The Gibbs free energy of formation for sodalite, Na4Al3Si3O12C], calculated from published data on its decomposition to nepheline and NaCl is:
T (deg. K) 1000 1100 1200 1300 1400
∆Gf (K cal/gfw) -1305.8 -1273.5 -1239.6 -1199.9 -1160.2
Sodalite stability is strongly controlled by silica activity. At high temperatures it will be found only in magmas whose silica activities are near or below the nepheline-albite equilibrium. At lower temperatures its stability field expands and it will be stable at higher silica activities, and will therefore precede nepheline in the crystallization sequence of phonolitic trachytes. If fcl2 is not more than 108 greater than fF2 villiaumite (NaF) or fluorite and nepheline will take the place of sodalite. ∆G° for nosean (Na8Al6Si6O24[SO4]) has also been estimated, and it similarly can precede nepheline in the crystallization order of phonolitic trachytes. The ratio of fso2/fcl2, of nosean coexisting with sodalite has been calculated for various temperatures and fo2. The stability of sulphate (nosean) rather than sulphide minerals is controlled not only by fO2, but also by silica activity and peralkalinity.
Values of fcl2 calculated from the sodalite-bearing trachytes of Mt. Suswa, Kenya, vary from 10-13.4 at 1400°K to 10-23.5 at 1000°K, and the upper limits of fF2 vary from 10-23 to 10-35 atmospheres. Estimated fugacities of HC1 and SO2 are several orders of magnitude lower than the partial pressures of the same components in fumarolic gases of a hyper-sthene-dacite, as is to be expected. Siliceous magmas rich in Cl will generate a fluid phase rich in Cl which is expelled as crystallization proceeds. Magmas of lower silica activities (trachytes, leucite-basanites) will retain their Cl or SO2, as sodalite or nosean are stable.