On the microstructure of geyserites and hyalites, natural hydrous forms ofsilica

The submicron structures of two natural hydrous forms of silica (geyserites from Iceland, hyalite from Valec CS) have been studied by scanning electro n microscopy, small-angle and wide-angle X-ray scattering. All geyserites i nvestigated are particulate with a nonuniform size distribution. These part icles are arranged in spherical agglomerates, in chains or in helical patte rns. The average diameters of the colloidal particles range between 200 nm and 2 mu m depending on the texture of the samples. SAXS experiments have r evealed the porous nature of the particles with inhomogeneities smaller tha n 10 nm. Based on the structural pecularities of the geyserites their origi n is discussed as the result of forced condensation of silicic acid in an a queous environment, particle agglomeration and sedimentation. The structura l development including the textures is determined by the dynamics of the u nderlying processes: fast decompression and cooling of the saturated parent solutions and their transport. By contrast, hyalite is characterized by a homogeneous microstructure.