Thermodynamic Properties of Fluorapophyllite-(K) and Hydroxyapophyllite-(K)

Thermochemical study of natural minerals of the apophyllite group: fluorapophyllite-(K) KCa 4 [Si 8 O 20 ]F . 8H 2 O (Maharashtra, India) (I) and hydroxylapophyllite-(K) KCa 4 [Si 8 O 20 ]OH . 8H 2 O (Norilsk, Russia) (II) were performed on a high-temperature heat-flux microcalorimeter Tian–Calvet “Setaram” (France) using the melt solution calorimetry method. The first data on the enthalpies of formation from the elements for the minerals studied are obtained: −13 205±13 kJ/mol (I) and −13 054±20 kJ/mol (II). The values of their standard entropies and Gibbs energies of formation are estimated.