Palygorskite in Miocene mudstones, palustrine limestones and calcretes from the Esquivias locality (Madrid Basin, Spain) has been analyzed by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and analytical electron microscopy to determine its characteristics and chemical composition. Other palygorskites from the literature are used as references. The mean structural formula obtained from the analysis of isolated particles is (Si7.87Al0.13)O20(Al1.04Fe0.203+Mg3.110.65)(OH)2(OH2)4(Ca0.02K0.05Na0.08). This palygorskite has the largest Mg content reported in the literature, and it seems that, chemically, it fills the ‘compositional gap’ existing between sepiolite and palygorskite. Infrared spectroscopy reveals the absence of trioctahedral Mg and therefore the possibility of the existence of magnesic clusters in the ribbons is discounted. An homogeneous distribution of the octahedral cations (Al, Fe3+ and Mg) along the ribbons is proposed.

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