Iron-free pigeonite, a phase distinct from clinoenstatite solid solution, is shown to bo stable at atmospheric pressure. A primary field of crystallization for iron-free pigeonite is delineated in the system forsterite-diopside-silica.
The stability of iron-free pigeonite is evident from the morphology and compositions of crystals synthesized close to the liquidus temperatures from glasses with compositions in the field previously labeled protoenstatite. Crystals with monoclinic morphology crystallize below 1432°C and with orthorhombic morphology above 1432°C. Microprobe analysis of the crystals indicates a compositional gap in CaO content between the monoclinic and orthorhombic crystals. The discontinuity of the solidus curve in the Mg-rich region of the system MgSiO3-CaMgSi2O6 is clearly shown.
Terrestrial pigeonites containing less than 20 mole percent FeSiO3 have not been reported. From consideration of the composition and the temperature of stability limit, such low-iron pigeonites may occur in the chondrules of the least recrystallized chondrites, such as Type II and III carbonaceous chondrites and the “unequilibrated” ordinary chondrites.