A thermogravimetric study of the alunites of sodium, potassium and ammonium

Article abstract of DOI:10.1016/j.tca.2005.12.019

Thermogravimetry in tandem with mass spectrometry has been used to characterise the thermal decomposition of synthetic alunites of potassium, sodium and ammonium. Three mechanisms of decomposition are observed (a) dehydration, (b) dehydroxylation and (c) desulphation. The thermal decomposition of the three alunites is different. For NH₄-alunite, an additional process of de-ammoniation is observed which occurs simultaneously with dehydration. Dehydroxylation takes place in a series of four steps. De-sulphation occurs for K-alunite at 680°C in a single step in comparison with Na and NH₄ alunites where de-sulphation is observed in a series of four steps. The temperature of desulphation is cation dependent. The thermal decomposition is not completed until around 800°C.

Full text of DOI:10.1016/j.tca.2005.12.019

Thermochimica Acta 443 (2006) 56–61
A thermogravimetric study of the alunites of
sodium, potassium and ammonium
Ray L. Frost∗, Daria L. Wain, Rachael-Anne Wills, Anthony Musemeci, Wayde Martens
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology,
GPO Box 2434, Brisbane, Qld 4001, Australia
Received 31 August 2005; received in revised form 13 December 2005; accepted 21 December 2005
Available online 27 January 2006
Abstract
Thermogravimetry in tandem with mass spectrometry has been used to characterise the thermal decomposition of synthetic alunites of potassium,
sodium and ammonium. Three mechanisms of decomposition are observed (a) dehydration, (b) dehydroxylation and (c) desulphation. The thermal
decomposition of the three alunites is different. For NH4-alunite, an additional process of de-ammoniation is observed which occurs simultaneously
with dehydration. Dehydroxylation takes place in a series of four steps. De-sulphation occurs for K-alunite at 680 ◦C in a single step in comparison
with Na and NH4 alunites where de-sulphation is observed in a series of four steps. The temperature of desulphation is cation dependent. The
thermal decomposition is not completed until around 800 ◦C.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Alunite; Natroalunite; Ammonioalunite; Jarosite; Thermogravimetry; Sulphate
1. Introduction
One of the difficulties associated with the analysis of alunites
is that they are often poorly crystalline, making detection using
XRD techniques difficult. Another problem associated with the
study of alunites is their thermodynamic stability [15]. Often the
minerals are formed from acid-sulfate rich environments such
as acid mine drainage and acid-sulfate soils and as such their
solubility is controlled by climatic conditions, in particular the
temperature.
Interest in the chemistry of alunites stems from a number of
reasons. Firstly there is the possible discovery of alunites on
Mars [1,2]. Such a find implies the presence of water either at
present or at some time in the planetary past [3,4]. Interest in
such minerals and their thermal stability rests with the possible
identification of these minerals and dehydrated paragenetically
related minerals on planets [5]. There have been many stud-
ies on related minerals such as the Fe(II) and Fe(III) sulphate
minerals [6–11]. The importance of alunite formation and its
decomposition depends upon its presence in soils, sediments
and evaporite deposits [12]. These types of deposits have formed
in acid soils where the pH is less than 3.0 pH units [13]. Such
acidification results from the oxidation of pyrite which may be
from bacterial action or through air-oxidation. Thirdly alunites
are important from an environmental point of view. Alunites
are minerals which can function as collectors of heavy metals
and low concentrations can be found in the natural alunites.
Such minerals can act as a significant environmental sink [14].
Alunites are a group of minerals which form part of the
alunite supergroup [16]. The general formula is given by
DG3(TO4)2(OH,H2O)6 where the D sites are occupied by
either monovalent cations such as K, Na, NH4, H3O+ and oth-
ers, or divalent cations such as Ca, Ba, Sr, Pb, or trivalent
cations, for example, Bi. G is the trivalent cation either Al or
Fe3+ and T is S6+, As5+ or P5+. Alunites can be divided into
alunites and jarosites simply depending on whether the con-
centration of Al is >Fe (alunites) or Fe > Al (jarosites) [15].
Of course solid solution formation can exist across a wide
range of concentrations and substitutions. Common members
of the alunite group are alunite KAl3(SO4)2(OH)6, natroalunite
NaAl3(SO4)2(OH)6, ammonioalunite NH4Al3(SO4)2(OH)6,
sclossmacherite (H3O+,Ca2+)Al3(SO4)2(OH)6. The structure of
alunites is well known [17–19] and is trigonal with a 6.990, c
∗
Corresponding author. Tel.: +61 7 3864 2407; fax: +61 7 3864 1804.
˚
16.905 A space group R3m, with Z = 3.
E-mail address: r.frost@qut.edu.au (R.L. Frost).
0040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2005.12.019

R.L. Frost et al. / Thermochimica Acta 443 (2006) 56–61
57
Interestingly there have been few recent studies on the ther-
mal analysis of alunites [20]. The reason for this is unclear.
Perhaps it is because of the variation in composition which
occurs with natural minerals. Such variation can be overcome
with the use of synthetic minerals. The first recorded thermal
analysis of alunite occurred in 1919 [21]. Early studies focussed
on the effect of alunite impurity on the thermal treatment of other
chemicals [22–26]. Many studies occurred when new equipment
became available [27–40]. Recently, thermal analysis has been
used to enhance the understanding of the stability of minerals
which may be found on planets [5,41–51]. In this work, as part
of our studies of secondary mineral formation and their stabil-
ity, we report the comparison of the thermal analysis of selected
synthetic alunites.
Band component analysis of the DTG curves was undertaken
using the Jandel ‘Peakfit’ software package, which enabled the
type of fitting function to be selected and allows specific param-
eters to be fixed or varied accordingly. Band fitting was done
using a Gauss–Lorentz cross-product function with the mini-
mum number of component bands used for the fitting process.
The Gauss–Lorentz ratio was maintained at values greater than
0.7 and fitting was undertaken until reproducible results were
obtained with squared correlations of r2 greater than 0.995.
2.3. X-ray diffraction
X-ray diffraction (XRD) patterns were recorded using Cu
˚
K␣ radiation (n = 1.5418 A) on a Philips PANalytical X’ Pert
PRO diffractometer operating at 45 kV and 35 mA with 0.125◦
divergence slit, 0.25◦ anti-scatter slit, between 3 and 65◦ (2θ) at
a step size of 0.0167◦.
2. Experimental
2.1. Minerals
3. Results and discussion
The alunites were synthesised according to the following
recipes.
3.1. X-ray diffraction
2.1.1. Synthesis procedure of ammonium alunite
The XRD patterns of potassium, sodium and ammonium alu-
nite are shown in Fig. 1a–c. The figures compare the XRD
patterns with the published data. The XRD patterns show some
background in the 20–30◦ two theta. This simply means that
some of the synthesised product shows some amorphicity.
5.16 g of (NH4)2SO4 was dissolved in 40 ml of water. The
solution was transferred to an autoclave and 3 g of crystalline
Al(OH)3 added over a 5 min period. The autoclave was then
heated to 120 ◦C for 2 days. The crystalline white solid formed
was collected under vacuum filtration and dried. Samples were
dried in a vacuum desiccator at room temperature. 2.86 g of the
product was collected and this was identified by X-ray diffrac-
tion as ammonium alunite.
3.2. Thermogravimetric analysis
3.2.1. Potassium alunite
The thermogravimetric analysis of potassium alunite is
shown in Fig. 2. The mass spectra of evolved gases are reported
in Fig. 3. Three major processes occur for the thermal decompo-
sition of K-alunite. These are dehydration, dehydroxylation and
desulphation. The details of these processes are listed below.
2.1.2. Synthesis procedure of sodium alunite
0.82 g Na2SO4 and 5.9 g Al2(SO4)3 were dissolved in 40 ml
of water. This solution was covered and heated on a hotplate
at 150 ◦C for 4 h. The small amount of white precipitate was
collected under vacuum and dried for 4 days. The sample was
identified as sodium alunite by X-ray diffraction.
3.3. Mechanism for decomposition of potassium alunite
2.1.3. Synthesis procedure of potassium alunite
Step 1 Dehydration
Temperature up to ∼220 ◦C
One gram of K2SO4 and 5.9 g of Al2(SO4)3 were dissolved in
40 ml of water. The solution was heated at 150 ◦C in an autoclave
for 19 h. A white crystalline precipitate was collected and dried
under vacuum. The sample mass was 0.93 g. The product was
identified by X-ray diffraction as potassium alunite.
KAl3(SO4)2(OH)6·xH2O → KAl3(SO4)2(OH)6 + xH2O
This step represents the dehydration step and occurs over a
longtemperaturerangestartingat∼50 ◦C. Thewidetemperature
range for dehydration shows that water is strongly adsorbed to
the alunite surfaces.
2.2. Thermal analysis
Step 2 Dehydroxylation
Step 2a At 265 and 284 ◦C
Thermal decompositions of alunites were carried out in a
TA® Instruments incorporated high-resolution thermogravimet-
ric analyzer (series Q500) in a flowing nitrogen atmosphere
(80 cm3/min). Approximately 50 mg of sample was heated in
an open platinum crucible at a rate of 2.0 ◦C/min up to 1000 ◦C.
The TGA instrument was coupled to a Balzers (Pfeiffer) mass
spectrometer for gas analysis. The following gases/units were
analyzed: O2, S, SO3, SO2, OH and H2O. Mass/charge ratios
are measured for example O2 is 32/1 and 32/2.
KAl3(SO4)2(OH)6 → KO1/2Al3(SO4)2(OH)5 + (1/2) · H2O
This step represents the loss of one hydroxyl unit from the
K-alunite structure.
The MW of alunite is 414 and the % mass of OH units is
24.6%. Thus, there is 4.1% mass loss per OH unit. The mass
loss at 265 ◦C is 4.8% which represents the loss of one OH unit.

58
R.L. Frost et al. / Thermochimica Acta 443 (2006) 56–61
Fig. 2. Thermogravimetric analysis and differential thermogravimetric analysis
of potassium alunite.
in the MS curves corresponds with the mass loss in the TG
curves.
Step 2c At 489 ◦C
KOAl3(SO4)2(OH)4 → KO2Al3(SO4)2(OH)2 + H2O
There is a mass loss of 8% which represents the mass of two
OH units. The ion current curves for the evolved gases show for
m/z = 18 and 17 a mass gain at 490 ◦C. The mass gain in the MS
curves corresponds with the mass loss in the TG curves.
Step 2d At 586 ◦C
KO2Al3(SO4)2(OH)2 → KO3Al3(SO4)2 + H2O
Fig. 1. (a) X-ray diffraction pattern of synthetic potassium alunite and the potas-
sium alunite reference pattern (00-047-1884); (b) X-ray diffraction pattern of
synthetic sodium alunite and the sodium alunite reference pattern (00-041-1467)
and (c) X-ray diffraction pattern of synthetic ammonium alunite and the ammo-
nium alunite reference pattern (00-042-1430).
The ion current curves for the evolved gases show for m/z = 18
and 16 a mass gain at 281 ◦C. The mass gain in the MS curves
corresponds precisely with the mass loss in the TG curves.
Step 2b At 403 ◦C
KO1/2Al3(SO4)2(OH)5 → KOAl3(SO4)2(OH)4 + (1/2) · H2O
The mass loss at this step is 3.68% which may be equated
to the loss of one OH unit. In the equation above the oxygen
have been associated with the K. This is a matter of conve-
nience. The ion current curves for the evolved gases show
for m/z = 18 and 17 a mass gain at 406 ◦C. The mass gain
Fig. 3. Ion current curves for the evolved gases from the decomposition of
potassium alunite.

R.L. Frost et al. / Thermochimica Acta 443 (2006) 56–61
59
Fig. 4. Thermogravimetric analysis and differential thermogravimetric analysis
of sodium alunite.
Atthis step thereis amasslossof 3.43%which mustrepresent
the loss of the final OH units. No distinct peak is observed in the
ion current curves for m/z = 18, 17 and 16 at 586 ◦C. However,
the peaks at 490 ◦C is broad and there is a higher temperature
tail to this peak. It is proposed that the OH units for K-alunite
come off in a series of four steps at ∼265, 403, 489 and 586 ◦C.
Step 3 Desulphation
Loss of sulphate
Temperature 680 ◦C
KO3Al3(SO4)2 → 1/2K2SO4 + SO3 + 1.5Al2O3 + 2.5O2
The TG and DTG curves show a large mass loss of 24.6%
at 680 ◦C. The ion current curves for m/z = 64, 48 and 32 all
show a peak at 682 ◦C confirming the loss of sulphate at this
temperature.
Fig. 5. Ion current curves for the evolved gases from the decomposition of
sodium alunite.
3.3.1. Sodium alunite
The MW of Na-alunite is 398 and the % mass of OH units is
25.63%. Thus, there is 4.3% mass loss per OH unit. The mass
loss at 161 and 234 ◦C is 5.77% which represents the loss of one
and a half OH units.
The thermogravimetric analysis of sodium alunite is shown in
Fig. 4. The mass spectra of evolved gases are reported in Fig. 5.
Just as for K-alunite three major processes occur for the thermal
decomposition of K-alunite. These are dehydration, dehydroxy-
lation and desulphation. The details of these processes are listed
below.
Step 2b At 396 ◦C
NaO0.5Al3(SO4)2(OH)5 → NaO2Al3(SO4)2(OH)2 + 1.5H2O
3.4. Mechanism for decomposition of potassium alunite
The mass loss at this step is 11.25% which may be equated to
the loss of three OH units. The ion current curves for the evolved
gases show for m/z = 18 and 17 a mass gain at 396 ◦C. The mass
gain in the MS curves corresponds precisely with the mass loss
in the TG curves
Step 1 Dehydration
Temperature up to ∼120 ◦C
NaAl3(SO4)2(OH)6·xH2O → NaAl3(SO4)2(OH)6 + xH2O
Step 2c At 552 ◦C
This step represents the dehydration step and occurs over a
long temperature range starting at room temperature. The wide
temperature range for dehydration shows that water is strongly
adsorbed to the Na-alunite surfaces.
Step 2 Dehydroxylation
Step 2a At 161 and 234 ◦C
NaO2Al3(SO4)2(OH)2 → NaO3Al3(SO4)2 + H2O
There is a mass loss of some 33.6% which represents the
mass of two OH units. The ion current curves for the evolved
gases show for m/z = 18 and 17 a mass gain at 540 ◦C. The mass
gain in the MS curves corresponds with the mass loss in the TG
curves.
NaAl3(SO4)2(OH)6 → NaO0.5Al3(SO4)2(OH)5 + 1/2H2O
Step 3 Desulphation
Loss of sulphate
This step represents the loss of one hydroxyl unit from the
Na-alunite structure.

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R.L. Frost et al. / Thermochimica Acta 443 (2006) 56–61
The steps in the thermal decomposition of NH4-alunite are
reported below. As for the other alunites three basic processes
are involved dehydration, dehydroxylation, and desulphation.
In the case of NH4-alunite the process of de-ammoniation also
occurs.
3.5. Mechanism for decomposition of ammonium alunite
Step 1 De-ammoniation
Temperature 170 ◦C
NH4Al3(SO4)2(OH)6·xH2O → HAl3(SO4)2(OH)6
+ NH3 + xH2O
Fig. 6. Thermogravimetric analysis and differential thermogravimetric analysis
of ammonium alunite.
This step up to 170 ◦C involves the loss of adsorbed water
and the loss of ammonia. There is a 7.31% mass loss by 100 ◦C.
The ion current curves for m/z = 15, 28 show a mass gain at
∼121 ◦C. The m/z = 15 curve shows that some NH3 is retained
up to 582 ◦C. The m/z = 17 is not shown as it corresponds to both
NH3 and OH, both = 17. The total mass loss for NH3 is 4.5%.
This suggests that the mass loss due to dehydration is 3.81%.
It is proposed that as ammonia is lost, the ammonium ion is
replaced with a proton, thus making an H-alunite (protonated
alunite). There is no evidence from thermal analysis that this
protonated alunite exists. However, there is some spectroscopic
datathatbothprotonatedalunitesandprotonatedjarositescannot
not only be synthesised but also exist naturally [52–56].
Step 2 Dehydroxylation
Temperature 680 ◦C
NaO3Al3(SO4)2 → 1/2Na2SO4 + SO3 + 1.5Al2O3 + 2.5O2
The TG and DTG curves show a large mass loss of 33.6%
at 680 ◦C. The ion current curves for m/z = 64, 48 and 32 all
show a peak at 682 ◦C confirming the loss of sulphate at this
temperature.
3.4.1. Ammonium alunite
The TG/DTG curves for the thermal decomposition of NH4-
alunite are shown in Fig. 6. The ion current curves for selected
evolved gases are shown in Fig. 7.
Temperature 366 and 443 ◦C
HAl3(SO4)2(OH)6 → HAl3O3(SO4)2 + 3H2O
These steps are principally the dehydroxylation steps. For the
three steps at 170, 366 and 443 ◦C, there is a total mass loss of
21.77%. The theoretical mass loss of OH units is 25.9%.
Step 3 Desulphation
Loss of sulphate
Temperature 640, 689, 728 and 799 ◦C
2HAl3O2(SO4)2 → 4 · SO3 + 3Al2O3 + H2O
SO3 → SO2 + 1/2O2
Four steps are observed in the DTG curves at 640, 689, 728
and 799 ◦C. The ion current curves for m/z = 64, 48 and 32 show
that sulphate is decomposing over a wide temperature range
from 520 to 840 ◦C. The maximum for the ion current curves is
at 734 ◦C.
4. Conclusions
The thermal decomposition of alunite is of an archeochem-
istry importance as alunite was used as a source of potassium.
Further it has been suggested that the mineral exists on the planet
Mars. If so then the wide ranging temperatures of Mars would
affect the chemical composition of the alunite. Thus it is impor-
tant to understand the thermal decomposition of alunite.
Alunites decompose in a series of steps based upon dehydra-
tion, dehydroxylation and desulphation. The temperatures for
Fig. 7. Ion current curves for the evolved gases from the decomposition of
ammonium alunite.

R.L. Frost et al. / Thermochimica Acta 443 (2006) 56–61
61
these processes are dependent upon the cation present. Dehy-
droxylation takes place in a series of three to four steps. The
temperatures of the dehydroxylation show some cation depen-
dence. Desulphation for K-alunite occurs at 680 ◦C, Na-alunite
at 644 and 733 ◦C and for NH4 alunite at 640, 689 and 799 ◦C.
[22] P.F. Kerr, J.L. Kulp, American Mineralogist 33 (1948) 387.
[23] Y. Asada, Rikagaku Kenkyusho Iho 20 (1941) 757.
[24] T. Ao, Dainippon Yogyo Kyokai Zasshi 47 (1939) 399.
[25] S. Hirano, Kogyo Kagaku Zasshi 42 (1939) 27.
[26] S.C. Ogburn, Jr., H.B. Stere, J. Ind. Eng. Chem. (Washington, DC) 24
(1932) 288.
[27] G. Cocco, Periodico di Mineralogia 21 (1952) 103.
[28] G. Popa, D.N. Todor, Revue Roumaine de Chimie 16 (1971) 381.
[29] N.N. Poprukailo, V.P. Malyshev, G.L. Akhmetova, E.A. Buketov, Trudy
Khimiko-Metallurgicheskogo Instituta, Akademiya Nauk Kazakhskoi
SSR 15 (1970) 103.
[30] S. Cohen Arazi, T.G. Krenkel, Am. Mineral. 55 (1970) 1329.
[31] G.I. Veres, Zavodskaya Laboratoriya 35 (1969) 1482.
[32] L.J. Alias Perez, F. Girella, Estudios Geologicos (Madrid) 24 (1968) 79.
[33] A.K. Zapol’skii, V.S. Sazhin, N.N. Zakharova, A.I. Volkovskaya, Zhur-
nal Prikladnoi Khimii (Sankt-Peterburg, Russian Federation) 41 (1968)
1916.
Acknowledgments
The financial and infra-structure support of the Queensland
University of Technology Inorganic Materials Research Pro-
gram of the School of Physical and Chemical Sciences is grate-
fully acknowledged. The Australian Research Council (ARC) is
thanked for funding the thermal analysis facility.
References
[34] J.L.M. Vivaldi, F.G. Vilchez, Revista de Ciencia Aplicada 17 (1963)
320.
[1] M.E.E. Madden, R.J. Bodnar, J.D. Rimstidt, Nature 431 (2004) 821.
[2] D.T. Vaniman, D.L. Bish, S.J. Chipera, C.I. Fialips, J. William Carey,
W.C. Feldman, Nature (London, United Kingdom) 431 (2004) 663.
[3] J.-P. Bibring, Y. Langevin, F. Poulet, A. Gendrin, B. Gondet, M. Berthe,
A. Soufflot, P. Drossart, M. Combes, G. Bellucci, V. Moroz, N. Mangold,
B. Schmitt, Nature (London, United Kingdom) 428 (2004) 627.
[4] B.M. Hynek, Nature (London, United Kingdom) 431 (2004) 156.
[5] R.L. Frost, R.-A. Wills, M.L. Weier, A.W. Musumeci, W. Martens, Ther-
mochim. Acta 432 (2005) 30.
[35] I. Treiber, Studia Univ. Babes-Bolyai, Ser. 2 (1960) 73.
[36] N.S. Bayliss, D.F.A. Koch, Aust. J. Appl. Sci. 6 (1955) 312.
[37] N.S. Bayliss, D.F.A. Koch, Aust. J. Appl. Sci. 6 (1955) 298.
[38] A.I. Tsvetkov, E.P. Val’yashikhina, Doklady Akademii Nauk SSSR 89
(1953) 1079.
[39] T. Suzuki, M. Nakagawa, Kogyo Kagaku Zasshi 55 (1952) 71.
[40] G.M. Gad, Trans.Brit. Ceram. Soc. 50 (1951) 328.
[41] R.L. Frost, K.L. Erickson, M.L. Weier, A.R. Mckinnon, P.A. Williams,
P. Leverett, Thermochim. Acta 427 (2005) 167.
[6] M.S.R. Swamy, T.P. Prasad, B.R. Sant, J. Therm. Anal. 16 (1979) 471.
[7] M.S.R. Swamy, T.P. Prasad, B.R. Sant, J. Therm. Anal. 15 (1979) 307.
[8] S. Bhattacharyya, S.N. Bhattacharyya, J. Chem. Eng. Data 24 (1979)
93.
[42] R.L. Frost, J. Kristof, M.L. Weier, W.N. Martens, E. Horvath, J. Therm.
Anal. Calorim. 79 (2005) 721.
[43] R.L. Frost, A.W. Musumeci, T. Bostrom, M.O. Adebajo, M.L. Weier,
W. Martens, Thermochim. Acta 429 (2005) 179.
[9] M.S.R. Swami, T.P. Prasad, J. Therm. Anal. 19 (1980) 297.
[10] M.S.R. Swamy, T.P. Prasad, J. Therm. Anal. 20 (1981) 107.
[11] A.C. Banerjee, S. Sood, Therm. Anal. [Proc. Int. Conf.] 7th 1 (1982)
769.
[12] T. Buckby, S. Black, M.L. Coleman, M.E. Hodson, Mineral. Magazine
67 (2003) 263.
[44] R.L. Frost, A.W. Musumeci, J. Bouzaid, M.O. Adebajo, W.N. Martens,
J.T. Kloprogge, J. Solid State Chem. 178 (2005) 1940.
[45] R.L. Frost, K.L. Erickson, J. Therm.Anal. Calorim. 76 (2004) 217.
[46] R.L. Frost, K.L. Erickson, J. Therm. Anal. Calorim. 78 (2004) 367.
[47] R.L. Frost, S.J. Mills, K.L. Erickson, Thermochim. Acta 419 (2004)
109.
[13] P. A. Williams, Oxide Zone Geochemistry, Ellis Horwood Ltd., Chich-
ester, West Sussex, England, 1990.
[14] C. Drouet, D. Baron, A. Navrotsky, Am. Minera. 88 (2003) 1949.
[15] R.E. Stoffregen, C.N. Alpers, J.L. Jambor, Rev. Mineral. Geochem. 40
(2000) 453.
[48] R.L. Frost, M.L. Weier, Thermochim. Acta 409 (2004) 79.
[49] R.L. Frost, M.L. Weier, Neues Jahrbuch fuer Mineralogie, Monatshefte
(2004) 27.
[50] R.L. Frost, M.L. Weier, M.O. Adebajo, Thermochim. Acta 419 (2004)
119.
[16] J.E. Dutrizac, J.L. Jambor, Rev. Mineral. Geochem. 40 (2000) 405.
[17] G. Giuseppetti, C. Tadini, Neues Jahrbuch fuer Mineralogie, Monatshefte
(1980) 401.
[18] K. Okada, J. Hirabayashi, J. Ossaka, Neues Jahrbuch fuer Mineralogie,
Monatshefte (1982) 634.
[51] R.L. Frost, M.L. Weier, K.L. Erickson, J. Therm. Anal. Calorim. 76
(2004) 1025.
[52] G.P. Brophy, E.C. Scott, R.A. Snellgrove, Am. Mineral. 47 (1962)
112.
[53] J.E. Dutrizac, S. Kaiman, Can. Mineral. 14 (Pt. 2) (1976) 151.
[54] M. Hartman, V. Vesely, K. Jakubec, Collect. Czechoslovak Chem. Com-
mun. 52 (1987) 939.
[19] J. Ossaka, J. Hirabayashi, K. Okada, R. Kobayashi, T. Hayashi, Am.
Mineral. 67 (1982) 114.
[20] S. Tsutsumi, R. Otsuka, Therm. Anal. [Proc. Int. Conf.] 5th (1977)
456.
[21] E.S. Simpson, Chem. Eng. Mining Rev. 11 (1919) 297.
[55] J. Kubisz, Przeglad Geologiczny 17 (1969) 583.
[56] J.A. Ripmeester, C.I. Ratcliffe, J.E. Dutrizac, J.L. Jambor, Can. Mineral.
24 (1986) 435.