Siam Physics Congress (SPC) 2020 IOP Publishing
Journal of Physics: Conference Series 1719 (2021) 012002 doi:10.1088/1742-6596/1719/1/012002
appeared in tektite as well as strong evidence on origin of the Australasian tektites. Thus, this research
aims to identify characteristic of certain chemical composition that may be related to color-center in
tektites as well as being connected to the origin of the tektites.
Methods that have been used widely to characterize oxidation state of elements is to study the
spectrum of X-ray Absorption Near Edge Structure (XANES) retrieved from X-ray Absorption
Spectroscopy using synchrotron light [5-6]. The cause of color in material can also be studied by
calculations from the UV-Vis-NIR spectral data [7].
2. Materials and methods
The tektite samples were collected from 3 different environments including three samples deposited in
Khon Kaen province (KK), three samples purchased from gem market in Thailand (TH) and three
samples purchased from gem market in Vietnam (Vet). The physical properties of the samples were first
investigated prior to being cut into approximately 1x1x0.5 cm3 specimen. Microtextures of the tektites
were studied under the Scanning Electron Microscope (SEM). Chemical compositions were
investigated, prior to the X-ray Absorption Spectroscopy analysis, using X-ray Fluorescence (EDXRF).
The X-ray absorption spectral data were collected at the BL1.1W station at the Synchrotron Thailand
Central Lab, SLRI. The XANES were collected from Fe K-edge and Ti K-edge. The Extended X-ray
Absorption Fine Structure (EXAFS) were also collected from Fe K-edge and Ti K-edge for further
investigations of the Fe and Ti environments such as neighboring atoms, bond length, and bond angle
related with multiple scattering signal. The EXAFS spectra analysis is an on-going research, which will
be presented in our future work. The Fe foil and Ti foil standard could be used for energy calibration at
7112 eV and 4966 eV, respectively. The UV-Vis absorption spectra of the specimens were also
investigated. The energy band gap (Eg) was calculated with the Tauc Plot method of UV-Vis spectra
using a direct allowed transition [8].
3. Results and discussion
3.1. Physical and chemical properties
The results show that all nine tektite specimens consist of minute crystals in dark brown fine-grained
matrix having similar composition (figure 1). There is a slight difference in carbon content between the
matrix and crystals. The matrix of KK and TH tektites have less carbon, while the matrix of Vet tektites
has more carbon than the crystals. These may be a cause of slightly different texture of the matrix
between TH and KK samples and Vet samples. According to the EDXRF results, all 9 samples have
similar chemical composition in which Si and Al are the two most abundant elements with average
percentage of 76.95% and 13.58%, respectively. Other elements present in the tektite samples (from the
most to least abundant) are Fe, Mg, K, Ca, Na, Ti, Mn, Cr, and Ni, in which Fe content ranges from
3.65% to 3.96%.
Figure 1. SEM images and chemical compositions analyzed by EDS of a) TH011 and b) Vet004.
3.2. X-ray Absorption Near Edge Structure (XANES)
The XAS using synchrotron light was used to investigate the oxidation state of Fe in the tektites sample
via analysis of the XANES spectra. The oxidation state was determined by comparing binding energy
2
Siam Physics Congress (SPC) 2020 IOP Publishing
Journal of Physics: Conference Series 1719 (2021) 012002 doi:10.1088/1742-6596/1719/1/012002
(E0) of the samples to standards. The binding energy was calculated from the first derivative method of
the normalized XANES absorption spectra using the Athena software. The results show that all tektites
have similar XANES spectra with very close absorption edge, as shown in figure 2. The absorption
edges range from 7118.59 to 7119.19 eV, which are in consistent with the absorption edge of FeO
standard (7118.99 eV). The matching edge between samples and FeO standard suggests that Fe in the
tektites appear as Fe2+.
a) b)
E0 = E0 =
7118.99 eV 7119.00 eV
c)
E0 =
7118.59 eV
Figure 2. Fe K-edge XAS spectra of a) tektites deposited in Khon Kaen (KK1), b) tektites from
market in Thailand (TH001) and c) tektites from gem market in Vietnam (Vet001).
In addition to Fe, the x-ray absorption data of Ti has been studied due to its potential cause of color
when pairing with Fe [7]. Figure 3 shows the absorption edge of Ti at 4981.19 eV, which correlates with
absorption edge of Ti in TiO2 standard (4979.20 eV for rutile and 4983.56 eV for anatase) suggesting
Ti4+ in the tektites. Since the absorption edge of Ti is in between the edge of rutile and anatase, it is
possible that Ti can be presented in a form of rutile or anatase, or a mixture between the two phases.
3
Siam Physics Congress (SPC) 2020 IOP Publishing
Journal of Physics: Conference Series 1719 (2021) 012002 doi:10.1088/1742-6596/1719/1/012002
a) b)
E0 = E0 =
4981.20 eV 4981.40 eV
c)
E0 =
4981.20 eV
Figure 3. Ti K-edge XAS spectra of a) tektites deposited in Khon Kaen (KK1), b) tektites from
market in Thailand (TH001) and c) tektites from gem market in Vietnam (Vet001).
3.3. Ultraviolet Visible Near Infrared Spectrometer (UV-Vis-NIR)
In addition to the XANES study, the samples were investigated under the UV-Vis-NIR spectrometer in
order to provide another support for the presence of reduced Fe in the tektites. Due to high concentration
of Fe in the tektites, UV-Vis spectrum shows noisy background. However, the broad band absorption
spectrum was clearly noticed within a wavelength range of visible light, approximately 499-539 nm.
The absorption wavelength corresponds to the UV-Vis absorption spectrum of Fe2+ in tektite samples.
3.4. Energy band model
The energy band gap (Eg) of the samples are ranging from 1.35 eV to 1.76 eV corresponded to a black
color of tektite. Figure 4 shows the representative energy band model of a tektite sample (KK1) with a
specific energy band gap (Eg = 1.76 eV). The cause of black color in tektite can be explained by Fe2+
defected-color center as a donor state in the tektite structure which is related to the UV-Vis absorption
band. There are two Fe2+ broad absorption bands at 1.52 eV and 2.60 eV as well as Fe2+ single energy
state at 2.48 eV. For this model, the presence of absorption pattern showing at 1.52 eV is an evidence
of Fe2+ shallow donor state inside an energy band gap. The electron from this state is donated to
conduction band relating to the presence of absorption signal. Moreover, the other Fe2+ energy states at
2.48 eV and 2.60 eV are within the conduction band and it could be an indication of a cause of black
color of tektite samples.
4
Siam Physics Congress (SPC) 2020 IOP Publishing
Journal of Physics: Conference Series 1719 (2021) 012002 doi:10.1088/1742-6596/1719/1/012002
Figure 4. An energy band model of a representative tektite sample (KK1).
4. Conclusions
Tektites from various localities within Southeast Asia have similar chemical compositions in which the
Fe present as Fe2+ and Ti as Ti4+. The study suggests that color of tektite may relate to a transition of
electron within the energy band gap supported by the presence of Fe2+ energy states inside the conduction
band. Future study on other varieties of tektite, especially moldavite, would provide an insight into cause
of different color in tektites. Furthermore, the matching calculation for XANES spectrum of Ti in the
samples compared with TiO2 standard suggesting that Ti in the tektites is more likely to be present in a
form of rutile. However, fine structure x-ray absorption spectrum is being studied to better describe the
structure of Ti4+ in the tektites, which is possibly related to Fe2+ giving the color of the tektites.
Acknowledgement
This research was funded by Faculty of Science and Department of Earth Sciences, Kasetsart University,
with fully support from the Synchrotron Light Research Institute (SLRI) for an opportunity to perform
XAS analysis. We would like to extend our appreciation to Dr. Chatree Saiyasombat for his assistance
and suggestion in XAS study, and Dr. Ladda Tangwattananukul for providing tektite samples and her
advice on tektite deposit.
References
[1] Vand V 1965 Adv. Geophys. 11 1–114
[2] McCall G J H 2005 Encyclopedia of Geology (Amsterdam: Elsevier) pp. 443–55
[3] Sieh K et al. 2020 Proc. of the National Academy of Sciences (U.S.A.) 117(3) 1346–53
[4] Cohen A J 1958 Geochim. Cosmochim. Acta 14(4) 279–86
[5] Monarumit N, Wongkokua W and Satitkune S 2016 Procedia Comput. Sci. 86 180–3
[6] Giuli G, Eeckhout S G, Koeberl C, Pratesi G and Paris E 2008 Meteorit. Planet. Sci. 43(5) 981–
6
[7] Wongrawang P, Monarumit N, Thammajak N, Wathanakul P and Wongkokua W 2016 Mater.
Res. Express 3 026201
[8] Kusuma H H, Saidin M K and Ibrahim Z 2009 J. Fiz. UTM. 4 42–9
5
You can also read
