Thermochemical Study of Sodium and Sodium–Calcium Amphibols

Abstract

A thermochemical study of six natural sodium and sodium-calcium amphiboles was carried out using high-temperature melt solution calorimetry in a Tian–Calvet microcalorimeter. The enthalpies of formation from elements have been obtained for three arfvedsonite samples K_0.5(Na_1.5Ca_0.5)(Mg_4.0\({\text{Fe}}_{{0.9}}^{{3 + }}\)Al_0.1)[Si_8.0O₂₂](OH)₂ from the Inagli Massif, central Aldan Shield, Russia (–11626.6 ± 8.9 kJ/mol), (Na_0.5K_0.1)(Na_1.6Ca_0.4)(Mg_3.6\({\text{Fe}}_{{0.4}}^{{2 + }}\)\({\text{Fe}}_{{0.8}}^{{3 + }}\)Al_0.2)[Si_8.0O₂₂](OH)₂ from the Khibiny Massif, Kola Peninsula, Russia (–11520.8 ± 14.6 kJ/mol), and (Na_0.7K_0.3)Na_2.0(\({\text{Fe}}_{{4.0}}^{{2 + }}{\text{Fe}}_{{0.6}}^{{3 + }}\)Mn_0.1Ti_0.1Al_0.2)[Si_8.0O₂₂](OH)₂ from the Katuginskoe deposit, Transbaikalia, Russia (–11384.7 ± 17.1 kJ/mol); for two riebeckite samples Na_2.0(Mg_2.5\({\text{Fe}}_{{0.5}}^{{2 + }}{\text{Fe}}_{{1.5}}^{{3 + }}\)Al_0.5)Si₈O₂₂(OH)₂ from the Kumula deposit, Central Kazakhstan (–10791.0 ± 10.1 kJ/mol) and Na_2.0(Mg_0.9\({\text{Fe}}_{{2.1}}^{{2 + }}{\text{Fe}}_{{1.9}}^{{3 + }}\)Al_0.1)[Si₈O₂₂](OH)₂ from Krivoy Rog, Ukraine (–10260.8 ± 10.9 kJ/mol), and for one richterite sample (Na_0.7K_0.3)(Ca_1.2Na_0.8)(Mg_4.6\({\text{Fe}}_{{0.4}}^{{2 + }}\))[Si_7.8Al_0.2O₂₂](OH)₂ from the Kovdor Massif, Kola Peninsula, Russia (–12154.2 ± 9.7 kJ/mol). The values of the standard entropies, enthalpies, and Gibbs energies of formation are estimated for the end members of the isomorphic series arfvedsonite–magnesioarfvedsonite, riebeckite–magnesioriebeckite, and richterite–ferrorichterite.