Mineralogical and gemmological characteristics of garnets associated with xenoliths within trachyte dome, Hisarlıkaya (Ankara), Central Anatolia, Turkey

Abstract

Garnet-bearing xenoliths are observed within a trachytic dome extrusion in the Hisarlıkaya region (Ankara). These garnets exhibiting greenish-reddish-dark brown colour and ranging in sizes up to 1 cm were examined in terms of mineralogical, geochemical, and gemmological characteristics. Mineralogical studies indicate that these garnets (And88-93 Grs7-12) are linked to solid solution series, which are dominantly andradite with lower content of grossular. According to major, trace, and rare earth element (REE) analysis, the representative garnet crystal shows high CaO, Fe2O3, Al2O3, MgO, MnO, V, W, and light rare earth elements (LREE) concentrations. These high concentrations might indicate the mobility of these elements during skarn formation related with contact metamorphism, material exchange occurring via hydrothermal fluids, site geometry of the crystal, ionic radius of cations, and charge balance. The Hisarlıkaya garnets display dodecahedron-trapezohedron crystal habits with good translucency, glassy, transparent features, and also optical isotropic character. Their refractive indices are high (>1.78), and specific gravities range from 3.66 to 3.67 g/cm3. These garnets are not suitable for use as gemstones since they cannot be cut or processed considering all the mineralogical properties of garnets.