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Earth Systems 3209: Common Chemical Sedimentary Rocks
Evaporites
Halite
Rock Salt is a chemical sedimentary rock that forms from the evaporation of ocean or saline lake
waters. It is also known by the mineral name "halite". It is rarely found at Earth's surface, except in
areas of very arid climate. It is often mined for use in the chemical industry or for use as a winter
highway treatment. Some halite is processed for use as a seasoning for food.
Gypsum
Gypsum is an evaporite mineral most commonly found in layered sedimentary deposits in
association with halite, anhydrite, sulfur, calcite and dolomite. Gypsum (CaSO4.2H2O) is very similar
to Anhydrite (CaSO4). The chemical difference is that gypsum contains two waters and anhydrite is
without water. Gypsum is the most common sulfate mineral.
Sylvite
Sylvite is potassium chloride (KCl) in natural mineral form. It forms crystals very similar to
normal rock salt, halite (NaCl). Sylvite is colorless to white with shades of yellow and red. Sylvite has
a salty taste with a distinct bitterness. Sylvite is one of the last evaporite minerals to precipitate out
of solution. As such, it is only found in very dry saline areas. Its principal use is as a potassium
fertilizer
Earth Systems 3209: Common Chemical Sedimentary Rocks
Precipitates
Limestone
Limestone is a rock that is composed primarily of calcium carbonate. It can form organically from the
accumulation of shell, coral, algal and fecal debris. It can also form chemically from the precipitation
of calcium carbonate from lake or ocean water. Limestone is used in many ways. Some of the most
common are: production of cement, crushed stone and acid neutralization.
In a cave, droplets of water seeping down from above enter the cave through fractures or other pore
spaces in the cave ceiling. There they might evaporate before falling to the cave floor. When the
water evaporates, any calcium carbonate that was dissolved in the water will be deposited on the
cave ceiling. Over time this evaporative process can result in an accumulation of icicle-shaped
calcium carbonate on the cave ceiling. These deposits are known as stalactites. If the droplet falls to
the floor and evaporates there a stalagmite could grow upwards from the cave floor.
The limestone that makes up these cave formations is known as "travertine".
Dolomite
Dolomite (also known as "dolostone" and "dolomite rock") is a chemical sedimentary rock that is
very similar to limestone. It is thought to form when limestone or lime mud is modified by
magnesium-rich groundwater. The specimen shown above is about four inches (ten centimeters)
across.