Grain boundary processes and development of metamorphic plagioclase

Plagioclase from a progressively metamorphosed granodiorite changes as the metamorphic grade increases. Lower grade plagioclase are chemically inhomogeneous, with zoned rims containing distinct compositional levels of An_0?3, An₁₇, and An₂₅. As grade increases the plagioclase becomes more chemically homogeneous with An_0?3 rims dominating. Microcline inclusions are controlled by internal defects at lower grades and grain boundaries at higher grades. Myrmekite rims are developed at the highest grade. Rims are dependent on surface energy factors and occur at triple points, high angle lattice misfits and other high energy surfaces. At low grades, rims form at plagioclase-plagioclase contacts and at higher grades, at plagioclase-microcline contacts. These changes are due to impurity segregation and grain boundary migration, and an increase of the letter process at higher grades.