Sapphire Megacrysts In Syenite Pegmatites From the Ilmen Mountains, South Urals, Russia: New Mineralogical Data

Abstract

Abstract Sapphires in the Ilmen Mountains of Russia9s South Urals are found in syenite pegmatites where the mineral forms blue transparent to translucent megacrysts up to 6 cm in length. The corundum crystallized during the magmatic stage in equilibrium with columbite-(Fe), K-Na-feldspar, and ± perthite. There was a later metasomatic stage in which muscovite formed by the reaction corundum + orthoclase + H 2 O ↔ muscovite and diaspore precipitated. Highly distributed syn- and post-tectonic brittle deformation of the host rocks is responsible for the polysynthetic twinning and parting of blue sapphires along the r face, which decreases the gem quality of the raw material. The formation of the corundum syenite pegmatites is associated with the intrusion of miascites in the Ilmenogorsky complex of alkaline rocks. Columbite-(Fe), zircon, and minerals from the alkali feldspar group were identified as syngenetic inclusions within the blue sapphire. Monazite-(Ce), sub-micron grains of uraninite, muscovite and diaspore, and exsolved micron-sized needles of ilmenite were also found. The chemical composition of the sapphires fits well with those of magmatic sapphires from the literature with 10,000Ga/Al above 2.7, Ga/Mg > 29, Fe/Mg > 429, Cr/Ga 19. Solid inclusions within the sapphires of the Ilmen syenite pegmatites, as well as their chemistry, provide possible genetic linkages with sapphires from alluvial and eluvial deposits of alkali basaltic terrains in Houai Sai (northern Laos) and several mines in Chantaburi province (Thailand).