Thermal analysis of synthetic reevesite and cobalt substituted reevesite (Ni,Co)6Fe2(OH)16(CO3) ? 4H2O

Article abstract of DOI:10.1007/s10973-009-0145-x

The mineral reevesite and the cobalt substituted reevesite have been synthesised and studied by thermal analysis and X-ray diffraction. The d(003) spacings of the minerals ranged from 7.54 to 7.95 A. The maximum d(003) value occurred at around Ni:Co 0.4:0.6. This maximum in interlayer distance is proposed to be due to a greater number of carbonate anions and water molecules intercalated into the structure. This increase in carbonate anion content is attributed to an increase in surface charge on the brucite like layers. The maximum temperature of the reevesite decomposition occurs for the unsubstituted reevesite at around 220 °C. The effect of cobalt substitution results in a decrease in thermal stability of the reevesites. Four thermal decomposition steps are observed and are attributed to dehydration, dehydroxylation and decarbonation, decomposition of the formed carbonate and oxygen loss at ~807 °C. A mechanism for the thermal decomposition of the reevesite and the cobalt substituted reevesite is proposed.

Full text of DOI:10.1007/s10973-009-0145-x

J Therm Anal Calorim (2010) 100:125–131
DOI 10.1007/s10973-009-0145-x
Thermal analysis of synthetic reevesite and cobalt substituted
reevesite (Ni,Co)6Fe2(OH)16(CO3) ꢀ 4H2O
Kathleen H. Bakon Æ Sara J. Palmer Æ
Ray L. Frost
Received: 3 February 2009 / Accepted: 3 April 2009 / Published online: 19 June 2009
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract The mineral reevesite and the cobalt substituted
reevesite have been synthesised and studied by thermal
analysis and X-ray diffraction. The d(003) spacings of the
structure (Mg(OH)2) in which e.g. Al3? or Fe3? (pyroaurite-
2?
sjogrenite) substitutes a part of the Mg [2, 5–7]. This
¨
substitution creates a positive layer charge on the hydroxide
layers, which is compensated by interlayer anions or anionic
complexes. This surface charge is dependent upon the degree
of substitution. In hydrotalcites a broad range of composi-
˚
minerals ranged from 7.54 to 7.95 A. The maximum d(003)
value occurred at around Ni:Co 0.4:0.6. This maximum in
interlayer distance is proposed to be due to a greater number
of carbonate anions and water molecules intercalated into
the structure. This increase in carbonate anion content is
attributed to an increase in surface charge on the brucite like
layers. The maximum temperature of the reevesite decom-
position occurs for the unsubstituted reevesite at around
220 °C. The effect of cobalt substitution results in a decrease
in thermal stability of the reevesites. Four thermal decom-
position steps are observed and are attributed to dehydration,
dehydroxylation and decarbonation, decomposition of the
formed carbonate and oxygen loss at *807 °C. A mecha-
nism for the thermal decomposition of the reevesite and the
cobalt substituted reevesite is proposed.
Â
Ã
tions are possible of the type M21ꢁþxMx3þðOHÞ2
ꢀ yH2O,
x=n
where M2? and M3? are the di- and trivalent cations in the
octahedral positions within the hydroxide layers with x
normally between 0.17 and 0.33. An- is an exchangeable
interlayer anion [8]. There exists in nature a significant
number of hydrotalcites which are formed as deposits from
ground water containing Ni2? and Fe3? [9]. These are based
upon the dissolution of Ni-Fe sulphides during weathering.
Normally the hydrotalcite structure based upon takovite
(Ni,Al) and hydrotalcite (Mg,Al) has basal spacings of
˚
*8.0 A where the interlayer anion is carbonate. If the car-
bonate 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
sulphate the minerals honessite and hydrohonessite are
obtained. Among the naturally occurring hydrotalcites are
carrboydite and hydrohonessite [10, 11]. These two hydro-
talcites are based upon the incorporation of sulphate into the
Keywords Hydrotalcite ꢀ Brucite ꢀ Raman microscopy ꢀ
Carrboydite ꢀ Hydrohonessite ꢀ Takovite ꢀ Mountkeithite
Introduction
˚
interlayer with expansions of 10.34–10.8 A.
Hydrotalcites have been known for an extended period of
time [1–3]. Hydrotalcites, or layered double hydroxides
(LDH) are fundamentally known as anionic clays [4]. The
structure of hydrotalcite can be derived from a brucite
Our interest in reevesite originates from the fact that
reevesite and related minerals have been found in a wide
range of deposits in Western Australia and also have been
found in Shale balls resulting from the impact of a meteor
with earth at Wolfe Creek (W.A.). When a meteorite cra-
shed into earth, the impact can have devastating changes to
the environment. Such a meteor of some estimated 50,000
tonnes crashed into a remote area of Western Australia
some 300,000 years ago. The crater is some 875 m across
and approximately 65 m deep. The impact of the meteor
K. H. Bakon ꢀ S. J. Palmer ꢀ R. L. Frost (&)
Inorganic Materials Research Program, School of Physical
and Chemical Sciences, Queensland University of Technology,
GPO Box 2434, Brisbane, QLD 4001, Australia
e-mail: r.frost@qut.edu.au
123

126
K. H. Bakon et al.
can cause changes to the mineralogy of the district and
surrounds [12–15]. In the case of the Wolfe creek meteorite
fragments were scattered for over 4 km from the impact
site. The impact can cause the formation of new minerals
as a result of the high pressure and temperatures [16–18].
The meteor was originally an iron meteor containing both
phosphorus and nickel [19]. Weathering of the meteor
fragments has resulted in the formation of a range of new
minerals due to oxidation and corrosion [19]. The meteor
before impact in all probability contained the mineral
schreilbersite which with oxidation and hydration has
completely changed to several phosphate minerals includ-
ing lipscombite, cassidyite and collinsite [19]. Much of the
original meteorite has weathered to goethite. The presence
of Fe and Ni in the iron meteorite has resulted in the for-
mation of a hydrotalcite reevesite. In the interior of some of
the shale balls mineral formation of some of these above
minerals has occurred. Faust et al. [20] published infor-
mation on the formation of the mineral pecoraite [21, 22] in
the crevices of the Wolfe creek shale balls. Faust et al.
claimed the pecoraite was formed during mechanochemical
weathering. This however seems unlikely and the question
arises as to whether the mineral was formed during some
form of extraterrestrial weathering. The question arises as
to wether the weathering has occurred from the inside of
the shale ball or if thee weathering occurred through cracks
and crevices from the outside.
made: solution 1 contained 2 M NaOH and 0.2 M Na2CO3,
whilst solution 2 contained FeCl3 ꢀ 6H2O, NiCl2 ꢀ 6H2O
and CoCl2 ꢀ 6H2O in ratios shown in Table 1. Solution 1
was added to solution 2 drop wise using a burette under
vigorous stirring. The mixed solutions were left to stir for
24 h, and then vacuum filtered and washed with deionised
water to remove any residual salts. The resulting paste was
dried in an oven at 80 °C overnight.
X-ray diffraction
X-Ray diffraction patterns were collected using a Philips
X’pert wide angle X-Ray diffractometer, operating in step
˚
scan mode, with Cu Ka radiation (1.54052 A). Patterns
were collected in the range 3–90° 2h with a step size of
0.02° and a rate of 30 s per step. Samples were prepared as
a finely pressed powder into aluminium sample holders.
The Profile Fitting option of the software uses a model that
employs twelve intrinsic parameters to describe the profile,
the instrumental aberration and wavelength dependent
contributions to the profile.
Thermal analysis
Thermal decomposition of the reevesite was carried out in
a TAÒ Instruments incorporated high-resolution thermo-
gravimetric 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 500 °C. The TGA instrument was coupled to a
Balzers (Pfeiffer) mass spectrometer for gas analysis. Only
selected gases such as water and carbon dioxide were
analyzed.
Thermal analysis has been applied to the study of
hydrotalcites [23–31]. An important study of these hydro-
talcites is the thermal decomposition as this information
provides concepts on the thermal stability of reevesite. This
thermal stability is important in the assessment of whether
the reevesite was formed in the shale balls before impact or
whether the minerals are formed post-impact.
Results and discussion
Experimental
X-ray diffraction
Preparation of reevesite and Co doped reevesite
The X-ray diffraction patterns of the synthesised reevesites
and their d(003) spacings together with standard reference
patterns are shown in Fig. 1. As the cobalt component of
Reevesite and several Co doped reevesites were synthes-
ised by the co-precipitation method. Two solutions were
Table 1 Table of the ratios of
Ratio Ni:Co
Mole of NiCl2 ꢀ
Moles of CoCl2 ꢀ
Moles of FeCl3 ꢀ
Ni and Co compounds for the
synthesis of reevesite and cobalt
doped reevesite
6H2O
6H2O
6H2O
1:0
0.075
0.060
0.045
0.030
0.015
0.000
0.000
0.015
0.030
0.045
0.060
0.075
0.025
0.025
0.025
0.025
0.025
0.025
0.8:0.2
0.6:0.4
0.4:0.6
0.2:0.8
0:1
123

Thermal analysis of synthetic reevesite
127
move to higher 2 theta positions. The width of the d(003)
peak increases with cobalt substitution. This is indicative of
a decrease in crystallinity.
Thermal analysis
The thermogravimetric analysis of the synthesised reeve-
site without any substitution is shown in Fig. 2. A summary
of the thermoanalytical results is provided in Table 2. Four
thermal decomposition steps are observed at 52, 216, 515
and 807 °C. These mass loss steps are attributed to dehy-
dration, dehydroxylation and decarbonation, decomposi-
tion of the formed carbonate and oxygen loss at 807 °C.
The mass loss at these steps is 10.13%, 15.2%, 3.54% and
3.12%. Figure 3 displays the TG and DTG curves for
reevesite with 20% Co substitution. Four mass loss steps
are again observed at 45, 219, 550 and 791 °C with mass
losses of 12.06%, 15.55%, 1.23% and 1.89%. Figure 4
shows the TG and DTG curves for reevesite with 40% Co
substitution. The formula is (Ni0.8,Co0.2)6Fe2(OH)16(CO3) ꢀ
4H2O. Four mass loss steps are observed at 65, 123, 226,
540 and 770 °C with mass losses of 10.69%, 16.89%,
1.23% and 3.32%. An additional mass loss step is observed
at 123 °C. This step was not observed in Figs. 2 and 3. This
decomposition is assigned to interlayer water held in the
cobalt substituted reevesite. Figure 5 reports the TG and
DTG curves for reevesite with 60% Co substitution. The
formula is given by (Ni0.4,Co0.6)6Fe2(OH)16(CO3) ꢀ 4H2O.
Four mass loss steps are observed at *72, 203, 500 and
812 °C with mass losses of 9.16%, 14.22%, 1.80% and
8.86%. Figure 6 records the TG and DTG curves for
reevesite with 80% Co substitution (Ni0.2,Co0.8)6Fe2(OH)16
(CO3) ꢀ 4H2O. Four mass loss steps are observed at 63,
123, 193 and 872 °C with mass losses of 8.56%, 12.19%
and 14.28%. Figure 7 displays the thermal analysis of
cobalt reevesite (Co)6Fe2(OH)16(CO3) ꢀ 4H2O. Four mass
loss steps are observed at 65, 116, 183 °C and two over-
lapping steps at 796 and 821 °C. This thermal analysis
pattern resembles the TG-DTG patterns given in Figs. 4
and 6.
Fig. 1 X-ray diffraction of reevesite and cobalt doped reevesite and
standards
the synthesised reevesites increased, X-ray diffraction
peaks became sharper, indicating that the level of crystal-
linity of the mineral had also increased. The d(003) values
also varied with composition, finding a maximum for
Ni:Co = 0.4:0.6 reevesite, indicating that at this ratio the
greatest interlayer distance is obtained. This increase in
interlayer distance is proposed to be due to a greater
number of carbonate anions and water molecules are
intercalated into the structure. This increase can only be
brought about through an increase in positive surface
charge on the brucite-like surfaces. If some oxidation of
Co(II) to Co(III) occurred then this would provide a
mechanism for the increased positive surface charge. The
Ni:Co = 0.6:0.4 also had a relatively high interlayer dis-
tance. It is proposed that at the 0.4:0.6 ratio, the nickel and
cobalt interfere with each other’s packing patterns,
increasing the interlayer distance. The minimum d(003)
value occurred for Ni:Co = 1:0, where the disappearance
of cobalt caused the mineral layers to pack more closely
together. However the interlayer distance also decreased
from the 0.4:0.6 ratio with the reduction of nickel. With an
increasing cobalt concentration, the d(003) peak appears to
Mechanism of thermal decomposition
The following steps show the possible mechanism for the
decomposition of reevesite mass loss step 1 at 52 °C:
Ni6Fe2CO3ðOHÞ16 ꢀ 4H2O ! Ni6Fe2CO3ðOHÞ16 þ 4H2O
ð1Þ
Mass loss step 2 at 216 °C:
Ni6Fe2CO3ðOHÞ16! ðNiOÞ6Fe2CO5 þ 8H2O
Mass loss step 3 at 515 °C:
ð2Þ
123

128
K. H. Bakon et al.
Fig. 2 Thermogravimetric
analysis of reevesite (100% Ni)
Table 2 Summary of the results of the Thermal analysis of reevesite and cobalt doped reevesite
Ni:Co 1:0
Ni:Co 0.8:0.2
Ni:Co 0.6:0.4
Ni:Co 0.4:0.6
Ni:Co 0.2:0.8
Ni:Co 0:1
Temperature Mass
Temperature Mass
range (°C)
Temperature Mass
loss % range (°C)
Temperature Mass
loss % range (°C)
Temperature Mass
loss % range (°C)
Temperature Mass
loss % range (°C)
loss % range (°C)
loss %
29–35
3.13 30–36
10.13 36–145
15.20 145–426
3.54 426–646
3.12 646–889
6.58 31–37
12.06 37–85
15.55 85–150
1.23 150–447
1.89 447–873
2.72 31–37
3.68 37–134
7.01 134–380
16.89 380–626
3.32 626–885
5.80 31–38
9.16 38–82
2.75 32–38
2.13 38–73
8.56 73–133
12.19 133–348
14.28 348–666
666–892
2.71
1.74
9.80
13.11
1.14
5.67
35–137
137–407
407–669
669–929
14.22 82–140
1.80 140–367
8.86 367–859
Fig. 3 Thermogravimetric
analysis of 20% cobalt doped
reevesite (80% Ni)
123

Thermal analysis of synthetic reevesite
129
Fig. 4 Thermogravimetric
analysis of 40% cobalt doped
reevesite (60% Ni)
Fig. 5 Thermogravimetric
analysis of 60% cobalt doped
reevesite (40% Ni)
Implications for the formation of reevesite in meteorite
shale balls
ðNiOÞ6Fe2CO5 ! 6 NiO þ Fe2O3 þ CO2
ð3Þ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
Further reactions
The fact that reevesite is found in the inside of shale balls
raises the question as to weather the mineral was formed
(a) during the space travel of the meteor (b) during impact
(c) post-impact. Thermal analysis clearly shows that the
thermal stability of reevesite does not surpass around
220 °C. If the mineral was formed pre-impact or during
impact, it is unlikely that the mineral would still be present.
It is concluded that the reevesite was formed post-impact.
The question arises as to how the reevesite was formed.
The mineral is present in many parts of Western Australia
NiO þ CO2 ! NiCO3
and
Fe2O3 þ CO2 ! Fe2CO3
Mass loss step 4 at 807 °C:
NiCO3 ! NiO þ CO2
And
Fe2CO3 ! Fe2O3 þ CO2
123

130
K. H. Bakon et al.
Fig. 6 Thermogravimetric
analysis of 80% cobalt doped
reevesite (20% Ni)
Fig. 7 Thermogravimetric
analysis of 100% cobalt doped
reevesite (0% Ni)
as there are many nickel containing mineral deposits. If the
shale balls contained cracks, then moisture, carbon dioxide
could penetrate the interior of the shale ball. The interior of
the shale ball will corrode and a range of minerals
including the oxides of Fe and Ni could be formed. It is
suggested that the mineral reevesite was formed from
solution in the interior of the shale balls.
was confirmed by X-ray diffraction. The maximum inter-
layer distance occurred when there is maximum substitu-
tion of Co for Ni.
Thermogravimetric analysis proves there are at least four
decomposition steps attributed to dehydration, dehydroxy-
lation and decarbonation, decomposition of the formed car-
bonate. The maximum thermal stability occurs for the
unsubstituted reevesite and is around 220 °C. A mechanism
for the thermal decomposition of reevesite is proposed.
Conclusions
Acknowledgements The financial and infra-structure support of
the Queensland University of Technology Inorganic Materials
Research Program is gratefully acknowledged. The Australian
Research Council (ARC) is thanked for funding the Thermal
Analysis Facility.
In order to determine the thermal stability of natural
reevesite, reevesite and the cobalt substituted reevesites
were synthesised. The layered structure of the reevesite
123

Thermal analysis of synthetic reevesite
131
metastable (Mg,Fe)2SiO4 polymorph in Martian meteorites. Earth
Planet Sci Lett. 2007;261:469–75.
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