Calorimetric measurement of the enthalpy of formation, dehydration, and dehydroxylation of vermiculite

Abstract

Vermiculite, the widespread phyllosilicate, is usually formed by hydration of trioctahedral micas during weathering, with preservation of micalike structure. It is made up of three talclike 2:1 sheets and interlayer exchange cations surrounded by water molecules. A small charge of threesheet layer and weak electrostatic interaction between sheets and interlayer cations sur� rounded by water shell define the low degree of struc� tural perfection of vermiculite and its cationexchanged properties. Thus, vermiculite is the natural inorganic ionexchanger, whose cationexchange capacity is highest among all phyllosilicates. The exchange cations in natural vermiculite is Mg or Ca, Na, Ni, and Cu sub� stituting it in the interlayer; in synthetic vermiculite, exchange cations are additionally supplemented by Sr, Cs, Li, Rb, and other cations. Dehydrated vermiculite is characterized by excellent thermal insulating proper� ties, since it does not burn, is not decomposed, extremely durable, and can be used up to 900°С. The wide practical use of vermiculite in the production of fireresistant, heat and soundinsulating materials, as well as catalyst, sorbents and ion exchangers, and in many other technological processes stimulated active mining of natural vermiculite deposits, synthesis of its new modifications, as well as the study of different physicochemical properties of this unique mineral. The composition, structure, and properties of natural and synthetic vermiculites have been studied by Xray, IR spectroscopic, and EPR methods; its behavior during heating and cooling was studied by thermal analysis, and ionexchange properties were investigated by dif� ferent methods of direct determination of cation� exchange capacity of the mineral (1). The thermal decomposition of vermiculite was studied in detail by different thermal analytical methods, including high� temperature Xray imaging and highpressure differen� tial thermal analysis (2, 3). The last work was dedicated to the determination of kinetic parameters of vermicu� lite dehydration. The values of the Gibbs free energy of the formation of two vermicultes devoid of interlayer water on the basis of experimental data on their solubil� ity were determined in the work (5), and they were also estimated using data on the electrical negativity of its constituent cations. In this work, the enthalpies of formation, dehydra� tion, and dehydroxylation were thermochemically determined for vermiculite samples from two known Russian economic deposits using hightemperature Calvet microcalorimentry.