Sum of all processes, chiefly chemical, that produce changes in a sediment after its deposition but before its final lithification. Usually, not all the minerals in a sediment are in chemical equilibrium, so changes in interstitial water composition or in temperature or in both will lead to chemical alteration of one or more of the minerals present. Diagenesis is considered a relatively low-pressure, low-temperature alteration process that involves such processes as cementation, reworking, replacement, crystallization, and leaching.
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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.
The study of diagenesis in rocks is used to understand the tectonic history they have undergone; the nature and type of fluids that have circulated through them. From a commercial standpoint, such studies aid in assessing the likelihood of finding various economically viable mineral and hydrocarbon deposits.
The composite nature of bone, comprising one third organic (mainly protein collagen) and two thirds mineral (calcium phosphate mostly in the form of hydroxyapatite) renders its diagenesis more complex. Alteration occurs at all scales from molecular loss and substitution, through crystallite reorganization, porosity and microstructural changes, and in many cases, to disintegration of the complete unit. Three general pathways of the diagenesis of bone have been identified:
They are as follows:
It is generally accepted that hydrocarbons are formed by the thermal alteration of these kerogens (the biogenic theory). In this way, given certain conditions (which are largely temperature-dependent) kerogens will break down to form hydrocarbons through a chemical process known as cracking, or catagenesis.
Cleavages developed in mudstone during diagenesis and deformation: an example from the Carboniferous (Tournaisian), southeastern New Brunswick, Canada.(Report)
Jan 01, 2009; ABSTRACT The Weldon Formation of the Lower Carboniferous (Tournaisian) Sussex Group of southeastern New Brunswick consists of red...