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- Author or Editor: M. Weier x
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Abstract
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. Humboldtine as the natural iron(II) oxalate mineral is a classic example. Thermogravimetry coupled to evolved gas mass spectrometry shows dehydration takes place in two steps at 130 and 141°C. Loss of the oxalate as carbon dioxide occurs at 312 and 332°C. Dehydration is readily followed by Raman microscopy in combination with a thermal stage and is observed by the loss of intensity of the OH stretching vibration at 3318 cm-1. The application of infrared emission spectroscopy supports the results of the TG-MS. Three Raman bands are observed at 1470, 1465 and 1432 cm-1 attributed the CO symmetric stretching mode. The observation of the three bands supports the concept of multiple iron(II) oxalate phases. The significance of this work rests with the ability of Raman spectroscopy to identify iron(II) oxalate which often occurs as a film on a host rock.
Summary
Jarosites are a group of minerals formed in evaporite deposits and form a component of efflorescence. As such the minerals can function as cation and heavy metal collectors. Thermogravimetry coupled to mass spectrometry has been used to study three Australian jarosites which are predominantly K, Na and Pb jarosites. Mass loss steps of K-jarosite occur over the 130 to 330 and 500 to 622C temperature range and are attributed to dehydroxylation and desulphation. In contrast the behaviour of the thermal decomposition of Na-jarosite shows three mass loss steps at 215 to 230, 316 to 352 and 555 to 595C. The first mass loss step for Na-jarosite is attributed to deprotonation. For Pb-jarosite two mass loss steps associated with dehydroxylation are observed at 390 and 418C and a third mass loss step at 531C is attributed to the loss of SO3. Thermal analysis is an excellent technique for the study of jarosites. The analysis depends heavily on the actual composition of the jarosite.
Thermal decomposition of liebigite
A high resolution thermogravimetric and hot-stage Raman spectroscopic study
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 υ 1 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.
Thermal decomposition of sabugalite
A controlled rate thermal analysis study
The 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.
Abstract
The thermal decompositions of hydrotalcites with hexacyanoferrate(II) and hexacyanoferrate(III) in the interlayer have been studied using thermogravimetry combined with mass spectrometry. X-ray diffraction shows the hydrotalcites have a d(003) spacing of 11.1 and 10.9 which compares with a d-spacing of 7.9 and 7.98 for the hydrotalcite with carbonate or sulphate in the interlayer. XRD was also used to determine the products of the thermal decomposition. For the hydrotalcite decomposition the products were MgO, Fe2O3 and a spinel MgAl2O4. Dehydration and dehydroxylation take place in three steps each and the loss of cyanide ions in two steps.
