The most advanced geochemical stage of diagenesis (process of chemical and physical changes in deposited sediment during its conversion to rock), characterized by the replacement of clays by micas, feldspars, and chlorites.

Diagenesis refers to changes that take place in a sediment as a result of increased temperatures and pressures, causing solid rock to form, e.g. as sand becomes sandstone.

Mica is a group of chemically and physically related aluminum silicate minerals, common in igneous and metamorphic rocks, characteristically splitting into flexible sheets used in insulation and electrical equipment.

Feldspars are any of a group of abundant rock-forming minerals occurring principally in igneous, plutonic, and some metamorphic rocks, and consisting of silicates of aluminum with potassium, sodium, calcium, and, rarely, barium. About 60 percent of the earth's outer crust is composed of feldspar.

Chlorites consist of any of a group of green soft secondary minerals consisting of the hydrated silicates of aluminium, iron, and magnesium in monoclinic crystalline form; common in metamorphic rocks.