The bearing of the system CaMgSi2O6CaAl2SiO6CaFeAlSiO6 on fassaitic pyroxene

Abstract The system CaMgSi 2 O 6 CaAl 2 SiO 6 CaFeAlSiO 6 has been studied in air at 1 atm. The phase assemblage at subsolidus temperatures in the CaMgSi 2 O 6 -rich portion is Cpx + An + Mel and that in the CaMgSi 2 O 6 -poor portion Cpx + An + Mel + Sp. At subsolidus temperatures the sigle-phase field of clinopyroxene increases with an increase in the CaFeAlSiO 6 component of the system. The Al 2 O 3 content of clinopyroxene, however, continues to increase beyond the single-phase field and attains at least 16.04 wt.% Al 2 O 3 with 3.9 wt.% Fe 2 O 3 . The stability field of fassaite in the system over a range of pressures and oxygen fugacities has been estimated from data in the literature as well as the present data. The CaFeAlSiO 6 content of fassaite is dependent on oxygen fugacity, but is not influenced by pressure. The stability field is strongly influenced by oxygen fugacity at low and high pressure, and decreases with decreasing oxygen fugacity. Clinopyroxenes in both volcanic and metamorphic rocks from various localities, when plotted on the CaMgSi 2 O 6 CaAl 2 SiO 6 CaFeAlSiO 6 triangle, show that there is no compositional gap between diopside and fassaitic pyroxene in metamorphic rocks, and that the fassaitic pyroxene in alkalic rocks becomes richer in both CaAl 2 SiO 6 and CaFeAlSiO 5 components as crystallization proceeds. These results agree with those obtained in the experimental study.