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Clays and Clay Minerals; October 2002; v. 50; no. 5; p. 657-666
© 2002 Clay Minerals Society
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STABILITY OF MAGADIITE BETWEEN 20 AND 100°C

Martin Dietzel1,* and Ilse Letofsky-Papst2

1 Institute of Engineering Geology and Applied Mineralogy, University of Technology, A-8010 Graz, Austria
2 Research Institute for Electron Microscopy, Steyrergasse 17, A-8010 Graz, Austria

* E-mail address of corresponding author: dietzel{at}egam.tu-graz.ac.at

New experimental data with respect to the solubility of natural and synthesized magadiite at elevated temperatures and in alkaline solutions are presented. The results show that the solubility of magadiite increases according to the expression ln(Kmag) = –8146·T(K)–1 – 5.71 from 20 to 100°C ({Delta}G0R(Kmag) = 19.6±0.3 and {Delta}H0R(Kmag) = 16.2 ± 0.4 kcal mol–1). The experimental results and the related data from the literature suggest that the formation of magadiite may be favored by a decrease of temperature and pH (pH >9) as well by large amounts of Na+ ions and low ionic strength. These effects are related to the value of {Delta}H0R(Kmag), the distribution of dissolved silica species, the stoichiometry of magadiite, and the occurrence of negatively-charged species, respectively.

Key Words: Alkaline Solutions • Amorphous Silica • Magadiite • Monosilicic Acid • Polysilicic Acids • Solubility






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