An ab initio determination of the structure of sepiolite after acid treatment (HCl 0.5 n for 24 h) was carried out using X-ray powder diffraction data. After acid treatment, the sections normal to the a and c axes presented discontinuities, ~2.25 Å wide, parallel to the (010) plane, with no electronic density maxima, thus suggesting that adjacent planes are joined by van der Waals-like residual links. Partial dissolution was detected on both octahedral and tetrahedral sheets, beginning by breaking the ribbons not along the edges, but in the centre, thus creating a 5.20×6.79 Å tunnel along the a axis. By interrupting the tetrahedral sheet, this mechanism changes the phyllosilicate-like nature of the sepiolite to an inosilicate-like structure.

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