A thermodynamic model for feldspars in KAlSi ₃ O ₈ −NaAlSi ₃ O ₈ −CaAl ₂ Si ₂ O ₈ for mineral equilibrium calculations

Activity–composition (a-x) relations for feldspars petrological calculations in KAlSi3O8−NaAlSi3O8−CaAl2Si2O8 are calibrated using literature data (i) plagioclase cation exchange experiments at 600 and 700°C, (ii) experimental ternary feldspar pairs 880 − 900°C, (iii) the alkali solvus, (iv) dry melting loop plagioclase. The results tested against calorimetric heat of solution pressures coexisting with grossular, wollastonite, quartz. As aim is to produce a-x suitable modelling, we do not attempt model complex structural phenomena feldspar, except where they appear contribute significantly energetics high-temperature mixing. Seven models were investigated, various formulations entropy mixing together van Laar non-ideal (4TR) that most satisfactorily reproduces one which Al Si partially order onto four tetrahedral sites, approximated by reducing site a factor 4. When combined an ordered albite, this allows peristerite gap closing 600°C albite composition. resulting activity-composition should be applicable phase equilibrium over large range geologically relevant pressure temperature.