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Sepiolite is a lightweight porous mineral used in cat litter and other applications. The extraordinary properties of this clay make it a highly sought after mineral, despite its scarcity in the Earth‘s crust: only a few mines worldwide extract it, several of them clustered near Madrid in Spain, the world’s biggest exporter of this material. Sepiolite has been known since Roman times when it was used to filter and purify wine, but our understanding at the atomic scale of how these tiny crystals absorb enormous amounts of liquid has remained elusive until now. A team of scientists from Spain and France has obtained for the first time single-crystal X-ray diffraction images of sepiolite, opening the path to industrial synthesis and further improvement of its properties. The results will be published in the October 2011 issue of the journal American Mineralogist.
The team included scientists from the Universities of Madrid and Salamanca in Spain, of the Institut Laue-Langevin (ILL), the European Synchrotron Radiation Facility (ESRF), and the Spanish CRG Beamline at the ESRF (SpLine), all in Grenoble (France).
No other mineral is known to absorb more water or other liquids as efficiently as sepiolites. The reasons are its structural nanoporosity due to tunnels in the crystals, and the fact that the elongated, needle-shaped sepiolite crystals pack very loosely into a lightweight porous material. The surface area ranges between 75 and 400 m2/g, meaning that 20g of mineral have an internal surface equivalent to that of a football court. This is why sepiolite can absorb 2.5 times its weight in water. The tunnels in the crystal structure along with the empty space between the needles form a capillary network through which liquids can easily flow deep inside the bulk where the molecules attach to the surface of the crystals.
The tiny size of these crystals—they measure a few micrometres in length and as little as some dozen atoms across—has been the main obstacle to their being studied with single-crystal diffraction techniques. For this experiment, the scientists collected samples of sepiolite fibres from twenty different deposits around the world. These fibres, each made of many crystals, were first imaged with electron-microscopy and then studied using X-ray powder diffraction.
However, the most accurate technique to resolve the three-dimensional structure of a crystal is single-crystal diffraction with either X-rays or electrons as probe. “To study very small crystals, the ESRF uses an X-ray beam with just 2 by 5 micrometres cross section. In the end, we collected X-ray diffraction data for two fibres”, says Manuel Sanchez del Rio from the ESRF, “but the data were not easy to interpret, and needed extensive computer simulations to confirm and refine the information gathered by electron diffraction experiments done in parallel at the University Complutense of Madrid”.
The wide variety of sepiolites studied is now enabling the team to correlate between the physical and chemical properties of a given type with its atomic structure. “Today, no synthetic clay surpasses natural sepiolite. This is about to change as our understanding of their atomic structure will guide the synthesis of sepiolites from other, more abundant clay minerals and the design of completely new materials for use in catalysis and batteries”, says Mercedes Suárez from the University of Salamanca.
“The future of sepiolites in the household is outside the litterbox. Already today, they absorb liquid spillages and odours and stabilise aqueous products like paints, resins and inks. In synthetic form, they could bind food products and stabilise drugs, extending their shelf life and making sepiolite an edible product”, concludes Manuel Sanchez del Rio.