Solubility of CaMgSi3O8 and .SQU.Si4O8 endmembers in anorthite.

Significant amounts of MgO, and excess SiO2 and Al2O3 in anorthites from the island of Miyakejima are estimated by EPMA and X-ray diffraction analysis. The role in the crystal structure establishes Mg, Si and Al cations as CaMgSi3O8, ?? Si4O8 and AlAl3SiO8 endmembers for the anorthite solid solution. Syntheses under dry conditions of anorthites along the joins CaAl2Si2O8- ?? Si4O8 and CaAl2Si2O8-CaMgSi3O8 conduced to ascertaining the partial solubility limits of these endmembers in the solid solutions, compared with the anomalous anorthites from Miyakejima. The maximum limit of partial solubility of CaMgSi3O8 is about 25 mol% to CaAl2Si2O8, and that of ?? Si4O8 is nearly 8 mol%. Nevertheless inhomogeneous distribution of Mg cation within this single crystal has an implication for the unstable behavior of Mg cation in the anorthite structure. Chemical shift of MgKαI in the synthetic anorthites affords infallible discrimination of solubility of CaMgSi3O8 endmember rather than MgAl2Si2O8. Condensation of natural occurrences and syntheses of plagioclases with small amounts of CaMgSi3O8 and ?? Si4O8 components purifies that anorthites from Miyakejima may have been originally crystallized by super-cooling in SiO2-saturated and Mg-rich host magma at the hightemperature more than 1, 000°C, and that the phenomenon in anorthite similar to Tschermak substitution, Mg+Si⇔2Al, goes to prove the charge-coupled substitution of Mg+Si in only tetrahedral sites. Emphasis is placed upon experimental verification and comparison to observed data is made where possible.