COMPLEX EXSOLUTION IN GLAUCOPHANE FROM TILLOTSON PEAK, NORTH-CENTRAL VERMONT

Complex exsolution microstructures in zoned glaucophane from blueschis ts at Tillotson Peak, north-central Vermont, have been studied using t ransmission and analytical electron microscopy (TEM, AEM). In all case s, the exsolution lamellae are submicroscopic, ranging from 5 to 100 n m in thickness, and are coherently intergrown with the host. Abundant cummingtonite exsolution lamellae developed in Ca-Fe-Mg-rich zones in glaucophane, nearly parallel to (281BAR) and (281BAR) planes of the am phibole structure (space group C2/m). The observed orientations differ from more common ''100'' and ''101BAR'' exsolution lamellae in clinoa mphiboles and are optimal phase boundaries. AEM analyses indicate that Ca is preferentially partitioned into the cummingtonite lamellae, pro ducing metastable, high-Ca cummingtonite compositions that lie well wi thin the cummingtonite-actinolite miscibility gap. In many cases, narr ow, periodic lamellae of actinolite have subsequentially exsolved from the metastable cummingtonite lamellae along (100). Calculations of di mensional misfit and elastic strain energy using EPLAG indicate that t he formation of ''100'' actinolite lamellae within the cummingtonite l amellae results in a highly non-optimal phase boundary between glaucop hane and actinolite. Evidence for this is seen by high strain contrast in TEM images at the interfaces between actinolite and glaucophane. T he periodic nature of this secondary exsolution microstructure can be explained as the result of the minimization of the total elastic strai n energy associated with the glaucophane - cummingtonite - actinolite intergrowth. Evidence has also been found for multiple stages of exsol ution involving winchite, cummingtonite, and actinolite, leading to co mplex, metastable, five-amphibole assemblages.