In an attempt to determine the nature and conditions of formation of the aluminium silicate precipitates present at the core of senile plaque in the brains of patients with Alzheimer's disease, aluminosilicate precipitates have been prepared and equilibrated for up to one year in physiological saline conditions at 40°C and a pH near 7·4. The structure and composition of these precipitates, and the associated concentrations of free silicic acid, were similar to those found in systems buffered with CaCO3 at 20–25°C. Imogolite-like products with Si : Al ratios near 0·5 were obtained at silicic acid concentrations <100 µm, and hydrous feldspathoid products with ratios ≥1·0 became dominant at silicic acid concentrations greater than ∼150 µm. These products do not match senile plaque precipitates, which have Si : Al ratios ≥1·0, but form in an environment with <100 µm Si. The relative stability of aluminium hydroxides, proto-imogolite allophane and hydrous feldspathoids are estimated from equilibrated silicic acid concentrations.

Chemical, thermal, X-ray and statistical analyses were carried out on the clay fraction of the Bonarelli Horizon (Umbrian region, Central Italy) in order to investigate its origin. The vertical lithological sequence (organic carbon, siliceous and clay layers of varying colour and thickness) seems to change regularly, allowing the horizon to be subdivided into five sub-horizons (A to E). Sub-horizon B is clearly distinguishable from the others on the basis of the clay fractions. The presence of organic material and authigenic sulphates, constant chemical composition within a single sub-horizon, and regular repetition of the layers, show that the Bonarelli Horizon is the result of an alternation of anoxic events in a continental margin facies. A substantial contribution of volcanoclastic material is suggested for sub-horizon B.