Thermal decomposition of synthetic argentojarosite - Implications for silver production in medieval times

Article abstract of DOI:10.1016/j.tca.2005.06.008

The thermal decomposition of argentojarosite has been studied by thermogravimetric, spectroscopic and infrared emission techniques. Dehydroxylation 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. Changes in the structure of argentojarosite are also followed by the infrared emission spectra. The intensity of hydroxyl stretching vibration approaches zero by 400 °C confirming the loss of hydroxyl units by this temperature. At this temperature changes in the infrared spectra of the sulphate units occurs. The intensity of the sulphate stretching vibrations approaches zero by 750 °C. Thermal decomposition of argentojarosite is important in understanding silver production in ancient and medieval times. This work shows that temperatures of around 750 °C are required to produce metallic silver.

Full text of DOI:10.1016/j.tca.2005.06.008

Thermochimica Acta 437 (2005) 30–33
Thermal decomposition of synthetic argentojarosite—Implications
for silver production in medieval times
Ray L. Frost∗, Rachael-Anne Wills, Matt L. Weier, Wayde Martens
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology,
GPO Box 2434, Brisbane, Qld. 4001, Australia
Received 14 February 2005; received in revised form 29 May 2005; accepted 8 June 2005
Available online 15 July 2005
Abstract
The thermal decomposition of argentojarosite has been studied by thermogravimetric, spectroscopic and infrared emission techniques.
Dehydroxylation 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. Changes in the structure of argentojarosite
are also followed by the infrared emission spectra. The intensity of hydroxyl stretching vibration approaches zero by 400 ◦C confirming the
loss of hydroxyl units by this temperature. At this temperature changes in the infrared spectra of the sulphate units occurs. The intensity of
the sulphate stretching vibrations approaches zero by 750 ◦C. Thermal decomposition of argentojarosite is important in understanding silver
production in ancient and medieval times. This work shows that temperatures of around 750 ◦C are required to produce metallic silver.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Argentojarosite; Jarosite; Plumbojarosite; Dehydration; Dehydroxylation; Raman spectroscopy; Infrared emission spectroscopy; Thermogravimetric
analysis
1. Introduction
of water [10]. The process is apparently kinetically driven.
Water loss can occur at low temperatures over extended peri-
ods of time [10]. It is probable that in nature low temperature
environments would result in the decomposition of jarosite.
The products of the decomposition depend upon the jarosite
be it K, Na or Pb etc., but normally goethite and hematite are
formed together with soluble sulphates [11].
Argentojarosite (AgFe33+(SO4)2(OH)6)was first identi-
fied 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 suffi-
cient abundance as to be a silver bearing ore [2]. Argento-
jarosite 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].
2. Experimental
The thermal decomposition of jarosites has been stud-
ied for some considerable time [5–9]. However, no thermal
studies on the pure end member of argentojarosite have
been forthcoming, nor has there been studies on argento-
plumbojarosites [6]. It has been stated that the thermal
decomposition of jarosite begins at 400 ◦C with the loss
2.1. Synthesis of the argentojarosite
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 diffraction proved
the correct structure was obtained.
∗
Corresponding author. Tel.: +61 7 3864 2407; fax: +61 7 3864 1804.
E-mail address: r.frost@qut.edu.au (R.L. Frost).
0040-6031/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2005.06.008

R.L. Frost et al. / Thermochimica Acta 437 (2005) 30–33
31
2.2. Thermal analysis
cell. A description of the cell and principles of the emission
experiment have been published elsewhere [19–25]. Approx-
imately, 0.2 mg of the aregentojarosite mineral was spread
as a thin layer (approximately 0.2 ␮m) on a 6 mm diameter
platinum surface and held in an inert atmosphere within a
nitrogen-purged cell during heating. Emission spectra were
collected at intervals of 50 ◦C over the range 200–750 ◦C.
The time between scans (while the temperature was raised
to the next hold point) was approximately 100 s sufficient
time for the heating block and the powdered sample to reach
thermal equilibrium. Spectra were acquired by 1064 scans
over the temperature range 100–300 ◦C and 128 scans over
the range 350–900 ◦C (approximate scan time 45 s), with a
nominal resolution of 4 cm−1. Spectroscopic manipulation
such as baseline adjustment, smoothing and normalisation
was performed with the GRAMS® software package (Galac-
tic Industries Corporation, Salem, NH, USA).
Thermal decomposition of the argentojarosite was carried
out in a TA® Instruments (series Q500) in a nitrogen atmo-
sphere (80 cm3/min). 34.4 mg of sample with a heating rate
of 5 ◦C/min, resolution of 6 to 1000 ◦C. Several replicates
were analysed. The products of the thermal decomposition
were analysed by PXRD. The furnace temperature was regu-
lated 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 for
the determination of water vapour, sulphur dioxide, sulphur
trioxide, carbon dioxide and oxygen.
2.3. Raman spectroscopy
The crystals of the argentojarosite were oriented on the
stage of an Olympus BHSM microscope, equipped with 10×
and 50× objectives and part of a Renishaw 1000 Raman
microscope system. Raman spectra were excited by a HeNe
laser (633 nm) at a resolution of 2 cm−1 in the range between
100 and 4000 cm−1. Repeated acquisition using the highest
magnification was accumulated to improve the signal to noise
ratio. Spectra were calibrated with the 520.5 cm−1 line of a
silicon wafer. To ensure that the correct spectra are obtained,
the incident excitation radiation was scrambled. Previous
studies by the authors provide more details of the experi-
mental technique [12–18].
3. Results and discussion
3.1. Thermogravimetric analysis and mass
spectrometric analysis
The TG analysis of the synthetic argentojarosite together
with the DTG analysis is shown in Fig. 1. The ion current
2.4. Infrared spectroscopy
Infrared spectra were obtained with a Nicolet Nexus 870
FTIR spectrometer with a smart endurance single bounce
diamond ATR cell. Spectra over the 4000–525 cm−1 range
were obtained by the co-addition of 64 scans with a resolution
of 4 cm−1 and a mirror velocity of 0.6329 cm/s. Spectra were
co-added to improve the signal to noise ratio.
FTIR emission spectroscopy was carried out on a Nico-
let spectrophotometer equipped with a TGS detector, which
was modified by replacing the IR source with an emission
Fig. 1. Thermogravimetric and differential thermogravimetric analysis of
argentojarosite.
Fig. 2. Ion current curves (a) for evolved water vapour as a function of tem-
perature and (b) for oxygen and sulphur dioxide as a function of temperature.

32
R.L. Frost et al. / Thermochimica Acta 437 (2005) 30–33
curves for mass 18 are shown in Fig. 2a. The ion current
curves for masses 32 and 64 are shown in Fig. 2b. The small
mass loss at 31.7 ◦C is ascribed to the loss of physically
adsorbed water. Differences in the temperatures of the mass
loss, TG and the ion current of the evolvedgasesresultedfrom
the TG and mass spectrometry analyses being undertaken on
different samples. Nevertheless, the ion current of the evolved
water vapour clearly shows the mass loss step at 228 ◦C is
due to dehydroxylation. The mass loss is 3.4%, which cor-
responds to the loss of one water molecule, theoretical mass
loss 3.16%. The second mass loss at 383.4 ◦C corresponds
to a mass loss of 2.5 moles of water due to dehydroxylation.
The mass loss of 1.58% at 463 ◦C does not correspond to
the loss of sulphate as sulphur dioxide or sulphur trioxide.
The ion current curve of evolved water vapour does show a
broad feature at 490 ◦C, which appears to correspond with
the 463 ◦C mass loss. The reason for the different tempera-
tures was that the TG and evolved ion current analyses were
conducted at different rates. The ion current experiment was
conducted at a considerable faster rate than the thermal anal-
ysis.
with the theoretical mass loss of 25.38%. The mass loss at
790 ◦C corresponds to loss of oxygen. The ion current curve
shows a temperature of 778 ◦C. The products of the ther-
mal decomposition of argentojarosite as determined by XRD
analysis are hematite and silver metal.
3.2. Infrared emission spectroscopy
The use of infrared emission spectroscopy enables the
changes in the structure of the argentojarosite to be followed
in situ at elevated temperatures. As the material is heated, the
infrared radiation is emitted and is detected by the infrared
detector. The infrared emission spectra of Ag-jarosite in the
OH stretching region are shown in Fig. 3 over the 100–400 ◦C
temperature range. No intensity remains in this band above
400 ◦C. showing dehydroxylation of the Ag-jarosite is com-
plete by this temperature. The IE spectra of the sulphate
stretching region are shown in Fig. 4. In the IE spectrum at
100 ◦C, four bands are observed at 1205, 1091, 1024 (shoul-
der) and 1010 cm−1. The bands show a slight red shift with
temperature increase. No changes are observed in the spectral
profile until after 350 ◦C at which point dehydroxylation has
taken place. The definition and resolution of the bands is lost
at 550 ◦C and no intensity remains in the spectra at 750 ◦C;
The biggest mass loss occurs at 548.7 ◦C with a mass loss
of 24.3%. The ion current curves show this mass loss is due
entirely to the loss of sulphur dioxide, compares favourably
Fig. 3. Infrared emission spectra of the hydroxyl-stretching region of argen-
tojarosite.
Fig. 4. Infrared emission spectra of the 650–2250 cm−1 region of argento-
jarosite.

R.L. Frost et al. / Thermochimica Acta 437 (2005) 30–33
33
thus showing that the loss of sulphate has occurred by this
temperature.
[4] J.E. Dutrizac, J.L. Jambor, Rev. Miner. Geochem. 40 (2000) 405.
[5] S. Nagai, N. Yamanouchi, Nippon Kagaku Kaishi 52 (1949) 83.
[6] J.L. Kulp, H.H. Adler, Am. J. Sci. 248 (1950) 475.
[7] G. Cocco, Periodico di Mineralogia 21 (1952) 103.
[8] A.I. Tsvetkov, E.P. Val’yashikhina, Doklady Akademii Nauk SSSR
89 (1953) 1079.
4. Conclusions
[9] A.I. Tsvetkov, E.P. Val’yashikhina, Doklady Akademii Nauk SSSR
93 (1953) 343.
[10] J.E. Dutrizac, J.L. Jambor, Chapter 8 Jarosites and Their Application
in Hydrometallurgy, 2000, 405.
[11] P.S. Thomas, D. Hirschausen, R.E. White, J.P. Guerbois, A.S. Ray,
J. Therm. Anal. Calorim. 72 (2003) 769.
[12] R.L. Frost, J. Raman Spectrosc. 35 (2004) 153.
[13] R.L. Frost, Spectrochim. Acta, Part A: Mol. Biomol. Spectrosc. A
60 (2004) 1439.
[14] R.L. Frost, O. Carmody, K.L. Erickson, M.L. Weier, D.O. Henry, J.
Cejka, J. Mol. Struct. 733 (2004) 203.
[15] R.L. Frost, L. Duong, M. Weier, Spectrochim. Acta, Part A: Mol.
Biomol. Spectrosc. 60 (2004) 1853.
Argentojarosite was exploited at Rio Tinto, Spain from
Roman and even pre-Roman times for silver production [26].
Argentojarosites had an important influence on the accumu-
lation of wealth of both Europe and South America [26].
Thermal analysis shows the relatively low temperatures at
which argentojarosite can be converted to metallic silver. The
existence of large quantities of argentojarosite and argento-
plumbojarosite resulted in the formation of large quantities
of metallic silver.
[16] R.L. Frost, M. Weier, Neues Jahrbuch fuer Mineralogie, Monatshefte
(2004) 445.
[17] R.L. Frost, M.L. Weier, J. Raman Spectrosc. 35 (2004) 299.
[18] R.L. Frost, M.L. Weier, M.O. Adebajo, Thermochim. Acta 419
(2004) 119.
[19] R.L. Frost, A.M. Vassallo, Clays Clay Miner. 44 (1996) 635.
[20] R.L. Frost, G.A. Cash, J.T. Kloprogge, Vib. Spectrosc. 16 (1998)
173.
[21] R.L. Frost, J.T. Kloprogge, Tijdschr Klei Glas Keram. 19 (1998) 11.
[22] R.L. Frost, H. Ruan, J.T. Kloprogge, Internet J. Vib. Spectrosc. 4
(2000) 1.
Acknowledgments
The financial and infra-structure support of the Inorganic
Materials Research Program of the School of Physical and
Chemical Sciences, Queensland University of Technology is
gratefully acknowledged. The Australian Research Council
(ARC) is thanked for funding.
[23] R.L. Frost, K.L. Erickson, Thermochim. Acta 421 (2004) 51.
[24] R.L. Frost, N.K. Roberts, Springer Series Surf. Sci. 23 (2003) 203.
[25] R.L. Frost, Z. Ding, H.D. Ruan, J. Therm. Anal. Calorim. 71 (2003)
783.
[26] J.E. Dutrizac, J.L. Jambor, Appl. Miner., Proc. Int. Congr. Appl.
Miner. Miner. Ind. 2 (1985) 507.
References
[1] W.T. Schaller, J. Wash. Acad. Sci. 13 (1923) 233.
[2] C.A. Schempp, Am. J. Sci. 6 (1923) 73.
[3] J.E. Dutrizac, J.L. Jambor, J.B. O’reilly, CIM Bull. 76 (1983) 78.