Characterization,correlation and kinetics of dolomite samples as outlined by thermal methods

In this study, the decomposition behavior of four dolomite samples of different origin was studied by X-ray diffractometer and simultaneous TG-DTA experiments. Three different decomposition regions were determined known as dehydration of inter-particle water, formation of MgO and calcite and decomposition of calcite. Kinetic analysis of these decomposition regions was analyzed and the related activation energies are determined. It was observed that the activation energies were in the range of 48–137 kJ mol⁻¹.     

Thermal Characterization of Dolomites

The decomposition behaviour of dolomite samples has been carried out by simultaneous DTA-TG experiments. Dolomite samples could be classified broadly into two categories as regular and iron-bearing dolomites based on DTA decomposition temperatures. The mass% FeO of all the dolomite samples was identified by DTA and the results are confirmed by atomic absorption spectrophotometer experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal studies on thenardite

Two natural thenardite (Na₂SO₄) single crystals, e.g. from Wadi El-Natron and Searles Lake, and one synthetic thenardite were studied by combined thermal techniques (e.g. DTA, TG, TMA, TOA and MS) mainly with regard to the occurring phase transitions and in order to determine the reason for the observed differences in the transition behaviour of the studied samples. The synthetic thenardite which was chemically very pure (especially free of water) showed its phase transitions during heating and cooling for be much closer together and (1st heating) a little shifted to higher temperatures compared with the natural samples which contained remarkable amounts of water and carbon, respectively. Simultaneously with the phase transitions, both natural samples lost some weight, showed very sharp, clear changes in the TMA curves and released water and CO₂ during the first heating. The thermal transitions started at distincts spots within the single crystals, easily recognizable at different interference colours when using crossed nicols in the thermomicroscope. These spots were interpreted as domains with a certain crystal physical defect character which partly enables earlier phase transitions at the defect domains. The spots seem to include very small particles of mirabilite, Na₂SO₄^√ 10 H₂O, or carbon flakes which are the reason for the defect domains and the observed release of water and CO₂.     

Differential thermal study of Mg-bearing clays from saline lakes of Southern Tunisia

Clays high in Mg content occur frequently in the high saline environment of salt lakes in southern Tunisia. The DTA curves of these clays show a striking endothermic-exothermic reaction in the temperature range of 800–820°C. A strong correlation is observed between the intensity of these coupled reactions and the Mg content of the initial clay sample. The initial endothermic reaction is interpreted as the melting/dehydroxylation of the Mg-bearing smectites. The subsequent exothermic peak is interpreted as caused by the crystallisation of the new Mg-silicate phase enstatite. Therefore, the DTA is considered as a suitable method for the identification and relative quantification of high Mg clay minerals (e.g. trioctahedral smectites). Variations of the Mg content of the studied samples were well detectable by means of DTA, disclosing a distinct distribution pattern of the salt lake clays. Clues to bulk chemical composition of the initial clay assemblage can also be found in the results of the X-ray analysis of the firing products.     

Progress of thermal analysis in earth sciences

The historical origins of the present state of thermal analysis investigations in geosciences are outlined. The majority of about 80 contributions to ESTAC 4 concerning geosciences dealt with applied and technical mineralogy and crystallography. Others are discussed with respect to new of improved techniques, minerals and rocks, thermodynamics and kinetics of minerals, and archeometry. Finally, subjects are suggested that could be treated at future meetings.

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Zusammenfassung Der historische Ursprung des gegenwärtigen Standes thermoanalytischer Untersuchungen auf dem Gebiet der Geowissenschaften wird diskutiert. Von den auf dem ESTAC-4 vorgestellten ca. 80 Beiträgen zu Geowissenschaften mit Schwerpunkt Angewandte und Technische Mineralogie und Kristallographie werden einige unter den Aspekten: Neue oder verbesserte Methoden, Mineralien und Gesteine, Thermodynamik und Kinetik von Mineralen, Archäometrie diskutiert. Abschliessend werden Vorgeschläge für Themen künftiger Treffen gemacht.

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. 80 ESTAS 4 , . , , , , . , .

     

Thermal characterization of lacustrine dolomites from the Sambhar Lake playa,Thar desert,India

The Sambhar Lake playa sediments in Thar desert, western India, host a variety of evaporite minerals. Carbonates form a major proportion of the evaporites in a 23 m deep core and several dolomite-rich horizons are recorded. Our XRD and DTA based investigations show that these dolomites are non-stoichiometric and disordered (proto-dolomites) except at one level (4.08 m) from where a well-ordered dolomite is reported. We also record frequent variation in the characteristics of dolomites with depth in terms of structural ordering, Ca/Mg ratio and Fe content. Dolomites reported form the Sambhar Lake playa are interpreted to be authigenic due to the absence of any carbonate rock in the catchment and seem to be derived from a precursor calcite under (sub-) evaporitic conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Application of differential thermal analysis in mineralogy

In mineralogical research differential thermal analysis can be applied as either a single or a combined method for three purposes:

1. for the qualitative identification of minerals and the (semi-)quantitative determination of the components of rocks and soils,
2. for the characterization of crystal-physical and crystal-chemical properties, including the study of kinetics and the determination of thermodynamic data, phase and reaction equilibria,
3. for special petrogenetic investigations concerning the interrelation of mineralogical properties with the formation, decomposition or recrystallization of minerals.

The presented results were obtained under strongly standardized and reproducible conditions of analysis and by means of some methods recently developed by the author. They include the construction of key diagrams ofΔT versus peak temperatures to identify unknown minerals; the dependence of dehydration and decomposition peak temperatures on the partial pressure of gaseous reaction products (= PA curves); the determination of chemical and crystal-chemical parameters of minerals by using the dependence of dehydroxylation peak temperatures on solid solution (chlorites, smectites) or by using the interrelation between the peak temperatures of structural transformations and crystal-chemical substitutions (sulfides, carbonates, phosphates, silicates); the determination of the degree of disorder of minerals formed in sedimentary environments by using the dehydration or inversion peak temperature and peak shape dependence on the defect character of these crystal structures (clay minerals, quartz, cristobalite, hydroxides); the subdivision of mineral groups by using the interrelation of crystal chemistry and dehydration behaviour (zeolites, mixed-layer clay minerals); the characterization of ferromagnetic minerals by studying the Curie temperatures as a function of the chemical composition; the development of a DTA method to measure the degree of weathering of granitic rocks, etc. Data of 150 minerals are given.     

The determination of authigenic quartz in porous sedimentary rocks by means of differential scanning calorimetry

The DSC method is used to characterize diagenetic transformations of porous sedimentary rocks by measuring the reversible high-low inversion of quartz crystals. For this, ten specimens of authigenic quartz from cavities in sand-and limestones have been studied, avoiding any influence of mechanical treatment on the inversion behaviour. The method shows to be very sensitive in determining disordered parts of quartz crystals, using the interrelationship between the degree of structural disorder and the decrease of the inversion temperature. The application of the method for the purpose of prospection for hydrocarbons, sulfides or phosphates is shortly discussed. Dedicated to Professor Lisa Heller-Kallai on the occasion of her 65^th birthday