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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.

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Neogene (Upper Miocene-Pliocene) lacustrine sediments of northern Konya consist of conglomerate, sandstone, marl, mudstone, claystone, clayey limestone,and limestone. Limestones are white, beige-creamcolored, fine-grainedand contain remnants of plant roots. Brecciation, caleretion, mud cracks, and dissolution voids are common. Claystonesare typical with white and green colors. Green claystonecroppingout at the lower part of the sequence is alternated with mudstoneand sandstone. White claystonealternatingwith carbonate units appears at the upper part of the sequence. Carbonateunits are found as intercalatedlayers and lenses of conglomerate, sandstone, and mudstone.Calcite,dolomite, feldspar, and quartzmineralsare dominantin the study area. They are accompanied by sepiolite,palygorskite, smectite, chlorite, and illite minerals hosted by white colored clayey limestone and claystone.In addition, on the green colored claystones, minerals such as chlorite, smectite, and illite are formed.SEM studies indicate that sepiolite-palygorskite fiber and fiber bundles cover calcite and dolomite. Calcite and dolomiteare of hexagonal and rhombic crystal types. These minerals show a meniscus type cement which characterizes a vadose zone.Paragenesis and textural features of the minerals determined in the study area indicate that precipitation should occur due to climate fluctuations ranging from arid, semi-arid, and wet conditions. Changesin climaticconditions does affect the lacustrinewaterchemistryand precipitationof carbonate and detrital units. Sepiolite and palygorskite form authigenically as a result of the calcretionof carbonate units in alkalineconditions, high Si and Mg activity, and low AI.

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