Amorphous silica materials in soils and sediments of the Ex-Lago de Texcoco, Mexico: An explanation for its subsidence

Abstract

Mexico City is built on the sediments of the former Texcoco lake (Ex-Lago de Texcoco), which are characterised by the presence of a siliceous deposit, up to 70 m in thickness, called “jaboncillo” (like soap). The jaboncillo is composed dominantly of amorphous silica and iron oxides and hydroxides, with minor amounts of phyllosilicates and detrital volcanic minerals. The mineralogy, the micromorphology and physical and chemical properties are discussed. The optical isotropic groundmass consists largely of amorphous silica and iron hydroxides. The bulk density, calculated for the dry material is about 0.26 g cm −3. Its water retention capacity is extremely high, up to 522% at field capacity, but this characteristic is partly lost after drying. Also the high solubility of Si, the high CEC and COLE are irreversibly reduced after drying, due to a denser packing of the originally randomly distributed hydrated silica spheres. Therefore, it is evident that drainage will irreversibly change physical characteristics, and increase risk of instability and subsidence of the sediments when used for construction.