A thermogravimetric and infrared emission spectroscopic study of alunite

Article abstract of DOI:10.1007/s10973-006-7979-2

Thermogravimetric and differential thermogravimetric analysis has been used to characterize alunite of formula [K₂(Al³⁺) ₆(SO₄)₄(OH)₁₂]. Thermal decomposition occurs in a series of steps (a) dehydration up to 225°C, (b) well defined dehydroxylation at 520°C and desulphation which takes place as a series of steps at 649, 685 and 744°C. The alunite minerals were further characterized by infrared emission spectroscopy (IES). Well defined hydroxyl stretching bands at around 3463 and 3449 cm^-1 are observed. At 550°C all intensity in these bands is lost in harmony with the thermal analysis results. OH stretching bands give calculated hydrogen bond distances of 2.90 and 2.84-7 A. These hydrogen bond distances increase with increasing temperature. Characteristic (SO₄)^2- stretching modes are observed at 1029.5, 1086 and 1170 cm^-1. These bands shift to lower wavenumbers on thermal treatment. The intensity in these bands is lost by 550°C.

Full text of DOI:10.1007/s10973-006-7979-2

Journal of Thermal Analysis and Calorimetry, Vol. 91 (2008) 1, 267–274
A THERMOGRAVIMETRIC AND INFRARED EMISSION
SPECTROSCOPIC STUDY OF ALUNITE
*
R. L. Frost and Daria Wain
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO
Box 2434, Brisbane Queensland 4001, Australia
Thermogravimetric and differential thermogravimetric analysis has been used to characterize alunite of formula
3
+
2 6 4 4
[K (Al ) (SO ) (OH)12]. Thermal decomposition occurs in a series of steps (a) dehydration up to 225°C, (b) well defined
dehydroxylation at 520°C and desulphation which takes place as a series of steps at 649, 685 and 744°C.
The alunite minerals were further characterized by infrared emission spectroscopy (IES). Well defined hydroxyl stretching
–
bands at around 3463 and 3449 cm are observed. At 550°C all intensity in these bands is lost in harmony with the thermal analysis
1
results. OH stretching bands give calculated hydrogen bond distances of 2.90 and 2.84–7 . These hydrogen bond distances in-
–1
2–
4
crease with increasing temperature. Characteristic (SO ) stretching modes are observed at 1029.5, 1086 and 1170 cm . These
bands shift to lower wavenumbers on thermal treatment. The intensity in these bands is lost by 550°C.
Keywords: alunite, ammonioalunite, jarosite, natroalunite, Raman spectroscopy, sulphate
Introduction
ronments such as acid mine drainage and acid-sulfate
soils and as such their solubility is controlled by the
climatic conditions in particular the temperature.
Alunites are a group of minerals which form part
of the alunite supergroup [15]. The general formula is
given by DG (TO ) (OH,H O) where the D sites are
Interest in the chemistry of alunites stems from a
number of reasons. Firstly, because of the possible
discovery of alunites on Mars [1, 2]. Such a find im-
plies the presence of water on Mars either at present
or at some time in the planetary past [3, 4]. Interest in
such minerals and their thermal stability rests with the
possible identification of these minerals and related
dehydrated paragenetically related mineral on planets
and on Mars. There have been many studies on re-
lated minerals such as the Fe(II) and Fe(III) sulphate
minerals [5–10]. Secondly, alunites are found in soils
and evaporate deposits. The importance of alunite
formation and its decomposition depends upon its
presence in soils, sediments and evaporate deposits
3
4 2
2
6
occupied by monovalent cations such as K, Na, NH ,
4
+
H O and others, divalent cations such as Ca, Ba, Sr,
Pb, trivalent cations for example Bi; and G is the tri-
3
3
valent cation either Al of Fe ; and T is S , As or
+
6+
5+
5
+
P . Alunites can be divided into alunites and jarosites
simply depending on whether the concentration of Al
is >Fe (alunites) or Fe>Al (jarosites) [14]. Of course
solid solution formation can exist across a wide range
of concentrations and substitutions. Common mem-
bers of the alunite group are alunite
KAl (SO ) (OH) , natroalunite NaAl (SO ) (OH) ,
[
11]. These types of deposits have formed in acid
3
4 2
6
3
4 2
6
soils where the pH is less than 3.0 pH units [12]. Such
acidification results from the oxidation of pyrite
which may be from bacterial action or through air-ox-
idation. Thirdly, alunites are important from an envi-
ronmental point of view. Alunites are minerals which
can function as collectors of heavy metals and low
concentrations can be found in the natural alunites.
Such minerals can act as a significant environmental
sink [13]. One of the difficulties associated with the
analysis of alunites is that they are often poorly crys-
talline, making detection using XRD techniques diffi-
cult. Another problem associated with the study of
alunites is their thermodynamic stability [14]. Often
the minerals are formed from acid-sulfate rich envi-
ammonioalunite NH Al (SO ) (OH) , sclossma-
4 3 4 2 6
+
2+
cherite (H O ,Ca )Al (SO ) (OH) . The structure of
3
3
4 2
6
alunites is well known [16–18]. The structure of
alunites is trigonal with a 6.990, c 16.905  space
group R3m, with Z=3.
There have been a significant number of vibra-
tional spectroscopic studies of alunites [19–26]. The
number of Raman studies have been few [27–29]. Ac-
cording to Ross the infrared spectrum of alunite con-
–
1
sists of the n mode at 1030 cm , the n mode at
1
2
–
475 cm , the n mode at 1170, 1086 cm and the n
1
–1
3
4
–
mode at 632 and 605 cm [30]. The observation of
1
multiple bands for the n and n modes implies the
3
2
*
Author for correspondence: r.frost@qut.edu.au
1
388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©
2008 Akadémiai Kiadó, Budapest

FROST, WAIN
symmetry of the sulphate is reduced from TdÞC .
3
Infrared spectroscopy
v
Indeed vibrational spectroscopy has been used to re-
fine the space group of alunites [31]. Isomorphism in
alunites has been studied by infrared spectroscopy
Infrared spectra were obtained using a Nicolet
Nexus 870 FTIR spectrometer with a smart endurance
single bounce diamond ATR cell. Spectra over the
[
21]. Vibrational spectroscopy allows a better method
–1
4000–525 cm range were obtained by the co-addition
of 64 scans with a resolution of 4 cm and a mirror ve-
for the study of these minerals. Infrared spectroscopy
has been used to study jarosite minerals but failed to
detect cationic differences in the jarosite structures
–1
–1
locity of 0.6329 cm s . Spectral manipulation such as
baseline adjustment, smoothing and normalization was
[
27, 32–36]. Raman spectroscopy has been used to
®
performed using the GRAMS software package (Ga-
lactic Industries Corporation, Salem, NH, USA).
study jarosites but some previous studies failed to
study the complete spectra [27, 33, 37]. Further the
lack of crystallinity of the synthetic samples studied
may have made the collection of Raman spectral data
difficult. Most studies have involved the use of syn-
thetic jarosites [38].
Thermal analysis has proven extremely useful
for determining the stability of minerals [39–47]. In
this work, as part of our studies of secondary mineral
formation, we report the infrared emission and ther-
mal analysis of selected alunites.
FTIR emission spectroscopy was carried out on a
Nicolet Nexus 870 FTIR spectrometer equipped with
a TGS detector, which was modified by replacing the
IR source with an emission cell. A description of the
cell and principles of the emission experiment have
been published elsewhere [48–51]. Approximately
0
.2 mg of alunite was spread as a thin layer (approxi-
mately 0.2 microns) on a 6 mm diameter platinum sur-
face and held in an inert atmosphere within a nitro-
gen-purged cell during heating. The infrared emission
cell consists of a modified atomic absorption graphite
rod furnace, which is driven by a thyristor-controlled
AC power supply capable of delivering up to
Experimental
1
50 amps at 12 volts. A platinum disk acts as a hot
Minerals
plate to heat the smectite sample and is placed on the
graphite rod. An insulated 125-mm type R thermocou-
ple was embedded inside the platinum plate in such a
way that the thermocouple junction was <0.2 mm be-
low the surface of the platinum. Temperature control
of ±2°C at the operating temperature of the smectite
sample was achieved by using a Eurotherm
Model 808 proportional temperature controller, cou-
pled to the thermocouple. The design of the IES facil-
ity is based on an off axis paraboloidal mirror with a
focal length of 25 mm mounted above the heater cap-
turing the infrared radiation and directing the radia-
tion into the spectrometer. The assembly of the heat-
ing block, and platinum hot plate is located such that
the surface of the platinum is slightly above the focal
point of the off axis paraboloidal mirror. By this
means the geometry is such that approximately 3 mm
diameter area is sampled by the spectrometer. The
spectrometer was modified by the removal of the
source assembly and mounting a gold-coated mirror,
which was drilled through the centre to allow the pas-
sage of the laser beam. The mirror was mounted at
45°, which enables the IR radiation to be directed into
the FTIR spectrometer.
The alunites were obtained from the Mineralogical
Research Company. The minerals were phase ana-
lysed by powder X-ray diffraction and were analysed
by chemical composition by EDX methodology.
Methods
X-ray diffraction
X-ray diffraction (XRD) patterns were recorded using
CuK
PANalytical X’Pert PRO diffractometer operating at
0 kV and 40 mA with 0.125° divergence slit, 0.25°
a
radiation (n=1.5418 ) on
a
Philips
4
anti-scatter slit, between 3 and 15° (2q) at a step size
of 0.0167°. For low angle XRD, patterns were re-
corded between 1 and 5° (2q) at a step size of 0.0167°
with variable divergence slit and 0.5° anti-scatter slit.
Thermal analysis
Thermal decomposition of the alunite minerals was
®
carried out in a TA Instruments incorporated
high-resolution thermogravimetric analyzer (se-
ries Q500) in a flowing nitrogen atmosphere
3 –1
60 cm min ). Approximately 35 mg of each sample
(
underwent thermal analysis, with a heating rate of
In the normal course of events, three sets of spectra
are obtained: firstly the black body radiation over the
temperature range selected at the various temperatures,
secondly the platinum plate radiation is obtained at the
same temperatures and thirdly the spectra from the plati-
num plate covered with the sample. Normally only one
set of black body and platinum radiation is required. The
–
°C min , with resolution of 6 from 25 to 1000°C.
1
5
With the isothermal, isobaric heating program of the
instrument the furnace temperature was regulated pre-
cisely to provide a uniform rate of decomposition in
the main decomposition stage.
268
J. Therm. Anal. Cal., 91, 2008

A THERMOGRAVIMETRIC AND INFRARED EMISSION SPECTROSCOPIC STUDY OF ALUNITE
emittance spectrum at a particular temperature was cal-
culated by subtraction of the single beam spectrum of
the platinum backplate from that of the platinum+sam-
ple, and the result rationed to the single beam spectrum
of an approximate blackbody (graphite). This spectral
manipulation is carried out after all the spectral data has
been collected. The emission spectra were collected at
intervals of 50°C over the range 200–750°C. The time
between scans (while the temperature was raised to the
next hold point) was approximately 100 s. It was consid-
ered that this was sufficient time for the heating block
and the powdered sample to reach temperature equilib-
rium. The spectra were acquired by coaddition of
are reported in Figs 2a and 2b. Three major processes
occur for the thermal decomposition of alunite. These
are dehydration, dehydroxylation and desulphation. The
details of these processes are listed below.
Mechanism for decomposition of potassium alunite
Step 1 dehydration – temperature up to ~220°C
KAl (SO ) (OH) ·xH O®KAl (SO ) (OH) +xH O
3 4 2 6 2 3 4 2 6 2
This step represents the dehydration step and oc-
curs over a long temperature range starting at ~50°C
and continues up to 225°C. The wide temperature
range for dehydration shows that water is strongly ad-
sorbed to the alunite surfaces. There is a mass loss of
1.3% for this step.
6
4 scans for the whole temperature range (approximate
–1
scanning time 45 s), with a nominal resolution of 4 cm .
Good quality spectra can be obtained providing the sam-
ple thickness is not too large. If too large a sample is
used then the spectra become difficult to interpret be-
cause of the presence of combination and overtone
bands.
1
00
1.4
1.2
alunite
(Tolfa, Italy)
1
.3%
1
5%
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
7.8%
1
0
.0
.8
Spectral manipulation such as baseline adjust-
ment, smoothing and normalisation was performed
using the Spectracalc software package (Galactic In-
dustries Corporation, Salem, NH, USA).
1
7%
0.6
0.4
0.2
0.0
Results and discussion
25 125 225 325 425 525 625 725 825 925
Temperature/°C
Thermogravimetric analysis of potassium alunite
Fig. 1 Thermogravimetric and differential thermogravimetric
analysis of alunite from Italy
The thermogravimetric analysis of the alunite from Italy
is shown in Fig. 1. The mass spectra of evolved gases
a
520
alunite
Tolfa, Italy)
b
alunite
(Tolfa, Italy)
6
85
(
6
49
7
44
m/z=18
scaling factor=1.5
1
57
m/z=32
scaling factor=2.5
6
85
6
49
747
m/z=17
scaling factor=3
5
22
m/z=48
scaling factor=7
1
58
m/z=16
scaling factor=8
5
24
6
85
1
58
7
87
6
51
1
59
3
74
m/z=44
scaling factor=5
m/z=64
scaling factor=5
0
200
400
600
800
1000
0
200
400
600
800
1000
Temperature/°C
Temperature/°C
Fig. 2 Ion current curves of evolved water vapour a – m/z=16,17,18,44 and b – m/z=32,48,64
J. Therm. Anal. Cal., 91, 2008
269

FROST, WAIN
alunite
Kern County, California)
a
alunite
(Coquimbo District, Chile)
b
(
100°C
150°C
200°C
100°C
150°C
200°C
250°C
300°C
350°C
400°C
250°C
300°C
350°C
4
00°C
50°C
4
50°C
00°C
4
5
5
00°C
50°C
550°C
5
600°C
600°C
3100
3200
3300
3400
3500
3600
7600
3100
3200
3300
3400
3500
3600
7600
–1
–1
Wavenumber/cm
Wavenumber/cm
alunite
(Tolfa, Italy)
c
alunite
(Piute County, Utah)
d
1
00°C
50°C
1
00°C
50°C
1
1
2
00°C
50°C
2
00°C
50°C
2
2
3
00°C
50°C
3
00°C
50°C
3
3
4
00°C
50°C
4
00°C
50°C
4
4
5
00°C
50°C
5
500°C
550°C
600°C
3100
3200
3300
3400
3500
3600
7600
3100
3200
3300
3400
3500
3600
7600
–1
–1
Wavenumber/cm
Wavenumber/cm
Fig. 3 Infrared spectra of the hydroxyl stretching region from 100 to 600°C of alunites from a – California, b – Chile, c – Italy,
d – Utah
Step 2 dehydroxylation – at 520°C
The ion current curves for the evolved gases
show for m/z=18 and m/z=16 a mass gain at around
KAl (SO ) (OH) ®KO Al (SO ) +3H O
2
3
4 2
6
1/2
3
4 2
1
58°C (Fig. 4a). A further mass gain of water vapour
This step represents the loss of the hydroxyl
units from the alunite structure. Figure 1 shows two
mass loss steps at 520 and 685°C with mass losses of
occurs at 520°C. At these two temperatures OH units
are lost from the alunite structure. The mass gain in
the MS curves corresponds precisely with the mass
loss in the TG curves.
1
5.0 and 7.8% for a total of 22.8%. The MW of alunite
–
1
is 414 g mol and the theoretical mass% of OH units
is 24.6%. Thus there is ~4.0% mass loss per OH unit.
270
J. Therm. Anal. Cal., 91, 2008

A THERMOGRAVIMETRIC AND INFRARED EMISSION SPECTROSCOPIC STUDY OF ALUNITE
Step 3 desulphation – loss of sulphate – temperature
649, 685 and around 744°C
Kern County, California
3449
3463
3396
KO Al (SO ) ®1/2K SO +SO +1.5Al O +2.5O
2
3
3
4 2
2
4
3
2
3
K SO ®SO +K O
2
2
4
3
Piute County, Utah
3
483
and
3428
3
510
K O+Al O ®2KAlO
2
2
2
3
The TG and DTG curves show a large mass loss of
7.0% at 680°C. The ion current curves for m/z=64, 48
Tolfa, Italy
1
3471
and 32 all show peaks at 649, 685 and 744°C confirm-
ing the loss of sulphate at these temperatures.
3510
3432
Infrared emission spectroscopy of the OH stretching
region
Coquimbo District, Chile
3469
3422
The IES spectra of the OH stretching region of
alunites from (a) California, (b) Chile, (c) Italy and
3510
(
d) Utah are shown in Figs 3–d. A comparison of the
3
000
3100
3200
3300
3400
3500
3600
room temperature band component analysed infrared
spectra are displayed in Fig. 4. Alunite infrared spec-
tra in the OH stretching region show bands at 3463,
–1
Wavenumber/cm
Fig. 4 Band component analysis of the infrared spectra of the
hydroxyl stretching region of alunites from a – Califor-
nia, b – Chile, c – Italy, d – Utah at ambient temperature
-
1
3
3
3
449 and 3396 cm for the California alunite; 3510,
–
483 and 3428 cm for the Utah alunite; 3510, 3471,
1
–
432 cm for the alunite from Italy and 3510, 3469
1
–
1
and 3422 cm for the alunite from Chile (Fig. 2).
Serna et al. states that the alunite cell is rhombohedral
5
and that under the space group D , there should be
Infrared emission spectroscopy of the 650 to
–
1
3
d
1450 cm region
only two OH stretching vibrations [27]. Serna et al.
–
reported Raman bands at 3508 and 3481 cm for
1
Alunites contain sulphate and hydroxyl units which lend
themselves to study by vibrational spectroscopy.
Sulphates are readily detected by the range of tech-
niques offered by infrared spectroscopy. In aqueous sys-
–
K-al) 3488, 3452 cm (Na-al), 3492 cm (H O -al)
1
–1
+
(
[
3
27]. Studies have shown a strong correlation be-
tween OH stretching frequencies and both O···O bond
distances and H···O hydrogen bond distances
tems, the sulphate anion is of T
symmetry and is char-
d
–1
–1
acterised by Raman bands at 981 cm (n ), 451 cm
1
[
52–55]. Libowitzky (1999) showed that a regression
–
1
–1
(
n2), 1104 cm (n ) and 613 cm (n ). Reduction in
3 4
function can be employed relating the hydroxyl
stretching frequencies with regression coeffi-
cients better than 0.96 using infrared spectros-
symmetry in the crystal structure of sulphates such as
alunites will cause the splitting of these vibrational
modes. In 1937 Hendriks proposed the space group
R3m for alunite [57]. Later studies based upon refine-
ment of the X-ray crystallography of both synthetic and
copy [56]. The function is described as: n =
1
–
d(O–O)
.1321
0
–1
cm . Thus OH---O hydrogen
(3592-304) ×1 09
natural alunites gave the space group as R3barm with a
5
bond distances may be calculated using the
Libowitzky empirical function. The values of the OH
stretching bands lead to an estimation of hydrogen
bond distances. For alunite the bands at 3510 and
D
space group [58, 59]. Thus six sulphate fundamen-
3d
tals should be observed. Ross in Farmer (Chapter 18) re-
ports the results of the infrared spectra of alunite [30].
–1
–
1
Multiple n (1170 and 1086 cm ) and n (632 and
3 4
3
483 cm lead to hydrogen bond distances of 2.909
–1
605 cm ) bands are observed showing the loss of sym-
metry in the alunite structure. The symmetric stretching
and 2.872 . Slight differences in hydrogen bond dis-
tances are observed for the other alunites. These hy-
drogen bond distances appear to fall into two groups
–
1
n mode of the sulphate units was found at 1030 cm
1
–1
and the n mode at 475 cm . Ross lists three bending
2
at around 2.90 and 2.84–2.87 . X-ray crystallo-
graphic studies give a hydrogen bond distance of
–1
modes at 802, 780 and 605 cm . Other studies of the in-
frared spectra of alunites have been undertaken [60, 61].
Many studies compared the spectra of alunites with that
of jarosites and it was suggested that there was no
cationic influence [61]. However such a conclusion is
2
.92  which is in excellent agreement with the val-
ues estimated from the Raman OH stretching
wavenumbers. It is suggested that two hydrogen
bonds of slightly different strengths exist for alunites.
J. Therm. Anal. Cal., 91, 2008
271

FROST, WAIN
a
b
1
00°C
100°C
150°C
200°C
150°C
00°C
2
250°C
300°C
350°C
400°C
450°C
2
50°C
3
00°C
50°C
3
400°C
450°C
500°C
550°C
500°C
550°C
600°C
6
00°C
6
50°C
00°C
7
650°C
7
00°C
7
50°C
00°C
750°C
8
8
50°C
00°C
800°C
850°C
900°C
9
950°C
9
50°C
1
000°C
alunite
Kern County, California)
alunite
(Coquimbo District, Chile)
(
1000°C
6
50
850
1050
1250
1450
650
850
1050
1250
1450
–
1
–1
Wavenumber/cm
Wavenumber/cm
c
d
100°C
100°C
150°C
2
00°C
50°C
150°C
2
200°C
250°C
300°C
350°C
400°C
450°C
300°C
350°C
400°C
450°C
500°C
550°C
6
6
7
7
8
00°C
50°C
00°C
50°C
00°C
5
00°C
550°C
600°C
650°C
700°C
750°C
8
50°C
00°C
9
800°C
8
50°C
00°C
9
950°C
9
50°C
alunite
(Tolfa, Italy)
alunite
(Piute County, Utah)
1
000°C
1000°C
6
50
850
1050
1250
1450
650
850
1050
1250
1450
–
1
–1
Wavenumber/cm
Wavenumber/cm
–
1
Fig. 5 Infrared spectra of the 650 to 1450 cm region over the temperature range 100 to 600°C of alunites from a – California, b
Chile, c – Italy, d – Utah
–
questionable. Other workers basically gave a catalogue
of infrared spectra many of which were incomplete [60].
The infrared emission spectra in the 650 to
proaches zero by 500°C. Two bands are observed at
2–
–
1095 and 1165 cm and are assigned to the (SO4)
1
antisymmetric stretching modes. Infrared bands for
alunite for these vibrations were reported by Farmer
–
1
1
(
450 cm region of alunites from (a) California,
b) Chile, (c) Italy, (d) Utah are shown in Figs 5a–d.
–
1
to be at 1086 and 1170 cm . The broad band at
–
1
2–
–1
1240 cm is ascribed to OH deformation modes. It is
The band at 1029.5 cm is assigned to the (SO4)
symmetric stretching mode. Farmer reports the band
at 1030 cm for K-alunite . The band shows a shift to
lower wavenumbers upon thermal treatment. The
–
band is observed at 1028.9 cm at 200°C and at
noted the intensity of these bands decreases with in-
creased temperature rise until 500°C where the spec-
tral profile changes significantly. The band at
–
1
1
–1
2–
686 cm is assigned to the (SO
)
bending mode.
The intensity in this band is lost by 550°C. Farmer re-
4
–
027.8 cm at 400°C. The intensity of the band ap-
1
1
272
J. Therm. Anal. Cal., 91, 2008

A THERMOGRAVIMETRIC AND INFRARED EMISSION SPECTROSCOPIC STUDY OF ALUNITE
–
ported a band at 632 cm and attributed this band to
1
B. Schmitt, Nature (London, United Kingdom),
28 (2004) 627.
4 B. M. Hynek, Nature (London, United Kingdom),
31 (2004) 156.
–
the n bending mode [30]. A band at 685 cm was ta-
1
4
4
2
–
bled for the n bending mode of (SO ) for jarosite.
4
4
4
So it appears that this value for alunite is in error or a
misprint. Over the temperature range 550 to 700°C,
the spectral profile of the thermally treated alunites is
a broad almost featureless spectrum; yet after 700°C a
spectrum of the thermally decomposed alunite shows
greater intensity. These peaks are attributed to the fi-
nal decomposition product of the alunite. The IE
spectra of the four alunites are identical over the tem-
perature range studied.
5
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Alunites show characteristic thermogravimetric pat-
terns with thermal decomposition steps (a) dehydra-
tion up to 225°C (b) well defined dehydroxylation at
520°C and desulphation which takes place as a series
of steps at 649, 685 and 744°C.
2 P. A. Williams, Oxide Zone Geochemistry, Ellis Horwood
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13 C. Drouet, D. Baron and A. Navrotsky, Am. Mineral.,
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8
4 R. E. Stoffregen, C. N. Alpers and J. L. Jambor, Rev. Min-
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5 J. E. Dutrizac and J. L. Jambor, Rev. Mineral. Geochem.,
The alunite mineral group can be characterised
by their infrared emission spectra. Intensity loss in the
OH stretching modes by 550°C is in harmony with the
thermogravimetric
dehydroxylation at 520°C.
It is very important to be able to thermally char-
acterise minerals such as alunite which may be found
on planets such as Mars. The existence of alunites on
Mars would confirm the presence of water at some
time in the past as such minerals are only formed from
solution. The thermal stability of alunites is most im-
portant as there is a need to find the temperature range
over which the minerals are stable, since wide tem-
perature ranges are likely on planets.
40 (2000) 405.
6 G. Giuseppetti and C. Tadini, Neues Jahrbuch für
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results
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7 K. Okada, J. Hirabayashi and J. Ossaka, Neues Jahrbuch
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18 J. Ossaka, J. Hirabayashi, K. Okada, R. Kobayashi and
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1
2
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9 S. Aslanyan and R. Petrova, Geokhim., Mineral. Petrol., 3
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(
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3 K. I. Petrov, V. G. Pervykh and N. K. Bol’shakova,
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4 N. N. Poprukailo and T. B. Shkodina, Khim.-Metall. Inst.,
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The financial and infra-structure support of the Queensland
University of Technology Inorganic Materials Research Pro-
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gratefully acknowledged. The Australian Research Council
5 N. T. Uklonskaya, Zapiski Uzbekistanskogo Otdeleniya
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(
ARC) is thanked for funding.
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