Geological, mineralogical and geochemical features of the bentonite occurrences the Kapıkaya Eskişehir)

Numerous bentonite occurrences derived from the alteration of volcanic rocks are found between Kapıkaya and Beyköy (Eskişehir). These occur through alteratin of volcanic rocks such as lav (group 1), aglomerate (group 2) and tuff (group 3). The first group are characteristic distinguished with exfoliation alteration which is shown in the bentonite occurrences. Lateral zonation in the third group bentonite occurrences is also evident, and distinguished as physical, chemical and mineralogical properties. The concordances between distribution of bentonite deposits and strikes of faults in study area show that solutions caused by alteration are fault-related hydrothermal solutions. While first and second groups of bentonites are neutral (Na-Ca), group 3 of bentonites is Na bentonite in composition and are composed mainly of dioctahedral smectite. Illite is other clay mineral in bentonites. Non clay minerals in bentonites are gypsum, cristobalite/opal-CT, quartz, feldspar, dolomite and calcite. Cornflakes-shaped montmorillonite crystals are distinguished by means of scanning electron microscopy (SEM). The results of chemical analysis show that bentonites and parent rocks of group 1 and 2 are andesitic composition and bentonites and parent rocks of group 3 are also rhyodacitic/dacitic composition. During the alteration the MgO a show differences againts Fe2O3 and TiO2, while first group is not difference SiO2 and Al2O3, K2O to decrease during alteration. According to parent rocks, the triple enrichments of MgO, Fe2O3 and TiO2 indicated precipitation with secondary phases. According to parent rocks, the enrichments of MgO, CaO and Fe2O3 in bentonites are olso related to the composition of hydrothermalsolutions ascending along ophiolitic rocks. The difference of behavior between HREE and LREE during the alteration is due to the variety of the composition of the parent rock in all groups.2008 ? 96 pageKey Words: Bentonite, simectite, Eskişehir, Kapıkaya.