A thermodynamic model for feldspars in KAlSi3O8−NaAlSi3O8−CaAl2Si2O8 for mineral equilibrium calculations

Abstract

Activity–composition (a-x) relations for feldspars for petrological calculations in KAlSi3O8−NaAlSi3O8−CaAl2Si2O8 are calibrated using literature data for (i) plagioclase cation exchange experiments at 600 and 700°C, (ii) experimental ternary feldspar pairs at 880 − 900°C, (iii) the alkali feldspar solvus, and (iv) the dry melting loop for plagioclase. The results are tested against the calorimetric heat of solution data and the experimental pressures for plagioclase coexisting with grossular, wollastonite, and quartz. As the aim is to produce a-x relations suitable for petrological modelling, we do not attempt to model complex structural phenomena in feldspar, except where they appear to co..