Abstract Raman spectroscopy complimented with infrared ATR spectroscopy has been used to characterise a halotrichite FeSO 4 ·Al 2 (SO 4 ) 3 ·22H 2 O from The Jaroso Ravine, Almeria, Spain. Halotrichites form a continuous solid solution series with pickingerite and chemical analysis shows that the jarosite contains 6% Mg 2+ . Halotrichite is characterised by four infrared bands at 3569.5, 3485.7, 3371.4 and 3239.0 cm −1 . Using Libowitsky type relationships, hydrogen bond distances of 3.08, 2.876, 2.780 and 2.718Å were determined. Two intense Raman bands are observed at 987.7 and 984.4 cm −1 and are assigned to the ν 1 symmetric stretching vibrations of the sulphate bonded to the Fe 2+ and the water units in the structure. Three sulphate bands are observed at 77 K at 1000.0, 991.3 and 985.0 cm −1 suggesting further differentiation of the sulphate units. Raman spectrum of the ν 2 and ν 4 region of halotrichite at 298 K shows two bands at 445.1 and 466.9 cm −1 , and 624.2 and 605.5 cm −1 , respectively, confirming the reduction of symmetry of the sulphate in halotrichite

Fernando Rull

J Theo Kloprogge

Jesus Martinez-Frias

Matt L Weier

Ray L Frost

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Spectrochimica Acta Part A 62 (2005) 176–180
Raman spectroscopy of halotrichite from Jaroso, Spain
Ray L. Frosta,∗, Matt L. Weiera, J. Theo Kloproggea, Fernando Rullb, Jesus Martinez-Friasb
a Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology,
GPO Box 2434, Brisbane, Qld 4001, Australia
b Cristalografia y Mineralogia, Unidad Asociada al Centro de Astrobiologia INTA-CSIC, Universidad de Valladolid, Valladolid 47006, Spain
Received 24 November 2004; received in revised form 10 December 2004; accepted 10 December 2004
Abstract
Raman spectroscopy complimented with infrared ATR spectroscopy has been used to characterise a halotrichite FeSO4·Al2(SO4)3·22H2O
from The Jaroso Ravine, Almeria, Spain. Halotrichites form a continuous solid solution series with pickingerite and chemical analysis shows
that the jarosite contains 6% Mg2+. Halotrichite is characterised by four infrared bands at 3569.5, 3485.7, 3371.4 and 3239.0 cm−1. Using
Libowitsky type relationships, hydrogen bond distances of 3.08, 2.876, 2.780 and 2.718Å were determined. Two intense Raman bands are
observed at 987.7 and 984.4 cm−1 and are assigned to theν1 symmetric stretching vibrations of the sulphate bonded to the Fe2+ and the water
u −1 he
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ules.
lums
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nits in the structure. Three sulphate bands are observed at 77 K at 1000.0, 991.3 and 985.0 cmsuggesting further differentiation of t
ulphate units. Raman spectrum of theν2 andν4 region of halotrichite at 298 K shows two bands at 445.1 and 466.9 cm−1, and 624.2 an
05.5 cm−1, respectively, confirming the reduction of symmetry of the sulphate in halotrichite.
2004 Elsevier B.V. All rights reserved.
eywords: Sulphate; Halotrichite; Jarosite; Raman spectroscopy
. Introduction
Sulphate efflorescences have been known for some consi-
erable time[1–3]. These often occur in tailings impound-
ents (see[4, p. 322]). The sulphate formation results
rom the oxidation of pyrite. Halotrichites are formed close
o pyrite and are often found with copiapites and related
inerals[5]. The minerals are found in efflorescences in
eothermal fields[6]. Halotrichite is of formula FeSO4·
l 2(SO4)3·22H2O and forms an extensive solid solution se-
ies with pickingerite MgSO4·Al2(SO4)3·22H2O [6–9]. The
inerals are related to the alums R2SO4·M2(SO4)3·24H2O
r RM(SO4)2·12H2O, where R represents an atom of a uni-
alent ion such as ammonium, lithium, sodium, potassium
nd cesium and where M represents a trivalent metal such
s aluminium, iron, chromium, gallium, manganese, cobalt,
hodium and trivalent thallium. When a divalent atom is
∗ Corresponding author. Tel.: +61 7 3864 2407; fax: +61 7 3864 1804.
E-mail addresses: r.frost@qut.edu.au (R.L. Frost), rull@fmac.uva.es
F. Rull).
introduced instead of the monovalent atom, such as
ganese, ferrous iron, cobalt, zinc and magnesium will f
double sulphates. These sulphates form the halotric
mineral series. These minerals are not isomorphous wit
univalent alums. The minerals are all isomorphous and
tallise in the monoclinic space groupP21/c In the structure
of the pseudo-alums, four crystallograpically indepen
sulphate ions are present[10]. One acts as a unidenta
ligand to the M2+ ion, and the other three are involved
complex hydrogen bond arrays involving coordinated w
molecules to both cations and to the lattice water molec
Ross reports the interpretation of the infrared spectr
potassium alum asν1, 981 cm−1; ν2, 465 cm−1; ν3, 1200,
1105 cm−1; ν4, 618 and 600 cm−1 [11]. Water stretchin
modes were reported at 3400 and 3000 cm−1, bending mode
at 1645 cm−1, and librational modes at 930 and 700 cm−1
[12]. In the structure of alums, six water molecules surro
each of the two cations. This means the sulphate ions ar
tant from the cations and coordinate to the water molec
Ross also lists the infrared spectra of the pseudo-a
formed from one divalent and one trivalent cation. Halo386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
oi:10.1016/j.saa.2004.12.023
R.L. Frost et al. / Spectrochimica Acta Part A 62 (2005) 176–180 177
chite has infrared bands atν1, 1000 cm−1; ν2, 480 cm−1;
ν3, 1121, 1085, 1068 cm−1; ν4, 645, 600 cm−1. Pickerin-
gite the Mg end member of the halotrichite-pickeringite se-
ries has infrared bands atν1, 1000 cm−1; ν2, 435 cm−1; ν3,
1085, 1025 cm−1; ν4, 638, 600 cm−1 [11]. These minerals
display infrared water bands in the OH stretching, 3400 and
3000 cm−1 region; OH deformation, 1650 cm−1 region; OH
libration, 725 cm−1 region. Ross also reports a weak band
at ∼960 cm−1 which is assigned to a second OH librational
vibration [11]. As with the infrared spectra, Raman spec-
tra of alums are based on the combination of the spectra
of the sulphate and water. Sulphate typically is a tetrahe-
dral oxyanion with Raman bands at 981 cm−1 (ν1), 451 cm−1
(ν2), 1104 cm−1 (ν3) and 613 cm−1 (ν4) [13]. Some sulphates
have their symmetry reduced through acting as monodentate
and bidentate ligands[13]. In the case of bidentate behaviour
both bridging and chelating ligands are known. This reduc-
tion in symmetry is observed by the splitting of theν3 and
ν4 into two components underC3v symmetry and into three
components underC2v symmetry.
In this work we report the Raman and infrared spectra of
a halotrichite from Jaroso, Spain.
2. Experimental
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R nm)
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U
3. Results and discussion
The infrared spectrum and the Raman spectrum of halotri-
chite at 298 and 77 K in the 850–1200 cm−1 region are shown
in Fig. 1 and the results of the spectral analyses reported in
Table 1. The infrared spectra are broad but show two low in-
tensity bands at 981.0 and 992.7 cm−1. In the Raman spectra
at 298 K an intense band is observed at 984.4 cm−1 with a
shoulder at 987.7 cm−1. In the 77 K spectrum of halotrichite
two intense bands are observed at 991.3 and 1000.0 cm−1
with a third band at 985 cm−1. These bands are sharp in com-
parison with the infrared bands. These bands are assigned to
the ν1 symmetric stretching mode of the SO42− units. In
the crystal structure of the pseudo-alums there are four in-
dependent sulphate units, with one sulphate unidenate bond-
ing to the divalent cation and the other three to the water
molecules. Thus spectroscopically two symmetric stretching
bands would be predicted and this is what is found in the
298 K Raman spectrum. The ratio of the intensities of the
984.4 cm−1/987.7 cm−1 bands is approximately 25%. These
values would suggest the 984.4 cm−1 band is due to the sul-
phate bonded to water and the 987.7 cm−1 band attributed to
sulphate bonded to the divalent cation. The Raman spectra
at 77 K imply three different non-equivalent sulphate units.
X-ray diffraction at low temperatures would be needed to
prove this concept. From a spectroscopic point of view, the
s erals
Fig. 1. Raman spectra at 298 and 77 K and the IR-ATR spectrum of halotri-
chite from Jaroso, Spain in the 850–1200 cm−1 region..1. Minerals
The minerals used in this study were supplied by on
he authors (FR). The mineral halotrichite originated f
aroso, Spain. The minerals were analysed by X-ray dif
ion for phase purity and by electron probe using energy
ersive techniques for quantitative chemical compositio
.2. Raman spectroscopy
The crystals of halotrichite were placed and oriente
he stage of an Olympus BHSM microscope, equipped
0× and 50× objectives and part of a Renishaw 1000
an microscope system, which also includes a mono
ator, a filter system and a charge coupled device (C
aman spectra were excited by a HeNe laser (633
t a resolution of 2 cm−1 in the range between 100 a
000 cm−1. Repeated acquisition using the highest ma
cation was accumulated to improve the signal to nois
io. Spectra were calibrated using the 520.5 cm−1 line of
silicon wafer. In order to ensure that the correct spe
re obtained, the incident excitation radiation was sc
led. Previous studies by the authors provide more de
f the experimental technique[14–17]. Spectra at elevate
emperatures were obtained using a Linkam thermal s
Scientific Instruments Ltd., Waterfield, Surrey, UK). Sp
ral manipulation such as baseline adjustment, smoo
nd normalisation was performed using the GRAMS® soft-
are package (Galactic Industries Corporation, Salem,
SA).ituation is complex, since all these isomorphous min
178 R.L. Frost et al. / Spectrochimica Acta Part A 62 (2005) 176–180
Table 1
Results of the infrared and Raman spectra of halotrichite
ATR-IR Raman (298 K) Raman (77 K)
Centre FWHM % Centre FWHM % Centre FWHM %
3569.5 72.9 0.5 3565.1 29.3 0.7
3545.5 110.5 2.1
3485.7 150.7 1.2 3498.0 21.8 0.4
3425.4 146.5 29.1 3412.8 55.5 3.2
3412.3 231.4 20.2
3371.4 89.1 0.5
3269.1 232.2 31.9
3239.0 349.7 25.4 3249.2 107.1 7.0
3189.9 245.2 22.7
2958.2 207.9 13.3
2906.8 373.6 26.7
2484.2 363.4 10.2
1646.8 74.9 2.8 1662.0 83.1 0.9 1650.9 91.7 0.9
1597.6 152.3 2.3 1568.2 73.2 0.1
1455.9 151.7 0.9
1138.1 46.1 1.3 1146.5 27.1 2.9 1146.5 33.4 1.2
1107.0 28.4 2.1 1115.4 20.7 1.1
1084.1 41.9 3.0 1076.5 32.1 1.8
1055.9 73.4 16.1 1052.1 10.9 0.2
1042.5 18.3 0.5
1000.0 5.9 7.3
992.7 9.6 0.2 987.7 22.1 3.1 991.3 6.6 4.3
984.4 4.7 12.3 985.0 4.3 0.9
983.1 22.4 2.0
981.0 12.7 0.2 981.9 4.6 2.9 971.8 20.1 5.3
950.7 29.5 0.7
947.2 96.6 5.4
880.1 25.0 0.4
744.7 45.4 0.3
704.5 47.6 0.7
633.3 53.9 1.0
624.2 34.9 1.1 620.7 20.1 1.3
612.3 14.0 0.1 605.5 54.9 3.0 606.9 7.5 0.3
593.5 21.7 0.4
572.1 75.9 2.1
534.1 10.8 0.2
481.7 15.1 0.5
466.9 17.9 1.9 466.7 25.4 2.9
445.1 21.9 1.2 435.2 12.5 0.1
422.1 14.2 0.1 424.2 9.7 0.8
380.1 25.5 0.2
361.5 21.3 0.4 353.7 7.4 0.1
317.3 10.6 0.4
301.9 7.3 0.1
264.4 5.9 0.0
244.6 28.6 0.9 247.2 17.9 0.4
220.5 23.0 0.7 227.1 13.5 0.4
204.2 11.5 0.2 199.3 13.7 0.1
181.8 6.8 0.0 185.9 9.5 0.1
163.1 7.1 0.1
116.0 1.6 0.0
crystallise in a monoclinic space groupP21/c and four in-
dependently sulphate ions are present. In the unit cell, there
are 16 anions in the primitive cell each located in theC1
position. So additional numerous bands due to crystal effects
are expected. There are four crystallographically independent
sulphate ions. This will result in numerous band splittings,
which are more readily observed at 77 K. Hence it would be
expected that each independent sulphate anion would have
its individual Raman spectrum.
The infrared spectrum ofFig. 1shows a broad profile with
three bands observed at 1055.9, 1107.0 and 1138.1 cm−1.
These bands are assigned to theν3 antisymmetric stretching
vibrations. Ross reported two infrared bands for pickeringite
at 1085 and 1025 cm−1 and three bands for halotrichite at
R.L. Frost et al. / Spectrochimica Acta Part A 62 (2005) 176–180 179
Fig. 2. Raman spectra at 298 and 77 K and the IR-ATR spectrum of halotri-
chite from Jaroso, Spain in the 100–800 cm−1 region.
1121, 1085 and 1068 cm−1 [11]. In the Raman spectrum at
298 K two bands are observed at 1084.1 and 1146.5 cm−1
and are attributed to the sulphate antisymmetric stretching
vibrations. Four bands are observed in this region in the 77 K
spectrum at 1052.1, 1076.5, 1115.4 and 1146.5 cm−1. There
are two different types of sulphates in the structure of halotri-
chite and it would be expected that two sets of antisymmetric
stretching bands would be obtained. Thus in correspondence
with the two symmetric stretching bands at 77 K, two sets of
bands are obtained. One set is 1052.1 and 1115.4 cm−1 and
the second set 1076.5 and 1146.5 cm−1.
The low wavenumber regions of halotrichite at 298 and
77 K are shown inFig. 2. The Raman spectrum of theν2
region of halotrichite at 298 K shows two bands at 445.1
and 466.9 cm−1. In the 77 K spectrum four bands are found
at 435.2, 466.7, 481.7 and 534.1 cm−1. A previous study
showed halotrichite showed two bands at 468 and 424 cm−1
for halotrichite[13]. Halotrichites are known for forming a
continuous solid solution with pickingerite. This results from
the substitution of Mg2+ for Fe2+. In some ways this means
that every example of halotrichite may be of a different com-
position and may give different Raman spectra. Ross reported
one infrared band for halotrichite at 480 cm−1 and a band for
pickeringite at 435 cm−1 [11].
The infrared spectrum shows a broad band at 572.1 cm−1
w e
R and
Fig. 3. Raman spectra at 298 and 77 K and the IR-ATR spectrum of halotri-
chite from Jaroso, Spain in the 2000–3800 cm−1 region.
624.2 cm−1. Ross reports twoν4 modes in the infrared spectra
at 645 and 600 cm−1 for halotrichite and 638 and 600 cm−1
for pickeringite[11]. The infrared spectra of halotrichite show
two bands at 612 and 572 cm−1. The Raman spectrum at liq-
uid nitrogen temperature shows better band separation and
bands are observed at 593.5, 606.9, 620.7 and 633.3 cm−1.
Some sulphates have their symmetry reduced through the
formation of monodentate and bidentate ligands. In the case
of bidentate formation both bridging and chelating ligands
are known. This reduction in symmetry is observed by the
splitting of theν3 and ν4 into two components underC3v
symmetry and into three components underC2v symmetry.
The observation that three or four bands are observed in the
ν4 region of halotrichites is attributed to the reduction of sym-
metry toC2v or less, and the fact that four crystallographically
distinct sulphate groups are present in the structure, one acts
as a unidentate towards the M2+ ion.
The Raman spectrum of halotrichite in the 2800–
3800 cm−1 region is shown inFig. 3. The infrared spec-
trum displays four bands at 3239.0, 3371.4, 3485.7 and
3569.5 cm−1. Some selected studies have shown a strong
correlation between OH stretching frequencies and both
the O· · ·O bond distances and the H· · ·O hydrogen bond
distances[18–21]. The elegant work of Libowitsky showed
that a regression function can be employed relating the above
c 0.96
[
0 hehich is ascribed to the sulphateν4 bending mode. In th
aman spectrum at 298 K two bands are found at 605.5orrelations with regression coefficients better than
22]. The function isν1 = 3592− 304× 109 exp(−d(O O)/
.1321) cm−1. Two types of OH units are identified in t
180 R.L. Frost et al. / Spectrochimica Acta Part A 62 (2005) 176–180
structure and the known hydrogen bond distances used to pre-
dict the hydroxyl stretching frequencies. By using the band
positions of the OH stretching bands in the infrared spec-
trum hydrogen bond distances using the formula above can
be calculated. The values obtained are 2.718Å (3239 cm−1),
2.780Å (3371.4 cm−1), 2.876Å (3485.7 cm−1) and 3.081̊A
(3569.5 cm−1). The hydroxyl stretching modes of weak hy-
drogen bonds occur in the 3580–3500 cm−1 region and the
hydroxyl stretching modes of strong hydrogen bonds occurs
below 3485 cm−1. When the water is coordinated to the cation
in the clays as occurs in certain minerals, then the water OH
stretching frequency occurs at 3220 cm−1. A simple observa-
tion can be made that as the water OH stretching frequency
decreases then the HOH bending frequency increases. The
3239 cm−1 band corresponds to an ice-like structure with
O H· · ·O bond distances of 2.718̊A. In the infrared spec-
trum of halotrichite a strong band is found at 1647 cm−1.
The Raman band is observed at 1662 cm−1 at 298 K and at
1650 cm−1 at 77 K. The water hydroxyl stretching and the
water HOH bending 1610 cm−1 frequencies provide a mea-
sure of the strength of the bonding of the water molecules.
Likewise the position of the water bending vibration also
provides a measure of this strength of water hydrogen bond-
ing. Bands that occur at frequencies above 1650 cm−1 are
indicative of coordinated water and chemically bonded wa-
ter. Bands that occur below 1630 cm−1 are indicative of water
m hy-
d
4
ous
s tion
o ool
f f the
c trum
o stal
s hate
u one
t sul-
p stal
s that
s ching
7 rved
as well as multiple bending modes suggesting a reduction in
symmetry of the sulphate in the halotrichite structure. The
variation in the Raman spectrum of halotrichite with increas-
ing substitution of Fe2+ by Mg needs to be further explored.
Acknowledgments
The financial and infra-structure support of the Queens-
land University of Technology Inorganic Materials Research
Program of the School of Physical and Chemical Sciences is
gratefully acknowledged. The Australian Research Council
(ARC) is thanked for funding.
References
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[2] F. Wirth, Angew. Chem. 26 (1913) 81.
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[ c. 35
[
[ ge,
[
[
[
[
[
olecules that are not as tightly bound. In this case the
rogen bonding is weaker as the frequency decreases.
. Conclusions
Halotrichite is a pseudo-alum which can form a continu
eries of solid solutions with pickingerite by the substitu
f Mg for Fe2+. Raman spectroscopy is a very powerful t
or the study of these types of minerals and because o
ompositional variation differences in the Raman spec
f halotrichites of different origins are observed. The cry
tructure of halotrichites shows four non-equivalent sulp
nits, three of which are bonded to water molecules and
o the divalent cation. Raman spectroscopy identifies two
hate symmetric stretching vibrations in line with the cry
tructure. At 77 K, three bands are observed implying
ome structural change in the halotrichite occurs on rea
7 K. Multiple antisymmetric stretching bands are obse[7] A.D. Cody, T.R. Grammer, NZ J. Geol. Geophys. 22 (1979) 49
[8] M.C. Brandy, Am. Min. 23 (1938) 669.
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Society, London, 1974, 423 pp. (Chapter 18).
12] S.D. Ross, Inorganic Infrared and Raman Spectra, European C
istry Series McGraw-Hill, London, 1972.
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Spectrosc. 31 (2000) 1083.
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(2004) 28.
16] R.L. Frost, M.L. Weier, J. Raman Spectrosc. 35 (2004) 299.
17] R.L. Frost, P.A. Williams, W. Martens, P. Leverett, J.T. Kloprog
Am. Min. 89 (2004) 1130.
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19] H. Lutz, Struct. Bonding (Berlin, Germany) 82 (1995) 85.
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Spectrochimica Acta Part A 62 (2005) 176-180 Raman spectroscopy of halotrichite from Jaroso, Spain

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Spectrochimica Acta Part A 62 (2005) 176-180 Raman spectroscopy of halotrichite from Jaroso, Spain

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