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Fast detection of sulphate minerals (gypsum, anglesite, baryte) by a portable Raman spectrometer

Authors:
  • Global Change Research Institute, Academy of Sciences of the Czech Republic, v. v. i. (CzechGlobe)

Abstract and Figures

Well-resolved Raman spectra of gypsum, anglesite and baryte were detected using a portable Raman instrument (Ahura First Defender XL) in the laboratory and outdoor under atmospheric conditions. Spectra were obtained using a 785-nm excitation. The portable spectrometers display generally lower spectral resolution compared with the laboratory confocal instrument but permit the fast, unambiguous detection of minerals under field conditions. Portable Raman instruments can be advocated as excellent tools for field geological, environmental as well as exobiological applications. A miniaturized Raman instrument will be included in the Pasteur analytical package of the ESA ExoMars mission and interesting research applications can now be proposed for in situ field planetary studies. Additionally, portable Raman instruments represent an ideal tool for demonstrating possible applications of Raman spectroscopic techniques outdoor. In geosciences this approach represents a new field which could completely change classical field work. Copyright © 2009 John Wiley & Sons, Ltd.
Content may be subject to copyright.
1082
Research Article
Received: 25 September 2008 Accepted: 28 January 2009 Published online in Wiley Interscience: 3 March 2009
(www.interscience.wiley.com) DOI 10.1002/jrs.2246
Fast detection of sulphate minerals (gypsum,
anglesite, baryte) by a portable Raman
spectrometer
J. Jehliˇ
cka,aP. Vítek,aH.G.M. Edwards,bM. D. Hargreaves,band T. ˇ
Capounc
Well-resolved Raman spectra of gypsum, anglesite and baryte were detected using a portable Raman instrument (Ahura First
Defender XL) in the laboratory and outdoor under atmospheric conditions. Spectra were obtained using a 785-nm excitation.
The portable spectrometers display generally lower spectral resolution compared with the laboratory confocal instrument but
permit the fast, unambiguous detection of minerals under field conditions. Portable Raman instruments can be advocated as
excellent tools for field geological, environmental as well as exobiological applications. A miniaturized Raman instrument will
be included in the Pasteur analytical package of the ESA ExoMars mission and interesting research applications can now be
proposed for in situ field planetary studies. Additionally, portable Raman instruments represent an ideal tool for demonstrating
possible applications of Raman spectroscopic techniques outdoor. In geosciences this approach represents a new field which
could completely change classical field work. Copyright c
2009 John Wiley & Sons, Ltd.
Keywords: Raman spectra; portable instruments; outdoor measurements; sulphates; gypsum
Introduction
Sulphates represent a large group of important minerals that
occur under different environments on Earth. Under exogenic
conditions, they appear as products of marine sediment forma-
tion or as products of the weathering of sulphides of different
origin.Alternatively, they can crystallize from hydrothermal solu-
tions to appear in mineral veins in the frame of hydrothermal
deposits.[1,2]
Several hydrated sulphates can be used as unequivocal probes
for previous presence of water in rocks. Sulphate associations
can occur under specific oxidation conditions in preexisting
sedimentary environments and the detection of such phases
can provide very strong evidence that these conditions were
once present in these sediments, either on Earth or Mars. For
example, due to the recent identification of evaporites on Mars
by the National Aeronautics and Space Administration (NASA)
rovers Spirit and Opportunity,[3,4] it is reasonable to assume that
halophilic microorganisms could have flourished in these matrices
in the Martian environment as happened on Earth, and possibly
have left a record of their presence there. There are also indications
that various hydrated sulphates of iron occur on the Mars surface.[4]
Field geologists usually identify minerals under outcrop
conditions by carefully observing the physical properties (shape,
color, hardness, cleavage) of these materials. In the laboratory, the
optical properties of minerals can be obtained by observing thin
sections using an optical polarizing microscope. However, this is
generally not practical outdoors in the field.
Raman spectroscopy permits us to obtain molecular informa-
tion which is not easy to obtain by other methods. This technique
allows the unambiguous identification of minerals without the ne-
cessity of separation of the individual parts of complex samples, in
this context, rocks. Its main advantage is the possibility to evaluate
the spectral characteristics of a specimen nondestructively, nonin-
vasively and rapidly. Working with Raman microspectrometers, an
optical microscope, allows the spectrally resolved identification of
individual minerals of complex geological materials and samples
at the micrometer level. Grains or inclusions of about 1 µm3can
currently be identified using these bench instruments.
Raman spectroscopic studies on sulphate secondary minerals
have been published by Frost and colleagues.[3 – 10] More recently,
Raman spectra have been collected and included in different
database systems. Such databases are very important and present
a prerequisite for the future application of this technique as
the identification tool in geology. Actually, specialized databases
already exist in areas as different as synthetic substances i.e. drugs
of abuse, explosives, toxic compounds, mineralogy or gemmology.
The constitution of large databases of Raman spectra of minerals
and biomarkers is necessary for astrobiological purposes as well
and represents a challenge because of the wealth of mineralogical
information required to properly define geological specimens.
Mobile Raman spectroscopy has been used recently for the
identification of several classes of compounds, such as pigments
for museological purposes.[11] Instruments used for art object
studies consist generally of a Raman probe head connected to
the spectrometer with optical fibers. However, the weight of such
typical mobile systems is of the order of 10–30 kg (Renishaw
RA100, Renishaw RX210, MartA)[11] which can be prohibitive for
Correspondence to: J. Jehliˇ
cka, Charles University in Prague, Institute of
Geochemistry, Mineralogy and Mineral Resources, Prague, Czech Repu blic.
E-mail: jehlicka@natur.cuni.cz
aCharles University in Prague,Institute of Geochemistry, Mineralogy and Mineral
Resources, Prague, Czech Republic
bCentre for Astrobiology and Extremophile Research, Division of Chemical and
Forensic Sciences, University of Bradford,Bradford BD7 1DP, UK
cPopulation Protection Institute, L´
aznˇ
eBohdane
ˇ
c, Czech Republic
J. Raman Spectrosc. 2009,40, 1082 –1086 Copyright c
2009 John Wiley & Sons, Ltd.
1083
Fast detection of sulphate minerals using a portable Raman spectrometer
current use in field conditions for geological applications especially
for inaccessible sites. The feasibility of in situ Raman measurements
in museums, as well as for historical architecture was recently
estimated.[11 – 13]
Raman spectroscopy shows promise to become a field method
of detection of mineral phases, which is not only important for
geological applications but also in the field of astrobiology. Within
the payload designed by European Space Agency (ESA) and NASA
for future missions focusing on the Mars’s astrobiology, Raman
spectroscopy will be a key nondestructive analytical tool for the
in situ identification of both organic and inorganic compounds
relevant to life detection on the Martian surface or near-
subsurface.Forplanetaryexplorationtwo different approaches can
be identified, namely, telescopic Raman remote measurements
methods[14,15] or Raman detection in situ, close to the outcrops
using miniaturized field instruments.[16,17]
Here, we present the application of portable Raman instru-
mentation to estimate the potential for use in the unequivocal
identification of sulphates under indoor and outdoor conditions.
Experimental
Minerals
Three current sulphates were investigated in this study: gypsum
(CaSO4·2H2O), baryte (BaSO4), and anglesite (PbSO4). The
specimens originate from the Mineralogical Collection of Charles
University in Prague and have been selected because of their
relevance to Martian geology as determined by current and recent
NASA missions. Whereas it is not possible to carry out an exhaustive
and comprehensive survey of all relevant sulphate minerals in
this paper, it is intended that, following this ‘proof-of-principle’
exercise the specimen ban will be extended. Raman spectra of
these minerals were measured outdoors during a geological field
trip at Vlastˇ
ejovice, Bohemian Massif (May 2008), and indoors
in the spectroscopic laboratory at the Institute of Geochemistry,
Charles University in Prague which also contains the laboratory
Raman microspectrometer.
Instrumental
One of several currently distributed portable and relatively low-
cost Raman spectroscopic instruments was tested here, namely
the First Defender XL by Ahura Wilmington, MA, USA. Originally
this instrument was developed for defence and security purposes.
The instrument (1.8 kg) is equipped with a 785-nm diode laser
for excitation and a thermoelectrically cooled charged-coupled-
detector (CCD) detector operating at 50 C. The maximum
laser output is 300 mW at source. This laser output was used
in this study. The instrument is equipped with a rechargeable
7.4-V internal lithium ion battery allowing practically about 5 h
of measurements in the field. The instrument provides Raman
data over the wavenumber range of 250–2875 cm1. Here, for
the purpose of sulphate mineral detection we have limited this
to the 1200– 200 cm1spectral wavelength range. The effective
spectral resolution of this instrument during current application
lies between 7 and 10 cm1.
All measurements were carried out by encasing the specimen
and instrument contact area in black cloth to improve shadow
conditions for Raman spectral data collection. Typical data analysis
times were 10 s; optimization of the number of accumulations to
improve the signal-to-noise ratio was carried out by the software
automatically.
Laboratory Raman spectra were obtained using a Renishaw
inViareflexspectrometer(Wotton-under-Edge,UK), operating with
a high power (320 mW at source) near-infrared (NIR) diode laser
emitting at 785 nm and a thermoelectrically cooled CCD (400 ×575
pixels), coupled to a Leica DMLM microscope using 50×(NA 0.75),
20×(NA 0.40), and 5×(NA 0.12) microscope objectives which
provided a spectral footprint at the specimen of approximately
2–5 µm. The diffraction grating (1200 lines/mm) gives the spectral
wavelength range 3200–100 cm1with a spectral resolution of
2cm
1. Daily calibration of the wavenumber axis is achieved by
recording the Raman spectrum of a silicon wafer (1 accumulation,
10 s) in static mode. If necessary, an offset correction is performed
to ensure that the position of the silicon band is 520.5±0.1cm
1.
Spectra were recorded between five and ten scans; to effect signal-
to-noise enhancement, a ten seconds exposure time was used.
The spectrometer was controlled by a personal computer (PC)
with commercial instrument control software (Renishaw WiRE 2
Service Pack 9). Multiple-spot analyses were carried out in replicate
on different areas of the same sample to check for spectral
reproducibility, which was necessary for the characterization of
heterogeneous specimens.
Raman spectra from all instruments were exported into the
Galactic.SPC format. Spectra were then compared using GRAMS AI
(Version 8.0, Thermo Electron Corp, Waltham, MA, USA); the Raman
spectra were normally not subjected to any data manipulation or
processing techniques and are reported generally as collected,
although in several cases baseline subtraction was effected to
illustrate spectral imprint as shown in Fig. 1; clearly, use of this
will decrease the speed of data processing and eventual mineral
identification.
Results and Discussion
Gypsum, anglesite, and baryte were successfully analyzed in
the laboratory and under field conditions by the mobile Raman
instrument, First Defender XL (Ahura). The Raman spectra obtained
were compared to those obtained by the laboratory Raman
microspectrometer at the same specimen. The Raman spectra
obtained are given in Figs 1– 3 and all observed Raman bands are
summarized in the Tables 1–3.
Gypsum
Inthecaseofgypsum,thesulfatestretchingmode(ν1)was
observed at 1008 cm1using the portable instrument outdoors
which agree with the band wavenumber position observed
in the spectrum by the laboratory instrument. The bands of
medium intensity assigned to the ν2sulfate vibrational mode
were registered at 415 and 494 cm1. The bands at 621, 670 (ν4),
and 1134 cm1(ν3) were observed as weak bands by the mobile
Raman spectrometer. Other weak bands in the region 1000–1200
were not recognized by the mobile instrument (Fig. 1).
Anglesite
Good quality Raman spectra of the lead sulphate mineral
anglesite were obtained using the mobile Raman instrument
(Fig. 2). These are in good agreement with the laboratory
Raman microspectrometer, as well as with published literature
J. Raman Spectrosc. 2009,40, 1082 –1086 Copyright c
2009 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/jrs
1084
J. Jehlicka et al.
Figure 1. Raman spectra of gypsum. (a): raw spectrum, portable Raman
spectrometer First Defender XL (Ahura), (b): treated spectrum, (c): bench
microspectrometer inVia Reflex (Renishaw).
data.[18] The sulphate stretching vibration (ν1) is registered
as the strong band at 978 cm1on both the laboratory and
mobile spectrometers. Two ν2vibrational modes around 439 and
451 cm1were registered by the First Defender XL. Two bands
due to ν3vibrations were registered at 1060 and 1156 cm1,as
well as bands of weak-to-medium intensity at 609 and 641 cm1
corresponding to ν4vibrational modes.
Baryte
The sulphate stretching mode (ν1) of baryte was observed at
988 cm1outdoors by the First Defender portable instrument,
which agree with the band position observed in the spectra by the
laboratory instrument. The band of medium intensity assigned to
the ν2sulphate vibrational mode[18] was registered at 461 cm1.
The bands at 619, 650 (ν4), and 1142 cm1(ν3) were observed as
weak or very weak bands by mobile Raman spectrometer. Other
weak bands in the region 1000 –1200 were not recognized by the
mobile instrument (Fig. 3).
Raman spectra of the minerals studied were obtained using laser
beam excitation without filtering. The stability of these sulphates is
sufficient and excellent reproducibility in the spectra was obtained.
However, care must be taken with less-stable mineral forms which
Figure 2. Raman spectra of anglesite. (a): raw spectrum, portable Raman
spectrometer First Defender XL (Ahura), outside; (b): raw spectrum,
portable Raman spectrometer First Defender XL (Ahura), inside; (c): bench
microspectrometer inVia Reflex (Renishaw).
are known to heat or degrade under laser excitation (Jehlicka in
prep.).
Raw spectral data obtained by the portable instrument
frequently display a higher noise compared with the laboratory
instrument. This is related especially to the short accumulation
time required for data collection. However, careful treatment and
sometimes baseline correction of the spectrum using GRAMS
software improves the Raman spectra, for the purposes of cross-
referencing with other laboratory instruments.
Several issues have to be addressed when measuring Raman
spectra outdoors, especially in full solar illumination and with
respect to instrument positioning. We were able, at the time of our
in situ field measurements, to collect data without full shielding of
the spectrometer. However, all measurements were done using
black cloth to improve shadow conditions at the interface of
sample and specimen for Raman spectral collection.
The working distance between the front lens of the tested
portable instrument was optimized for the best signal-to-noise
spectrum recording. The specimen distance corresponds to
2–10 mm between the specimen surface and the optical head
window.
In another report[21] that was undertaken, two portable Raman
instruments, the Ahura First Defender XL and the Inspector
www.interscience.wiley.com/journal/jrs Copyright c
2009 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2009,40, 1082 –1086
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Fast detection of sulphate minerals using a portable Raman spectrometer
Figure 3. Raman spectra of baryte (a): raw spectrum, portable Raman
spectrometer First Defender XL (Ahura), outside; (b): raw spectrum,
portable Raman spectrometer First Defender XL (Ahura), inside; (c): bench
microspectrometer inVia Reflex (Renishaw).
Raman (Delta Nu) were compared for the measurement of the
Raman spectra of selected minerals. No significant difference was
observed in the Raman signals obtained with the indoors and
outdoors measurements of only anglesite and baryte performed
by both these mobile instruments. Also, the general agreement
between both the instruments over limited range was excellent.
Conclusions
In this paper, the successful application of a portable Raman
spectrometer to detect sulphate minerals in the field is described.
Table 1. Raman bands of gypsum (CaSO4·2H2O)
First Defender XL (Ahura),
outdoor InVia reflex (Renishaw) Assignmenta,b
125 w
134 w
146 w
165 w
182 mw
211 w
415 m 415 m ν2
495 m 494 m ν2
583 w br
621 m 620 m ν4
670 m 671 m ν4
1008 s 1009 s ν1
1134 m 1136 m ν3
The Raman bands between 125 and 211 cm1correspond to modes of
vibration involving mainly the relative motion of the Ca2+and SO42
ions.
aRef. [18].
bRef. [19].
Table 2. Raman bands of anglesite (PbSO4)
First Defender XL (Ahura)
Outdoor Indoor InVia reflex (Renishaw) Assignmenta,b
440 s 440 s 439 ms ν2
451 ms sh 450 ms sh 450 s ν2
609 mw 609 mw 608 w ν4
643 w 644 w 642 w ν4
978 s 978 s 978 s
1056 w 1056 w 1061 w ν3
1160 w 1161 w 1158 w ν3
aRef. [20].
bRef. [6].
Table 3. Raman bands of baryte (BaSO4)
First Defender XL (Ahura)
Outdoor Indoor InVia reflex (Renishaw) Assignmenta
461 m 461 m 453 m ν2
618 w 619 w 617 w ν4
648 w 650 w 647 w ν4
988 s 988 s 988 s ν1
1084 w
1104 w ν3
1138 w 1142 w 1138 w ν3
1167 vw 1170 vw 1166 w ν3
aRef. [20].
J. Raman Spectrosc. 2009,40, 1082 –1086 Copyright c
2009 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/jrs
1086
J. Jehlicka et al.
Gypsum, baryte, and anglesite were detected unequivocally using
their most-intense Raman bands which were found at their correct
reported wavenumber peak positions, corresponding to published
literature values. Excellent reliability and satisfactory spectral
resolution(7– 10 cm1)was observed for portableinstrumentation
in the wavelength range 200– 2000 cm1. Portable Raman
instrumentscanberecommendedfor the fast and robustdetection
of minerals in the field. Work is in progress to compare compact
instruments with those equipped with field microscopes in an
extension of this project.
Portable Raman instruments represent an ideal tool for chemical
or geoscience areas, for demonstrating possible applications
of Raman spectroscopic techniques outdoors. In geosciences,
this approach represents a completely new field which could
dramatically change the concept of classical field work.
Acknowledgement
This work was supported by a grant MSM0021620855 from the
Ministry of Education of the Czech Republic.
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www.interscience.wiley.com/journal/jrs Copyright c
2009 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2009,40, 1082 –1086
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... Between successive pairs of layers, the water molecules are located in such a way that they are hydrogen-bonded to oxygens of SO4 groups [45]. Each Ca ion is co-ordinated by six oxygens of sulphate groups and by two water molecules [30]. ...
... In particular, the highest band at 1008 cm −1 corresponds to the sulfate symmetric stretching mode (v1), while the bands of medium intensity, assigned to the ν2 sulfate symmetric bend mode, were registered at 415 and 493 cm −1 . The band at 1134 cm −1 is linked to the sulfate antisymmetric stretching mode (ν3) while the bands at 619 and 668 cm −1 correspond to the sulfate antisymmetric bending mode (ν4) [28,30,35; RRUFF ID R0400299]. ...
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... The low values of chromium in the XRF semi-quantification, belonging to lead chromate, was justified with the high absorption coefficient both of lead of the same pigment and of lead white used in the mixture. Additionally, by Raman spectroscopy other minor compounds were detected in that blue-green layer, such as gypsum (Raman bands at 412, 492, 617, 668 1008, and 1134 cm −1 , Figure S7b) [28] and anatase (β-TiO 2 , Raman bands at 140, 192, 393, 512, and 635 cm −1 , Figure S7c) [29]. ...
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... Assignments for the Raman spectra of the minerals that were identified in the studied fulgurite are given in the Supplementary Material (Figures S1-S7), according to the literature [23][24][25][26][27][28][29][30][31][32][33][34][35]. ...
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