ENTHALPIES OF FORMATION AND DEHYDRATION O F NATURAL ANALCIME

The enthalpies of formation and dehydration of natural analcime NaAlSi 2O6 . H2O and its anhydrous form were determined using Calvet high-tem perature calorimetry. The standard enthalpies of formation of analcime from binary oxides and elements at 298.15 K are -138.3+/-2.6 and -329 0.8+/-3.4 kJ/mol, respectively. These values agree with those deduced from HF solution calorimetry (Barany, 1962) but contradict the values recommended by Hemingway and Robie (1977). The enthalpy of dehydration at 298:15 K of analcime is 41.7+/-1.9 kJ/mol. Its value correlates li nearly with the water content.