A “conditional spinodal” within the peristerite miscibility gap of plagioclase feldspars

Some of the arguments over whether the peristerite miscibility gap in plagioclase feldspars is a true solvus or a binary loop can be resolved by using free energy curves and assuming that the Al/Si order/disorder transformation in albite is other than first order in character. If the disordered plagioclase sohd solution behaves nearly ideally in the sodium rich range and the substitution of excess Al (in the form of the CaAl₂Si₂O₈) in low albite is energetically highly unfavorable, then a miscibility gap appears which contains a “conditional spinodal” between ordered albite and disordered oligoclase. The thermodynamic driving force for exsolution is considered to be entirely due to the ordering of Al and Si in albite so that the miscibility gap must peak below the transformation temperature in pure albite (∼575–625°C). A three dimensional diagram of free energy, composition and degree of order allows a rationalization of the compositions and occurrence of coexisting albite + oligoclase in low grade metamorphic rocks based on variations in the degree of order achieved during crystal growth.