On the Microstructure of Geyserites and Hyalites, Natural Hydrous Forms of Silica

The submicron structures of two natural hydrous forms of silica (geyserites from Iceland, hyalite from Valec CS) have been studied by scanning electron microscopy, small-angle and wide-angle X-ray scattering. All geyserites investigated are particulate with a nonuniform size distribution. These particles are arranged in spherical agglomerates, in chains or in helical patterns. The average diameters of the colloidal particles range between 200 nm and 2 μm depending on the texture of the samples. SAXS experiments have revealed the porous nature of the particles with inhomogeneities smaller than 10 nm. Based on the structural pecularities of the geyserites their origin is discussed as the result of forced condensation of silicic acid in an aqueous environment, particle agglomeration and sedimentation. The structural development including the textures is determined by the dynamics of the underlying processes: fast decompression and cooling of the saturated parent solutions and their transport. By contrast, hyalite is characterized by a homogeneous microstructure.