The mineral stichtite was synthesised and its thermal decomposition measured using thermogravimetry coupled to an evolved
gas mass spectrometer. Mass loss steps were observed at 52, 294, 550 and 670�C attributed to dehydration, dehydroxylation
and loss of carbonate. The loss of carbonate occurred at higher temperatures than dehydroxylation.
The thermal analysis of euchroite shows two mass loss steps in the temperature range 100–105 °C and 185–205 °C. These mass
loss steps are attributed to dehydration and dehydroxylation of the mineral. Hot-stage Raman spectroscopy (HSRS) has been
used to study the thermal stability of the mineral euchroite, a mineral involved in a complex set of equilibria between the
copper hydroxy arsenates: euchroite Cu2(AsO4)(OH)·3H2O → olivenite Cu2(AsO4)(OH) → strashimirite Cu8(AsO4)4(OH)4·5H2O → arhbarite Cu2Mg(AsO4)(OH)3. HSRS inolves the collection of Raman spectra as a function of the temperature. HSRS shows that the mineral euchroite decomposes
between 125 and 175 °C with the loss of water. At 125 °C, Raman bands are observed at 858 cm−1 assigned to the ν1 AsO43− symmetric stretching vibration and 801, 822, and 871 cm−1 assigned to the ν3 AsO43− (A1) antisymmetric stretching vibrations. A distinct band shift is observed upon heating to 275 °C. At 275 °C, the four Raman
bands are resolved at 762, 810, 837, and 862 cm−1. Further heating results in the diminution of the intensity in the Raman spectra, and this is attributed to sublimation of
the arsenate mineral. HSRS is the most useful technique for studying the thermal stability of minerals, especially when only
very small amounts of mineral are available.
Bayer hydrotalcites prepared using the seawater neutralisation (SWN) process of Bayer liquors are characterised using X-ray
diffraction and thermal analysis techniques. The Bayer hydrotalcites are synthesised at four different temperatures (0, 25,
55, and 75 °C) to determine the effect of synthesis temperature on the thermal stability of the Bayer hydrotalcite structures
and the mineralogical phases that form. The interlayer distance increased with increasing synthesis temperature, up to 55 °C,
and then decreased by 0.14 Å for Bayer hydrotalcites prepared at 75 °C. The three mineralogical phases identified in this
investigation are; (1) Bayer hydrotalcite, (2), calcium carbonate species, and (3) hydromagnesite. The DTG curve can be separated
into four decomposition steps; (1) the removal of adsorbed water and free interlayer water in hydrotalcite (30–230 °C), (2)
the dehydroxylation of hydrotalcite and the decarbonation of hydrotalcite (250–400 °C), (3) the decarbonation of hydromagnesite
(400–550 °C), and (4) the decarbonation of aragonite (550–650 °C).
Thermal analysis and differential thermal analysis offers a novel means of studying the desorption of acids such as stearic
acid from clay surfaces. Both adsorption and chemisorption can be distinguished through the differences in the temperature
of mass losses. Increased adsorption is achievable by adsorbing onto a surfactant adsorbed montmorillonite. Stearic acid sublimes
at 179 °C but when adsorbed upon montmorillonite sublimes at 207 and 248 °C. These mass loss steps are ascribed to the desorption
of the stearic acid on the external surfaces of the organoclays and from the de-chemisorption from the surfactant held in
the interlayer of the montmorillonite.
Authors:Ray Frost, János Kristóf, and Erzsébet Horváth
Controlled rate thermal analysis (CRTA) technology offers better resolution and a more detailed interpretation of the decomposition
processes of a clay mineral such as sepiolite via approaching equilibrium conditions of decomposition through the elimination
of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition
processes of non-isothermal nature reveal changes in the sepiolite as the sepiolite is converted to an anhydride. In the dynamic
experiment two dehydration steps are observed over the ~20–170 and 170–350 °C temperature range. In the dynamic experiment
three dehydroxylation steps are observed over the temperature ranges 201–337, 337–638 and 638–982 °C. The CRTA technology
enables the separation of the thermal decomposition steps.
The desorption of benzoic acid and stearic acid from sodium and calcium montmorillonites has been studied using thermogravimetric and differential thermogravimetric analysis. Desorption of benzoic acid from sodium montmorillonites occurs at 140 °C and from calcium montmorillonites at 179 °C. This increase in temperature is attributed to the benzoic acid bonding to the calcium in the interlayer. A lowering of the dehydroxylation temperature of montmorillonites is observed with acid adsorption. Stearic acid desorbs at 218 °C as observed by the DTG curves. The desorption pattern differs between the sodium montmorillonites and the calcium montmorillonites.
Thermogravimetric analysis has been used to determine the thermal stability of the mineral stercorite H(NH4)Na(PO4)·4H2O. The mineral stercorite originated from the Petrogale Cave, Madura, Eucla, Western Australia. This cave is one of many caves in the Nullarbor Plain in the South of Western Australia. The mineral is formed by the reaction of bat guano chemicals on calcite substrates. Upon thermal treatment the mineral shows a strong decomposition at 191 °C with loss of water and ammonia. Other mass loss steps are observed at 158, 317 and 477 °C. Ion current curves indicate a gain of CO2 at higher temperature and are attributed to the thermal decomposition of calcite impurity.
Authors:Laure-Marie Grand, Sara Palmer, and Ray Frost
Hydrotalcites based upon gallium as a replacement for aluminium in hydrotalcite over a Mg/Al ratio of 2:1 to 4:1 were synthesised.
The d(003) spacing varied from 7.83 Å for the 2:1 hydrotalcite to 8.15 Å for the 3:1 gallium containing hydrotalcite. A comparison
is made with the Mg/Al hydrotalcite in which the d(003) spacing for the Mg/Al hydrotalcite varied from 7.62 Å for the 2:1 Mg hydrotalcite to 7.98 Å for the 4:1 hydrotalcite.
The thermal stability of the gallium containing hydrotalcite was determined using thermogravimetric analysis. Four mass loss
steps at 77, 263–280, 485 and 828 °C with mass losses of 10.23, 21.55, 5.20 and 7.58% are attributed to dehydration, dehydroxylation
and decarbonation. The thermal stability of the gallium containing hydrotalcite is slightly less than the aluminium hydrotalcite.
The transition of cubic indium hydroxide to cubic indium oxide has been studied by thermogravimetric analysis complimented
with hot-stage Raman spectroscopy. Thermal analysis shows the transition of In(OH)3 to In2O3 occurs at 219 °C. The structure and morphology of In(OH)3 synthesised using a soft chemical route at low temperatures was confirmed by X-ray diffraction and scanning electron microscopy.
A topotactical relationship exists between the micro/nano-cubes of In(OH)3 and In2O3. The Raman spectrum of In(OH)3 is characterised by an intense sharp band at 309 cm−1 attributed to ν1 In–O symmetric stretching mode, bands at 1137 and 1155 cm−1 attributed to In-OH δ deformation modes, bands at 3083, 3215, 3123 and 3262 cm−1 assigned to the OH stretching vibrations. Upon thermal treatment of In(OH)3, new Raman bands are observed at 125, 295, 488 and 615 cm−1 attributed to In2O3. Changes in the structure of In(OH)3 with thermal treatment is readily followed by hot-stage Raman spectroscopy.
Authors:Laure-Marie Grand, Sara Palmer, and Ray Frost
The hydrotalcite based upon manganese known as charmarite Mn4Al2(OH)12CO3·3H2O has been synthesised with different Mn/Al ratios from 4:1 to 2:1. Impurities of manganese oxide, rhodochrosite and bayerite
at low concentrations were also produced during the synthesis. The thermal stability of charmarite was investigated using
thermogravimetry. The manganese hydrotalcite decomposed in stages with mass loss steps at 211, 305 and 793 °C. The product
of the thermal decomposition was amorphous material mixed with manganese oxide. A comparison is made with the thermal decomposition
of the Mg/Al hydrotalcite. It is concluded that the synthetic charmarite is slightly less stable than hydrotalcite.