Limestone, Chert, Evaporites — and Organic Rocks (Coal)

While clastic rocks are built from transported fragments, another major family forms in place through precipitation or biological accumulation. Geologists divide these into three categories:

• Chemical rocks — minerals precipitate directly from water.
• Biochemical rocks — composed of organism remains (shells, tests).
• Organic rocks — formed from compressed organic matter (coal).

Limestone is mostly calcite (CaCO₃). It forms in two ways:

• Biochemically: microscopic organisms (foraminifera, coccolithophores) and larger shelled creatures die and pile up on the seafloor. Chalk — like the White Cliffs — is almost pure microscopic shells.
• Chemically: warm, shallow seawater saturated in calcium carbonate can precipitate oolites — tiny spherical grains that accrete like snowballs as they roll in the current.

Limestone therefore signals warm, shallow marine environments with clear water and abundant life.

Chert is a hard, microcrystalline silica rock (SiO₂). It forms from the slow accumulation of microscopic silica shells — radiolarians in the deep ocean and diatoms in surface waters. When these organisms die, their silica dissolves and reprecipitates as a dense, hard layer. Chert can also replace other rocks (such as limestone) by silica-rich groundwater — a process called silicification.

Evaporites crystallise when restricted bodies of water (lagoons, inland seas, salt flats) evaporate faster than they are replenished. As water volume drops, dissolved salts become concentrated and precipitate in a predictable sequence:

• Carbonates (calcite, dolomite) precipitate first.
• Sulfates (gypsum, CaSO₄·2H₂O) form as brine concentrates further.
• Finally, halides (halite, NaCl) and potash salts crystallise from the most concentrated brine.

Thick evaporite deposits are evidence of arid climates and restricted marine basins.

Coal is unique among sedimentary rocks because it forms from compressed plant matter rather than mineral grains. In swampy environments, dead plants accumulate in thick layers of peat. As burial deepens, heat and pressure drive off water and volatile compounds, increasing the carbon content through a progression of ranks:

• Peat — loose, wet, low carbon.
• Lignite — brown coal, still moist, low energy.
• Sub-bituminous — darker, higher carbon.
• Bituminous — the most commonly mined coal, high energy.
• Anthracite — metamorphosed by extreme heat/pressure, highest carbon, cleanest burn.