Thermal analysis of halotrichites

Article abstract of DOI:10.1016/j.tca.2007.04.015

Four halotrichites from different origins were analysed by thermogravimetric and differential thermogravimetric analysis. The halotrichites were analysed by powder X-ray diffraction and were phase pure. The chemical composition was analysed using EDX techniques. The formula of the halotrichite minerals were determined as halotrichite (Fe_0.75²⁺,Mg_0.25)SO₄·Al₂(SO₄)₃·22H₂O, apjohnite (Mn_0.64²⁺,Mg_0.28,Zn_0.08)SO₄·Al₂(SO₄)₃·22H₂O, pickeringite (Fe_0.22²⁺,Mg_0.78)SO₄·Al₂(SO₄)₃·22H₂O, wupatkiite as (Co_0.45,Fe_0.55²⁺)SO₄·Al₂(SO₄)₃·22H₂O. Three low temperature decomposition steps (a) between 0 and 44 °C, 50 and 76 °C, 72 and 88 °C were attributed to dehydration. An additional dehydration step at around 317-330 °C was confirmed by in situ mass spectrometry. The higher temperature decomposition steps between 516 and 738 °C are attributed to the decomposition of sulphate to sulphur dioxide and oxygen as confirmed by mass spectrometry. A comparison of the thermal decomposition of jarosites is made.

Full text of DOI:10.1016/j.tca.2007.04.015

Thermochimica Acta 459 (2007) 64–72
Thermal analysis of halotrichites
Ashley J. Locke, Wayde N. Martens, Ray L. Frost∗
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology,
GPO Box 2434, Brisbane, Queensland 4001, Australia
Received 15 February 2007; received in revised form 18 April 2007; accepted 20 April 2007
Available online 29 April 2007
Abstract
Four halotrichites from different origins were analysed by thermogravimetric and differential thermogravimetric analysis. The halotrichites
were analysed by powder X-ray diffraction and were phase pure. The chemical composition was analysed using EDX techniques. The for-
mula of the halotrichite minerals were determined as halotrichite (Fe0.752+,Mg0.25)SO4·Al2(SO4)3·22H2O, apjohnite (Mn0.642+,Mg0.28,Zn0.08)SO4·
Al2(SO4)3·22H2O, pickeringite (Fe0.222+,Mg0.78)SO4·Al2(SO4)3·22H2O, wupatkiite as (Co0.45,Fe0.552+)SO4·Al2(SO4)3·22H2O. Three low temper-
ature decomposition steps (a) between 0 and 44 ◦C, 50 and 76 ◦C, 72 and 88 ◦C were attributed to dehydration. An additional dehydration step at
around 317–330 ◦C was confirmed by in situ mass spectrometry. The higher temperature decomposition steps between 516 and 738 ◦C are attributed
to the decomposition of sulphate to sulphur dioxide and oxygen as confirmed by mass spectrometry. A comparison of the thermal decomposition
of jarosites is made.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Evaporite; Jarosite; Halotrichite; Sulphate; Raman spectroscopy
1. Introduction
ture of the pseudo-alums, four crystallographically independent
sulphate ions are present [11–15]. One acts as a unidentate lig-
and to the M2+ ion, and the other three are involved in complex
hydrogen bond arrays involving coordinated water molecules to
both cations and to the lattice water molecules [13].
The minerals in the halotrichite group have been known for
an extended period of time [1–6]. The minerals are monoclinic
sulphates of general formula AB2(SO4)4·22H2O where A is Mg,
Mn2+, Fe2+, Ni, Zn and some combination of these cations and B
may be Al, Cr3+ or Fe3+. These minerals are also known as pseu-
doalums. Metals such as manganese, ferrous iron, cobalt, zinc
and magnesium will form double sulphates. These sulphates are
related to the halotrichites mineral series. These alums are not
isomorphous with the univalent alums. Typically the end mem-
ber formulae are MgSO4·Al2(SO4)3·22H2O (pickeringite) or
FeSO4·Al2(SO4)3·22H2O (halotrichite), but other M2+ cations
substitute and solid solutions in the series are extensive. These
minerals are referred to as the pseudo-alums [7–9], and are
often found in nature as post-mining phases [7–9]. Apjohnite
is the manganese pseudo-alum. Wupatkiite is the cobalt ana-
logue, which may have some other divalent metal substitution
for the cobalt cation [10]. The minerals are all isomorphous and
crystallise in the monoclinic space group P21/c [11]. In the struc-
The thermal decomposition of jarosites has been studied for
some considerable time [16–20]. There have been many studies
on related minerals such as the Fe(II) and Fe(III) sulphate miner-
als [21–26]. Interest in such minerals and their thermal stability
restswiththepossibleidentificationofthesemineralsandrelated
dehydrated paragenetically related mineral on planets and on
Mars. The existence of these minerals on planets would give a
positive indication of the existence or at least pre-existence of
water. Further such minerals are formed through crystallisation
from solutions. It has been stated that the thermal decomposi-
tion of jarosite begins at 400 ◦C with the loss of water [27]. The
process is apparently kinetically driven. Water loss can occur at
low temperatures over extended periods of time [27]. It is proba-
ble that in nature low temperature environments would result in
the decomposition of jarosite. The products of the decomposi-
tion depend upon the jarosite be it K, Na or Pb, etc. but normally
goethite and hematite are formed together with soluble sulphates
[28]. Recently thermogravimetric analysis has been applied to
some complex mineral systems and it is considered that TG–MS
∗
Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804.
E-mail address: r.frost@qut.edu.au (R.L. Frost).
0040-6031/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2007.04.015

A.J. Locke et al. / Thermochimica Acta 459 (2007) 64–72
65
analyses may also be applicable to the jarosite minerals [29–34].
Raman spectroscopy has proven very useful for the study of
minerals [35–37]. Indeed Raman spectroscopy has proven most
useful for the study of diagentically related minerals as often
occurs with carbonate minerals [38–42]. Some previous studies
have been undertaken by the authors using Raman spectroscopy
to study complex secondary minerals formed by crystallisation
from concentrated sulphate solutions [43]. To the best of the
authors knowledge no thermoanalytical studies of halotrichites
have been undertaken; although differential thermal analysis of
some related minerals has been published [18,44–46]. In this
work we report the thermogravimetric analysis of a selection of
natural halotrichites.
a full standards quantitative procedure on the JEOL 840 SEM
using a JEOL-2300 energy-dispersive X-ray microanalysis sys-
tem (JEOL Ltd., Tokyo, Japan). Oxygen was not measured
directly but was calculated using assumed stoichiometry of the
other elements analysed. Table 1 is in at.%. The total at.% is
added and the subtracted from 100.0% to obtain the oxygen
at.%.
2.3. X-ray diffraction
XRD analyses were performed on a PANalytical X’Pert
PRO® X-ray diffractometer (radius: 240.0 mm). Incident X-ray
radiation was produced from a line focused PW3373/10 Cu X-
ray tube, operating at 45 kV and 35 mA. The incident beam
passed through a 0.04 rad, Soller slit, a 1/2◦ divergence slit, a
15 mm fixed mask and a 1◦ fixed anti scatter slit. After inter-
action with the sample, the diffracted beam was detected by an
X’Celerator RTMS detector fitted to a graphite post-diffraction
monochrometer. The detector was set in scanning mode, with
an active length of 2.022 mm. Samples were analysed utilising
Bragg–Brentano geometry over a range of 3–75◦ 2θ with a step
size of 0.02◦ 2θ, with each step measured for 200 s.
2. Experimental
2.1. Minerals
The halotrichite minerals used in this work were supplied by
theMineralogicalResearchCompanyandTheMuseumofSouth
Australia. The minerals were analysed by X-ray diffraction for
phase purity and by electron probe using energy dispersive tech-
niques for quantitative chemical composition. Table 1 lists the
minerals their origin and composition as determined by EDX.
The values are an average of not less than five measurements.
2.4. Thermal analysis
Thermal decomposition of the halotrichites was carried out
in a TA® Instruments incorporated high-resolution thermogravi-
metric analyzer (series Q500) in a flowing nitrogen atmosphere
(60 cm3/min). Approximately 35 mg of sample underwent ther-
mal analysis, with a heating rate of 5 ◦C/min, resolution of 6, to
1000 ◦C. With the quasi-isothermal, quasi-isobaric heating pro-
gram of the instrument the furnace temperature was regulated
precisely to provide a uniform rate of decomposition in the main
decomposition stage.
The TGA instrument was coupled to a Balzers (Pfeiffer) mass
spectrometer for gas analysis. Only water vapour, carbon diox-
ide and oxygen were analysed. In the MS figures, e.g. Fig. 9, a
background of broad peaks may be observed. This background
occurs for all the ion current curves. The background becomes
more prominent as the scale expansion is increased. It is consid-
ered that this background may be due to the loss of chemicals
2.2. SEM and EDX
Mineral samples were coated with a thin layer of evapo-
rated carbon and secondary electron images were obtained using
a scanning electron microscope (FEI Quanta 200 SEM, FEI
Company, Hillsboro, Oregon, USA). For X-ray microanalysis
(EDX), three samples were embedded in Araldite resin and
polished with diamond paste on Lamplan 450 polishing cloth
using water as a lubricant. The samples were coated with a
thin layer of evaporated carbon for conduction and examined
in a JEOL 840A analytical SEM (JEOL Ltd., Tokyo, Japan)
at 25 kV accelerating voltage. Preliminary analyses of the nat-
ural halotrichite mineral samples were carried out on the FEI
Quanta SEM using an EDAX X-ray microanalyser, and micro-
analyses of the clusters of fine crystals were carried out using
Table 1
EDX analysis of four minerals of the halotrichite group
Atom
Apjohnite [Tariano, Italy]
(average 0.05 at.%)
Halotrichite [Corral Hollow, California,
USA] (average 0.05 at.%)
Pickeringite [San Bernadino,
California] (average 0.05 at.%)
Wupatkiite [Cloncurry, Queensland,
Australia] (average 0.05 at.%)
O
64.34
1.74
9.09
22.97
1.39
0.47
0.00
0.00
0.00
82.26
0.55
4.68
10.45
0.00
0.00
0.00
2.06
0.00
69.22
2.87
7.29
19.96
0.00
0.00
0.00
0.66
0.00
71.88
0.00
6.66
18.19
0.00
0.00
0.00
1.81
1.46
Mg
Al
S
Mn
Zn
As
Fe
Co
Total
100.0
100.0
100.0
100.0

66
A.J. Locke et al. / Thermochimica Acta 459 (2007) 64–72
Fig. 1. XRD patterns of the four natural halotrichites and the standard reference minerals.
in Table 2 is (Mn0.392+,Mg0.48,Zn0.13)SO4·Al2(SO4)3·22H2O.
The pickeringite EDX analyses show the presence of both
Mg and Fe2+. The formula of the pickeringite is therefore
given as (Fe0.222+,Mg0.78)SO4·Al2(SO4)3·22H2O. The analy-
sis of the wupatkiite from Cloncurry, Queensland, Australia is
(Fe0.552+,Co0.45)SO4·Al2(SO4)3·22H2O.
which have deposited in the capillary which connects the TA
instrument to the MS.
3. Results and discussion
3.1. X-ray diffraction and EDX analysis
The powder X-ray diffraction patterns of the four selected
minerals halotrichite, apjohnite, pickeringite and wupatkiite
together with standard XRD patterns are shown in Fig. 1. The
XRD patterns of the minerals show identical patterns to the stan-
dards. The XRD pattern of the pickeringite mineral shows an
impurity of gypsum together with a second unknown impurity
which may be jarosite.
3.2. Thermogravimetric analysis and mass spectrometric
analysis
The summary of the thermogravimetric and differential ther-
mogravimetric analyses are reported in Table 2.
EDX analyses are reported in Table 1. The halotrichite corre-
sponds to an formula (Fe0.752+,Mg0.25)SO4·Al2(SO4)3·22H2O.
The EDX analysis if the apjohnite from Italy shows the pres-
ence of not only Zn and Mg but some Mn is observed
as well. The formula of the apjohnite based upon the data
3.2.1. Apjohnite
The thermogravimetric and differential thermogravimetric
analysis of apjohnite are shown in Fig. 2. The associated mass
spectrometric analysis is reported in Fig. 3. Six major thermal
decomposition steps are observed.
Fig. 2. TG and DTG of apjohnite.

A.J. Locke et al. / Thermochimica Acta 459 (2007) 64–72
67
Table 2
Halotrichite thermogravimetric analysis summary
Decomposition stage
Mineral
Halotrichite
Apjohnite
Pickeringite
Wupatkiite
1st
42 ◦C
53 ◦C
14.28%
9.20%
18.91%
3.18%
1.69%
19.03%
7.72%
7.21%
31 ◦C
50 ◦C
8.25%
17.69%
15.27%
3.29%
44 ◦C
83 ◦C
19.45%
20.35%
51 ◦C
76 ◦C
17.10%
10.49%
7.55%
2nd
3rd
4th
5th
6th
7th
8th
72 ◦C
88 ◦C
180 ◦C
317 ◦C
330 ◦C
546 ◦C
625 ◦C
697 ◦C
738 ◦C
317 ◦C
11.30%
516 ◦C
631 ◦C
663 ◦C
730 ◦C
4.37%
8.46%
5.73%
10.46%
643 ◦C
710 ◦C
778 ◦C
20.42%
9.06%
5.26%
612 ◦C
640 ◦C
22.69%
4.55%
Steps 1, 2 and 3 temperature 50, 88 and 317 ◦C
loss of 3.29% at 317 ◦C is observed. Some water molecules are
retained to significantly high temperatures, perhaps because the
water is trapped inside the apjohnite structure. Based upon the
formula above the total theoretical mass loss is 44.95%. The total
mass loss of the decomposition steps attributed to water loss is
44.50%. The theoretical and experimental mass losses are in
excellent agreement. A small mass loss of <0.5% at 500 ◦C is
observed in Fig. 2. This mass loss is attributed to the presence
of a trace of jarosite in the natural mineral.
(Mn0.392+, Mg0.48, Zn0.13)SO4·Al2(SO4)3·22H2O
→ (Mn0.392+, Mg0.48, Zn0.13)SO4·Al2(SO4)3 + 22H2O
These steps represent the loss of water and are confirmed by the
ion current curves for H2O and OH (Fig. 3). The maxima for the
ion current curves for OH and H2O are observed at 50 and 88 ◦C.
The low temperature mass loss is due to adsorbed water [47].
The mass loss in the 50–88 ◦C is attributed to weakly hydrogen
bonded water. The mass loss at 50 ◦C is 17.69% and at 88 ◦C
is 15.2%. There is an initial mass loss up to 50 ◦C of 8.25%
which is also attributed to the water mass loss. A small mass
Steps 4, 5 and 6 temperatures of 643, 710 and 778 ◦C.
The thermal decomposition steps at 643, 710 and 778 ◦C
are attributed to the decomposition of the sulphate anions
in the apjohnite. Such decomposition is confirmed by the
ion current curves of SO2 where maxima at 643, 710 and
778 ◦C are observed. Similar maxima are observed in the ion
current curves of oxygen. The following decomposition is
proposed
(Mn0.642+, Mg0.28, Zn0.08)SO4·Al2(SO4)3
→ Al2O3 + 0.64MnO + 0.28MgO + 0.08ZnO + 4SO3
and
1
2
4SO3 → 4SO2 + O2
Thecalculatedmasslossbasedupontheformulaforapjohnite
above is 36.3% which compares well with the measured mass
loss of 34.74%.
3.2.2. Halotrichite
The thermogravimetric and differential thermogravimetric
analysis of halotrichite are shown in Fig. 4. The associated mass
spectrometric analysis is reported in Fig. 5. Seven major thermal
decomposition steps are observed.
Steps 1, 2, 3 and 4 temperature 53, 72 and 330 ◦C.
A mass loss of 14.28% is observed at quite low temperatures
around or up to 42 ◦C. A further mass loss of 9.20% occurs at
53 ◦C and 18.91% up to 300 ◦C. A small mass loss of 3.18% is
observed at 330 ◦C. Each of these thermal decomposition steps
is attributed to dehydration of the halotrichite. The ion current
curves for halotrichite of H2O and OH units show maxima at 72
and 330 ◦C. Using the formula.
(Fe0.752+,Mg0.25)SO4·Al2(SO4)3·22H2O the total theoretical
mass loss for water is 44.90%. The total experimental mass loss
is 45.57% which is in good agreement. The water mass loss
Fig. 3. Evolved gas analysis for apjohnite.

68
A.J. Locke et al. / Thermochimica Acta 459 (2007) 64–72
Fig. 4. TG and DTG of halotrichite.
steps appear to be at slightly lower temperatures for halotrichite
in comparison to that of apjohnite.
O2. The mass losses at these temperatures are 1.69, 19.03, 7.72
and 7.21% making a total of mass loss of 35.65%. The theoreti-
cal mass loss calculated using the above formula is 36.29%. The
measured mass loss is in agreement with the theoretical mass
loss. This agreement confirms the formula of halotrichite. If the
formula was different then the numbers would not be in such
close agreement. These decomposition steps are confirmed by
the ion current curves of the evolved gases SO2 and O2 where
ion current maxima at 546, 625, 697 and 738 ◦C are observed.
The maxima for evolved SO3 are small but are observed at the
same temperatures.
Steps 4, 5, 6, 7 and 8 temperatures of 546, 625, 697 and
738 ◦C.
Four thermal decomposition steps are observed for halotri-
chite of the above formula at 546, 625, 697 and 738 ◦C. Each of
these steps is assigned to the decomposition of sulphate anions,
the formation of the metal oxides and the evolution of SO2 and
3.2.3. Pickeringite
The thermogravimetric and differential thermogravimetric
analysis of pickeringite are shown in Fig. 6. The associated mass
spectrometric analysis is reported in Fig. 7. Seven major thermal
decomposition steps are observed.
Steps 1, 2,3 and 4 temperature 44, 83 and ∼300 ◦C.
For pickeringite two sharp thermal decomposition steps are
observed in the DTG curves at 44 and 83 ◦C. A very small
decomposition step at around 300 ◦C is also observed. The
mass losses at 44 and 83 ◦C are 19.45 and 20.35%. All these
three steps are ascribed to the dehydration of the pickerin-
gite. The ion current curves for H2O and OH, i.e. m/z ratios
of 18 and 17, respectively, show ion current maxima at 44
and 83 ◦C confirming the loss of water at these temperatures.
By using the formula calculated from the EDX measurements
of (Fe0.222+,Mg0.78)SO4·Al2(SO4)3·22H2O a total theoretical
mass loss of 45.78% is estimated. The total mass loss due to
dehydration is 39.80% which is in reasonable agreement with
the theoretical mass loss. Any impurities in the pickingerite min-
eral sample such as gypsum and jarosite will affect these values.
In particular their presence will be reflected in lower values that
that predicted from the theoretical values.
Steps 5, 6, 7 and 8 temperatures of 516, 631, 663 and 730 ◦C.
Four higher temperature mass loss steps at 516, 631, 663 and
730 ◦C are observed for pickeringite. Mass losses of 4.37, 8.46,
5.73 and 10.46% are found for these steps making a total mass
Fig. 5. Evolved gas analysis for halotrichite.

A.J. Locke et al. / Thermochimica Acta 459 (2007) 64–72
69
Fig. 6. TG and DTG of pickeringite.
loss of 29.02%. This value is low compared with the theoretical
mass loss of total sulphate as SO3 of 36.99%.
The total mass loss for sulphate decomposition can be calcu-
lated from the equation and the proposed formula from
through a number of factors (a) the mineral is a natural mineral,
(b) the mineral contains low amounts of impurities and (c) the
mass loss of adsorbed water is not taken into account in the total
measured mass loss.
2SO3 → 2SO2 + O2
3.2.4. Wupatkiite
The thermogravimetric and differential thermogravimetric
analysis of wupatkiite are shown in Fig. 8. The associated mass
spectrometric analysis is reported in Fig. 9. Five major thermal
decomposition steps are observed. The DTG plots for wupatki-
ite appear similar to that of the other halotrichites in the low
temperature region but are different in the higher temperature
range.
The total measured mass loss is 68.82 whilst the theoreti-
cal mass loss is 82.77%. This difference may be accounted for
Steps 1, 2, 3 and 4 temperature 51, 76 and 317 ◦C.
Thermal decomposition steps at 51, 76 and 317 ◦C are
observed with mass losses of 17.10, 10.49 and 11.30%. The ion
current curves confirm the loss of water at these temperatures.
There is a gradual mass loss of 7.55% over the 120–250 ◦C tem-
perature range. The total mass loss attributed to dehydroxylation
is 46.44%. The theoretical mass loss according to the formula
(Fe0.552+,Co0.45)SO4·Al2(SO4)3·22H2O is 44.44%. The experi-
mentally determined result and the theoretical value are in good
agreement.
Steps 5, 6, 7 and 8 temperatures of 612 and 640 ◦C.
In contrast to the thermal decomposition of the other halotri-
chites studied in this work one sharp DTG peak is observed at
612 with a second peak at 640 ◦C. The mass losses at these two
temperatures are 22.69 and 4.55%. The theoretical mass loss for
wupatkiite is 35.91%. The ion current curves for evolved SO2
show maxima at 614 and 640 ◦C. These two temperatures are
supported by the ion current curves for O2 where another ion
current SO2 peak is observed at 570 ◦C; a small DTG peak is
observed at this temperature.
3.3. Comparison with paragenically related minerals
A comparison may be made with jarosites [48–53]. Mass
loss steps of K jarosite occur over the 130–330 and 500–622 ◦C
temperature range and are attributed to dehydroxylation and
Fig. 7. Evolved gas analysis for pickeringite.

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A.J. Locke et al. / Thermochimica Acta 459 (2007) 64–72
Fig. 8. TG and DTG of wupatkiite.
desulphation. The thermal decomposition of Na-jarosite shows
three mass loss steps at 215–230, 316–352 and 555–595 ◦C. For
Pb-jarosite two mass loss steps associated with dehydroxylation
are observed at 390 and 418 ◦C and a third mass loss step at
531 ◦C is attributed to the loss of SO3. For argentjarosite, dehy-
droxylation occurs in three stages at 228, 383, 463 ◦C with the
loss of 2, 3 and 1 hydroxyl units. Loss of sulphate occurs at
548 ◦C and is associated with a loss of oxygen. At 790 ◦C loss
of oxygen only leaves metallic silver and hematite.
The decomposition steps for lead jarosite (as an example of
jarosites) may be given as:
Step 1 Temperature 390 ◦C
PbFe6(SO4)4(OH)12 → PbO2Fe6(SO4)4(OH)8 + 2H2O
Step 2 Temperature 418 ◦C
PbO2Fe6(SO4)4(OH)8 → PbO6Fe6(SO4)4 + 4H2O
Step 3 Temperature 531 ◦C
PbO6Fe6(SO4)4 → PbSO4 + 3SO3 + 3Fe2O3
Step 4 Temperature 759 ◦C
PbSO4 → PbO + SO3
An early study using TGA methods showed that the plumbo-
jarosite decomposed at 500 ◦C [17]. Another study suggested
that the jarosite was completely dehydrated by 450 ◦C [54]. It
was found that at temperatures above 550 ◦C further decompo-
sition occurred with the loss of water and sulphur trioxide [54].
The thermal decomposition was complete by 950 ◦C. The final
products were a mixture of hematite and PbO [54].
It is obvious that the halotrichites decompose at significantly
higher temperatures than jarosites.
Evaporite minerals such as are found in the El Jaroso Ravine,
Sierra Almagrera, Spain include halotrichites and jarosites as
well as the sulphates of Fe, K, Ca and others. The importance
of halotrichite and jarosite formation and its decomposition
depends upon its presence in mine tailings, soils, sediments and
evaporitedeposits [55]. Thesetypesof depositsforminacidsoils
where the pH is less than 3.0 pH units [56]. Such acidification
results from the oxidation of pyrite which may be from bac-
terial action or through air-oxidation. The Mars mission rover
known as Opportunity has been used to discover the presence
Fig. 9. Evolved gas analysis for wupatkiite.

A.J. Locke et al. / Thermochimica Acta 459 (2007) 64–72
71
of jarosite on Mars, thus providing evidence for the existence or
pre-existence of water on Mars. Interest in evaporite minerals
and their thermal stability rests with the possible identification
of these minerals and related dehydrated paragenetically related
minerals on planets. The existence of these minerals on planets
would give a positive indication of the existence or at least pre-
existence of water on Mars. Further such minerals are formed
through crystallization from solutions.
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(1953) 1079.
4. Conclusions
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p. 769.
[27] J.E. Dutrizac, J.L. Jambor, Chapter 8 Jarosites and their application in
hydrometallurgy, 2000, p. 405.
Thermogravimetric and differential analysis of mineral
known as pseudoalums including halotrichites, apjohnite,
pickeringite and wupatkiite. EDX analysis shows the chem-
ical formula of the minerals to be (Fe0.752+,Mg0.25)SO4·Al2
(SO4)3·22H2O,(Mn0.642+,Mg0.28,Zn0.08)SO4·Al2(SO4)3·22H2O,
(Fe0.222+,Mg0.78)SO4·Al2(SO4)3·22H2O, (Co0.45,Fe0.552+)SO4
·Al2(SO4)3·22H2O, respectively. X-ray diffraction showed the
minerals to be phase pure except for pickingerite which showed
the presence of gypsum.
The halotrichite minerals showed in general three low tem-
perature thermal decomposition steps attributed to dehydration.
A fourth dehydration step at around 317 ◦C was observed and
was assigned to water trapped within the halotrichite structure.
Depending on the halotrichite 2, 3 or 4 higher temperature ther-
mal decomposition steps are observed. For halotrichite thermal
decomposition steps are observed at 546, 625, 697 and 738 ◦C
and are attributed to the decomposition of sulphate anions to SO3
and consequentially to SO2 and 1/2O2. A comparison with the
thermal decomposition of jarosites shows that the halotrichites
decompose at higher temperatures.
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Acknowledgments
[37] R.L. Frost, A.W. Musumeci, J.T. Kloprogge, M.O. Adebajo, W.N. Martens,
J. Raman Spectrosc. 37 (2006) 733.
[38] R.L. Frost, J. Cejka, M. Weier, W.N. Martens, J. Raman Spectrosc. 37
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[39] R.L. Frost, M.L. Weier, J. Cejka, J.T. Kloprogge, J. Raman Spectrosc. 37
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The financial and infrastructure 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 Thermal Analysis Facilty.
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