Mineralogy and crystal-chemistry of trioctahedral smectites from Turkey

Abstract

This study focused on a detailed mineralogical and crystal-chemical analysis of Mg-smectites from four bentonite samples from Turkey. Mg-rich smectites, mainly associated with alkaline and evaporitic depositional conditions, are formed in environments such as salt lakes, brine springs, and sabkhas, as well as in hydrothermal systems, in some cases by transformation from other phyllosilicates. Saponite has also been documented on the surface of Mars. The systems that produce Mg-smectites are less common than those that produce dioctahedral Al-smectites and consequently Mg-rich smectites are less abundant than dioctahedral smectites. For this reason, information on nanoscale mineralogy and crystal chemistry of Mg-smectites is relatively lacking. In this study, X-ray diffraction, thermal analysis and electron microscopy were used to study Mg-smectites. The crystal chemistry of single crystals determined with analytical electron microscopy in transmission electron microscopy (AEM-TEM) revealed that all samples had notable variability in the composition of individual crystals, such that no point analysis resulted in ideal structural formulae for saponite, stevensite, sepiolite, or palygorskite. They contain SiO2 content greater than that corresponding to a Mg-smectite, even stevensite, and often are intermediate to Mg-smectites and the sepiolite-palygorskite series. Meanwhile, the number of octahedral cations is small for fibrous clay minerals. Neither the point analysis of smectitic particles nor the mean structural formula fit properly for Mg-smectites showing crystallochemistry complexity. The results of these point analyses, in which no contamination has been observed, suggest that these smectites have intermediate compositions between trioctahedral smectites and sepiolite-palygorskite, indicating nanometer-scale intergrowths of these minerals in Mg-rich clay deposits.