Driving forces for the weathering and alteration of silica in the regolith: Implications for studies of prehistoric flint tools

A range of geochemical and mineralogical factors trigger the weathering and influence the stability of various silica phases under near-surface temperatures and pressures. Based on the current knowledge of the crystallochemical basis of silica behaviour in near-surface environments, the mineralogical and petrographical transformations of silica under weathering conditions in the regolith are reviewed using data from wide-ranging and detailed investigations of siliceous materials. Some of the investigations are of siliceous materials that evolved in near-surface environments over geological time and thus the weathering and alteration features are relatively clear and well expressed. Examples include weathering of tightly cemented sandstones, the formation of silica duricrusts (pedogenetic silcret es), and the weathering of flints. The various micromorphological and mineralogical forms of silica as determined by high resolution petrographic and electron-optical techniques can be related to geochemical and hydrological conditions in particular microenvironments. However, new studies have demonstrated that silica alteration and transformation mechanisms can also occur within prehistoric and historic time intervals. These are also described but are more difficult to observe, principally because of the microcrystalline character of the alteration phases and the infra-millimetric scale on which they develop. Of many potential applications of knowledge about the near-surface alteration of silica in regolith materials and the environmental conditions under which it occurs, our studies.