Mineralogical and chemical characterization of sepiolite occurrences at Karapinar (Konya Basin, Turkey)

The Konya region in central Anatolia is covered by Pliocene-Late Pleistocene sediments and volcanites related to the sediments NNW of Karapinar, Turkey. In the area, the Upper Miocene-Quaternary Uzecek Dagi and Karacadag volcanites are generally of the same age and formed from magmas of similar composition. The Karapinar formation is brown to whitish-beige, partly fossiliferous and consists of limestone, marl, claystone and, locally, sandy layers. Silica-rich lenses, nodules and layers are observed in the upper strata which locally contain sepiolite-rich layers. The mineralogical composition of sepiolite samples taken from the area was determined by powder X-ray diffractometry, while the abundance of major-element oxides was measured by X-ray fluorescence spectrometry. The crystallographic and morphological properties of samples were determined by means of scanning electron microscopy and energy dispersive spectroscopy. Samples were taken from three sections and from random locations. Mineral assemblages in the same stratigraphic position are generally similar in the three sections, while the thickness of the individual beds varies between the sections. Dolomite and calcite are the main carbonate minerals in the sections. Sepiolite occurs primarily with dolomite and, locally, dolomite and calcite, and less commonly with just calcite. Generally, quartz, feldspar and mica are found, especially in the upper parts of the sections where tuff is abundant. CaO and MgO dominate the major-element oxides. The CaO content is between 1 and 30% while MgO is 3-21%. Al2O3 and SiO2 are generally higher in the sepiolitic and tuffitic layers. Al2O3 is <3% and SiO2 is between 15-18% in the sepiolitic layers. The average structural formula of sepiolite was calculated as: (Mg7.00Al0.44Fe0.18)(Si11.71Al0.29)O-30 (OH)(4)(OH2)(4)Ca0.13K0.09Na0.01. Sepiolite occurs as fibers and dolomite as subhedral or euhedral crystals. It is considered that sepiolite was formed either by conversion of dolomite or by direct precipitation from solution under alkaline and saline conditions in the Karapinar paleolake. The paleolake was saturated with respect to Mg, Ca and Si derived from groundwater that percolated along fracture systems.