Thermal analysis of beaverite in comparisonwith plumbojarosite

Article abstract of DOI:10.1007/s10973-007-8702-7

The thermal decomposition of beaverite and plumbojarosite was studied using a combination of thermogravimetric analysis coupled to a mass spectrometer. The mineral beaverite Pb(Fe,Cu)₃(SO₄)₂(OH) ₆ decomposes in three stages attributed to dehydroxylation, loss of sulphate and loss of oxygen, which take place at 376 and 420, 539 and 844°C. In comparison three thermal decomposition steps are observed for plumbojarosite PbFe₆(SO₄)₄(OH)₁₂ at 376, 420 and 502°C attributed to dehydroxylation; loss of sulphate occurs at 599°C; and loss of oxygen and formation of lead occurs at 844 and 953°C. The temperatures of the thermal decomposition of the natural plumbojarosite were found to be less than that for the synthetic jarosite. A comparison of the thermal decomposition of plumbojarosite with argentojarosite is made. The understanding of the chemistry of the thermal decomposition of minerals such as beaverite, argentojarosite and plumbojarosite and related minerals is of vital importance in the study known as 'archeochemistry'.

Full text of DOI:10.1007/s10973-007-8702-7

Journal of Thermal Analysis and Calorimetry, Vol. 92 (2008) 3, 887–892
THERMAL ANALYSIS OF BEAVERITE IN COMPARISON WITH
PLUMBOJAROSITE
R. L. Frost*, A. J. Locke and W. Martens
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology
GPO Box 2434, Brisbane Queensland 4001, Australia
The thermal decomposition of beaverite and plumbojarosite was studied using a combination of thermogravimetric analysis cou-
pled to a mass spectrometer.
The mineral beaverite Pb(Fe,Cu)3(SO4)2(OH)6 decomposes in three stages attributed to dehydroxylation, loss of sulphate and
loss of oxygen, which take place at 376 and 420, 539 and 844°C. In comparison three thermal decomposition steps are observed for
plumbojarosite PbFe6(SO4)4(OH)12 at 376, 420 and 502°C attributed to dehydroxylation; loss of sulphate occurs at 599°C; and loss
of oxygen and formation of lead occurs at 844 and 953°C. The temperatures of the thermal decomposition of the natural
plumbojarosite were found to be less than that for the synthetic jarosite. A comparison of the thermal decomposition of
plumbojarosite with argentojarosite is made. The understanding of the chemistry of the thermal decomposition of minerals such as
beaverite, argentojarosite and plumbojarosite and related minerals is of vital importance in the study known as ‘archeochemistry’.
Keywords: argentojarosite, beaverite, dehydration, dehydroxylation, plumbojarosite, TG
Introduction
Fe(III) sulphate minerals [27–32]. Interest in such
minerals and their thermal stability rests with the pos-
sible identification of these minerals and related de-
hydrated paragenetically related minerals on planets
and on Mars. The existence of these minerals on plan-
ets would positively identify existence or at least pre-
existence of water on Mars [33]. Further such miner-
als are formed through crystallisation from solutions.
It has been stated that the thermal decomposition of
jarosite begins at 400°C with the loss of water [34].
The process is apparently kinetically driven. Water
loss can occur at low temperatures over extended pe-
riods of time [34]. It is probable that in nature low
temperature environments would result in the decom-
position 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 to-
gether with soluble sulphates [35]. Recently
thermogravimetric analysis has been applied to some
complex mineral systems and it is considered that
TG-MS analyses may also be applicable to the
jarosite minerals [36–41]. Differential thermal analy-
sis of some related minerals has been published
[24, 42–44]. In this work we report the thermogravi-
metric analysis of beaverite and make a comparison
with plumbojarosite.
The mineral beaverite [Pb(Fe,Cu)3(SO4)2(OH)6] is one
mineral of the many jarosite minerals [1] and was first
discovered in 1911 [2]. Beaverite is like plumbo-
jarosite in that it forms in close proximity to Fe–Pb–Cu
sulphides [1, 3]. The mineral is related to jarosites and
alunites and is trigonal. Beaverite forms a solid solu-
tion with plumbojarosite with replacement of the Fe3+
by Cu2+ or Zn2+ or both [4]. Beaverite is related to the
mineral plumbojarosite. Lead jarosite (plumbojarosite)
(PbFe6(SO4)4(OH)12) was identified in relation to
jarosite in 1902 [5]. Plumbojarosite is often found in
cationic mixed jarosites [6–15]. Such minerals are of
importance in medieval and archaeological science
[16, 17]. The minerals are found in mine drainage sites
both ancient and modern [9, 17, 18]. Such formation of
jarosites has been occurring since before the Bronze
Age [19]. The importance of jarosite formation and its
decomposition depends upon its presence in soils, sed-
iments and evaporate deposits [20]. These types of de-
posits have formed in acid soils where the pH is less
than pH 3.0 units [21].
The thermal decomposition of jarosites has been
studied for some considerable time [22–26]. The au-
thors have reported recent studies of jarosites both
natural and synthetic [10–15]. There have been many
studies on related minerals such as the Fe(II) and
*
Author for correspondence: r.frost@qut.edu.au
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

FROST et al.
Experimental
instrument was coupled to a Balzers (Pfeiffer) mass
spectrometer for gas analysis. Only water vapour, car-
bon dioxide and oxygen were analyzed. Previous
studies by the authors have reported these experimen-
tal methods [10–14].
Minerals
The mineral beaverite was synthesised according to
the method given by Jambor and Dutrizac [45]. A
40 mL chloride solution was prepared. It contained
0.25 g PbCl2, 5 mL of 1.23 M FeCl3 and 12 mL of
saturated LiCl solution with the remaining volume be-
ing water. A separate sulphate solution was also pre-
pared. This contained 2.5 g Fe2(SO4)3 and 4.62 g CuSO4
dissolved in a minimal volume of water. The sulphate
solution was added dropwise to the chloride solution
over a 5 min time period. The solution was transferred to
an autoclave and enough water was added to bring the
final volume of the solution to approximately 0.1 L. The
autoclave was heated at 120°C for 3 days. After this
time, the brown precipitate was collected, washed and
dried under vacuum. 1.33 g of sample was obtained and
identified by X-ray diffraction as beaverite.
Results and discussion
The XRD pattern of beaverite and the reference pat-
tern are shown in Fig. 1. The synthesised mineral
shows an XRD pattern identical to that of the stan-
dard. Some sulphates of lead and copper are present
as impurities but are estimated to be in low concentra-
tions. and Dutrizac showed that substitution of Pb2+
by Cu2+ results in the formation of a probable solid so-
lution series between beaverite and plumbojarosite
[4]. The authors discussed the lack of studies on this
topic and the lack of fundamental knowledge of such
minerals and on their solid solutions [4].
The mineral plumbojarosite has been synthesized
by the authors [10]. Natural plumbojarosite originated
from Teutonic Bore Mine, Western Australia. The
beaverite and plumbojarosite were analysed by X-ray
diffraction for phase purity and by electron probe using
energy dispersive techniques for quantitative chemical
composition. Natural plumbojarosite contained 5% po-
tassium. No other cations were found.
TG and DTG curves for beaverite are shown in
Fig. 2 and the mass spectrometric ion current curves
in Fig. 3. The ion current curves are shown for mass/
charge ratios of 44, 17, 18, 16, 32, 48 and 64 repre-
Methods
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 fo-
cused PW3373/10 Cu X-ray tube, operating at 45 kV
and 35 mA, providing Ka1wavelength of 1.540596 .
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 interaction with the
sample, the diffracted beam was detected by an
X’Celerator RTMS detector. The detector was set in
scanning mode, with an active length of 2.022 mm.
Samples were analysed utilising Bragg–Brentano ge-
ometry over a range of 3–75° 2q with a step size of
0.02° 2q, with each step measured for 200 s.
Fig. 1 XRD pattern of beaverite
Thermal analysis
Thermal decomposition of beaverite and plumbo-
jarosite was carried out in a TA® Instruments incorpo-
rated high-resolution thermogravimetric analyzer (se-
ries Q500) in a flowing nitrogen atmosphere
(60 cm3 min–1). Approximately 35 mg of sample un-
derwent thermal analysis, with a heating rate of
5°C min–1, with high resolution, to 1000°C. The TG
Fig. 2 TG and DTG curves of beaverite
888
J. Therm. Anal. Cal., 92, 2008

THERMAL ANALYSIS OF BEAVERITE IN COMPARISON WITH PLUMBOJAROSITE
Stage 2 Temperatures of 539 and 570°C
[Pb(Fe,Cu)3O3(SO4)2]®
[Pb(Fe,Cu)3O7]+2SO2
Theoretical mass loss of 12.68% is predicted.
Experimental mass loss for the synthesised
beaverite is 15.5%. In the ion current curves (Fig. 3)
mass losses of 32, 48 and 64 corresponding to S, SO
and SO2 are observed.
Stage 3 Temperatures of 767, 833 and 860°C
2[Pb(Fe,Cu)3O7]®
2Pb+3Fe2O3+3CuO+O2
Theoretical mass loss of 5.7%.
Experimental mass loss for the synthesised
beaverite is 7.2%. The ion current curves show in-
creased current for m/z=16 at 767, 833 and 860°C.
Mechanism of thermal decomposition of plumbojarosite
Thermogravimetric analysis
The TG and DTG curves for natural lead jarosite are
Fig. 3 Ion current curves of the evolved gases from the thermal
shown in Fig. 4. The ion current curves for the
evolved gases water vapour and SO2 from the plum-
bojarosite are shown in Figs 5a and b. There is an ap-
parent lowering of the temperatures of decomposition
for the natural plumbojarosite as compared with a
synthetic plumbojarosite. This may be caused by the
presence of other elements in the natural plumbo-
jarosite. EDX analysis shows the presence of around
5% K in the natural plumbojarosite.
decomposition of beaverite
senting the gas evolution attributed to CO2, OH, H2O,
O, O2, SO2 and SO3. M/z ratio of 44 is analysed to
check to see if any CO2 is involved with the combus-
tion and thermal decomposition of the beaverite.
Mechanism of thermal decomposition of beaverite
It is proposed that the thermal decomposition of
beaverite [Pb(Fe,Cu)3(SO4)2(OH)6] occurs in three
stages as follows: (a) dehydroxylation, (b) loss of sul-
phate, (c) loss of oxygen.
Stage 1 Temperatures of 219, 250 and 374°C
[Pb(Fe,Cu)3(SO4)2(OH)6]®
[Pb (Fe,Cu)3O3(SO4)2]+3H2O
Theoretical mass loss of 9.56% is predicted.
Experimental mass losses of 3.5% at 219 and
250°C are observed. A further mass loss attributed to
dehydroxylation of 7.7% at 374°C is found.
The total mass loss for dehydroxylation of
11.2% is observed.
Fig. 4 TG-DTG curves of Pb-jarosite
The ion current curves for evolved water
vapour show water is evolved over a broad tempera-
ture range from 350 to 450°C. Mass losses that occur
at 376, 420 and 502°C for the synthetic plumbo-
jarosite and 391 and 418°C for the natural sample are
attributed to dehydroxylation. The theoretical mass
loss for dehydroxylation is 9.56% based upon the
formula (PbFe6(SO4)4(OH)12) with the loss of
Ion current curves show gas evolution of water at
228 and 379°C which corresponds to the mass losses
at 219, 250 and 374°C.
J. Therm. Anal. Cal., 92, 2008
889

FROST et al.
Stage 1
PbFe6(SO4)4(OH)12®
PbFe6(SO4)4O6+3H2O
Theoretical mass loss of 9.56%.
Experimental mass loss is 11.8%.
Temperature of mass loss occurs at 376 and 420°C.
Stage 2
Pb(FeO)6(SO4)4®
PbO10(FeO)6+4SO2
Theoretical mass loss of 22.68%.
Experimental mass loss for the synthesised
Plumbojarosite is 25%.
Temperature of mass loss is 599°C.
Stage 3
PbO10(FeO)6®
Pb+3Fe2O3+3.5O2
Theoretical mass loss of 5.7%.
Experimental mass loss for the Pb-jarosite is 5.6%.
Temperature of mass loss is 844 and 953°C.
Fig. 5 Ion current curves of a – m/z=17 and 18; b – m/z=44, 48
Karoleva et al. reported the DTA patterns for a
number of jarosites and suggested that the sublima-
tion of lead may be a problem in the thermal analysis
of lead jarosite [46]. There is a mass loss around
1000°C which may be ascribed to the lead sublima-
tion. Only single endothermic peaks were observed
by these authors. In comparison the TG patterns of
plumbojarosite show a considerable number of ther-
mal decomposition steps. Kulp and Adler reported
similar DTA results [23]. Ozacar et al. researched the
kinetics of the thermal decomposition of plumbo-
jarosite and basically stated there were three decom-
position stages (a) up to 760°C, (b) up to 1000 and up
to 1250°C. Such a study does not differentiate be-
tween the chemical processes of dehydroxylation and
desulphation of the plumbojarosite. There have been
no reported fundamental studies of the TG and
TG-MS of plumbojarosite and beaverite.
and 64 for plumbojarosite as a function of temperature
6 hydroxyl units as 3H2O. The experimentally deter-
mined mass losses for synthetic and natural plumbo-
jarosites are 11.8 and 11.2%.
The ion current curve for evolved SO2 (Fig. 5b)
shows that the mass loss for the synthetic plumbo-
jarosite at 605°C is due to the loss of sulphate as SO2.
The temperature for the natural plumbojarosite is
531°C. The difference in temperature is attributed to a
‘depression of freezing point effect’ i.e. a lowering of
the temperature of the loss of sulphate by the presence
of impurities in the natural sample. The experimental
mass loss is 25%. If all 4 sulphate units are lost simulta-
neously then the theoretical mass loss would be 22.68%.
Higher temperature mass losses are observed at 953 and
844°C for plumbojarosite and at 812 and 759°C for the
natural sample. These mass loss steps are attributed to
the loss of oxygen. XRD shows the decomposition
products of plumbojarosite are lead and hematite.
Comparison with the thermal decomposition of
argentojarosite
Mechanism of thermal decomposition
To date there have been few studies of the thermal de-
composition of argentojarosite [12]. The thermal de-
composition of argentojarosite takes place with steps
similar to that for beaverite and plumbojarosite.
Dehydroxylation of argentojarosite 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.
It is proposed that the thermal decomposition of
plumbojarosite occurs in three stages as follows:
(a) dehydroxylation, (b) loss of sulphates, (c) loss of
oxygen.
890
J. Therm. Anal. Cal., 92, 2008

THERMAL ANALYSIS OF BEAVERITE IN COMPARISON WITH PLUMBOJAROSITE
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The thermal decomposition of a natural and synthetic
plumbojarosite has been studied using thermogravi-
metric analysis and mass spectrometry and the thermal
decomposition compared with that of beaverite. The
temperatures of the thermal decomposition of the natu-
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