Lecture Notes on Chemical and Biochemical Sedimentary Rocks

Chemical Sedimentary Rocks

• Formation

  • Formed when minerals crystallize out of a solution due to evaporation.
  • Sediments are ions dissolved in solution.

• Types of Chemical Sedimentary Rocks

  1. Rock Salt
     • Composed of the mineral halite.
     • Forms when sodium and chlorine ions crystallize after evaporation.
     • Example: Dead Sea (high rate of evaporation, high salt content).
  2. Rock Gypsum
     • Composed of the mineral gypsum.
     • Forms large masses of crystals, referred to as rock gypsum.
  3. Limestone (Chemical)
     • Most common and widespread.
     • Composed of the mineral calcite (calcium carbonate).
     • Forms when calcium ions and carbonate ions crystallize post-evaporation.

• Characteristics

  • Inorganic formation process: evaporation of solution causing ionic bonds.
  • Referred to as evaporites due to formation from evaporation.

Biochemical Sedimentary Rocks

• Formation

  • Formed from chemicals derived from fossils.
  • Sediments are fossils.

• Types of Biochemical Sedimentary Rocks

  1. Fossiliferous Limestone
     • Derived from fossilized coral reefs.
     • Coral extracts calcium carbonate from ocean water.
     • Recrystallizes calcite full of fossils.
  2. Coquina
     • Composed of fragments of shells cemented together, usually with calcite.
     • Shells are made of calcite, derived from ocean water.
     • Undergo lithification, cemented by calcite.
  3. Chalk
     • Formed from tiny single-celled algae called coccolithophores.
     • Algae extract calcite from seawater to form shells.
     • Massive deposits formed over geologic time.

• Coal

  • Formed from fossilized plant material.
  • Occurs in swampy environments where plant material becomes compacted.
  • Coal is a biochemical sedimentary rock, distinct in being derived from plants rather than animals.

Key Points

• Chemical sedimentary rocks form inorganically through evaporation.
• Biochemical sedimentary rocks form from organic materials such as fossils.
• Both types involve the crystallization of minerals, but the source of the minerals (inorganic vs organic) differentiates them.