The thermal stability and thermal decomposition pathways for synthesized composite iowaite/woodallite have been determined
using thermogravimetry analysis in conjunction with evolved gas mass spectrometry. Dehydration of the hydrotalcites occurred
over a range of 56–70�C. The first dehydroxylation step occurred at around 255�C and, with the substitution of more iron(III)
for chromium(III) this temperature increased to an upper limit of 312�C. This trend was observed throughout all decomposition
steps. The release of carbonate ions as carbon dioxide gas initialised at just above 300�C and was always accompanied by loss
of hydroxyl units as water molecules. The initial loss of the anion in this case the chloride ion was consistently observed
to occur at about 450�C with final traces evolved at 535 to 780�C depending of the Fe:Cr ratio and was detected as HCl (m/z=36). Thus for this to occur, hydroxyl units must have been retained in the structure at temperatures upwards of 750�C. Experimentally
it was found difficult to keep CO2 from reacting with the compounds and in this way the synthesized iowaite-woodallite series somewhat resembled the natural
Evidence for the existence of primitive life forms such as lichens and fungi can be based upon the formation of oxalates.
These oxalates form as a film like deposit on rocks and other host matrices. The anhydrous oxalate mineral moolooite CuC2O4 as the natural copper(II) oxalate mineral is a classic example. Another example of a natural oxalate is the mineral wheatleyite
High resolution thermogravimetry coupled to evolved gas mass spectrometry shows decomposition of wheatleyite at 255°C. Two
higher temperature mass losses are observed at 324 and 349°C. Higher temperature mass losses are observed at 819, 833 and
857°C. These mass losses as confirmed by mass spectrometry are attributed to the decomposition of tennerite CuO. In comparison
the thermal decomposition of moolooite takes place at 260°C. Evolved gas mass spectrometry for moolooite shows the gas lost
at this temperature is carbon dioxide. No water evolution was observed, thus indicating the moolooite is the anhydrous copper(II)
oxalate as compared to the synthetic compound which is the dihydrate.
The thermal decomposition of beaverite and plumbojarosite was studied using a combination of thermogravimetric analysis coupled
to a mass spectrometer.
The mineral beaverite Pb(Fe,Cu)3(SO4)2(OH)6 decomposes in three stages attributed to dehydroxylation, loss of sulphate and loss of oxygen, which take place at 376 and
420, 539 and 844°C. In comparison three thermal decomposition steps are observed for plumbojarosite PbFe6(SO4)4(OH)12 at 376, 420 and 502°C attributed to dehydroxylation; loss of sulphate occurs at 599°C; and loss of oxygen and formation of
lead occurs at 844 and 953°C. The temperatures of the thermal decomposition of the natural plumbojarosite were found to be
less than that for the synthetic jarosite. A comparison of the thermal decomposition of plumbojarosite with argentojarosite
is made. The understanding of the chemistry of the thermal decomposition of minerals such as beaverite, argentojarosite and
plumbojarosite and related minerals is of vital importance in the study known as ‘archeochemistry’.
The precursors of carbonate minerals have the potential to react with greenhouse gases to form many common carbonate minerals.
The carbonate bearing minerals, magnesite, calcite, strontianite and witherite, were synthesised and analysed using a combination
of thermogravimetry and evolved gas mass spectrometry. The DTG curves show that as both the mass and the size of the metal
cationic radii increase, the inherent thermal stability of the carbonate also increases dramatically. It is proposed that
this inherent effect is a size stabilisation relationship between that of the carbonate and the metal cation. As the cationic
radius increases in size, the radius approaches and in the case of Sr2+ and Ba2+ exceeds that of the overall size of the carbonate anion. The thermal stability of these minerals has implications for the
geosequestration of greenhouse gases. The carbonates with the larger cations show significantly greater stability.
Authors:R Frost, R Wills, J Kloprogge, and W Martens
with mass spectrometry has been used to study the thermal decomposition of
a synthetic hydronium jarosite. Five mass loss steps are observed at 262,
294, 385, 557 and 619C. The mass loss step at 557C is sharp and
marks a sharp loss of sulphate as SO3 from the hydronium
jarosite. Mass spectrometry through evolved gases confirms the first three
mass loss steps to dehydroxylation, the fourth to a mass loss of the hydrated
proton and a sulphate and the final step to the loss of the remaining sulphate.
Changes in the molecular structure of the hydronium jarosite were followed
by infrared emission spectroscopy. This technique allows the infrared spectrum
at the elevated temperatures to be obtained. Infrared emission spectroscopy
confirms the dehydroxylation has taken place by 400 and the sulphate loss
by 650C. Jarosites are a group of minerals formed in evaporite deposits
and form a component of the efflorescence. The minerals can function as cation
and heavy metal collectors. Hydronium jarosite has the potential to act as
a cation collector by the replacement of the proton with a heavy metal cation.
A combination of high resolution thermogravimetric analysis coupled to a gas evolution mass spectrometer has been used to study the thermal decomposition of liebigite. Water is lost in two steps at 44 and 302°C. Two mass loss steps are observed for carbon dioxide evolution at 456 and 686°C. The product of the thermal decomposition was found to be a mixture of CaUO4 and Ca3UO6. The thermal decomposition of liebigite was followed by hot-stage Raman spectroscopy. Two Raman bands are observed in the 50°C spectrum at 3504 and 3318 cm-1 and shift to higher wavenumbers upon thermal treatment; no intensity remains in the bands above 300°C. Three bands assigned to the υ1 symmetric stretching modes of the (CO3)2- units are observed at 1094, 1087 and 1075 cm-1 in agreement with three structurally distinct (CO3)2- units. At 100°C, two bands are found at 1089 and 1078 cm-1. Thermogravimetric analysis is undertaken as dynamic experiment with a constant heating rate whereas the hot-stage Raman spectroscopic experiment occurs as a staged experiment. Hot stage Raman spectroscopy supports the changes in molecular structure of liebigite during the proposed stages of thermal decomposition as observed in the TG-MS experiment.
Authors:R. Frost, J. Bouzaid, A. Musumeci, J. Kloprogge, and W. Martens
stability and thermal decomposition pathways for synthetic iowaite have been
determined using thermogravimetry in conjunction with evolved gas mass spectrometry.
Chemical analysis showed the formula of the synthesised iowaite to be Mg6.27Fe1.73(Cl)1.07(OH)16(CO3)0.336.1H2O
and X-ray diffraction confirms the layered structure. Dehydration of the iowaite
occurred at 35 and 79C. Dehydroxylation occurred at 254 and 291C.
Both steps were associated with the loss of CO2. Hydrogen
chloride gas was evolved in two steps at 368 and 434C. The products of
the thermal decomposition were MgO and a spinel MgFe2O4.
Experimentally it was found to be difficult to eliminate CO2
from inclusion in the interlayer during the synthesis of the iowaite compound
and in this way the synthesised iowaite resembled the natural mineral.
Iron doped boehmite nanofibres with varying iron content have been prepared at low temperatures using a hydrothermal treatment
in the presence of poly(ethylene oxide) surfactant. The resultant nanofibres were characterized by X-ray diffraction (XRD),
and transmission electron microscopy (TEM). TEM images showed the resulting nanostructures are predominantly nanofibres when
Fe doping is no more than 5%; in contrast nanosheets were formed if Fe doping was above 5%. For the 10% Fe doped boehmite,
a mixed morphology of nanofibres and nanosheets were obtained. Nanotubes instead of nanofibres were observed in samples with
20% added iron. The Fe doped boehmite and the subsequent nanofibres/nanotubes were analysed by thermogravimetric and differential
thermogravimetric methods. Boehmite nanofibres decompose at higher temperatures than non-hydrothermally treated boehmite and
nano-sheets decompose at lower temperatures than the nanofibres.
Authors:R. Frost, J. Kristóf, W. Martens, M. Weier, and E. Horváth
mineral sabugalite (HAl)0.5[(UO2)2(PO4)]2⋅8H2O, has been studied using a combination of energy
dispersive X-ray analysis, X-ray diffraction, dynamic and controlled rate
thermal analysis techniques. X-ray diffraction shows that the starting material
in the thermal decomposition is sabugalite and the product of the thermal
treatment is a mixture of aluminium and uranyl phosphates. Four mass loss
steps are observed for the dehydration of sabugalite at 48°C (temperature
range 39 to 59°C), 84°C (temperature range 59 to 109°C), 127°C
(temperature range 109 to 165°C) and around 270°C (temperature range
175 to 525°C) with mass losses of 2.8, 6.5, 2.3 and 4.4%, respectively,
making a total mass loss of water of 16.0%. In the CRTA experiment mass loss
stages were found at 60, 97, 140 and 270°C which correspond to four dehydration
steps involving the loss of 2, 6, 6 and 2 moles of water. These mass losses
result in the formation of four phases namely meta(I)sabugalite, meta(II)sabugalite,
meta(III)sabugalite and finally uranyl phosphate and alumina phosphates. The
use of a combination of dynamic and controlled rate thermal analysis techniques
enabled a definitive study of the thermal decomposition of sabugalite. While
the temperature ranges and the mass losses vary due to the different experimental
conditions, the results of the CRTA analysis should be considered as standard
data due to the quasi-equilibrium nature of the thermal decomposition process.
Authors:R. L. Frost, W. Martens, and M. O. Adebajo
Summary High resolution TG coupled to a gas evolution mass spectrometer has been used to study the thermal properties of a chromium based series of Ni/Cu hydrotalcites of formulae NixCu6-xCr2(OH)16(CO3)×4H2O where x varied from 6 to 0. The effect of increased Cu composition results in the increase of the endotherms and mass loss steps to higher temperatures. Evolved gas mass spectrometry shows that water is lost in a number of steps and that the interlayer carbonate anion is lost simultaneously with hydroxyl units. Differential scanning calorimetry was used to determine the heat flow steps for the thermal decomposition of the synthetic hydrotalcites. Hydrotalcites in which M2+ consist of Cu, Ni or Co form important precursors for mixed metal-oxide catalysts. The application of these mixed metal oxides is in the wet catalytic oxidation of low concentrations of retractable organics in water. Therefore, the thermal behaviour of synthetic hydrotalcites, NixCu6-xCr2(OH)16CO3×nH2O was studied by thermal analysis techniques in order to determine the correct temperatures for the synthesis of the mixed metal oxides.