By Christina Sloane | Updated Aug 30, 2022

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There are three primary rock classes: igneous, sedimentary, and metamorphic. Within sedimentary rocks, the chemical sedimentary subset—sometimes called orthochemical rocks—forms when minerals precipitate directly from solution. These rocks can arise through organic or inorganic processes and are pivotal in the Earth's mineral and fossil fuel inventories.

Types of Sedimentary Rocks

The sedimentary family is divided into four distinct categories:

• Clastic (detrital) rocks build from the physical weathering and transport of pre‑existing rock fragments. Particle size and texture define subtypes such as mudstone, siltstone, and sandstone.
• Biochemical rocks form when organisms precipitate minerals during biological activity. Common examples are limestone and certain cherts that record biological carbonates.
• Organic rocks result from the accumulation of dead plant or animal material, the most well‑known being coal, which forms over millennia in peat‑rich swamps.
• Chemical rocks develop when ions in solution precipitate out, producing deposits such as evaporites (halite, gypsum), dolostone, and iron formations. Although some literature lumps all chemically formed rocks together, the term also includes those produced through biological activity.

Regardless of terminology, chemical sedimentary rocks share a common origin: precipitated material once dissolved in water.

Organic vs. Inorganic Formation

Organic processes involve carbon‑bearing organisms and result in rocks that contain biological material, whereas inorganic processes are independent of life. For example, two limestone types illustrate this distinction:

• Biochemical limestone—coral reefs and chalk—comprise the skeletal remains of marine organisms.
• Inorganic limestone (travertine)—formed when calcium carbonate precipitates from mineral‑rich waters in caves or hot springs—has no biological contribution.

Evaporites such as halite and gypsum, and many cherts, are purely inorganic, forming by evaporation or chemical precipitation.

Where Sedimentary Rocks Form

Sedimentary deposition occurs in three principal environments:

• Continental (terrestrial) settings—lakes, rivers, alluvial fans, deserts, and glacial outwash plains—collect and compact sediment on land.
• Marine settings—beaches, reefs, continental shelves, and both shallow and deep ocean basins—support carbonate and clastic deposition.
• Transitional zones—barrier islands, tidal flats, deltas, and lagoons—where land meets sea, provide calm conditions ideal for sediment accumulation.

In all cases, the key factor is the delivery and deposition of sediment particles, whether by water currents, wind, or gravity.