Stages of Diagenesis of Sandstone

On the basis of mineral replacement and intergrowths, four oxide series are recognized among the sandstones –

1. alumina, lime-magnesia, iron oxide
   • Reactions : hematite + calcite + Mg siderite + ferro-dolomite + dolomite
2. silica, lime-magnesia, iron oxide
   • Reactions : hematite + clay minerals + silica + chlorite + greenalite
3. silica, alumina, iron oxide
   • Reactions : diaspore + silica clay minerals
   • diaspore + silica + k-glauconite + clay minerals
   • during late burial illite + Na + K + Ca = muscovite + biotite + feldspar
4. silica, alumina, lime-magnesia
   • Reactions : illite or glaauconite = chlorite + muscovite
   • calcite + Mg + illite = micas + feldspar + dolomite
   • deep and late burial; clay minerals = chlorite or sericite,
   • k-feldsapr = sericite ; plagioclase = chlorite + chert

Stages of diagenesis

From the type of reaction and the principal sequence of occurence three stages of diagenesis are established as follows –

1. Redoxomorphic : which dominates the episodes of deposition and early burial. Reactions principally are reduction and oxidation which form the basis for the name of the stage.
2. Locomorphic : Involves replacement of one mineral by another, the two minerlas not entering into mutual reaction. Such reactions aretypical of the early burial stage and are an important part of the process of lithification.
3. Phyllomorphic : Follows the principal locomorphic replacements and involves development of micas, principally from clays.

Among sandstones, the progress of diagenesis may be arrested at either the locomorphic or phyllomorphic stages, in more or less parallel fashion to metamorphic grades. Also, the phyllomorphic stage overlaps the zeolite and chlorite grades of metamorphism.

Reactions in sedimentary diagenesis