A detailed study of crystal-chemical properties of clay minerals is constrained by difficulties related to their complex structure and large number of isomorphic impurities. One way to solve this problem is physical-chemical modeling. This paper presents the results of a study of the most common representatives of clay minerals – montmorillonites and illites. We used data of using melt-calorimetry method as a test to verify the accuracy of the method we are developed. Our method allows us to calculate the mineral composition and the thermodynamic properties. It describes complex natural clay minerals with the use of solid-solution models and determine the thermodynamic properties of these minerals by dual decisions using the method of free energy minimization implemented in a universal program complex “Selector”. An example of the calculation of the formulas and thermodynamic properties of illites from Lake Baikal sediments is given. The intervals of precipitation formed in warm and cold climatic periods are studied. It has been found that illites of warm periods have a higher content of potassium, sodium, and calcium. The chemical features of the layered silicates accumulated in the bottom sediments make it possible to determine the prevailing weathering conditions in the catchment basin, since their composition significantly changes in warm and cold climatic episodes. The thermodynamic potentials of illites from warm periods are lower. Illites form in soils and are more stable during warm climatic periods. A similar approach can be successfully used for sedimentary rocks.

Oshchepkova Anastasia Vladimirovna, Junior Researcher, Institute of Geochemistry SB RAS, 1a, Favorsky st., Irkutsk, 664033, Russian Federation; Senior Lecturer, Irkutsk State University, 1, K. Marx st., Irkutsk, 664003, Russian Federation, tel.: 8 (3952) 42-70-79, e-mail: oshepkova-anasta@mail.ru 

Bychinsky Valerii Alexeevich, Candidate of Sciences (Geology and Mineralogy), Senior Researcher, Vinogradov Institute of Geochemistry SB RAS, 1a, Favorsky st., Irkutsk, 664033, Russian Federation; Associate Professor, Irkutsk State University, 1, K. Marx st., Irkutsk, 664003, Russian Federation, tel.: 8(3952)42-70-79, e-mail: val@igc.irk.ru 

Chudnenko Konstantin Vadimovich, Doctor of Sciences (Geology and Mineralogy), Head, Laboratory of Environmental Geochemistry and Physico-Chemical Modeling, Institute of Geochemistry SB RAS, 1a, Favorsky st., Irkutsk, 664033, Russian Federation, tel.: 8(3952)42-70-79, e-mail: chud@igc.irk.ru 

Sasim Sergey Alexandrovich, Candidate of Sciences (Geology and Mineralogy), Associate Professor, Head, Department of Mineral Resources, Irkutsk State University, 1, K. Marx st., Irkutsk, 664003, Russian Federation, tel.: 8(3952)24-32-80, e-mail: sasimserg@mail.ru

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