F-OH exchange equilibria between mica-amphibole mineral pairs

Abstract

Fluoride-hydroxyl exchange equilibria between phlogopite-pargasite and phlogopite-tremolite mineral pairs were experimentally determined at 1,173K, 500 bars and 1,073–1,173 K, 500 bars respectively. The distribution of fluorine between phlogopite and pargasite was found to favor phlogopite slightly, ΔG ^._ex (1,173 K)=−1.71 kJ anion⁻¹, while in the case of phlogopite-tremolite, fluorine was preferentially incorporated in the mica, ΔG ^._ex (1,073)=− 5.67 kJ anion⁻¹ and ΔG ^._ex (1,173K)=−5.84 kJ anion⁻¹. These results have yielded new values of entropy and Gibbs energy of formation for fluortremolite, ΔS ^∘_f =−2,293.4±16.0JK⁻¹ mol⁻¹ and ΔG ^∘_f = −11,779.3±25.0 kJ mol⁻¹, respectively. In addition, F-OH mineral exchange equilibria support a recent molten oxide calorimetric value for the Gibbs energy of fluorphlogopite, ΔG ^∘_f =−6,014.0±7.0 kJ mol⁻¹, which is approximately 40 kJ mol⁻¹ more exothermic than the tabulated value.