Thermal stability of the soil minerals destinezite and diadochite Fe 3+2(PO4)(SO4)(OH)·6H 2O - Implications for soils in bush fires

Article abstract of DOI:10.1016/j.tca.2011.04.014

Thermogravimetry combined with evolved gas mass spectrometry has been used to ascertain the stability of the soil minerals destinezite and diadochite Fe³⁺₂(PO₄)(SO₄)(OH)·6H ₂O. These two minerals are identical except for their morphology. Diadochite is amorphous whereas destinezite is crystalline. Both minerals are found in soils. It is important to understand the stability of these minerals because soils are subject to bush fires especially in Australia. The thermal analysis patterns of the two minerals are similar but not identical. Subtle differences are observed in the DTG patterns. For destinezite, two DTG peaks are observed at 129 and 182 °C attributed to the loss of hydration water, whereas only a broad peak with maximum at 84 °C is observed for diadochite. Higher temperature mass losses at 685 °C for destinezite and 655 °C for diadochite, are due to sulphate decomposition, based upon the ion current curves. This research has shown that at low temperatures the minerals are stable but at high temperatures, as might be experienced in a severe bush fire, the minerals decompose.

Full text of DOI:10.1016/j.tca.2011.04.014

Thermochimica Acta 521 (2011) 121–124
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Thermal stability of the soil minerals destinezite and diadochite
3
+
Fe (PO )(SO )(OH)·6H O – Implications for soils in bush fires
2
4
4
2
∗
Ray L. Frost , Sara J. Palmer
Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Thermogravimetry combined with evolved gas mass spectrometry has been used to ascertain the sta-
Received 9 February 2011
Received in revised form 12 April 2011
Accepted 14 April 2011
3+
bility of the soil minerals destinezite and diadochite Fe 2(PO4)(SO4)(OH)·6H2O. These two minerals are
identical except for their morphology. Diadochite is amorphous whereas destinezite is crystalline. Both
minerals are found in soils. It is important to understand the stability of these minerals because soils are
subject to bush fires especially in Australia. The thermal analysis patterns of the two minerals are similar
but not identical. Subtle differences are observed in the DTG patterns. For destinezite, two DTG peaks are
Available online 6 May 2011
Keywords:
Thermogravimetry
Destinezite
◦
observed at 129 and 182 C attributed to the loss of hydration water, whereas only a broad peak with
◦
◦
maximum at 84 C is observed for diadochite. Higher temperature mass losses at 685 C for destinezite
◦
and 655 C for diadochite, are due to sulphate decomposition, based upon the ion current curves. This
Diadochite
Phosphate
Sulphate
Water of hydration
Soil minerals
Bush fires
research has shown that at low temperatures the minerals are stable but at high temperatures, as might
be experienced in a severe bush fire, the minerals decompose.
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
There are many minerals found in soils, many of which are
and acceptors to oxygen atoms of adjacent slabs. The mineral has a
clay-like appearance under a scanning electron microscope (SEM).
The amorphous form of the mineral is called diadochite and is com-
monly regarded as a colloidal mineral [1–4]. This no doubt defines
why the composition of the mineral is open to question at least
in terms of the number of waters of hydration. Indeed it is not
known whether the two minerals are identical or not. The mineral
is yellowish brown to reddish to brownish black and all colours in-
between. The mineral may ion exchange with many cations and as
a consequence the colour varies.
formed from aqueous solutions. Some of the minerals remain
as the so-called colloidal minerals [1–4]. Two such minerals are
destinezite and diadochite Fe3 (PO )(SO )(OH)·6H O. The name
+
2
4
4
2
“
destinezite” is given to samples that are triclinic and crystalline,
while diadochite is poorly ordered and X-ray amorphous. Both min-
erals have the chemical composition: Fe3 (PO )(SO )(OH)·6H O.
+
2
4
4
2
The mineral destinezite is a hydrated hydroxy phosphate of fer-
ric iron with some sulphate substitution, with the iron in the
ferric state [3,5–7]. The mineral [6] is triclinic with cell param-
The mineral is diagenetically related to delvauxite
CaFe3+ (PO ,SO ) (OH) ·4-6H O, which is also amorphous or
4
4
4
2
8
2
◦
◦
eters = 9.584 A˚ , b = 9.748 A˚ , c = 7.338 A˚ , ˛ = 93.07 , ˇ = 95.78 , and
ꢀ
consists of infinite chains of Fe(O,OH,H O)6 octahedra, sulphate
tetrahedra and phosphate tetrahedra linked by a unique system
of vertex sharing. The chains are weakly bonded into layers by
possibly triclinic [8–10]. These minerals are formed through the
reaction of acid sulphate solutions with already formed phosphate
minerals. Because of the known and ill-defined structure of these
minerals, it is very important to undertake structural studies.
Due to the amorphous nature of diadochite, the application of
thermoanalytical techniques is very important. The minerals are
found in soils and caves [6,11].
◦
= 105.32 . According to Peacor et al. [6] the crystal structure
2
hydrogen bonding between OH and H O of the Fe(III) octahedra
and oxygen ions of the sulphate tetrahedra [6]. Layers of tetra-
2
hedral/octahedral chains alternate with sheets of H O molecules.
Peacor et al. [6] states that the structure resembles hydrated clay
The minerals destinezite and diadochite act as a sink for both
sulphur and phosphorus in soils. Further, the minerals have been
found in old or ancient burial sites. There is a vital need to study
the thermal stability of these minerals in more detail and to ascer-
tain whether the minerals are stable in a bush fire environment. To
the best of the authors’ knowledge no such studies have ever been
undertaken.
2
minerals, with H O molecules that act as hydrogen bond donors
2
∗ Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804.
E-mail address: r.frost@qut.edu.au (R.L. Frost).
0
040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2011.04.014

1
22
R.L. Frost, S.J. Palmer / Thermochimica Acta 521 (2011) 121–124
Fig. 1. Thermogravimetric analysis of destinezite.
◦
2
. Experimental
1000 C at high resolution. The TGA instrument was coupled to a
Balzers (Pfeiffer) mass spectrometer for gas analysis. Only selected
gases such as water and sulphur dioxide were analysed. X-Ray
diffraction patterns were collected using a Philips X’pert wide angle
2.1. Minerals
The mineral destinezite originated from Argenteau, Visé, Liège
X-Ray diffractometer, operating in step scan mode, with Cu K␣
radiation (1.54052 A˚ ).
Province, Belgium and was supplied by The Mineralogical Research
Company as was the diadochite which originated from Alum Cave
Bluff, Great Smoky Mts, Sevier Co., Tennessee, USA.
The minerals have been analysed and the data published [12].
3. Results and discussion
2
.2. Thermogravimetric analysis
The results of thermogravimetric analysis of destinezite and
diadochite are displayed in Figs. 1 and 2, respectively. The ion cur-
rent curves of the evolved gases of these two minerals are shown in
Figs. 3 and 4. The DTG curve of destinezite displays mass loss steps at
Thermal decomposition of the destinezite and diadochite was
®
carried out in a TA Instrument incorporated high-resolution
thermogravimetric analyser (series Q500) in a flowing nitrogen
atmosphere (80 cm /min). Approximately 68 mg of sample was
◦
129, 182, 576, 604 and 685 C with 9.48, 16.75, 3.44 and 15.45% mass
3
losses. For diadochite, three mass loss steps are observed in the DTG
◦
◦
heated in an open platinum crucible at a rate of 5.0 C/min up to
curve at 84, 124 (shoulder), 289, 655 C, and 723 (shoulder) with
Fig. 2. Thermogravimetric analysis of diadochite.

R.L. Frost, S.J. Palmer / Thermochimica Acta 521 (2011) 121–124
123
Fig. 3. Ion current curves of the evolved gases resulting from the thermal decom-
Fig. 4. Ion current curves of the evolved gases resulting from the thermal decom-
position of destinezite.
position of diadochite.
mass losses of 18.95, 5.55 and 17.4%. There is a strong resemblance
between the two thermal analysis patterns, which goes some way
to confirm that the two minerals are closely related. Destinezite
is the crystalline product of the diadochite gel. The crystallinity
of destinezite appears to cause the dehydration of the structure to
occur at more defined temperatures, compared to diadochite where
broad peaks are observed.
also due to dehydroxylation. The following decomposition step is
proposed:
+
3+
4
2
Fe3 (PO )(SO )(OH) → 2Fe
O(PO )(SO ) + H O
2
4
4
4
4
2
2
If the formula Fe3+2(PO4,SO4)(OH)·6H2O with 6 moles of water
◦
Mass losses observed below 200 C are attributed to the dehy-
is used, the formula mass is 428 amu. The total loss of water is 25.2%.
◦
◦
dration of the mineral structures. The mass loss at 189 C is
The total mass loss for destinezite at 129 and 182 C is 26.23%, which
◦
approximately double the mass loss observed at 129 C. Therefore,
is in harmony with the theoretical result. The total mass loss for
diadochite is 24.9%, which again is close to the theoretical result. It is
the mass loss dehydration steps of destinezite are given by the
following equations:
◦
not clearly understood why there is a mass loss at 289 C for diado-
chite, which is not observed for destinezite. However, it is proposed
that the poor crystallinty of diadochite traps some water molecules
in its disordered structure, which then requires additional energy
Fe3 (PO )(SO )(OH)·6H O → Fe
+
3+
2
(PO )(SO )(OH)·4H O
2
4
4
2
4
4
2
◦
+
2H O at 129 C
2
(heat) to remove.
◦
A mass loss step is found at 576 C for destinezite. There are no
maximums in the ion current curves for any of the expected evolved
Fe3 (PO )(SO )(OH)·4H O → Fe
+
3+
(PO )(SO )(OH)
◦
2
4
4
2
2
4
4
gases, except for a peak for m/Z = 44 at 565 C which is attributed
◦
◦
4H O at 182 C
2
to CO2 evolution. Thus, the 3.44% mass loss over the 530–630 C
+
temperature range is attributed to the presence of some carbonate
impurity. These peaks are not found in the thermal analysis patterns
of diadochite. This seems to support the concept that the colloidal
mineral diadochite is purer than destinezite.
For diadochite, these two mass loss steps overlap, and the DTG
maximum occurs at 84 C. The DTG curve of diadochite is strongly
asymmetric on the higher temperature side and a peak may be
resolved at 123 C. The maximum in the DTG curve at 84 C is
assigned to the loss of water from the diadochite gel. In addition, a
mass loss of 5.55% is observed at 289 C. The ion current curves
show that water is the evolved gas at around this temperature
295 C). The formula of destinezite contains OH units. It is proba-
ble that the mass loss at 182 C is not only due to dehydration but
◦
◦
◦
◦
There is a high temperature mass loss for destinezite at 685 C
with a mass loss of 15.45%, while for diadochite a mass loss of
◦
◦
17.40% at 655 C occurs. In the ion current curves for destinezite,
◦
maxima are observed for SO , PO2 and related gases at 685 C. The
2
◦
◦
(
maximum for diadochite for SO is 660 C with a second maximum
at 720 C. There is a small shoulder observed at 725 C observed in
2
◦
◦
◦

1
24
R.L. Frost, S.J. Palmer / Thermochimica Acta 521 (2011) 121–124
the DTG curves. The following reaction is proposed to occur:
a sink for the free migration of phosphate into surroundings. The
implication is that these minerals are stable even if exposed to bush
fires.
Fe3 O(PO ) (SO ) → Fe O + 2FePO +2SO
+
4
4
2
4
2
3
4
4
3
4
. Conclusions
The two minerals diadochite and destinezite have the same
Acknowledgments
The financial and infra-structure support of the Queens-
land University of Technology, Chemistry discipline is gratefully
acknowledged. The Australian Research Council (ARC) is thanked
for funding the instrumentation.
3
+
formula Fe (PO )(SO )(OH)·6H O. Destinezite is the crystalline
2
4
4
2
material, whereas diadochite has been described as a colloidal min-
eral and displays amorphicity. The question must be asked: are
the two minerals identical. Such a question cannot be answered by
XRD. Thermal analytical techniques go someway to showing some
similarity in the two minerals. However, this is not definitive as
the thermal analysis patterns of the two minerals show significant
differences. Therefore, the question remains unanswered.
References
[
[
1] M. Foldvari, B. Nagy, Diadochite and destinezite from Martansentimre
Hungary], Foldtani Kozlony 115 (1985) 123–131.
2] M.Z. Fursova, Diadochite from the oxidation zone of the Karagaily polymetallic
deposit, Izvestiya Akademii Nauk Kazakhskoi SSR, Seriya Geologicheskaya 22
[
◦
For destinezite, two DTG peaks are observed at 129 and 182 C,
(
1965) 74–76.
and based upon the ion current curves are attributed to the loss
of water of hydration. For diadochite, a broad peak at 84 C is
attributed to this water mass loss. The thermal analysis patterns
of destinezite show small DTG peaks at 576 C attributed to a car-
bonate impurity. The thermal analysis patterns of diadochite show
an additional peak at 289 C, which is assigned to the loss of water
molecules trapped in the disordered structure. The higher temper-
ature mass losses at 685 C for destinezite and 655 C for diadochite
based upon the ion current curves are due to sulphate and phos-
phate decomposition. However, more sulphate anions are in the
structures, illustrated by the smaller multiplicity numbers used in
preparing the mass spectra.
[
3] L.D. German, Destinezite in the oxidation zone of the pyrite deposits of Blyava,
S. Ural, Zapiski Vserossiiskogo Mineralogicheskogo Obshchestva 85 (1956)
574–577.
◦
◦
[4] A.I. Ginzburg, The phosphates of granite pegmatites, Trudy Mineralogicheskogo
Muzeya, Akademiya Nauk SSSR (1952) 36–63.
[
5] V. Bouska, E.K. Lazarenko, Y.M. Melnik, E. Slanski, Destinezite, Acta Universitatis
Carolinae: Geologica (1960) 127–152.
◦
[
6] D.R. Peacor, R.C. Rouse, T.D. Coskren, E.J. Essene, Destinezite (“diadochite”),
Fe2(PO4)(SO4)(OH)·6H2O: its crystal structure and role as a soil mineral at Alum
Cave Bluff, Tennessee, Clays and Clay Minerals 47 (1999) 1–11.
◦
◦
[7] R. Sitzia, Infrared spectra of some natural phosphates, Rendiconti del Seminario
della Facolta di Scienze dell’Universita di Cagliari 36 (1966) 105–115.
[
8] E. Dittler, The origin of delvauxite, Chemie und Industrie der Kolloide 5 (1910)
5.
3
[9] K.A. Vakusevich, Mineral of the delvauxite type from recent sediments of the
Moscow Basin, Zapiski Vserossiiskogo Mineralogicheskogo Obshchestva 76
What this work has shown is that the two stoichiometrically
(
1947) 271–272.
related minerals destinezite and diadochite are quite stable up to
[
10] R. van Tassel, Autunite, apatite, delvauxite, evansite, and fluellite from the
Vis.acte.e region, Bulletin de la Societe Belge de Geologie, de Paleontologie et
d’Hydrologie 68 (1959) 226–248.
◦
2
00 C. These minerals are found in soils and function as a phos-
phate and sulphate sink. The origin of relatively stable destinezite
from diadochite gel-like medium in clayey parts of mineral dumps
and similar media including soils, thus represents the possibility of
phosphate/sulphate capture in the solid-solution phase and makes
[
[
11] W. Heyer, History and geology of the diadochite caves near Saalfeld (Saale),
Zeitschrift fuer Praktische Geologie 47 (1939) 165–168.
12] J.W. Anthony, R.A. Bideaux, K.W. Bladh, M.C. Nichols, Handbook of Mineralogy,
Mineral Data Publishing, Tuscon, Arizona, USA, 2000.