Characterization of a Mg-rich and low-charge saponite from the Neogene lacustrine basin of Eskişehir, Turkey
Abstract: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 d001 values varying between 16.55 and 17.32 Å. In oriented and air-dried samples, this reflection occurs between 14.45 and 16.42 Å and is fairly symmetrical with FWHM of 2.7°2y. Oriented and ethylene glycol-solvated samples produce a rational series of basal reflections, where 001 occurs at ∼17.8 Å as an intense, narrow (1.1°2) and fairly symmetrical reflection. Upon solvation with glycerol, the 001 reflection shifts to ∼18.7 Å.
The chemical composition of this saponite is similar to stevensite. However, the structural formula of Na0.114Ca0.013K0.003(Mg2.957Al0.004Fe0.028Ti0.004)(Si3.826Al0.174)O10(OH)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.
Document Type: Research Article
Publication date: December 1, 2007