The intermediate plagioclase structure: An explanation from interface theory

The orientation of the plane of modulation of the e-plagioclase superstructure appears to be defined by minimization of interface energy during spinodal-like unmixing of preexisting high plagioclase. Computations with strain-free high-temperature lattice parameters show that the plane of minimization of interface energy for the unmixing of anorthite from calcic plagioclase lies in the zone of s vectors 34, 8, 22] (Ab cell) and is rotated about 15° beyond the orientation of the An 75 superstructure. In contrast, the corresponding plane for unmixing of high albite from sodic plagioclase is rotated about 15° beyond the orientation of the An 25 superstructure. The orientation of individual intermediate plagioclase superstructures is the resultant of the proportional constraints acting on anorthiteplagioclase and albite-sodic plagioclase interfaces. The modulation period (S=1/‖s‖) at any composition is approximately equal to the modulation translation distance parallel to ?21, 39, ?55] resolved onto the modulation direction. The structural consequences of the present study are consistent with the essential features of the crystal structure model of Kitamura and Morimoto. However, the e-plagioclase superstructure is considered to represent a category of unmixing phenomena, rather than atomistic ordering, and it seems inappropriate to attempt to describe it with a composition-related supercell.