CEMENTATION

in geology, hardening and welding of clastic sediments (those formed from preexisting rock fragments) by the precipitation of mineral matter in the pore spaces. It is the last stage in the formation of a sedimentary rock. The cement forms an integral and important part of the rock, and its precipitation affects the porosity and permeability of the rock. Many minerals may become cements; the most common is silica (generally quartz), but calcite and other carbonates also undergo the process, as well as iron oxides, barite, anhydrite, zeolites, and clay minerals. It is unclear just how and when the cement is deposited. Part seems to originate within the formation, and part seems to be brought in from outside by circulating waters. The reverse process, called decementation, also is thought to occur. There is evidence that decementation has occurred in calcareous sandstones, in which case the calcareous cement or grains are dissolved in the same manner as the solution of limestones. The frosted and etched surfaces of quartz grains in some friable and loosely cemented sandstones seem to indicate the former presence of a carbonate cement that has been leached.