Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al ₂ SiO ₅ polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey

Abstract

Regionally metamorphosed, muscovite-bearing quartzites from Sivrihisar, Turkey, contain coexisting andalusite, kyanite, and sillimanite. Kyanite is the most abundant polymorph and defines a lineation along with prismatic sillimanite, andalusite, staurolite, and elongate quartz. Andalusite is the most Fe-rich of the polymorphs (0.9-1.6 wt% Fe2O3, compared with 0.6-0.9 wt% for kyanite and sillimanite), and was ductilely deformed. Staurolite has partially pseudomorphed kyanite, and occurs intergrown with sillimanite. Garnet occurs in some metaquartzites and interlayered mica schists. Mica schists lack Al2SiO5 polymorphs. Porphyroblasts in mica schists are chloritoid, chloritoid + staurolite ± garnet, or staurolite ± garnet with inclusions of chloritoid and staurolite.