Nanostructural investigation of slightly altered rhyolitic volcanic glass

Silica rich volcanic glass is investigated by selected area electron diffraction (SAED) and high resolution electron microscopy (HRTEM), and nanostructural characterization is presented. In freshly crushed glass shards the presence of amorphous and randomly oriented nanocrystalline component was detected, the latter one enriched in iron with respect to the amorphous regions. To interpret the iron containing nanocrystalline structure, model clusters of different size, shape and composition were constructed and used to calculate scattered electron intensity. According to our calculations, already a single ring of six interconnected SiO4 tetrahedra results a separate broad peak at ~ 5 Å which reflects nicely the main diffraction feature of amorphous silica. Inserting two dimensional iron array between two neighbouring silica layers produces a less compact structure and the first diffraction peak shifts towards larger values with respect to the pure silica. For smaller clusters (number of atoms < 250) first peak position shows a remarkable dependence on the structure and size of the cluster. The pair distribution analysis of SAED and HRTEM data indicates a certain degree of ordering with a domain size of ~ 2 nm.