Thermodynamic Properties of Fe-Rich Smectite—Nontronite

摘要

The results of thermochemical studies are reported for nontronite samples from the Pinares-de-Majari (Eastern Cuba) (Sample I) and Kempirsai serpentine massif (South Urals, Kazakhstan) (Sample II). The enthalpies of formation of dehydrated hydroxyl-bearing nontronites from elements were determined by melt dissolution calorimetry using high-temperature heat-flux Tiana-Calvet microcalorimeter: Δ_fH_(el)° (298.15 K): -4958 ± 13 kJ/rnol for Mg_(0.4)(Fe_(1.5)~(3+)Mg_(0.4)Ni_(0.1))[Si_(3.7)Al_(0.3)O_(10)] (OH)2 (I) and -5003.6 ± 8.0 kJ/mol for Mg_(0.3)Na_(0.1)Ca_(0.1)Fe_(1.4)~(3+)Mg_(0.5)Ni_(0.1))[Si_(3.7)Al_(0.3)O_(10)](OH)2 (II). It was determined experimentally that the enthalpy of dehydration (removal of molecular adsorption and interlayer water) of the studied nontronites is 6 + 2 kJ per 1 mole H2O. The enthalpy of formation of nontronite of theoretical composition Mg_(0.15)Fe2~(3+)[Si_(3.7)Al_(0.3)]O_(10)(OH)2 was estimated at —4750 kJ/mol. The Gibbs free energies of formation of the nontronites were calculated.