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 m 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.     

Diffusion Structural Analysis for characterization of ceramics and glasses from gels: I. Principle of the method and its use in the study of silica gels preparation

The Diffusion Structural Analysis (DSA) is described as a new tool for characterization of the processes taking place during preparation of ceramics and glasses from gels. Examples of the DSA application in the silica gels preparation from TEOS are demonstrated. The DSA results are compared with the results of optical transparency and pH measurements during sol-gel transition.     

Synthesis and crystal structure of low ferrialuminosilicate sanidine

Iron-containing potassium feldspar crystals are prepared using the hydrothermal synthesis in an alkaline medium at temperatures ranging from 500 to 52°C. The crystal structure of the synthetic potassium feldspar is refined [Ital Structures diffractometer, MoKαradiation, 1327 unique reflections with Fs>σ(F), anisotropic approximation, R(F) = 0.044]. It is established that, under the given preparation conditions, the synthesis leads to the formation of the monoclinic modification with the following unit-cell parameters: a = 8.655(7) Å, b = 13.101(9) Å, c = 7.250(8) Å, β = 116.02(2)°, space group C2/m, and Z = 4. The cation distribution over crystallographically inequivalent tetrahedral positions T(1) and T(2) is determined and justified using X-ray diffraction data. According to this distribution, the iron-containing potassium feldspar is assigned to the low ferrialuminosilicate sanidine. The proposed structural formula K _{A = 0.99}(Si_1.2Fe_0.5Al_0.3)_{ΣT(1) = 2}(Si_1.81Al_0.19)_{ΣT(2) = 2}O₈ agrees well with the data of the electron microprobe analysis. It is revealed that iron occupies the T(1) position and manifests itself as a majority rather than minority impurity element with respect to aluminum.