Mineralogical characteristics of unusual “Anatolian” diaspore (zultanite) crystals from the İlbirdağı diasporite deposit, Turkey

Abstract

The İlbirdağı diasporic metabauxite (diasporite) deposit of the Milas (Muğla) region of Turkey is a unique deposit including both metamorphic (primary) and hydrothermal-remobilized (secondary) diaspore that formed during different geological periods. Microscopic diaspore crystals with a metamorphic origin are common and are the main constituent of the metabauxite ore, which was metamorphosed during from the Late Cretaceous to Late Paleocene Periods. However, secondary macroscopic diaspore crystals filling fracture zones that crosscut the metabauxite ore formed during the Late Paleocene, Eocene and Oligocene Periods as the result of later hydrothermal solutions that remobilized constituents of the metabauxite. The macroscopic diaspore crystals can be distinguished from the metamorphosed microscopic diaspore crystals, based on size, appearance, occurrence, and origin. Approximately 60% of the macroscopic diaspore crystals have an opaque appearance and pale green coloration and are not considered attractive. By contrast, the other 40% are gem quality and exhibit a marked change in color under different types of illumination. The crystals are mostly olive-green and soil-brown in daylight. A small number of the crystals display color-change, such as green in daylight or equivalent illumination and carmine in low-wattage tungsten lights. X-ray diffraction (XRD) study, using a comparative matching technique, reveals that these Anatolian diaspore (zultanite) crystals [AlO(OH)] are polycrystalline and that their X-ray spectrum includes the overlapped XRD peaks of some mineral inclusions consisting of donbassite [Al 5.33Si 3O 10(OH) 8] (di-di-octahedral sheeted Al-rich chlorite), corundum (Al 2O 3), boehmite [AlO(OH)], quartz (SiO 2), ilmenite (FeTiO 3), goethite [FeO(OH)] and chloritoid [(Fe, Mg, Mn) 2Al 4Si 2O 10(OH) 4]. We label these unusual inclusions as sub-microscopic inclusions, because they cannot be seen with a polarizing microscope. Polarizing microscope (PM), scanning electron microscope (SEM), optical absorption (OA), Fourier transform-infrared (FT-IR) and simultaneous differential-thermogravimetric analysis (DTA/TGA) investigations were performed to reveal the mineralogical characteristics of the Anatolian diaspore crystals.