Characterization of two forms of sepiolite and related Mg-rich clay minerals from Yenidogan (Sivrihisar, Turkey)

An Early Pliocene sedimentary succession in the Yenidogan area, Sivrihisar, Turkey, consists of sepiolite, stevensite, kerolite, dolomite and magnesite. The geology, mineralogy and geochemistry of the succession was examined by extensive field work along several trenches and a representative measured section, followed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermal, and chemical analyses. Structurally, two distinct forms of sepiolites were distinguished by XRD: (a) well crystallized sepiolite with a 110 reflection at 12.07-12.3 Aa, and (b) poorly crystallized sepiolite in which the 110 reflection occurs at 12.7-13.0 Aa (denoted as sepiolite-13Aa). Differences in crystal chemistry, thermal and morphological properties of these forms, the vibrational spectra and XRD characterization of the related phyllosilicates were also documented. Stevensite, kerolite and sepiolite were formed by direct precipitation from alkaline lake water rich in Mg and Si. Sepiolite-13Aa was probably formed by transformation from precursor smectite via dissolution-precipitation, more likely during early diagenesis. Environmental conditions such as ion concentration, salinity and variations in pH may have controlled the formation of the phyllosilicates.