Chemically and/or Organically Formed Sedimentary Rocks

1) Chemically formed:
            Some minerals, halite, gypsum (selenite), dolomite, and calcite are soluble in water. All seawater contains at least some of these minerals in dissolved form. Sometimes a sea or part of a sea may become cut off from the rest of the world's oceans perhaps as a result of tectonic events. Over a very long time this isolated body of water may evaporate. The minerals that were dissolved become more and more concentrated as the water evaporates and the sea becomes smaller. Eventually the remaining water can no longer hold the high concentrations of minerals which begin to precipitate (fall out of solution) onto the sea floor. These 'precipitates' form layers of solid minerals which may harden into rock. Such rocks are sometimes called evaporites. Halite forms Rock Salt. Gypsum forms Rock Gypsum. Dolomite forms Dolostone, Calcite forms Limestone.

             Do not confuse the minerals (composition column) with the rocks they form (rock name column).

            As minerals precipitate they often form crystals. This is how rock candy is made. Sugar is dissolved in water. The water is allowed to evaporate. The sugar becomes more and more concentrated until it begins to crystallize inside the container.

Badwater salt flats

Salt precipitates (Dead Sea)

 

2) Limestone may form as calcite precipitates as described above. Limestone that forms this way is an evaporite. But there is another way limestone may form. Many marine organisms grow shells. Their shells are made of calcite which they extract from sea water as the grow. When these organisms, oysters and clams for instance, die their shells pile up in the shallow bays where they lived. Waves and other forces may grind the shells into small pieces which may then be cemented together to form bioclastic limestone. "Bio" because the shells were of biologic origin. "Clastic" because the rock is formed from small particles cemented together. If the organisms were microscopic their shells form another bioclastic rock, chalk.

Bioclastic limestone

Bioclastic chalk cliffs (England)

 

3) Another type of bioclastic rock is formed not from calcite but from plant remains (mostly carbon). In warm, wet climates plants may grow at enormous rates. When they die they may pile up in swampy areas where they do not decompose because of the lack of oxygen in the swamp water. The layers of dead plant material get thicker and thicker and become compressed under their own weight. Eventually the material becomes peat, and given more time and more pressure the peat may become compressed into bituminous coal, a bioclastic rock.

Peat layer (Ireland)

Bituminous coal mine

Bituminous coal

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