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Mineralogical and geochemical properties of messinian gypsum occurence in Polatlı Sazılar region, Ankara

Çakmak, Hayriye
The objective of this study is to understand the petrographical, mineralogical and geochemical characteristics of Messinian (Upper Miocene) gypsum occurence in Polatlı Sazılar region, Ankara and to determine the impurities associated with gypsum. Moreover, it is aimed to interpret the usability of this raw material with impurities in plaster and plasterboard production based on industrial standards. Based on petrographical study, the first variety of gypsum which have different physical properties is white massive, micro crystalline, and translucent gypsum. Prismatic, transparent, idiomorphic and cleavage dominated selenite is the second variety. The gypsum rock is also identified that it is composed of sand sized selenite crystals and gypsum grains in a calcite and clay rich matrix. These impurity minerals also observed in thin beds alternating with gypsum. Scanning electron microscopic (SEM) studies are conducted to determine the relationship between the impurity mineral, gypsum and the selenite. X-ray diffractometry (XRD) studies were used to identify the type of clay minerals. In the samples, Ca-smectite is the essential impurity clay mineral. Rarely, mixed layer of illite-smectite is also identified. Geochemical studies also shows that as the amount of clay impurity increases, MgO, Al2O3, Fe2O3, Na2O, K2O (wt%) increases with decreasing SO3 (wt%). On the other hand, impurity of calcite controls the CaO (wt%) content in varying amounts. Five different groups of samples indicating different abundance of clay and calcite impurities are interpreted. The clay rich samples are enriched, in general, in vanadium, nickel, copper and chromium.