diagenesis, sum of all processes, chiefly chemical, by which changes in a sediment are brought about after its deposition but before its final lithification (conversion to rock). Because most sediments contain mineral mixtures in which not all the minerals are in chemical equilibrium with each other, changes in interstitial water composition or changes in temperature or both will usually lead to chemical alteration of one or more of the minerals present. Diagenesis is considered a relatively low-pressure, low-temperature alteration process, whereas metamorphism is considered to be a rock-alteration process occurring at relatively higher pressures and temperatures. An example of diagenesis is the chemical alteration of a feldspar to form a distinctly new mineral in its place, a clay mineral.