Thermoanalytical studies of silver and lead jarosites and their solid solutions

Article abstract of DOI:10.1007/s10973-009-0406-8

Dynamic and controlled rate thermal analysis has been used to characterise synthesised jarosites of formula [M(Fe)₃(SO₄) ₂(OH)₆] where M is Pb, Ag or Pb-Ag mixtures. Thermal decomposition occurs in a series of steps. (a) dehydration, (b) well defined dehydroxylation and (c) desulphation. CRTA offers a better resolution and a more detailed interpretation of water formation processes via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of water formation reveal the subtle nature of dehydration and dehydroxylation. CRTA offers a better resolution and a more detailed interpretation of the decomposition processes via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of non-isothermal nature reveal separation of the dehydroxylation steps, since in these cases a higher energy (higher temperature) is needed to drive out gaseous decomposition products through a decreasing space at a constant, pre-set rate.

Full text of DOI:10.1007/s10973-009-0406-8

J Therm Anal Calorim (2010) 101:73–79
DOI 10.1007/s10973-009-0406-8
Thermoanalytical studies of silver and lead jarosites
and their solid solutions
´
´
Ray L. Frost Æ Sara J. Palmer Æ Janos Kristof Æ
´
Erzsebet Horvath
´
Received: 21 June 2009 / Accepted: 29 July 2009 / Published online: 28 August 2009
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract Dynamic and controlled rate thermal analysis
has been used to characterise synthesised jarosites of for-
mula [M(Fe)3(SO4)2(OH)6] where M is Pb, Ag or Pb–Ag
mixtures. Thermal decomposition occurs in a series of
steps. (a) dehydration, (b) well defined dehydroxylation
and (c) desulphation. CRTA offers a better resolution and a
more detailed interpretation of water formation processes
via approaching equilibrium conditions of decomposition
through the elimination of the slow transfer of heat to the
sample as a controlling parameter on the process of
decomposition. Constant-rate decomposition processes of
water formation reveal the subtle nature of dehydration and
dehydroxylation. CRTA offers a better resolution and a
more detailed interpretation of the decomposition pro-
cesses via approaching equilibrium conditions of decom-
position through the elimination of the slow transfer of
heat to the sample as a controlling parameter on the
process of decomposition. Constant-rate decomposition
processes of non-isothermal nature reveal separation of the
dehydroxylation steps, since in these cases a higher energy
(higher temperature) is needed to drive out gaseous
decomposition products through a decreasing space at a
constant, pre-set rate.
Keywords Jarosite ꢀ Thermal analysis ꢀ
Controlled rate thermal analysis ꢀ Thermogravimetry
Introduction
Argentojarosite (AgFe33?(SO4)2(OH)6) was first identified
in 1923 from the Titanic Standard mine at Dividend, Utah,
USA [1]. It has since been identified in at least 14 other US
sites. The mineral in some localities is of sufficient abun-
dance as to be a silver bearing ore [2]. Argentojarosite was
exploited at Rio Tinto, Spain, from Roman or even pre-
Roman times [3]. Argentojarosites had an important
influence on the wealth of both Europe and South
America [4]. Lead jarosite also known as plumbojarosite
(PbFe6(SO4)4(OH)12) was identified in relation to jarosite
in 1902 [5]. Plumbojarosite is often found in cationic
mixed jarosites [6–9]. Such minerals are of importance in
medieval and archaeological science [10, 11] and are also
found in mine drainage sites both ancient and modern [9,
11, 12]. Such formation of jarosites has been occurring
since the Bronze Age [13]. The importance of jarosite
formation and its decomposition depends upon its presence
in soils, sediments and evaporate deposits [14]. These types
of deposits have formed in acid soils where the pH is less
than 3.0 pH units [15]. Such acidification results from the
oxidation of pyrite which may be from bacterial action or
through air-oxidation.
R. L. Frost (&) ꢀ S. J. Palmer
Inorganic Materials Research Program, School of Physical and
Chemical Sciences, Queensland University of Technology, 2
George Street, GPO Box 2434, Brisbane, QLD 4001, Australia
e-mail: r.frost@qut.edu.au
´
J. Kristof
Department of Analytical Chemistry, University of Pannonia,
´
PO Box 158, 8201 Veszprem, Hungary
´
E. Horvath
Department of Environmental Engineering and Chemical
Technology, University of Pannonia, PO Box 158, 8201
´
Veszprem, Hungary
The thermal decomposition of jarosites has been studied
for some considerable time [16–20]. However no thermal
123

74
R. L. Frost et al.
studies on the pure end member of argentojarosite have
been forthcoming, nor has there been studies on argento-
plumbojarosites [17]. It has been stated that the thermal
decomposition of jarosite begins at 400 °C with the loss of
water [21]. The process is apparently kinetically driven.
Water loss can occur at low temperatures over extended
periods of time [21]. It is probable that in nature low
temperature environments would result in the decomposi-
tion of jarosite. The products of the decomposition depend
upon the jarosite be it K, Na or Pb etc. but normally goe-
thite and hematite are formed together with soluble sul-
phates [22].
thermogravimetric (DTG) and differential thermal analysis
(DTA) curves simultaneously. The sample was heated in
a ceramic crucible in static air atmosphere at a rate of
5 °C/min.
Controlled rate thermal analysis experiment
Thermal decomposition of the jarosite was carried out in
the Derivatograph under static air at a pre-set, constant
decomposition rate of 0.1 mg/min. (Below this threshold
value the samples were heated under dynamic conditions at
a uniform rate of 1.0 °C/min). The samples were heated in
an open ceramic crucible at a rate of 1.0 °C/min-1 up to
300 °C. With the quasi-isothermal, quasi-isobaric heating
program of the instrument the furnace temperature was
regulated precisely to provide a uniform rate of decom-
position in the main decomposition stage.
In this work, as part of our studies of secondary mineral
formation [23–35] and their stability, we report the com-
parison of the thermal analysis of designed synthetic lead
and silver jarosites using both dynamic and controlled rate
thermal analysis techniques.
Experimental
Results and discussion
Synthesis of the lead jarosite and argentojarosite
The dynamic TG, DTG and DTA patterns of unground
and ground silver, lead and 50:50 and 25–75 lead–silver
mixes are shown in Figs. 1, 2, 3, 4 and 5, respectively.
Argentojarosite was synthesised by dissolving 4.8 g
Fe2(SO4)3 in 120 mL of water. 0.108 g of AgNO3 was
added to the solution followed by a further 30 mL of water.
The solution was heated in a covered Erhlenmeyer flask for
22 h at 95 °C. The resulting yellow-brown precipitate was
collected and dried under vacuum. Powder X-ray diffrac-
tion proved the correct structure was obtained.
To synthesise lead jarosite, a 50 mL chloride solution
was prepared. This solution contained 0.5 g PbCl2, 12 mL
of a saturated LiCl solution and 5 mL of 1.23 M FeCl3
solution. (The high chloride concentration was necessary
to prevent the precipitation of lead sulphate). 4.93 g
Fe2(SO4)3 was dissolved in the minimal volume of water
and slowly added drop wise to the chloride solution. The
final solution was heated for 21 h at 120 °C in an auto-
clave. A golden brown precipitate was collected under
vacuum and then dried at 100 °C for an hour. 1.25 g of
pure lead jarosite was obtained.
Fig. 1 TG, DTG and DTA of unground silver jarosite
The minerals were phase analysed by powder X-ray
diffraction and were analysed by chemical composition by
EDX methodology.
Thermal analysis
Conventional thermal analysis experiment
Thermal decomposition of the jarosite was carried out in a
Derivatograph PC type thermoanalytical equipment (Hun-
garian Optical Works, Budapest, Hungary) capable of
recording the thermogravimetric (TG), derivative
Fig. 2 TG, DTG and DTA of ground silver jarosite
123

Thermoanalytical studies of silver and lead jarosites
75
Fig. 6 CRTA of unground Ag jarosite
Fig. 3 TG, DTG and DTA of lead jarosite
Fig. 7 CRTA of ground Ag jarosite
Fig. 4 TG, DTG and DTA of lead–silver jarosite (50:50 ratio)
Fig. 8 CRTA of unground Pb jarosite
Fig. 5 TG, DTG and DTA of lead–silver jarosite (25:75 ratio)
Dynamic thermal analysis of silver jarosite
The results of the conventional constant rate heating
experiment are reported in Table 1. The controlled rate
thermal analysis results are reported in Table 2 and the
CRTA patterns of unground and ground silver, lead and
50:50 and 25–75 lead–silver mixes are shown in Figs. 6,
7, 8, 9 and 10.
The behavior of the thermal decomposition is different for
the silver and lead jarosites and their mixtures. The thermal
decomposition of silver jarosite takes place in five mass
loss steps as shown in Fig. 1. The mass loss values
belonging to the individual decomposition steps are sum-
marized in Table 1. DTG peak maxima are observed at
123

76
R. L. Frost et al.
Based on the TG curve, the exact amount of crystallization
water was 1.31 mole.
Step 2: dehydroxylation
Dehydroxylation occurs at 390 and 435 °C resulting in the
evolution of 3 moles of water:
AgFe3ðSO4Þ2ðOHÞ6 ! AgFe3O3ðSO4Þ6 þ 3H2O
(The theoretical mass loss is 13.0%, the observed loss is
11.6%).
Fig. 9 CRTA of Pb–Ag jarosite (50:50) ratio
Step 3: desulphation
The desulphation reaction occurs in two steps at 673 and
946 °C leading to the formation of Ag2SO4ꢀFe2O3 and iron
oxide as follows:
2AgFe3O3ðSO4Þ6 ! Ag2SO4 ꢀ Fe2O3 þ 2Fe2O3 þ 3SO3
(The theoretical mass loss in this reaction is 31.30%, the
actual loss is 30.6%).
There is a difference between the unground (Fig. 1) and
ground (Fig. 2). In the DTG curve in Fig. 2, peaks are
observed at 238, 378 390, 438, 666 and 945 °C. The effect
of the grinding on the Ag-jarosite caused an additional
peak at 390 °C. The effect of grinding on the DTG peak
positions appears to shift the dehydration peak to higher
temperatures and the dehydroxylation point to lower tem-
peratures. The first desulphation step is shifted to lower
temperatures and the temperature of the second step
remains the same. An additional DTA peak at 395 °C is
also observed.
Fig. 10 CRTA of Pb–Ag jarosite (25:75) ratio
229, 390, 435, 673 and 946 °C. The DTA patterns shows
minima at 230, 395 and 675 °C indicating endothermic
steps in the thermal decomposition process.
Step 1: dehydration
Dynamic thermal analysis of lead jarosite
The loss of crystallization water takes place at 229 °C
according to the following equation:
The thermal decomposition behaviour of the lead jarosite
(Fig. 3) appears significantly different to that of the silver
AgFe3ðSO4Þ2ðOHÞ6xH2O ! AgFe3ðSO4Þ2ðOHÞ6 þ xH2O
Table 1 Decomposition stages under dynamic conditions for jarosites
Sample: Ag jarosite unground Sample: Ag jarosite Sample: Pb jarosite
Sample: Pb, Ag jarosite Sample: Pb2 Ag8 jarosite
Temp
range/°C
Mass loss
(Sample
mass:
Temp
Mass loss Temp
Mass loss (Sample Temp
Mass loss
(Sample
mass:
Temp
range/°C mass:
55.13 mg)
Mass loss (Sample
range/°C (Sample
mass:
62.78 mg)
range/°C mass: range/°C
75.35 mg)
mg
75.83 mg)
89.07 mg)
mg
%
mg
%
%
mg
%
mg
%
204–333
333–416
416–502
502–736
875–978
2.5
3.3
8.6
200–314 1.6 2.5 242–445 8.2
314–414 5.9 9.4 445–530 0.7
414–487 0.7 1.1 530–693 17.6
487–736 14.9 23.7 693–994 1.4
883–992 3.8 6.1
10.9
0.9
30–463
463–728
728–994
8.1
6.4
4.8
9.1
7.2
5.4
68–369 3.5
369–473 1.7
473–695 7.7
695–996 3.1
6.3
6.5
1.0
3.1
14.0
5.6
1.3
23.4
1.9
18.3
4.9
24.1
6.5
123

Thermoanalytical studies of silver and lead jarosites
77
Table 2 Decomposition stages under controlled rate thermal analysis conditions for silver and lead jarosites
Sample: Ag jarosite
unground
Sample: Ag jarosite
Sample: Pb jarosite
Sample: Pb, Ag jarosite
Sample: Pb2Ag8 jarosite
Temperature Mass loss Temperature Mass loss Temperature Mass loss Temperature Mass loss Temperature Mass loss
range/°C
range/°C
(Sample
mass:
188.42 mg)
range/°C
(Sample
mass:
123.67 mg)
range/°C
(Sample
mass:
248.37 mg)
range/°C
(Sample
mass:
220.40 mg)
(Sample
mass:
238.98 mg)
mg
%
mg
%
mg
%
mg
%
mg
%
183-306
306-389
389-467
6.2 3.3 131–291
16.9 9.0 291–388
2.3 1.2 388–455
455–495
3.4 2.7 196–331
12.0 9.7 331–421
1.2 1.0 421–498
0.2 0.2 498–569
8.8 3.5
29–425
20.4 9.3
35–349
17.4 7.3
6.5 2.7
34.6 14.5
19.1 7.7 425–701
2.1 0.8
15.4 7.0 349–443
443–593
21.7 8.7
jarosite (Fig. 2). The three mass loss steps that appear in
the DTG curve at 411, 495 and 667 °C belong to the lib-
eration of water, while desulphation occurs in a single
process at 667 °C.
single phases. The thermal analysis patterns of the silver
lead jarosites of formula Ag0.5(Pb0.5 0.5Fe3(SO4)2(OH)6
xH2O and Ag075(Pb0.5 0.25Fe3(SO4)2(OH)6 xH2O are given
)
)
in Figs. 4 and 5. Three mass loss steps are observed over
the (a) 30 to 163 °C temperature range assigned to dehy-
dration, (b) the 463 to 728 °C temperature range assigned
to dehydroxylation and (c) 728 to 994 °C ascribed to
desulphation. For the lead-silver jarosite with equimolar
mixtures of silver and lead, the DTG peaks are broad;
whereas for the silver-lead jarosite with a 3:1 molar ratio,
the DTG peaks are better defined. DTG peaks are observed
at 337, 393 and 642 °C. Because silver is the predominant
metal in the jarosite, the thermal analysis pattern more
closely follows that of the silver jarosite. The effect of the
lead is to lower the temperatures of the decomposition
steps.
Step 1: dehydration
The loss of crystallization water takes place at temperatures
\300 °C according to the following equation:
Pb0:5Fe3ðSO4Þ2ðOHÞ6xH2O ! Pb0:5Fe3ðSO4Þ2ðOHÞ6
þ xH2O
Based on the TG curve, the exact amount of crystallization
water was 1.31 mole.
Step 2: dehydroxylation
Dehydroxylation occurs at 411 °C resulting in the evolu-
tion of 3 moles of water:
The controlled rate thermal analysis experiment
The thermal decomposition patters of the silver, lead and
silver-lead jarosites recorded under CRTA decompositions
are shown in Figs. 6, 7, 8, 9, 10. The mass loss data are
summarized in Table 2. The dehydration process of silver
jarosite (Fig. 6) under the slow heating rate of 1 °C min-1
can be resolved to a step between 100 and 200 °C, at
198 °C. During the dynamic experiment only two dehy-
dration steps could be identified in the DTG curve. The
controlling rate of 0.1 mg min-1 was not reached until
375 °C. Dehydroxylation took place in a homogeneous
process represented by an isothermal, constant rate of water
evolution at 375 °C. The CRTA of the ground Ag-jarosite
(Fig. 7) provided a different pattern. A non-isothermal step
assigned to dehydration is observed at 214 °C. In this case
the dehydroxylation took place at 400 °C in an isothermal
process.
Pb0:5Fe3ðSO4Þ2ðOHÞ6 ! Pb0:5Fe3O3ðSO4Þ6 þ 3H2O
(The theoretical mass loss is 13.0%, the observed loss is
11.6%).
Step 3: desulphation
The desulphation reaction occurs in a single step at 667 °C
leading to the formation of PbSO4ꢀFe2O3 and alumina as
follows:
2Pb0:5Fe3O3ðSO4Þ6 ! PbSO4 ꢀ Fe2O3 þ 2Fe2O3 þ 3SO3
(The theoretical mass loss in this reaction is 33.34%, the
actual loss is 36.37%).
Dynamic thermal analysis of silver–lead jarosite
Dehydration of lead jarosite (Fig. 8) under CRTA con-
ditions took place in two steps (similarly to the dynamic
experiment) up to 200 min and between 200 and 400 min.
It should be noted that lead and silver jarosites form a
continuous series of solid solutions and are identified as a
123

78
R. L. Frost et al.
The controlling level of 0.1 mg min-1 was reached in both
steps resulting in a quasi-isothermal pattern of water evo-
lution at 80 and 150 °C. Above 400 min, dehydroxylation
took place in four steps at 200, 249, 325 and 452 °C under
the slow heating (the rate of decomposition did not reached
the controlling level of 0.1 mg min-1). This experiment
revealed the subtle nature of dehydroxylation which was
not observed in the dynamic experiment.
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The thermal decomposition of the silver-lead mixed
jarosites (Figs. 9 and 10) under CRTA conditions results in
dehydration occurring in a series of non-isothermal steps up
to 300 °C. A quasi-isothermal steps is observed at 562 °C,
attributed to desulphation. For the Ag075(Pb0.5 0.25Fe3
)
(SO4)2(OH)6 xH2O jarosite two dehydroxylation steps are
observed similar to the two dehydroxylation steps observed
in the dynamic experiment. A pseudo-isothermal desulph-
ation step at 566 °C is observed (Fig. 10).
10. Amoros JL, Lunar R, Tavira P. Jarosite: a silver-bearing mineral
of the gossan of Rio Tinto (Huelva) and La Union (Cartagena,
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lurgy; 2000. p. 405–452.
Synthetic jarosites show characteristic thermogravimetric
patterns with thermal decomposition steps: (a) dehydration,
(b) well defined dehydroxylation and (c) desulphation.
While the patterns of dehydration and dehydroxylation (as
well as the amounts of crystallisation water) differ signif-
icantly depending on the nature of the cation, desulphation
is a rather homogeneous process (the DTG peak tempera-
tures fall in the range between 760 and 800 °C for all the
five samples).
CRTA offers a better resolution and a more detailed
interpretation of the decomposition processes via
approaching equilibrium conditions of decomposition
through the elimination of the slow transfer of heat to the
sample as a controlling parameter on the process of
decomposition. With this technique differences in the
dehydration and decomposition patterns can be revealed
suspecting a more complex pattern of decomposition as
revealed under dynamic hating.
It is very important to be able to thermally characterise
minerals such as jarosites which may be found on planets
such as Mars. The existence of jarosites 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 jarosites is most important as there is a need to
find the temperature range over which the minerals are
stable, since wide temperature ranges are likely on planets.
22. Thomas PS, Hirschausen D, White RE, Guerbois JP, Ray AS.
Characterization of the oxidation products of pyrite by thermo-
gravimetric and evolved gas analysis. J Therm Anal Calorim.
2003;72:769–76.
23. Frost RL, Hales MC, Martens WN. Thermogravimetric analysis
of selected group (II) carbonate minerals—implication for the
Acknowledgements This research was supported by the Hungarian
Scientific Research Fund (OTKA) under grant No. K62175. The
financial and infra-structure support of the Queensland University of
Technology Inorganic Materials Research Program is gratefully
acknowledged.
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Thermoanalytical studies of silver and lead jarosites
79
geosequestration of greenhouse gases. J Therm Anal Calorim.
2009;95:999–1005.
24. Palmer SJ, Spratt HJ, Frost RL. Thermal decomposition of hy-
drotalcites with variable cationic ratios. J Therm Anal Calorim.
2009;95:123–9.
30. Hales MC, Frost RL. Thermal analysis of smithsonite and
hydrozincite. J Therm Anal Calorim. 2008;91:855–60.
31. Palmer SJ, Frost RL, Nguyen T. Thermal decomposition of hy-
drotalcite with molybdate and vanadate anions in the interlayer. J
Therm Anal Calorim. 2008;92:879–86.
25. Carmody O, Frost R, Xi Y, Kokot S. Selected adsorbent materials
for oil-spill cleanup. A thermoanalytical study. J Therm Anal
Calorim. 2008;91:809–16.
32. Vagvoelgyi V, Daniel LM, Pinto C, Kristof J, Frost RL, Horvath
E. Dynamic and controlled rate thermal analysis of attapulgite. J
Therm Anal Calorim. 2008;92:589–94.
26. Frost RL, Locke A, Martens WN. Thermogravimetric analysis of
wheatleyite Na2Cu2?(C2O4)2ꢀ2H2O. J Therm Anal Calorim.
2008;93:993–7.
33. Vagvolgyi V, Frost RL, Hales M, Locke A, Kristof J, Horvath E.
Controlled rate thermal analysis of hydromagnesite. J Therm
Anal Calorim. 2008;92:893–7.
27. Frost RL, Locke AJ, Hales MC, Martens WN. Thermal stability of
synthetic aurichalcite. Implications for making mixed metal oxi-
des for use as catalysts. J Therm Anal Calorim. 2008;94:203–8.
28. Frost RL, Locke AJ, Martens W. Thermal analysis of beaverite in
comparison with plumbojarosite. J Therm Anal Calorim. 2008;
92:887–92.
29. Frost RL, Wain D. A thermogravimetric and infrared emission
spectroscopic study of alunite. J Therm Anal Calorim. 2008;91:
267–74.
34. Vagvolgyi V, Hales M, Martens W, Kristof J, Horvath E, Frost
RL. Dynamic and controlled rate thermal analysis of hydrozincite
and smithsonite. J Therm Anal Calorim. 2008;92:911–6.
35. Zhao Y, Frost RL, Vagvolgyi V, Waclawik ER, Kristof J,
Horvath E. XRD, TEM and thermal analysis of yttrium doped
boehmite nanofibres and nanosheets. J Therm Anal Calorim.
2008;94:219–26.
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