Characterization of a Mg-rich and low-charge saponite from the Neogene lacustrine basin of Eskisehir, Turkey

The saponite examined occurs as two 0.1 m thick layers in a Pliocene sequence consisting of dolomite and dolomitic marl. To characterize this material, mineralogical and structural analyses (XRD, SEM and FTIR), thermal analyses (DTA, TG) and chemical analyses (ICP-ES) were performed. From XRD patterns of randomly-oriented powder samples, the first basal reflection appears as an asymmetric and broad peak with d (sub 001) values varying between 16.55 and 17.32 Aa. In oriented and air-dried samples, this reflection occurs between 14.45 and 16.42 Aa and is fairly symmetrical with FWHM of 2.7 degrees 2theta . Oriented and ethylene glycol-solvated samples produce a rational series of basal reflections, where 001 occurs at approximately 17.8 Aa as an intense, narrow (1.1 degrees 2theta ) and fairly symmetrical reflection. Upon solvation with glycerol, the 001 reflection shifts to approximately 18.7 Aa. The chemical composition of this saponite is similar to stevensite. However, the structural formula of Na (sub 0.114) Ca (sub 0.013) K (sub 0.003) (Mg (sub 2.957) Al (sub 0.004) Fe (sub 0.028) Ti (sub 0.004) ) (Si (sub 3.826) Al (sub 0.174) )O (sub 10) (OH) (sub 2) indicates that vacancies in the octahedral sheet do not exist. The negative layer charge arises nearly entirely from the substitutions in the tetrahedral sheet, with the net layer charge of -0.148, smaller than for common smectites. Due to the XRD characteristics and particularly the layer-charge distribution, it was concluded that this mineral is a Mg-rich saponite with low layer charge. The saponite was formed by direct precipitation in an alkaline lake environment from Mg- and Si-rich solutions at high pH.