In the present paper the thermal behaviour of Mg-carbonates and -silicates is reviewed and discussed, based on own DTA investigations and data from the literature. Mg-bearing minerals in soils and sediments offer informations about the palaeoenvironment and palaeoclimatology of sediments and soils and - thus - the study of their crystal chemical compositions by means of (differential) thermal analysis informs about environmental conditions of (sedimentary Mg-) mineral (trans-) formations. The paper stresses (1) the interrelations between decomposition temperatures of (Mg-) carbonates and substitution processes and is (2) concerned with the interdependence between the dehydroxylation behaviour of Mg-bearing sheet silicates and their crystal chemical composition.
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 paper reviews studies on possible applications of the high-low quartz inversion in petrology. Since the first suggestions of Fenner and Tuttle, nearly fifty papers have dealt with the subject, including ten by the present authors. The detailed discussion of the preparative and instrumental factors which may influence the quartz inversion behaviour reveals that the DTA (DSC) runs have to be made under highly standardized conditions to permit measurement of the ‘material inherent factors’ (mainly crystal physical factors) which cause variations in shape and temperatures. The most striking variations in shape and temperatures of the inversion effect (e.g. including temperatures more than 70°C lower than the ‘textbook value’ of 573°C for the quartz inversion) are observed for microcrystalline quartz crystals. The literature and the authors' own investigation demonstrate that the most interesting field of application of the method is that of authigenic quartz formation in sediments and soils. The characterization of metamorphic quartz is more complicated. Four fields of study are chosen to demonstrate the possible application of the quartz inversion characterization for petrogenetic interpretations: (1) the differentiation between authigenic and inherited quartz crystals in sediments, (2) the characterization of the contact-metamorphic aureoles around granites, (3) special regional-metamorphic terrains and (4) (from a few so far unpublished studies) the differentiation between sedimentary and soil silcretes.
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.
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
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
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.