Stages/types of chemical changes in sedimentology

In sedimentary petrology and diagenesis, terms like neochemical, allochemical, protochemical and post chemical reactions are used to describe various stages or types of chemical changes.

1. Neochemical Reactions : Chemical processes that involve the formation of new minerals from solution during early diagenesis.
   • Processes :
     • Precipitation of new minerals (dolomite replacing calcite)
     • Authigenesis (formationfo minerals in place)
   • Environment : Often occurs in marine pore waters soon after deposition.
   • Example : Formation of galuconite or dolomite in marine sediment.
2. Allochemical Reactions : Reactions involving transported grains (allochems) like ooids, peloids, fossil fragments. It concerns the original composition and alteration of these transported grains.
   • Processes :
     • Cementation of ooids.
     • Micritization of bioclasts.
   • Environment : Can occur during early to late diagenesis.
   • Example : Alteration of shell fragments in a limestone.
3. Protochemical Reactions : Very early chemical changes occurring immediately after sediment deposition, often before lithification.
   • Processes :
     • Initial microbial decay.
     • Early mineral precipitation.
   • Importance : Sets the stage for later diagenetic reactions.
   • Example : Microbial sulfate reduction leading to early pyrite formation.
4. Post-chemical Reactions : Chemical changes that occur after lithification (hardening of sediment into rock), usually greater burial depths or during uplift.
   • Processes :
     • Pressure solution
     • Recrystallization
     • Mineral transformations due to T & P changes
   • Examples : Calcite recrystallizing into sparite during deep burial.