Abstract:
New experimental data on the heat capacities, enthalpies of dehydration and formation of some fibrous zeolites have been obtained. Natural samples of zeolites of the natrolite group including natrolite Na2Al2Si3O10 · 2H2O, tetranatrolite Na2Al2Si3O10 · 2H2O, scolecite CaAl2Si3O10 · 3H2O, mesolite Na2Ca2[Al2Si3O10] 3 · 8H2O and structural types of thomsonite NaCa2[Al5Si5O20] · 6H2O, edingtonite BaAl2Si3O10 · 4H2O and gonnardite Na0.16Ca1.95Al4Si6O20 · 6H2O were used for calorimetric study. The heat capacities of zeolites were measured by DSC from 120 K to dehydration temperatures and the resultant C0p(T) equations were calculated. The standard enthalpies of dehydration of mineral studied at 298.15 K were determined by two-step "transposed temperature drop" technique in a Calvet microcalorimeter: 115.3 ± 7.2 kJ/mol (natrolite), 114.6 ± 8.4 (tetranatrolite), 121.6 ± 8.8 (scolecite), 350.5 ± 18.6 (mesolite), 333.4 ± 23.2 (thomsonite), 88.6 ± 13.5 (edingtonite) and 315.8 ± 35.4 (gonnardite). The standard enthalpies of formation (in kJ/mol) from oxides and elements at T = 298.15 K were calculated using experimental data obtained by the "drop solution Calvet microcalorimetry": ΔHof(oxidcs) ΔHof(elements) Natrolite -372.6± 8.1 - 5769.9± 8.7 Tetranatrolite -349.4± 14.3 - 5743.7±14.6 Scolecite -200.3± 9.9 - 6100.7±10.5 Mesolite -748.9±25.0 -17943.9±27.2 ΔHof(oxides) ΔHof(elements) Thomsonit -529.4 ±22.2 -12464.8±23.0 Edingtonite -287.4±14.9 -12793.2±30.8 Gonnardite -396.4±19.7 -12198.6±20.9 The small anomalies in heat-capacity behaviour of thomsonite, edingtonite and gonnardite in the temperature interval of 120-210 K which are associated with phase transformations were observed by DSC method. The α-β metanatrolite transition at T = 836 ± 3 K was studied and the enthalpy was calculated (14.1 ± 10 kJ/mol). The enthalpy of the disordering reaction: natrolite - tetranatrolite was determined from experimental calorimetric data to be 23.2 kJ/mol.