Corundum in Biotite-Sillimanite Gneiss From near Polgahawela, Ceylon

Abstract Idiomorphic, pale blue corundum crystals (S.G. =4·02, n 0 =1·770 ± 0·002) up to one inch long occur as porphyroblasts in a finegrained biotite-sillimanite gneiss cut by pegmatite veins at Gangoda, 5½ miles south of Polgahawela. The field relations and petrography of the corundum-bearing rocks are described briefly. The corundum is thought to have crystallized by metamorphic differentiation in alumina-rich silica-poor bands in a semipelitic gneiss.