Thermodynamic Properties of Pectolite

— Pectolite H 1.00 Na 0.96 Ca 2.00 Mn_0.02^2 + Si 3.00 O 9 from nepheline−feldspathic veins in the Kovdor massif, Kola Peninsula, Russia, was studied by X-ray diffraction (XRD), IR and Raman spectrometry, microprobe analysis (EPMA), and thermal analysis. The enthalpy of formation of pectolite HNaCa 2 Si 3 O 9 .from elements ∆ f H ° (298.15 K) = –4651.0 ± 4.3 kJ/mol was determined by melt solution calorimetry on a Setaram Tian–Calvet (France) microcalorimeter. The enthalpy of formation of serandite HNaMn 2 Si 3 O 9 , the manganese end member of the pectolite–serandite isomorphic series, was determined to be Δ H (298.15 K) = −4052.6 ± 4.4 kJ/mol. The absolute entropies and Gibbs energies of pectolite and serandite were calculated.