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Impurities in minerals may be caused (a) by simple admixtures or (b) by crystal chemical substitutions. For the present study standardized differential thermal analyses were performed to investigate the interrelation between thermal effects and chemical composition, for example, to determine the influence of impurities on thermal effects. In the case of thermal decomposition reactions, which are controlled by the partial pressure of a gaseous reaction product (for example, carbon dioxide (CO2) in carbonates), type (a) impurities may be determined by applying special PA-curves (that show the dependence of decomposition temperatures on partial pressure). Small amounts of admixtures of organic matter or sulfides in clays can be detected by intensive oxidation effects. The transition behavior of thenardites (Na2SO4) is influenced by the presence of small amounts of hydrated sodium sulfate. Chemical substitution in copper sulfides may be found by measuring their transition temperatures. In the case of carbonate minerals small amounts of substitutions can be determined by their influence on shape and temperatures of the decomposition effects. The substitution of iron by aluminum in goethites lowers the dehydroxylation temperature and can be determined from a special calibration curve. Substitutions in magnetites lower the Curie temperatures of these ferromagnetic minerals. Because of restricting the investigations on the range of “impurities” in minerals solid solution phenomena are only treated briefly, for example, using the influence of solid solution on the dehydroxylation/decomposition temperatures of the phyllosilicates of the smectite and chlorite series.
impurities in minerals, admixtures, crystal chemical substitution, solid solution, standardized differential thermal analysis
Professor, Mineralogical Institute, University of Karlsruhe, Karlsruhe,