Mechanism and effect of chemical weathering of sedimentary rocks

摘要

The mineralogy, geochemistry and physical andmechanical properties of rocks from four weathering profiles ofMiocene to Pleistocene mudstones and sandstones in Japanshowed that chemical weathering of sedimentary rocks ischaracterized by sequential reaction between percolatinggroundwater and rock-forming minerals. Pyrite, a commonmineral contained in sedimentary rocks, is especially important inthese sequential reactions. Pyrite is oxidized by oxygen comingfrom the ground surface and sulfuric acid is generated at the basedof the oxidized zone. The sulfuric acid, in turn, dissolves rock-forming materials to make a dissolved zone. If the fluxes ofoxygen and water are in the same direction, sulfuric acid generatedat the oxidation front migrates farther and forms a dissolved zone.If these fluxes are in opposite directions, the oxidized anddissolved zones are not differentiated. Rocks in the dissolved zoneare caused to deteriorate by the acid leaching and are acidic ifbuffering minerals, such as calcite and zeolite, are absent. In theoxidized zone, sandstone is strengthened because of cementationby iron oxide or hydroxide, while mudstone is weakened because ithas greater clay fractions and larger specific surface areas than sandstone.