Complex exsolution in glaucophane from Tillotson Peak, north-central Vermont

Complex submicroscopic exsolution structures in zoned glaucophane from blueschists at Tillotson Peak, north-central Vermont, have been examined by TEM and AEM. The lamellae range from 5 to 100 nm in thickness, and are coherently intergrown with the host. Abundant cummingtonite lamellae developed in Ca-Fe-Mg-rich zones in the glaucophane, approximately (28<$E 1 back 40 up 40 ->) and (2<$E 8 back 40 up 40 -><$E 1 back 40 up 40 ->) planes of the amphibole structure (C2/m). The observed orientations differ from the more common '100' and '<$E 1 back 40 up 40 ->01' exsolution lamellae in clinoamphiboles and are optimal phase boundaries. Ca is preferentially partitioned into the cummingtonite lamellae, producing metastable, high-Ca cummingtonite compositions that lie well within the cummingtonite-actinolite miscibility gap. The second episode of exsolution formed a periodic microstructure that seems to have been controlled by buildup in strain energy at the non-optimal phase boundaries between glaucophane and actinolite.