Mechanism and effect of chemical weathering of sedimentary rocks

The mineralogy, geochemistry and physical and mechanical properties of rocks from four weathering profiles of Miocene to Pleistocene mudstones and sandstones in Japan showed that chemical weathering of sedimentary rocks is characterized by sequential reaction between percolating groundwater and rock-forming minerals. Pyrite, a common mineral contained in sedimentary rocks, is especially important in these sequential reactions. Pyrite is oxidized by oxygen earning from the ground surface and sulfuric acid is generated at the base of the oxidized zone. The sulfuric acid, in turn, dissolves rock-forming materials to make a dissolved zone. If the fluxes of oxygen and water are in the same direction, sulfuric acid generated at the oxidation front migrates farther and forms a dissolved zone. If these fluxes are in opposite directions, the oxidized and dissolved zones are not differentiated. Rocks in the dissolved zone are caused to deteriorate by the acid leaching and are acidic if buffering minerals, such as calcite and zeolite, are absent. In the oxidized zone, sandstone is strengthened because of cementation by iron oxide or hydroxide, while mudstone is weakened because it has greater clay fractions and larger specific surface areas than sandstone. (C) 1999 Elsevier Science B.V. All rights reserved.