The pressure-temperature conditions of formation of jadeite-bearing pyroxenes can be calculated from data on the reactions: NaAlSix3O8 + NaAlSiO4 = 2 NaAlSi2O6, NaAlSi3O8 = NaAlSi2O6 + SiO2, where feldspar, nepheline, and pyroxene occur as complex solid solutions. If it is assumed that (a) nepheline can be treated as a binary "sub-regular" solution, (b) pyroxene can be treated as a multicomponent "sub-regular" solution, and (c) the activity coefficient of albite in plagioclase coexisting with jadeitic pyroxene is 1.0, then experimental data yield a relationship between the compositions of coexisting minerals and their temperatures of equilibration. For nepheline-bearing assemblages this relation is: \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7...