Thermal decomposition of the natural hydrotalcites carrboydite and hydrohonessite

Article abstract of DOI:10.1016/S0040-6031(03)00262-4

A combination of high-resolution thermogravimetric analysis (HRTGA) coupled to a gas evolution mass spectrometer combined with infrared emission spectroscopy has been used to study the thermal decomposition of two Australian hydrotalcites carrboydite (Ni₆Al₂(SO₄,CO ₃)(OH)₁₆·4H₂O) and hydrohonessite (Ni₆Fe₂(SO₄,CO₃)(OH) ₁₆·7H₂O). High-resolution thermal analysis shows the decomposition takes place in five steps. Each step is related to the loss of water, hydroxyl units, carbonate and sulphate. Infrared emission spectroscopy clearly identifies the presence of these molecular species and the temperature at which they are lost. Minor amounts of carbonate are observed in the minerals. Water is in two states in the structure, namely weakly hydrogen bonded and strongly hydrogen bonded. The symmetry of the sulphate anion is reduced through this hydrogen bonding.

Full text of DOI:10.1016/S0040-6031(03)00262-4

Thermochimica Acta 407 (2003) 1–9
Thermal decomposition of the natural hydrotalcites
carrboydite and hydrohonessite
Ray L. Frosta,∗, Matt L. Weierb, Meagan E. Clissolda,
Peter A. Williamsa, J. Theo Kloproggeb
Centre for Industrial and Process Mineralogy, School of Science, Food and Horticulture,
a
University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia
Centre for Instrumental and Developmental Chemistry, Queensland University of Technology,
b
G.P.O. Box 2434, Brisbane, Qld 4001, Australia
Received 19 February 2003; received in revised form 11 April 2003; accepted 12 April 2003
Abstract
A combination of high-resolution thermogravimetric analysis (HRTGA) coupled to a gas evolution mass spectrometer
combined with infrared emission spectroscopy has been used to study the thermal decomposition of two Australian hydro-
talcites carrboydite (Ni6Al2(SO4,CO3)(OH)16·4H2O) and hydrohonessite (Ni6Fe2(SO4,CO3)(OH)16·7H2O). High-resolution
thermal analysis shows the decomposition takes place in five steps. Each step is related to the loss of water, hydroxyl units,
carbonate and sulphate. Infrared emission spectroscopy clearly identifies the presence of these molecular species and the
temperature at which they are lost. Minor amounts of carbonate are observed in the minerals. Water is in two states in the
structure, namely weakly hydrogen bonded and strongly hydrogen bonded. The symmetry of the sulphate anion is reduced
through this hydrogen bonding.
© 2003 Elsevier Science B.V. All rights reserved.
Keywords: Dehydration; Dehydroxylation; Hydrotalcite; Infrared emission spectroscopy; High-resolution thermogravimetric analysis
1. Introduction
uses of hydrotalcites. Hydrotalcites are important in
the removal of environmental hazards in acid mine
drainage [9,10]. Hydrotalcite formation also offers a
mechanism for the disposal of radioactive wastes [11].
Hydrotalcite formation may also serve as a means of
heavy metal removal from heavy metal contaminated
waters [12]. These hydrotalcites are readily synthe-
sised by a co-precipitation method [13–15].
Hydrotalcites, or layered double hydroxides (LDH)
are fundamentally anionic clays, and are less well
known and more diffuse in nature than cationic clays
like smectites [16,17]. The structure of hydrotalcite
can be derived from a brucite structure (Mg(OH)2)
in which, e.g. Al3+ or Fe3+ (pyroaurite–sjögrenite)
Interest in the study of hydrotalcites results from
their potential use as catalysts [1–5]. The reason for
the potential application of hydrotalcites as catalysts
rests with the ability to make mixed metal oxides at
the atomic level, rather than at a particle level. Such
mixed metal oxides are formed through the thermal
decomposition of the hydrotalcite [6,7]. Hydrotalcites
may also be used as components in new nano-materials
such as nano-composites [8]. There are many other
∗
Corresponding author.
E-mail address: r.frost@qut.edu.au (R.L. Frost).
0040-6031/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0040-6031(03)00262-4

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R.L. Frost et al. / Thermochimica Acta 407 (2003) 1–9
substitutes a part of the Mg2+. This substitution
creates a positive layer charge on the hydroxide
layers, which is compensated by interlayer anions
or anionic complexes [18,19]. In hydrotalcites a
position was determined by X-ray diffraction. Both
minerals were found to be phase pure.
2.2. Thermal analysis
broad range of compositions are possible of the type
[M1−x2+Mx3+(OH)2][A
]
x/n·yH2O, where M2+
n−
Thermal decomposition of the hydrotalcite was
carried out in a TA® Instruments incorporated high-
resolution thermogravimetric analysis (HRTG) (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 500 ◦C. With the quasi-isothermal, quasi-isobaric
heating program of the instrument, the furnace tem-
perature was regulated precisely 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. Only
selected gases were analysed.
and M3+ are the di- and trivalent cations in the oc-
tahedral positions within the hydroxide layers with
x normally between 0.17 and 0.33. A
n−
is an ex-
changeable interlayer anion [20]. There exists in
nature a significant number of hydrotalcites which
are formed as deposits from ground water containing
Ni2+ and Fe3+ [21]. These are based upon the disso-
lution of Ni–Fe sulphides during weathering. Among
these naturally occurring hydrotalcites are carrboy-
dite and hydrohonessite [22,23]. These two hydro-
talcites are based upon the incorporation of sulphate
into the interlayer with expansions of 10.34–10.8 Å.
Normally, the hydrotalcite structure based upon
takovite (Ni, Al) and hydrotalcite (Mg, Al) has basal
spacings of ∼8.0 Å where the interlayer anion is
carbonate. If the carbonate is replaced by sulphate
then the mineral carrboydite is obtained. Similarly
reevesite is the Ni, Fe hydrotalcite with carbonate
as the interlayer anion, which when replaced by sul-
phate the minerals honessite and hydrohonessite are
obtained.
The use of thermal analysis techniques for the study
of the thermal decomposition of hydrotalcites is com-
mon [24]. Heating sjögrenite or pyroaurite at <200 ◦C
caused the reversible loss of H2O. At 200–250 ◦C on
static heating, or 200–350 ◦C on dynamic heating, very
little H2O or CO2 were lost, but changes in the in-
frared spectrum and DTA effects were observed [24].
To date the number of thermal analysis studies of these
minerals is very limited. No studies of the thermal de-
composition of carrboydite and hydrohonessite have
been forthcoming.
2.3. Infrared emission spectroscopy
FTIR emission spectroscopy was carried out on a
Nicolet spectrometer equipped with a TGS detector,
which was modified by replacing the IR source with an
emission cell. A description of the cell and principles
of the emission experiment have been published else-
where. Approximately 0.2 mg of the hydrotalcite min-
eral 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. Details of the technique have been published
by the authors [27,28]. Spectral manipulation such
as baseline adjustment, smoothing and normalisation
was performed using the GRAMS® software package
(Galactic Industries Corporation, Salem, NH, USA).
3. Results and discussion
3.1. High-resolution thermogravimetric analysis
and mass spectrometric analysis
2. Experimental
2.1. Minerals
The HRTG of carrboydite is shown in Fig. 1.
Five principal weight loss steps are observed at 74,
99.6, 195, 407 and 703 ◦C. Thermal decomposition
causes the loss of water initially. The initial weight
loss including the steps at 74 and 99.6 ◦C is around
14%. Calculations for the weight loss based upon the
The minerals carrboydite and hydrohonessite were
obtained from CSIRO and originated from Western
Australia [25,26]. The composition of the minerals
was checked using an electron probe. The phase com-

R.L. Frost et al. / Thermochimica Acta 407 (2003) 1–9
3
Fig. 1. TG and DTG curves for carrboydite.
formula (Ni6Al2(SO4)(OH)16·4H2O) is 14.0%. The
• Decomposition Steps 3 and 4
broad weight loss step over the 100–250 ◦C is 6.5%.
The weight loss step at 407 ◦C is 12.0%. These two
steps are assigned to the dehydroxylation of the car-
rboydite. The experimentally determined weight loss
for the carrboydite is 18.5% which may be compared
with a theoretical weight loss of 35.1%. The final
weight loss step is 11.6% and is attributed to sulphate
loss as SO2. The theoretical weight loss step is 12.4%.
The mass spectrometric analyses together with the
DTGA patterns of carrboydite are shown in Fig. 2.
What may be clearly observed is that the first four
weight loss steps can be attributed to water loss either
through dehydration or dehydroxylation. The mass
gain of CO2 is minimal for carrboydite showing the
mineral formulation as (Ni6Al2(SO4)(OH)16·4H2O) is
good. The mass loss step at 703 ◦C is assignable to
the interlayer anion sulphate decomposition.
Ni6Al2(SO4)(OH)16 → Ni6Al2(SO4)O8 + 8H2O
temperature: 407 ◦C.
• Decomposition Step 5
Ni6Al2(SO4)O8 → 6NiO + Al2O3 + SO2.
The TG and DTGA patterns for hydrohonessite
are reported in Fig. 3. Hydrohonessite has a for-
mula Ni6Fe2(SO4,CO3)(OH)16·7H2O. The difference
between hydrohonessite and honessite simply rests
with the amount of hydration. Honessite has four
molecules of water per formula unit and hydrohones-
site approximately seven molecular units of water.
Honessite is related to the mineral reevesite. The dif-
ference is simply the interlayer anion. In the case of
honessite and hydrohonessite, it is the sulphate anion.
In the example of reevesite it is carbonate. In some
ways this imposes a difficulty for natural minerals
in that mixed species can be obtained, i.e. both car-
bonate and sulphate exist in the interlayer. Thus, the
natural mineral is some composition between the end
members of reevesite and hydrohonessite.
The following chemical reactions for the thermal
decomposition of carrboydite are proposed:
• Decomposition Steps 1 and 2
Ni6Al2(SO4)(OH)16 · 4H2O
→ Ni6Al2(SO4)(OH)16 + 4H2O
Three weight loss steps are observed which are at-
tributed to dehydration. These are observed at 71, 95.6
temperature: 95.6 ◦C.

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R.L. Frost et al. / Thermochimica Acta 407 (2003) 1–9
Fig. 2. MS and DTG curves for carrboydite.
and 137 ◦C with weight losses of 2.3, 1.56 and 5.2%.
The total weight loss ascribed to dehydration is 9.06%.
Based on the formula Ni6Fe2(SO4)(OH)16·7H2O the
theoretical weight loss for dehydration is 13.15%
based upon seven molecules of water in the formula. If
there are four water molecules in the formula unit then
the theoretical weight loss of 7.9% is obtained. Thus,
the experimental result for dehydration is between
Fig. 3. TG and DTG curves for hydrohonessite.

R.L. Frost et al. / Thermochimica Acta 407 (2003) 1–9
5
that of honessite and hydrohonessite. Such a result is
not unexpected as these minerals may dehydrate and
rehydrate quite readily. Two weight loss steps are ob-
served at 294 and 329 ◦C with a total weight loss of
10.5%. These two steps are attributed to dehydroxyla-
tion. The theoretical weight loss for dehydroxylation
is 28.39% based upon the hydrohonessite formula.
There is an apparent large difference between the
experimentally determined and theoretical calculated
results. This means that the number of hydroxyl units
in the formula is not 16 but some number less. Two
further weight loss steps are observed at 394 and
455 ◦C. The total for these two steps is 4.0%. This
weight loss step is attributed to the loss of carbonate.
The final weight loss step is observed at 646 ◦C, and
is attributed to loss of sulphate.
(b) More steps are involved in the thermal decompo-
sition of hydrohonessite.
(c) In general, the temperature of decomposition for
each step is less for hydrohonessite than that for
carrboydite.
The following mechanisms are suggested for the
thermal decomposition of hydrohonessite:
• Decomposition Step 1
Ni6Fe2(SO4, CO3)(OH)16 · 7H2O
→ Ni6Fe2(SO4, CO3)(OH)16 + 3H2O
temperature: 95.6 ◦C.
• Decomposition Step 2
Ni6Fe2(SO4, CO3)(OH)16 · 4H2O
→ Ni6Fe2(SO4, CO3)(OH)16 + 4H2O
The complexity of the DTGA patterns is reflected in
the mass spectrum of hydrohonessite. The mass spec-
trometric analyses together with the DTGA patterns
of hydrohonessite are shown in Fig. 4. The difference
between the MS patterns of carrboydite and hydro-
honessite are summarised as follows:
temperature: 137 ◦C.
• Decomposition Steps 3 and 4
Ni6Fe2(SO4, CO3)(OH)16
→ Ni6Fe2(SO4, CO3)O8 + 8H2O
temperatures: 294 and 329 ◦C.
(a) The hydrohonessite has significantly more carbon-
ate in the structure.
Fig. 4. MS and DTG curves for hydrohonessite.

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R.L. Frost et al. / Thermochimica Acta 407 (2003) 1–9
Fig. 5. Infrared emission spectra of the hydroxyl stretching region of carrboydite.
• Decomposition Step 5
niques [29–32]. The infrared emission spectra of car-
rboydite are shown in Fig. 5. The spectra at the lower
temperatures appear with low signal to noise. This is
because of the low energy of thermal emission at the
lower temperatures, namely 50, 75 and 100 ◦C. As the
temperature is raised the energy of emitted radiation is
increased and the signal to noise ratio improves. The
spectra in the hydroxyl stretching region are then band
fitted to find the position and relative intensity of the
bands.
Ni6Fe2(SO4, CO3)O8 → Ni6Fe2(SO4)O8 + CO2
temperatures: 394 and 455 ◦C.
• Decomposition Step 6
Ni6Fe2(SO4)O8 → 6NiO + Fe2O3 + SO2.
3.2. Infrared emission spectroscopy of carrboydite
and hydrohonessite
In the 100 ◦C spectrum, three overlapping bands
are observed at 3040, 3372 and 3578 cm−1. The first
two bands are assigned to the OH stretching modes
of water and the last band to the OH stretching vi-
bration of the hydroxyl units in the carrboydite struc-
ture. The intensity of the first two bands approaches
One method of following the thermal decomposi-
tion of these natural hydrotalcites is to use a combina-
tion of vibrational spectroscopy and a thermal stage.
Infrared emission spectroscopy is one of these tech-

R.L. Frost et al. / Thermochimica Acta 407 (2003) 1–9
7
Fig. 6. Infrared emission spectra of the hydroxyl stretching region of hydrohonessite.
zero by 400 ◦C and intensity remains in the hydroxyl
stretching vibration of the OH units up to 500 ◦C.
These bands show a red shit with heating. The bands
are observed at 3090, 3411 and 3588 cm−1 at 250 ◦C.
At 350 ◦C, the bands are observed at 3370, 3542 and
3617 cm−1. The infrared emission spectra of the hy-
droxyl stretching region of hydrohonessite are shown
in Fig. 6. The intensity of the hydroxyl stretching re-
gion is lost by 400 ◦C. The temperatures for the dehy-
droxylation are in harmony with the TG results. The
thermal decomposition of hydrohonessite takes place
at lower temperatures than carrboydite.
assigned to the hydroxyl stretching vibrations of water
and the band at 3540 cm−1 to the OH stretching vi-
bration of the OH units. In the 250 ◦C, spectrum these
bands are observed at 3406, 3521 and 3604 cm−1
.
The infrared emission spectra of carrboydite in the
SO4 and CO3 stretching region are shown in Fig. 7.
The spectra show the thermal decomposition steps
in line with the chemical reactions proposed above.
The spectrum is most interesting as it shows the fea-
tures that are observed in the HRTG and MS pat-
terns. The spectrum shows three bands at 1054, 1107
and 1159 cm−1, attributed to the SO4 antisymmetric
stretching vibrations. There is a small inflection in
the spectrum at 973 cm−1 which is assigned to the
SO4 symmetric stretching vibration. The observation
In the infrared emission spectrum of hydrohones-
site at 150 ◦C, three overlapping bands are observed
at 3101, 3371 and 3540 cm−1. The first two bands are

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R.L. Frost et al. / Thermochimica Acta 407 (2003) 1–9
Fig. 7. Infrared emission spectra of the SO4 stretching region of carrboydite.
of multiple antisymmetric stretching vibrations shows
the reduction in symmetry of the sulphate anion in
the interlayer. It is suggested that this reduction in
symmetry is brought about through hydrogen bond-
ing to both the water molecules in the interlayer and
to the surface hydroxyls. Two other bands of sig-
nificantly lower intensity are observed at 1405 and
1459 cm−1. These bands are assigned to the CO3 an-
tisymmetric stretching vibrations. The observation of
carbonate even though of very low concentrations is
in harmony with the MS results where some very mi-
nor evolved CO2 gas was observed. The spectra also
show two bands at 1612 and 1670 cm−1. These two
bands are assigned to the bending modes of water.
The fact that two bands are observed with different
wavenumbers rests with the differences in hydrogen
bonding strength. The band at 1670 cm−1 is attributed
to water which is strongly hydrogen bonded to the
hydroxyl units of Ni and Al. An alternative expla-
nation is that the water is strongly hydrogen bonded
to the sulphate anions. This explanation appears less
likely. The 1612 cm−1 band is assigned to weakly hy-
drogen bonded water. Such water fills the interlayer
and could be scribed to water bonding to other water
molecules.
The set of spectra clearly show the phase changes
involved in the thermal decomposition of this mineral.
As for the analysis of the IE spectra of carrboydite,
three sets of bands are observed. Firstly, the bands
observed at 978, 1049 and 1151 cm−1 are assigned

R.L. Frost et al. / Thermochimica Acta 407 (2003) 1–9
9
to the SO4 symmetric and antisymmetric stretching
vibrations. Secondly, a complex band profile is cen-
tred on 1446 cm−1 ascribed to the CO3 antisymmetric
stretching vibrations. Thirdly, the bands observed at
1614 and 1654 cm−1 are assigned to the water bend-
ing modes. In the IE spectrum at 200 ◦C, bands are
observed at 1030, 1096 and 1159 cm−1. These bands
are assigned to the SO4 antisymmetric stretching
modes. The broad profile at 1441 cm−1 is attributed
to the antisymmetric CO3 stretching modes. What
is noted is the absence of any HOH bending modes.
At this temperature no water is observed in the IE
spectra. The spectrum at 600 ◦C shows the absence
of both water and carbon dioxide. At this tempera-
ture the SO4 spectra become less complex. Only two
bands are observed at 972 and 1121 cm−1 assigned
to the SO4 symmetric and antisymmetric stretching
modes. The reason why the spectra have become sim-
ple is due to the lack of either water or OH units with
which the sulphate could hydrogen bond.
hydroxyls and the carbonate is lost. The sulphate is
lost at significantly higher temperatures.
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4. Conclusions
The HRTGA of the two related minerals carrboy-
dite and hydrohonessite have been studied. These hy-
drotalcite minerals show at least five weight loss steps
ascribed to (a) water desorption, (b) dehydration, (c)
dehydroxylation, (d) de-carbonate-ing (hydrohones-
site), and (e) de-sulphating.
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(a) The hydrohonessite has significantly more carbon-
ate in the structure.
(b) More steps are involved in the thermal decompo-
sition of hydrohonessite.
(c) In general, the temperature of decomposition for
each step is less for hydrohonessite than that for
carrboydite.
The thermal analysis results are complemented by
the infrared emission spectra. These spectra confirm
the presence of carbonate in both minerals but only
in very low concentrations. This suggests that the for-
mula of carrboydite and hydrohonessite as written is
correct, namely Ni6Al2(SO4,CO3)(OH)16·4H2O and
Ni6Fe2(SO4,CO3)(OH)16·7H2O, respectively. The IE
spectra show the temperatures at which the water, the
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scopic study of synthetic and natural pyrophyllite. Neues
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