The System MgSiO3--CaMgSi2O6

Subsolidus and liquidus phase relations along the join MgSiO₃—CaMgSi₂O₆have been determined from the results of dry and hydrothermalruns. Two-pyroxene mixtures which crystallize within the solvusin dry runs form cryptoperthites, and X-ray methods must beused to determine their compositions and locate the boundariesof the solvus. Mg-rich and Ca-rich pyroxenes can, however, bedistinguished in well-crystallized hydrothermal runs by opticaltechniques. The results obtained indicate that solid solution along thisjoin is more restricted than was found by Atlas (1952). Thesolvus intersects the solidus over a composition interval of42 wt per cent. Additional data on the rhombic enstatite protoenstatiteinversion are consistent with Atlas's value of 985??10?C. Thisinversion has proved to be very sensitive to pressure, and thedT/dP slope of the transition is 84??10?/kb. The crystallization of natural pyroxenes from basaltic magmasis reviewed in the light of the experimental data. The solidsolution shown by Fe-poor pyroxene pairs from layered intrusionssuch as the Skaergaard is remarkably restricted. The temperatureof crystallization of these pyroxenes as deduced from the solvuson the join MgSiO₃—CaMgSi₂O₆ is about 1000? C. In makingthis estimate it is assumed that any subsolidus exsolution hasnot proceeded beyond the stage of lamellae formation. This temperatureis below the dry solidus of basalt, and the result indicatesthat either the solid solution shown by these natural pyroxenesis influenced by impurities such as Al or Fe', or the meltinginterval of the magmas which formed the intrusions was loweredby a substantial vapor pressure of H₂O. It is suggested that pyroxenes which have crystallized fromextrusive basalts and which have compositions that plot in thecentral portion of the pyroxene quadrilateral are metastablesolid solutions formed by quick cooling. Available informationis not sufficient to clarify the relationship between pigeoniteand the various polymorphs of MgSiO₃. It is probable, however,that there is a first-order inversion between pigeonite andprotoenstatite, and possible phase relations between these formsin the system MgSiO₃—FeSiO₃ are discussed.