Theoretical calculation of Gibbs free energy of mixing of biotite: phlogopite-annite-eastonite-siderophyllite system

摘要: The Gibbs free energy of mixing (ΔGmix) can be represented as the sum ideal (ΔGid) and excess (Gex). For biotite in phlogopite (Ph)-annite (An)-eastonite (Es)-siderophyllite (Sd) system, ΔGid is estimated based on MOS (mixing site) model andGex obtained from theoretically calculated binding energies biotite. Previous experimental works suggested that Ph-An possibly Ph-Es pairs form solutions. In this study, however,Gex shows none binary solutions among Ph-An-Es-Sd end-members are ideal. ΔGmix indicates Es-Sd complete solutions, but An-Sd both mixed by Tschermak's substitution show miscibility gaps. limit a function mole fraction Fe trioctahedral site,X(Fe), temperature. A comparison solvus spinodal points with chemical compositions synthesized natural biotites implies Es Sd metastable limits determined not curves ones. Most fall fairly well within range defined at 800°C, although there some out-lying especially when theirX(Fe) between 0.5 0.7. general good agreement what study predicts suggests theoretical thermodynamic seems promising to applied for understanding behavior only also other similar solid particulary approach difficult.