The Middle to Upper Miocene volcano-sedimentary units in the Mihalıççık-Eskişehir area of Turkey consist mainly of altered tuff and claystone, with thin layers of dolomite and silica nodules and horizons. Ripple marks and desiccation cracks are common in the siliceous tuff and dolomite units. Green to yellowish green claystone exhibits either hard layered or friable features. Sepiolite and loughlinite are dominant in the claystone of the study area. These minerals are generally associated with opal-CT, analcime, feldspar, dolomite and calcite. At Mihalıççık, loughlinite and sepiolite are identified by d(110) reflections at 12.9 and 12.3 Å, respectively. Loughlinite and sepiolite occur as fibers in dissolution voids and along the sides of dissolved volcanic glass particles and casts of silicic nodules. The area is represented by a lacustrine environment of variable depth and salinity, controlled by synsedimentary step-faulting in arid and semi-arid climatic conditions. Under these conditions, volcanic glass as well as dolomite release Si and Mg, which are used for the formation of sepiolite. Increasing evaporation causes the dominance of Mg with Na and K in the depression zone in the Killik area, favoring the formation of loughlinite under alkaline conditions. Field observations and mineralogical determinations indicate that sepiolite and loughlinite are both formed authigenically and independently in different physicochemical environments rather than being the product of a transformation of one to the other.

The study area is located around the town of Hatunsaray, approximately 40 km away from the south-west of Konya. In this study, the petrological and geochemical characteristics of the Bulumya ignimbrite, Detse ignimbrite and Sadıklar ignimbrite observed in the region were revealed. The study area is basically Upper Miocene - Lower Pliocene aged Güneydere formation and overlying Bulumya ignimbrite, Detse ignimbrite, Sadıklar ignimbrite and Quaternary alluviums. All these units were formed in the Upper Miocene - Lower Pliocene aged fluvial and lake environment and have a lateral vertical transition with carbonate, clastic units. The gray colored Bulumya ignimbrite contains andesite-dacite rock fragments and large pumice grains. The Detse Ignimbrite is yellow in color and shows a well sorted lapilli tuff composition. The Sadıklar Ignimbrite, on the other hand, contains agglomeratic levels with yellow colored slightly fused lenses and wedge geometry. All ignimbrite samples have porphyric texture and were classified as “crystal-vitric tuff” and “crystal lithic-vitric tuff” in the glass-crystal-rock fragment classification. In petrographic investigations of ignimbrites, general mineral paragenesis consists of quartz, plagioclase, plagioclase microliths, biotite, amphibole, opaque minerals and rock fragments. When geochemical data are evaluated, it is observed that all ignimbrite samples are subalkaline, trachy-andesite, andesite-basaltic andesite and calc-alkaline in character. When the main oxide, trace and rare earth elements are evaluated, fractional crystallization is observed in ignimbrites mainly controlled by K-feldspar and plagioclase minerals. In addition, the high K and Rb content in the spider diagrams indicate crustal contamination. The distribution of the ignimbrite samples in the Rb/Y-Nb/Y diagram suggests that the magma source forming the samples is enriched by subduction and/or crustal components.