Studies on synthetic galloalunites AGa3(SO4) 2(OH)6: Synthesis, thermal analysis, and X-ray characterization

Article abstract of DOI:10.1016/j.tca.2011.04.013

Stoichiometric end member galloalunites of the general formula AGa ₃(SO₄)₂(OH)₆, with A = Na ⁺, K⁺, Rb⁺, H₃O⁺, and NH₄⁺ have been synthesized under hydrothermal conditions. These galloalunites were characterized by chemical methods, thermal analysis (DSC, TG coupled with mass spectroscopy), and powder X-ray diffraction (XRD). The stages of thermal decomposition of sodium, potassium and rubidium galloalunite show a common decomposition mechanism forming β-Ga ₂O₃ and A₂SO₄ (A = Na⁺, K⁺, and Rb⁺) while ammonium and oxonium galloalunite decompose under formation of pure β-Ga₂O₃. The thermogravimetric results confirmed the analytical results on the galloalunites and thereby verified the stoichiometry of these synthetic products. Galloalunites with different monovalent cations in A site (i.e. Na⁺, K⁺, Rb⁺, H₃O⁺ and NH ₄⁺) crystallize in the rhombohedral space group R3m (#166). The effects of substitution on the unit cell parameters are rationalized in terms of the structural arrangements in galloalunites. The unit cell parameter c increases with increasing effective ionic radii of the cation in the A site, whereas the parameter a changes to a much lesser degree.

Full text of DOI:10.1016/j.tca.2011.04.013

Thermochimica Acta 521 (2011) 112–120
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Thermochimica Acta
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Studies on synthetic galloalunites AGa3(SO4)2(OH)6: Synthesis, thermal analysis,
and X-ray characterization
Wolfram W. Rudolpha, Peer Schmidtb,∗
a TU Dresden, Medizinische Fakultät, Institut für Virologie im MTZ, Fiedlerstr. 42, D-01307 Dresden, Germany
b University of Applied Sciences Lausitz (FH), Dept. Biology, Chemistry and Process Technology, P.O. Box 101548, D-01958 Senftenberg, Germany
a r t i c l e i n f o
a b s t r a c t
Stoichiometric end member galloalunites of the general formula AGa3(SO4)2(OH)6, with A = Na+, K+, Rb+,
Article history:
+
H3O+, and NH4 have been synthesized under hydrothermal conditions. These galloalunites were char-
Received 10 February 2011
Received in revised form 12 April 2011
Accepted 14 April 2011
acterized by chemical methods, thermal analysis (DSC, TG coupled with mass spectroscopy), and powder
X-ray diffraction (XRD).
Available online 23 April 2011
The stages of thermal decomposition of sodium, potassium and rubidium galloalunite show a common
decomposition mechanism forming ␤-Ga2O3 and A2SO4 (A = Na+, K+, and Rb+) while ammonium and
oxonium galloalunite decompose under formation of pure ␤-Ga2O3. The thermogravimetric results con-
firmed the analytical results on the galloalunites and thereby verified the stoichiometry of these synthetic
products.
Keywords:
Synthesis
Galloalunites
X-ray diffraction
Unit cell parameters
Thermal analysis
Galloalunites with different monovalent cations in A site (i.e. Na+, K+, Rb+, H3O+ and NH4+) crystallize
in the rhombohedral space group R3m (#166). The effects of substitution on the unit cell parameters are
rationalized in terms of the structural arrangements in galloalunites. The unit cell parameter c increases
with increasing effective ionic radii of the cation in the A site, whereas the parameter a changes to a much
lesser degree.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
is mineralogical evidence for aqueous processes on Mars, probably
under acid-sulfate conditions [12]. Jarosite also plays an important
Minerals of the alunite-jarosite group have the general formula
role as an antiferromagnetic material with interesting magnetic
behaviour [13]. These are but a few reasons to study compounds
of this assemblage.
AB3(SO4)2(OH)6, where A is H3O+, Na+, K+, Rb+, Ag+, Tl+, NH4
,
+
1/2Ca2+ or 1/2 Pb2+ and B is Al3+ (alunite group) or Fe3+ (jarosite)
[1–4]. Members of this group are isostructural, belonging to the
hexagonal space-group R3m [5,6].
In this paper, we deal exclusively with members of the galloalu-
nite group AGa3(SO4)2(OH)6 expanding on our study on synthetic
alunites [14]. The synthetic alunites and jarosites [14–16] are prone
to form nonstoichimetric products on both the A and B sites. Oxo-
nium ions commonly replace alkali metal ions in the A site, while
deficiencies in the B site of Al3+ are charge balanced by protona-
tion of the hydroxyl groups to form water. This leads to occupancy
of the Al3+ site that is typically in the 82–97% range. Therefore,
special care was taken in applying synthesis conditions that allow
production of stoichiometric products [14,15]. After our studies on
synthetic alunites [14] we advance further to the investigation of
galloalunite end members with oxonium, Na+, K+, Rb+ and ammo-
nium as monovalent cations (A(I)). Galloalunite, KGa3(SO4)2(OH)6
sensu stricto, is the end member of gallium bearing alunites but,
for clarity, we will name it potassium galloalunite. The stoichiom-
etry of these galloalunite compounds has been determined by
chemical methods and thermal analysis. Structural studies have
been carried out using powder X-ray diffraction and Rietveld
refinement.
The basic gallium sulfate (H3O)Ga3(SO4)2(OH)6 described some
time ago, is not a naturally occurring mineral but its structure
is isostructural with alunite [7]. Gallium accompanies Fe3+ and
Al3+ in their compounds and recently natural jarosite samples
with approximately 1000–3000 ppm Ga3+ replacing Fe3+ in the B
site were described [8]. The isomorphous substitution of Al3+ and
Ga3+ ions in the alunite structure of synthetic compounds were
described as well [9]. These chemically and thermally stable mate-
rials are of wide interest in metallurgy [10] and for fixation of
toxic or radioactive ions for depositional purposes [11]. Further-
more, jarosite has been detected on Mars and because hydroxyl
ions are present in its structure its occurrence at Meridiani Planum
∗
Corresponding author. Tel.: +49 357385827.
E-mail addresses: wolfram.rudolph@tu-dresden.de (W.W. Rudolph),
peer.schmidt@hs-lausitz.de (P. Schmidt).
0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2011.04.013

W.W. Rudolph, P. Schmidt / Thermochimica Acta 521 (2011) 112–120
113
Table 1
Synthesis conditions for galloalunite end members (oxonium galloalunite (H3O-Ga-alunite), D3O-galloalunite (d-Ga-alunite-d6), Na-galloalunite, K-galloalunite, NH4-
galloalunite, Rb-galloalunite, and K-galloalunite-d6).
Run no.
Ga alunite
Sample
Starting solution compositions
Synthesis conditions
Yield (g)
pH super-natant
m
m
m
pH
ϑ (◦C)
p/bar
Time (h)
A2SO4#/g
Ga2(SO4)3/g
H2Ototal/g*
G01
H3O-Ga-alunite
G02
D3O-Ga-alunite-d6
G03
Na-Ga-alunite
G04
K-Ga-alunite
G05
NH4-Ga-alunite
G06
Rb-Ga-alunite
G07
0.147
0.102
0.715
0.805
0.725
1.590
0.760
1.617
1.605
1.936
1.983
2.010
1.860
1.956
29.215
28.620
15.450
14.850
22.200
14.100
14.100
0.89
–
170
170
150
150
150
150
150
15
904
904
344
344
344
342
324
1.19
0.30
–
12.9
12.9
12.9
12.9
12.9
12.9
1.13
0.85
0.87
0.87
0.86
–
1.627
1.583
1.482
3.866
1.550
0.21
0.12
0.11
0.12
–
K-Ga-alunite-d6
# A2SO4—G01: H2SO4; G02: D2SO4; G03: Na2SO4; G04: K2SO4; G05: (NH4)2SO4; G06: Rb2SO4; G07: K2SO4.
*D2O was used in case of compound # G02 and # G07.
2. Experimental
[18]. The galloalunites were dried for 12–14 h in a vacuum appa-
ratus under the following temperatures: oxonium galloalunites at
190 ◦C, ammonium galloalunites at 220 ◦C, potassium galloalunites,
sodium galloalunites and rubidium galloalunites at 250 ◦C. After
this drying procedure, the galloalunites contained only minute
amounts of excess water (<0.2 wt %). The solid products were cooled
in a desiccator and stored for further investigation. The synthesis
conditions for all of the galloalunites are given in Table 1.
Syntheses of oxonoium galloalunite ((H3O)Ga3(SO4)2(OH)6) and
potassium-galloalunites (KGa3(SO4)2(OH)6) were carried out twice
in order to check the synthesis procedure as well as the whole ana-
lytical procedure (from acid digestion to analytical procedure) and
to make sure stoichiometric compounds were produced.
2.1. Synthesis of galloalunite samples
Synthesis conditions are important in producing stoichiometric
samples and the hydrothermal synthesis applied has been pub-
lished in some detail elsewhere [14]. Galloalunites were prepared
from commercial grade Ga2(SO4)3·18H2O after further purification:
Ga2(SO4)3·18H2O was precipitated with p.a. ethanol from the aque-
ous solution, dried over CaCl2 and used to prepare a Ga2(SO4)3 stock
solution. The Ga3+ and sulfate content of the stock solution were
determined as described in detail by Rudolph and Pye [17]. The
stock solution was filtered through a 0.45 ␮m Millipore filter. The
concentration of the stock solution was 0.98 mol/L (1.012 mol/kg).
Commercial-grade RbSO4, K2SO4, Na2SO4, and (NH4)2SO4 (all salts
of p.a. quality, Merck, Darmstadt) were used for synthesis. The start-
ing solutions for galloalunite synthesis were prepared by weight.
The pH values of the sulfate solution prior to the precipitation of gal-
loalunite were about 0.90, but after complete precipitation, the pH
had dropped to between 0.30 and 0.10 in the supernatants (Table 1)
with the lowest pH for the alkali metal galloalunite end members
and a higher value for the H3O-galloalunite end member.
2.2. Analytical methods
The chemical analysis of galloalunite species was carried out on
samples digested in 4 mol/L HCl (Baker, ultrapure) in a Teflon lined
beaker mantled with a steel jacket at 120 ◦C for 12 h. The gallium,
potassium, sodium, rubidium and sulfur contents were determined
with inductively coupled plasma optical emission spectroscopy
(ICP-OES). The instrument used for all analyses of Ga, S, Na, K, and
Rb was an ARL 3520 sequential ICP-OES. Further details about the
wet chemical procedure and ICP-OES analysis may be found in Ref.
[14].
A fully deuterated D3O+-galloalunite-d6 has been synthesized
from a Ga2(SO4)3/D2O stock solution in D2O (99.9 wt% D; Merck,
Darmstadt) with ca. 0.1 g 96 wt% D2SO4 (99 wt% D; Aldrich) yielding
a product with a deuteration degree better than 95%. A deuterated
potassium galloalunite, KGa3(SO4)2(OD)6 has been synthesized
from potassium sulfate and a Ga2(SO4)3/D2O solution in D2O. The
potassium galloalunite-d6 had a deuteration degree better than
98%.
The synthesis was carried out under hydrothermal conditions
in quartz ampoules containing ca. 15 ml solution. The optimal tem-
perature conditions for producing stoichiometric galloalunite inter
alia alunite were determined and described in detail previously [14]
and therefore only a brief account is given. Synthesis temperatures
of 150–160 ◦C were chosen and the synthesis was maintained for
up to approximately 340 h in order to improve crystallinity and to
reach equilibrium. After quenching the reaction vessel in ice water,
the crystalline galloalunite samples were isolated and washed with
bi-distilled water and dried. In contrast, the deuterated galloalunite
samples were washed with D2O. From preliminary thermal analy-
sis studies, it became clear that the drying procedure is crucial to
eliminate the remaining amounts of adsorbed or occluded water. It
is known that oxonium alunite, for instance, adsorbs a few wt.-% of
water, which is responsible for the anomalous proton conduction
The thermogravimetric (TG) and thermal analysis (DSC) was
carried out in a simultaneous thermal analyser (STA 409 Luxx)
manufactured by NETZSCH. The gas outlet of the thermogravimet-
ric analysis device was directly coupled with a mass spectrometer
(Äolos, NETZSCH; (m/z)max = 300). Measurements were carried out
in the temperature range from ϑ = 25 to 1100 ◦C with a heating rate
of 10 K/min in an atmosphere of air. The decomposition temper-
ature, derived from the first decomposition step, was determined
at the onset of the corresponding thermal effect, where the mate-
rial starts to decompose into new solids and gaseous products. This
temperature has been reported to be the decomposition tempera-
ture which is lower than the maximum of the thermal peak of the
DSC curve; the decomposition temperatures of the galloalunites
are given below. The analysis of the gaseous products carried out
with the mass-spectrometer has been used to verify the sequence
of decomposition. The composition of the gas phase has been
detected in a first measurement run with an overall scan in the mass
range m/z = 1,. . .,300 and the fragments of the gaseous species with
m/z = 14 (N+), 15 (NH+) for the ammonium compound in addition to
m/z = 16 (O+, NH2+), 17 (OH+, NH3+), 18 (H2O+), 48 (SO+), 64 (SO2
)
+

114
W.W. Rudolph, P. Schmidt / Thermochimica Acta 521 (2011) 112–120
Fig. 2. TG-curve of the thermal decomposition of Na[Ga3(SO4)2](OH)6 (NETZSCH
STA 409 Luxx + Äolos, atmosphere: air, heating rate: 10 K min−1), measurement of
thermal effects (DSC) during the decomposition and MS trace of the ion current
of fragments of water (m/z = 18 (H2O+); m/z = 17 (OH+)) and of sulfur oxide species
(m/z = 48 (SO+); m/z = 64 (SO2+); m/z = 80 (SO3+)).
Fig. 1. Comparison of TG-curves of the thermal decomposition of galloalunites
AGa3(SO4)2(OH)6 (A = Na+, K+, Rb+, NH4+, and H3O+) (NETZSCH STA 409 Luxx + Äolos,
atmosphere: air, heating rate: 10 K min−1).
and 80 (SO3+) for all the galloalunites have been detected. No car-
bon containing species (CHx, CO, and CO2) could be ascertained. A
second run of the TG–MS measurement has been performed for all
samples in order to detect the temperature dependent intensities
of the corresponding fragments of the species.
Powder XRD patterns of the solid products of synthesized com-
pounds were collected with a SIEMENS D5000 X-ray diffractometer
using Cu K␣-radiation (reflexion mode) using an accelerating volt-
age of 40 kV and a filament current of 25 mA. The diffractometric
scans were obtained over the 2ꢀ range between 5◦ and 110◦ at a
scan speed of 0.875◦/min. The crystal structure parameters and lat-
tice constants have been refined using the program package Powder
Cell [19].
3. Results
Details on starting solutions and hydrothermal synthesis
conditions used for the preparation of these end mem-
bers: (H3O)Ga3(SO4)2(OH)6, KGa3(SO4)2(OH)6, NaGa3(SO4)2(OH)6,
(NH4)Ga3(SO4)2(OH)6, and RbGa3(SO4)2(OH)6 are presented in
Table 1. Analytical results of the end member galloalunites and the
determined stoichiometries are presented in Table 2. Thermal data
on these galloalunites are presented in Tables 3–7 for Na+-, K+-
, Rb+-, H3O+-, and NH4+-galloalunites respectively. Fig. 1 depicts
representative TG curves for comparison of thermal decomposi-
tion temperatures of different phases. Figs. 2–6 show always the
complete data set of thermal analysis for Na+, K+, Rb+, H3O+, and
Fig. 3. TG-curve of the thermal decomposition of K[Ga3(SO4)2](OH)6 (NETZSCH STA
409 Luxx + Äolos, atmosphere: air, heating rate: 10 K min−1), measurement of ther-
mal effects (DSC) during the decomposition and MS trace of the ion current of
fragments of water (m/z = 18 (H2O+); m/z = 17 (OH+)) and of sulfur oxide species
(m/z = 48 (SO+); m/z = 64 (SO2+); m/z = 80 (SO3+)).
+
NH4 galloalunites including TG curves, DSC measurements and
mass spectrometric (MS) traces of species resulting from water
(m/z = 17, 18) and sulfurous species (m/z = 48, 64, 80). XRD pat-
terns of gallo-alunites with A =: Na+, K+, Rb+, NH4+, and H3O+ are
presented in Fig. 7. The progress of reflexion positions is marked
for reflexes with h k l-indexes (0 0 3), (0 0 6), (0 0 9), (1 1 0), and
(2 2 0).
Unit cell parameters for the galloalunite end members are pre-
sented in Table 8 and Fig. 8.
3.1. Chemical characterization and thermal analysis studies on
galloalunites
Examination of the SEM photographs shows that the galloalu-
nite samples are polycystalline particles, ranging in size from 9
to 12 ␮m [11]. The absence of unidentified peaks in powder XRD
indicates that no other crystalline phases are present in the pre-
cipitate at detectable levels and only a single solid phase was
Fig. 4. TG-curve of the thermal decomposition of Rb[Ga3(SO4)2](OH)6 (NETZSCH
STA 409 Luxx + Äolos, atmosphere: air, heating rate: 10 K min−1), measurement of
thermal effects (DSC) during the decomposition and MS trace of the ion current
of fragments of water (m/z = 18 (H2O+); m/z = 17 (OH+)) and of sulfur oxide species
(m/z = 48 (SO+); m/z = 64 (SO2+); m/z = 80 (SO3+)).

W.W. Rudolph, P. Schmidt / Thermochimica Acta 521 (2011) 112–120
115
Table 2
Analytical results (ICP-OES) for the end-members and the galloalunite.
Sample
m% Ga
m% S
m% A
m% Ga
m% S
m% A
Formula (OH and excess H2O from TGA)a
Theoretical
Theoretical
Theoretical
Experiment
Experiment
Experiment
G01
H3O-Ga-alunite
G03
Na-Ga-alunite
G04
K-Ga-alunite
G05
NH4-Ga-alunite
G06
Rb-Ga-alunite
40.042
20.537
19.542
20.589
20.333
12.277
16.271
15.482
16.311
16.108
3.642
4.827
9.44
40.30
20.50
19.45
20.60
20.25
12.30
16.20
15.40
16.30
16.09
–
(H3O)Ga3.02(SO4)2.00(OH)6.00
<0.05 H2O
Na1.00Ga2.99(SO4)1.99(OH)6.10
<0.05 H2O
K1.00Ga2.99(SO4)1.99(OH)6.04
<0.10 H2O
(NH4)Ga2.97(SO4)2.00(OH)6.20
<0.05 H2O
–
9.40
–
4.588
5.775
5.75
Rb1.00Ga2.98(SO4)2.00(OH)6.04
<0.20 H2O
a
The formula represent the experimental values and allow an error estimation for the stoichiometry (they are therefore not stoichiometrically balanced). The error in
determining the OH content (and water content) from the TGA curves is unrealistically high because of the error in separating the individual steps. In TA experiments the total
weight loss with respect to the stoichiometric formula is more meaningful. We have carried out two or three independent TA experiments in order to verify our experimental
results and OH content and given is an average.
Table 3
TG results for decomposition reaction of sodium-galloalunite – NaGa3(SO4)2(OH)6; all temperatures in ◦C.
Product
Decomposition step
Tonset (DSC)
Tpeak (DSC)
Tpeak (DTG)
TG range (◦C)
Theor. step (%)
Exp. step (%)
Na[Ga3(SO4)2](OH)6
1/2Na2SO4 + 1/2Ga2(SO4)3 + Ga2O3
1/2Na2SO4 + 3/2Ga2O3
−3.0 H2O
−1.5 SO3
380(2)
587(2)
457(2)
653(2)
456(2)
653(2)
370–490
580–850
−10.3
−22.8
−33.1
−10.0(3)
−23.0(3)
−33.0(6)
Table 4
TG results for decomposition reaction of potassium-galloalunite – KGa3(SO4)2(OH)6; all temperatures in ◦C.
Product
Decomposition step
Tonset (DSC)
Tpeak (DSC)
(DSC)
Tpeak (DTG)
(DTG)
TG range (◦C)
Theor. step (%)
Exp. step (%)
/%
K[Ga3(SO4)2](OH)6
KGa(SO4)2 + Ga2O3
−3.0 H2O
−1.5 SO3
395(2)
576(2)exo
622(2)
482(2)
595(2)exo
685(2)
480(2)
687(2)
390–520
600–850
−10.0
−22.1
−10.3(3)
−22.4(3)
1/2K2SO4 + 3/2Ga2O3
−32.1
−32.7(6)
Table 5
TG results for decomposition reaction of Rb-galloalunite – RbGa3(SO4)2(OH)6; all temperatures in ◦C.
Product
Decomposition step
Tonset (DSC)
Tpeak (DSC)
Tpeak (DTG)
TG range (◦C)
Theor. step (%)
Exp. step (%)
Rb[Ga3(SO4)2](OH)6
RbGa(SO4)2 + Ga2O3
−3.0 H2O
−1.5 SO3
382(2)
589(2)exo
643(2)
453(2)
602(2)exo
704(2)
446(2)
370–495
590–850
−9.2
−9.5(5)
−20.4
−21.0(5)
705(2)
1/2Rb2SO4 + 3/2Ga2O3
−29.6
−30.5(10)
Table 6
TG results for decomposition reaction of oxonium–galloalunite – H3O[Ga3(SO4)2](OH)6; all temperatures in ◦C.
Product
Decomposition step
Tonset (DSC)
Tpeak (DSC)
Tpeak (DTG)
TG range (◦C)
200–470
Theor. step (%)
Exp. step (%)
−3.5 H2O
207(2)
405(2)
361(2)
427(2)
360(2)
424(2)
−12.1
H3O[Ga3(SO4)2](OH)6
−12.0(2)
−1.0 H2O
−0.5 SO3
−1.5 SO3
485(2)
749(2)
675(2)
690(2)
779(2)
671(2)
685(2)
772(2)
470–710
740–820
−11.1
7/6Ga2O3 + 2/3Ga(HSO4)3
−12.0(2)
Ga2O3 + 1/2Ga2(SO4)3
Ga2O3
−23.0
−46.2
−22.0(2)
−46.0(6)
produced (Fig. 7). The analytical and thermal results on these
end member galloalunites (see Table 2) show that only a minute
amount of additional water is present in the synthetic galloalu-
nites. A water content ∼0.2 wt% was detected. Thus the drying
procedure was crucial in order to obtain stoichiometric galloalu-
nites thereby driving off all occluded water. The thermal analysis
shows that adsorbed/occluded water driven off from the galloalu-
nites at temperatures of about 250 ◦C up to max. 340 ◦C for Na+-,
K+- and Rb+-galloalunite can be carried out without destroying
the structure, i.e. resulting in stoichiometric galloalunites without
significant “free” water content. An exception applying such high
drying temperatures constitutes the oxonium galloalunite which
would decompose at this temperature. Therefore this sample has
to be dried at temperatures not higher than 190 ◦C. The ammonium
galloalunite has been dried at 220 ◦C. Besides the minute amount
of excess water, the ratio A(I): Ga:S shows that our synthetic gal-
loalunites are stoichiometric and no gallium deficient compounds
resulted.
In contrast to our results Tananaev et al. [20] reported
chemical analysis data on galloalunites which showed severe non-
stoichiometric composition with respect to the A site and lack of
full gallium occupancy resulting in compounds with additional
water. Our synthetic basic gallium sulfate of the alunite type com-
pares well with the synthetic product described by Johansson [7].
However, non-stoichiometric compositions have been described
for synthetic alunites by Ripmeester et al. [21].

116
W.W. Rudolph, P. Schmidt / Thermochimica Acta 521 (2011) 112–120
Table 7
TG results for decomposition reaction of ammonium-galloalunite – NH4[Ga3(SO4)2](OH)6; all temperatures in ◦C.
Product
Decomposition step
Tonset (DSC)
Tpeak (DSC)
Tpeak (DTG)
TG range (◦C)
320–490
Theor. step (%)
Exp. step (%)
−3.0 H2O
−1.0 NH3
−13.6
NH4[Ga3(SO4)2](OH)6
325(2)
426(2)
425(2)
−13.5(4)
−0.5 H2O
−0.5 SO3
490–610
610–820
−9.4
−7.0a
Ga2O3 + GaSO4(HSO4)
510(2)
615(2)
579(2)
579(2)
−1.5
675(2)
737(2)
781(2)
674(2)
740(2)
782(2)
−23.0
−32.4
−25.9
Ga2O3 + 1/2
Ga2(SO4)3
SO3
−32.9(6)
Ga2O3
−46.0
−46.8(10)
a
Incomplete decomposition step; overlapped with next step.
Table 8
Unit cell parameters and the crystal data for the end member galloalunites.
Sample
Run no.
a (Å)
c (Å)
V (Å3)
c/a
D(x) (g/cm3)
H3O-Ga-alunite
Na-Ga-alunite
K-Ga-alunite
NH4-Ga-alunite
Rb-Ga-alunite
G01
G03
G04
G05
G06
7.184(1)
7.140(1)
7.138(1)
7.162(1)
7.164(1)
17.169(2)
767.59(6)
738.58(6)
764.42(6)
788.32(6)
792.18(6)
2.3896
2.3430
2.4270
2.4785
2.4879
3.390
3.550
3.535
3.295
3.703
16.729(2)
17.324(2)
17.751(6)
17.823(2)
Fig. 5. TG-curve of the thermal decomposition of H3O[Ga3(SO4)2](OH)6 (NETZSCH
STA 409 Luxx + Äolos, atmosphere: air, heating rate: 10 K min−1), measurement of
thermal effects (DSC) during the decomposition and MS trace of the ion current
of fragments of water (m/z = 18 (H2O+); m/z = 17 (OH+)) and of sulfur oxide species
(m/z = 48 (SO+); m/z = 64 (SO2+); m/z = 80 (SO3+)).
Fig. 7. XRD pattern (SIEMENS D5000; Cu K␣; reflexion mode) of gallo-alunites
A[Ga3(SO4)2](OH)6 with A = alkali metal ions: Na+, K+, Rb+ and NH4+, and H3O+. The
progress of reflexion positions is marked for reflexes with h k l-indexes (0 0 3), (0 0 6),
(0 0 9) and (1 1 0), (2 2 0).
Fig. 6. TG-curve of the thermal decomposition of NH4[Ga3(SO4)2](OH)6 (NETZSCH
STA 409 Luxx + Äolos, atmosphere: air, heating rate: 10 K min−1), measurement of
thermal effects (DSC) during the decomposition and MS trace of the ion current of
fragments of water and ammonia (m/z = 18 (H2O+); m/z = 17 (OH+; NH3+)) and of
sulfur oxide species (m/z = 48 (SO+); m/z = 64 (SO2+); m/z = 80 (SO3+))..
Fig. 8. Progress of lattice constants of gallo-alunites A[Ga3(SO4)2](OH)6 with
A = alkali metal ions: Na+, K+, Rb+ and NH4+, H3O+.

W.W. Rudolph, P. Schmidt / Thermochimica Acta 521 (2011) 112–120
117
+
The thermogravimetric measurements of dried powder samples
reinforce the analytical results in that there are no problems of
“non-stoichiometry” concerning “excess water” and lack of gallium
occupancy in the B site. The decomposition of galloalunites shows
two steps with the exception of oxonium and ammonium galloalu-
nite. The decomposition of the latter two galloalunite compounds
will be discussed separately. The first step reflects the theoretical
amount of water resulting from the OH-groups for all galloalunite
samples, according to Eq. (1). The second step reflects the loss of
sulfur oxides. The processes of thermal decomposition will be dis-
cussed in detail later but first the total mass loss of the galloalunite
decompositions will be discussed because it allows one to draw
conclusions about the stochiometry of the synthetic compounds.
The experimental total mass loss of H2O, (NH3), and SO3
reported in Tables 3–7 confirms the composition of the initial
products in accordance with their stoichiometry AGa3(SO4)2(OH)6
(A = Na+, K+, Rb+, H3O+ and NH4+). For Na-galloalunite a weight
loss of 33.0(6) wt% was detected which compares well with the
theoretical value of 33.1 wt%; similarly for K-galloalunite the exper-
imental value of 32.7(6) wt% compares well with the theoretical
value of 32.1; for Rb-galloalunite the found value is 30.5(10) and
the theoretical value is 29.6 wt%. For oxonium-galloalunite which
decomposes in a more complicated manner than the alkali metal
galloalunites the found the value at 46.0(8) wt% and compares
favorably with its theoretical value of 46.2 wt%. The same is true
for ammonium-galloalunite with a measured value of 46.8(10) wt%
and a theoretical one at 46.0 wt%. Therefore, the weight losses
compare favorably with the theoretical values of the galloalunite
compounds within the experimental uncertainty again verifying
the stoichiometry of the compounds.
The detected fragments are SO+ (m/z = 48), SO2 (m/z = 64) and
+
small amounts of SO3 (m/z = 80) (Figs. 2–4). The actual ratio of
molecules of decomposition cannot be experimentally quantified
from the ratio of the sulfoxide-species in the mass-spectrometer
due to their fragmentation in the ionization chamber. The equi-
librium constant Kp = 0.6 (at T = 1000 K) can be obtained from
thermodynamic equilibrium calculations using the data from [24],
and a theoretical ratio p(SO2) ≈ 2·p(SO3) results (3). Furthermore,
the signal m/z = 80 is accompanied by the signal m/z = 18 which
may suggest the formation of H2SO4. The mass spectrum of H2SO4
contains also m/z = 80.
The presence of SO in the gas is originated from the
destruction of SO2 molecules in the ionisation chamber of
the mass spectrometer. Although the release of SO(g) as
a
primary product of the decomposition of potassium alunite
K[Ga3(SO4)2](OH)6 was discussed [25], thermodynamic cal-
culations substantiate, that the equilibrium constant of the
formation of SO(g) = (2/3SO2(g) = 2/3SO(g) + 1/3O2(g): Kp ≈ 10−7) is
very low.
Finally, sublimation of alkali metal sulfates could not be
detected within the investigated temperature range; the equi-
librium partial pressures are calculated for compound data [24]:
p(A2SO4)1300 < 10−5 bar.
1/2Na2SO4(s) + 1/2Ga2(SO4)3(s) + Ga2O3(s) → 1/2Na2SO4(s)
+ 3/2Ga2O3(s) + 3/2SO3(g) (SO2(g) + 1/2O2(g)
)
(2a)
AGa(SO4)2(s) + Ga2O3(s) → 1/2A2SO4(s) + 3/2Ga2O3(s)
+ 3/2SO3(g) (SO2(g) + 1/2O2(g) (A = K, Rb)
The alkalimetal galloalunites (A = Na+, K+, Rb+; see Figs. 1–4;
Tables 3–5) decompose in two steps similar to the alunite coun-
terparts [14]. In the first step, the dehydration step, release of
water from the OH− groups and formation of A2SO4, Ga2(SO4)3
and Ga2O3 (1) is observed (Tables 3–5). The detected mass-
spectrometric fragments of the molecule H2O(g) are OH+ (m/z = 17)
and H2O+ (m/z = 18); Figs. 2–4. The onset temperatures of the first
(endothermic) step taken from the DSC curve occurs at 380(2) ◦C for
Na-galloalunite, at 395(2) ◦C for K-galloalunite, and at 382(2) ◦C for
Rb-galloalunite. The formal decomposition reaction may be sum-
marized by Eq. (1):
)
(2b)
(3)
2/3SO3(g) → 2/3SO2(g) + 1/3O2(g)
The decomposition of oxonium-galloalunite and ammonium-
galloalunite shows notably an earlier effect due to the thermody-
namic gain from the formation of the gaseous products such as
H2O and NH3, respectively; see Fig. 1. The onset temperatures of
the first (endothermic) step lie at 207(2) ◦C for H3O+-galloalunite
and 325(2) ◦C for NH4+-galloalunite. Basically, the decomposition
steps for (H3O)Ga3(SO4)2(OH)6 and NH4Ga3(SO4)2(OH)6 are more
strongly structured although the two steps characteristic of the
galloalunite decomposition, dehydration and de-sulfation are still
separated from each other. Dealing first with oxonium galloalunite
(Fig. 5, Table 6) reveals that the decomposition can be divided into
three steps:
A[Ga3(SO4)2](OH)6 → 1/2A2(SO4)(s) + 1/2Ga2(SO4)3(s)
+ Ga2O3(s) + 3H2O(g) (A = Na, K, Rb)
(1)
In the cases of potassium and rubidium the intermediate phases
A2SO4(s) + Ga2(SO4)3(s) combine in an exothermic reaction to form
AGa(SO4)2 (A = K, and Rb respectively [22]). The corresponding ther-
mal effect has been observed at ϑ = 576(2) ◦C for KGa(SO4)2 (Fig. 3,
Table 4) and ϑ = 589(2) ◦C for RbGa(SO4)2 (Fig. 4, Table 5) and the
phases formed have been identified by powder XRD. The XRD mea-
surements were taken after halting the thermal analysis after the
first decomposition step. These measurements were carried out in
an independent TG measurement. Similar exothermic effects have
been determined in the same manner for the alunite phases con-
taining potassium and rubidium [14,23]. The analogous compound
is unknown for sodium; accordingly no exothermic effects could be
detected for this system (Fig. 2, Table 3).
H3O[Ga3(SO4)2](OH)6 → 7/6Ga2O3 + 2/3Ga(HSO4)3 + 7/2H2O(g)
(4)
7/6Ga2O3 + 2/3Ga(HSO4)3 → Ga2O3 + 1/2Ga2(SO4)3 + H2O(g)
+ 1/2SO3(g) (SO2(g) + 1/2O2(g)
)
(5)
Ga2O3 + 1/2Ga2(SO4)3 → 3/2Ga2O3 + 3/2SO3(g) (SO2(g) + 1/2O2(g)
)
(6)
The second steps occur from 587(2) ◦C for Na-galloalunite,
622(2) ◦C for K-galloalunite, and 643(2) ◦C for Rb-galloalunite.
In the process the decomposition of intermediate phases
Na2SO4(s) + Ga2(SO4)3(s) or rather AGa(SO4)2 (A = K, Rb) proceeds
under release of sulfoxides (SO3(g), SO2(g)) and the formation of sta-
ble products A2SO4 and Ga2O3 (see Eq. (2a) for sodium, Eq. (2b) for
potassium, rubidium).
In the first step of the oxonium-galloalunite decomposition
ranging from 200 to 470 ◦C, of which the first interval is broad and
smeared out while the second one is sharp and 3.5 mole water
are lost resulting in the formation of Ga2O3 and Ga(HSO4)3. The
second step, directly preceding the third step, is actually a double
peak with maxima at 675 and 690 ◦C (DSC) resulting in the loss of
one mole of water simultaneously with small amounts of SO2/SO3

118
W.W. Rudolph, P. Schmidt / Thermochimica Acta 521 (2011) 112–120
(Figs. 5 and 6) stemming from 1/2 mole of SO3 (5). In the process
Ga(HSO4)3 decomposes forming the solid products Ga2O3 and 1/2
mol Ga2(SO4)3. Finally in the third step (6), starting above 740 ◦C
the remaining Ga2(SO4)3 is decomposed completely to form Ga2O3
instance shows that there is only a slight increase in c parameter
(ca. 0.2%) but a significant decrease in a parameter (ca. 2.5%).
4. Discussion
and gaseous products SO3(g), SO2(g), and O2(g)
.
In the case of ammonium-galloalunite – NH4[Ga3(SO4)2](OH)6
– three decomposition regions containing submaxima are observed
(Fig. 6; Table 7). The first broad peak (320–490 ◦C) for the
first decomposition step (7) occurs under release of stoi-
chiometric amounts of ammonia and water (dehydration and
de-ammoniation, respectively) namely 3H2O and one NH3 and the
intermediate products (8) formed are Ga2O3 and GaSO4(HSO4). The
second decomposition region (containing two peaks between 490
and 610 ◦C) reflects the incomplete decomposition of GaSO4(HSO4)
leading to the formation of anhydrous gallium sulfate Ga2(SO4)3
under release of water and SO3 (SO2 + 1/2O2). Above 610 ◦C three
peaks appear marking the third decomposition region (peaks at
675, 737 and 781 ◦C, DSC). In this decomposition step the comple-
tion of the second step occurs and also the release of 1.5 mole SO3
(SO2 + 1/2O2) under formation of Ga2O3 (see Fig. 6). The following
process is proposed:
4.1. Chemical composition and thermal analyses
Our synthesis conditions (Table 1) were designed to produce
stoichiometric galloalunites: very long reaction times and optimal
temperature ranges were applied. Recently, stoichiometric alunites
and jarosites with respect to the complete filling of the A(I) site as
well as avoiding the deficiency in the B site were reported. Rudolph
et al. [14] described the synthesis of stoichiometric alunites and
Baciano and Peterson [15] reported the synthesis of stoichiometric
jarosites.
The thermal analysis of the synthetic galloalunites was under-
taken to determine the stoichiometry and purity of the synthetic
products. From TG analysis of the end member galloalunites
(cf. Tables 3–7) it could be established that the galloalunites
are stoichiometric and show the typical decomposition mech-
anism also found in alunites [14,24]. Among the alkali metal
galloalunites, potassium galloalunite is most thermally stable,
NH4[Ga3(SO4)2](OH)6 → Ga2O3 + GaSO4(HSO4) + 3H2O(g) + NH3(g)
while Na-galloalunite and Rb-galloalunite are less stable. Clearly,
potassium fits most favorably in the galloalunite structure. NH4
(7)
+
-
galloalunite and, even less so, the oxonium galloalunite are the least
stable members of the end member galloalunites, Fig. 1.
Ga2O3 + GaSO4(HSO4) → Ga2O3 + 1/2Ga2(SO4)3
Thermal analysis for the end members A[Ga3(SO4)2](OH)6 with
A = H3O+, NH4+, Na+, K+, and Rb+ supply proof for an almost
common decomposition sequence of all prepared galloalunites:
two primary endothermic reactions occurred during the thermal
decomposition of galloalunites: first the dehydration, and second
the desulfurization. The regions could be assigned using coupled
mass spectrometry. A small exothermic step starting directly before
the second endothermic step occurs for potassium and rubidium
galloalunites. This step is due to the formation of double sulfates
such as KGa(SO4)2 and RbGa(SO4)2, respectively. The respective
decomposition of analogous alunite has been discussed recently
by Rudolph et al. [14]. The thermal decomposition data for oxo-
nium galloalunite reinforces the existence of the oxonium ion in
these structures. Thermal analysis on these pure galloalunites has
not yet been described in the literature.
+ 1/2H2O(g) + 1/2SO3(g) (SO2(g) + 1/2O2(g)
)
(8)
Ga2O3 + 1/2Ga2(SO4)3 → 3/2Ga2O3 + 3/2SO3(g) (SO2(g) + 1/2O2(g)
)
(9)
In the case of oxonium and ammonium galloalunite indepen-
dent X-ray powder diffraction experiments identified the stable
final product namely Ga2O3 as ␤-Ga2O3 [26] while in the case of
the alkali metal galloalunites, alkali metal sulfates, A2SO4 (A = Na+,
K+, and Rb+) and ␤-Ga2O3 are formed.
3.2. X-ray analysis and determination of unit cell parameters
The powder XRD patterns of all galloalunites presented (Fig. 7)
show phase pure samples with characteristic reflexes of the alu-
nite/jarosite type (rhombohedral space group R3m; #166). The
reflexes shift due to different unit cell parameters of the galloalu-
nites in correlation to the ionic radii of the alkali metal-, oxonium-
or ammonium-cation. Noticeably, reflexes with Miller indices (h k 0)
are almost not displaced while those with indices (0 0 l) shift con-
siderably from the highest diffraction angles (lower d-values) to the
lowest in the sequence: sodium – oxonium – potassium – ammo-
nium and rubidium. Hence the c parameter is changed significantly
(ca. 7%) by isomorphous replacement of the univalent cation (ꢁr(A)
ca. 21%). Changes in the a parameter, although in the same direc-
tion, are much smaller (ca. 0.4% difference between Na-galloalunite
and Rb-galloalunite). Oxonium galloalunite, which has the highest
a parameter is slightly out of sequence. The unit cell parameters a
and c, the ratio c/a, the volume of the unit cell and the crystallo-
graphic densities are presented in Table 8 for the galloalunite end
members. In Fig. 8 the progression of the lattice constants a and
c are given for the galloalunites as function of the ionic radius for
12-fold coordination.
4.2. Structural arrangements and unit cell parameters of
galloalunites
In order to facilitate discussion of the chemical and unit cell
parameter data we present a brief discussion of the alunite struc-
ture. The first crystal structure of alunite was assigned to the
acentric space group R3m (C3v5) on the basis of a positive pyro-
electric test by Hendricks [27], although the reported structural
coordinates displayed a centre of symmetry. Later, crystal structure
refinement data revealed that the space group for the alunite-
jarosite structure has to be assigned to a centrosymmetric space
group R3m (D3d5) [5,28] with Z = 3 (in the hexagonal unit cell).
The basic structure consists of layers of two-dimensional corner-
shared octahedra of GaO6 and isolated, distorted icosahedra AO12
between these layers. The icosahedra are formed with the alkali
metal-, oxonium- or ammonium-cations surrounded by twelve
anions, consisting of six sulfate- (O2) and six OH-groups (O3; Fig. 9)
which are nearly at the same distance from the cation. In con-
trast the slightly distorted GaO6-octahedra are coordinated by two
sulfate- (O2) and four OH-groups (O3; Fig. 9). Each gallium atom
lies in a centre of symmetry.
Substituting the trivalent cation Al3+ for Ga3+ in oxonium gal-
loalunite, results in the end member, oxonium alunite. Comparable
unit cell parameters have been recently published by Rudolph
et al. [14]. Comparing H3O+-galloalunite with the H3O+-alunite for
The sulfur atoms lie on the trigonal axis and are surrounded by
three basal oxygen atoms (O(2)) and the apical oxygen (O(1)) which

W.W. Rudolph, P. Schmidt / Thermochimica Acta 521 (2011) 112–120
119
Considering the data for potassium galloalunite Tananaev et al.
˚
˚
˚
[20] reported a parameter a of 7.24 A and a parameter c of 17.24 A.
˚
For Na-galloalunite the authors reported: a = 7.13 A and c = 16.66 A;
for ammonium galloalunite: a = 7.21 A and c = 17.78 A; and for Rb-
˚
˚
˚
˚
galloalunite: a = 7.21 A and c = 17.05 A. Comparison of our unit cell
parameters with the ones published by Tananaev et al. [20] shows
that their data were collected on material that was synthesized
under conditions that would not lead to a stoichiometric prod-
uct (e.g. Rb-galloalunite has an unusually low c parameter). It is
noteworthy that in samples which were grown under conditions
chosen to produce a stoichiometric alunite, and in which stoi-
chiometry was confirmed by chemical analysis, the results are
consistent [29]. The most common problem encountered in pro-
ducing stoichiometric alunite inter alia galloalunite is a lack of the
cation in the A site and a lack of gallium in the B site accord-
ing to ideal stoichiometry and subsequent incorporation of water.
The importance of synthesis conditions on resulting stoichiomet-
ric alunite products and consequently on obtaining reliable unit
cell parameters as well as Raman and infrared spectra has to be
stressed.The following data sets on oxonium galloalunite were
˚
˚
reported by Tananaev et al. [20]: a = 7.18 A and c = 16.96 A; while
˚
˚
Johansson reported: a = 7.18 A and c = 17.17 A. Our unit cell param-
eters on oxonium galloalunite are presented in Table 8 and compare
favorably with the values reported by Johannson [7]. As described
above, we took care to remove excess water by heating the sam-
ples to drive off slight amounts of non-stoichiometric water. It
is noteworthy that the oxonium galloalunite is one of the few
examples where the existence of H3O+ is unambiguous. (About
the problem of identifying oxonium ions in minerals see e.g. Ref.
[30]).
The data plot (data from Table 8) in Fig. 8 of the unit cell param-
eters a and c dependent on the ionic radii of the monovalent cations
shows a straight line with a correlation coefficient of 0.9962. This
implies an almost complete substitution of the alkali metal cations
in the A(I) site of the galloallunite structure and again underlying
the stoichiometry of the formed galloalunites.
The significant shift of c parameter results from the structural
arrangement: the isomorphous replacement in A position (Na+, K+,
NH4+, Rb+ and H3O+) resulting in a anisotropic widening of the 12-
fold coordination polyhedra. Since the AO12 icosahedra are linked
in the ab plane by six O(2) oxygen atoms of the sulfate groups (and
six O(3) from OH−) a flexible stacking of layers and a subsequent
increase of the inter-sheet distance only becomes possible along
c where the terminal oxygen atoms (O1) of the sulfate group lies.
Thus, the unit cell parameter c follows the increasing size of the
+
˚
ionic radii of the cations in twelvefold coordination (Na = 1.39 A
+
+
+
˚
˚
˚
[31], H3O = 1.70 A [33], K = 1.64 A [32], NH4 = 1.85 A [31] and
Rb = 1.86 A [32]). The influence on the length of a parameter is
+
˚
only slight.
The comparison of the unit cell parameters for galloalunites with
unit cell parameters of alunite [14] and jarosites [15] allows the
study of the influence of different B site cations, namely Ga3+, Al3+
or Fe3+. Comparing H3O+-galloalunite with the H3O+-alunite for
instance shows that there is only a slight increase in c parameter
(ca. 0.2%) but a significant decrease in a parameter (ca. 2.5%). If B is
Fig. 9. (a) Crystal structure of gallo-alunites A[Ga3(SO4)2](OH)6 with A = alkali metal
ions: Na+, K+, Rb+ and NH4+, H3O+. (b) Octahedral coordination of gallium atoms and
linkage with sulfate groups via O(2) position. (c) Icosahedral coordination of A ions
and linkage with sulfate groups via O(2) position.
Al3+, the parameter a is 7.008(1) A, while it lengthens to 7.170(2) A
if B is occupied by Ga3+ (compare our unit cell data in Table 8).
Comparing the data of H3O+-galloalunite with the data reported
˚
˚
also lies on the threefold axis. The apical oxygen, O(1) is not shared
with coordination polyhedra of other cations and forms H-bonds to
OH (cf. Ref. [5]). Wang et al. [28] described this S–O bond as almost
doubly bonded. This apical oxygen atom (O(1)) of sulfate groups
show alternate orientations (+/−) along the c-axis.
for H3O+-jarosite we observe a still larger widening of a parameter
+
˚
for H3O -jarosite namely a = 7.355 A [15]. This observation is due to
the different B-O bond lengths, namely the largest bond length is
˚
to be found for Fe–O(3)H (1.992 A) [15] but a smaller for Ga–O(3)H
˚
˚
There have been few measurements of galloalunite unit cell
parameters published in the past [7,20]. Our results for end mem-
ber galloalunites of sodium, potassium, ammonium and oxonium
are presented in Table 8 and Fig. 8.
(1.952 A) bond length and smallest for Al–O(3)H (1.879 A). On the
other hand, together with the B-OH variation the bond length B-
˚
˚
O(2) also varies, namely Al–O(2) = 1.947 A, Ga–O(2) = 2.017 A and
˚
Fe–O(2) = 2.015 A [15]. This means that the size effect of substitut-

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W.W. Rudolph, P. Schmidt / Thermochimica Acta 521 (2011) 112–120
ing the B3+ cations results mainly by widening and tilting of the BO6
octahedra along the ab plane via the oxygen O(3).
These results suggest that the substitutions in the A and B site
can be distinguished on the basis of their effects on the unit cell
parameters. Values in unit cell parameters react very sensitively
on change in chemical composition which has been recently found
in the case for alkali metal alunite compounds [14].
[9] S.O. Aslanian, D.E. Apostolov, Y.A. Yordanov, Isomorphous substitution of alu-
minum(3+) and gallium(3+) ions in the alunite structure, Dokl. Bolgarskoi Akad.
Nauk 35 (1982) 1093–1096.
[10] J.E. Dutrizac, J.L. Jambor, Jarosites and their application in hydrometallurgy,
in: P.H. Ribbe (Ed.), Reviews in Mineralogy and Geochemistry: Sulfate Miner-
als, vol. 40, Mineralogical Society of America, Washington, DC, 2000, pp. 405–
452.
[11] U. Kolitsch, A. Pring, Crystal chemistry of the crandallite, beudantite and alunite
groups: a review and evaluation of the suitability as storage materials for toxic
metals, J. Mineral. Petrolog. Sci. 96 (2000) 67–78.
[12] G. Klingelhöfer, R.V. Morris, B. Bernhardt, C. Schröder, D.S. Rodionov, P.A. de
Souza Jr., A. Yen, R. Gellert, E.N. Evlanov, B. Zubkov, J. Foh, U. Bonnes, E.
Kankeleit, P. Gütlich, D.W. Ming, F. Renz, T. Wdowiak, S.W. Squyres, R.E. Arvid-
son, Jarosite and hematite at Meridiani Planum from opportunity’s Moessbauer
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[13] A.S. Wills, A. Harrison, C. Ritter, R.I. Smith, Magnetic properties of pure and dia-
magnetically doped jarosites: model kagome antiferromagnets with variable
coverage of the magnetic lattice, Phys. Rev. B 61 (2000) 6156–6169.
[14] W.W. Rudolph, P. Schmidt, R. Mason, Synthetic alunites of the potassium-
oxonium solid solution series and some other members of the group: synthesis,
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5. Conclusions
1. Careful synthesis and chemical analysis are required in order to
prepare and characterize stoichiometric end member galloalu-
nites.
2. The stages of thermal decomposition of these galloalunites show
a common decomposition mechanism for all end members (two
main decomposition stages). Taking into account the final prod-
ucts of decomposition: Ga2O3 (for galloalunites with A = H3O+
and NH4+) or Ga2O3 and A2SO4 (with A = Na+, K+, and Rb+ for the
respective galloalunites) the composition of the samples may be
derived from the decomposition steps of the thermogravimetry.
3. The thermogravimetric results confirmed the analytical results
on the end member galloalunites and showed that these gal-
loalunites are stoichiometric.
4. Galloalunite, the pure potassium end member, shows the highest
decomposition temperature (ϑons = 395 ◦C) while sodium gal-
loalunite (380 ◦C) and rubidium galloalunite (382 ◦C) are less
stable followed by ammonium alunite (325 ◦C) and oxonium
alunite (207 ◦C).
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