<P> Diagenesis (/ ˌdaɪəˈdʒɛnɪsɪs /) is the change of sediments or existing sedimentary rocks into a different sedimentary rock during and after rock formation (lithification), at temperatures and pressures less than that required for the formation of metamorphic rocks . It does not include changes from weathering . It is any chemical, physical, or biological change undergone by a sediment after its initial deposition, after its lithification . This process excludes surface alteration (weathering) and metamorphism . These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture . There is no sharp boundary between diagenesis and metamorphism, but the latter occurs at higher temperatures and pressures . Hydrothermal solutions, meteoric groundwater, porosity, permeability, solubility, and time are all influential factors . </P> <P> After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution . Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis . Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization . </P>

During diagenesis the sediments may undergo ___ changes