Samples of glaucophane from eclcgite assemblages in northern Vermont have been studied using transmission and analytical electron microscopy (TEM, AEM). The results show that the glaucophane is exsolved on a submicroscopic scale and contains abundant lamellae of cummingtonite. AEM analyses indicate that the present pair of coexisting sodic-ferromagnesian amphiboles differs significantly from all previously reported pairs and represents the first report of coexisting glaucophane-cummingtonite from normal eclogitic assemblages.
The lamellae are disc shaped and are coherently intergrown with the host. They occur in two symmetrically related orientations, nearly parallel to (281) and (281) planes of the host. Optimal phase boundary calculations indicate that these orientations represent planes of best dimensional fit between the glaucophane and cummingtonite lattices. This orientation differs significantly from previously reported lamellar orientations for monoclinic amphiboles, which generally occur nearly parallel to (101) and (100) planes for the C2/m unit-cell setting.