Scientists from Spain and France have obtained the first single-crystal X-ray diffraction images of sepiolite, opening the path to the industrial synthesis and further improvement of its properties.
The team included scientists from the universities of Madrid and Salamanca in Spain, the Institut Laue-Langevin (ILL), the European Synchrotron Radiation Facility (ESRF) and the Spanish CRG Beamline at the ESRF (SpLine), all in Grenoble (France).
According to the ESRF, no other mineral is known to absorb more water or other liquids as efficiently as sepiolite — a lightweight porous mineral used in cat litter and other applications. The reasons are its structural nanoporosity owing to tunnels in the crystals and the fact that the elongated, needle-shaped sepiolite crystals pack very loosely into a lightweight porous material.
The tiny size of these crystals has been the main obstacle to them being studied with single-crystal diffraction techniques.
For their experiment, the scientists collected samples of sepiolite fibres from 20 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, an analytical technique used for the phase identification of a crystalline material.
However, the most accurate technique to resolve the 3D structure of a crystal is said to be 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 a 2 x 5um cross section,’ said Manuel Sanchez del Rio from the ESRF. ‘In the end, we collected X-ray diffraction data for two fibres, 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‚’ said Mercedes Suárez from Salamanca University. ‘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.’
Sanchez del Rio added: ‘The future of sepiolites in the household is outside the litter box. Already today, they absorb liquid spillages and odours and stabilise aqueous products such as 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.’
Results of the research will be published in the October 2011 issue of the journal American Mineralogist.