DEFINITION OF CHEMICAL SEDIMENT

Sediments deposited directly by precipitation

from solution, evaporation, or from deposition of

insoluble precipitates. They differ from the other main type of

sediments and sedimentary rocks, known as clastic sediments

and rocks, which are made of particles broken from another

source, transported, and deposited as the new sediment.

Common types of chemical sediments include limestones,

dolostones, evaporites, chert, phosphates, and ironstones.

Some chemical sediments are produced by a combination of

biochemical processes, especially varieties of limestone, dolostone,

some types of phosphates, and banded iron formations.

Limestones and dolostones are carbonate rocks composed

primarily of calcite and dolomite and produced by a

combination of accumulation of dead organisms above the

level that calcium carbonate is saturated in seawater (the calcium

carbonate compensation depth), and by biochemical

and chemical reactions that precipitate carbonate from solution.

Evaporites, commonly gypsum (CaSO4 • 2 H2O), anhydrite

(CaSO4), and halite (NaCl), form most commonly by

the evaporation of seawater or shallow restricted lake basins

in arid environments, known as playas. In some of these

examples, rainwater flows through older rock deposits, dissolving

the evaporite minerals from the older rocks, then

reprecipitating the minerals as new evaporites in restricted

basins. Chert is a rock composed of nearly pure silica, consisting

of microcrystalline or cryptocrystalline quartz. Chert

may form from the inorganic or biologic precipitation from

seawater. Phosphates commonly form nodules on the

seafloor, as layered sediments in shallow water environments,

and as guano accumulations, especially from sea birds. Manganese

forms nodules on the seafloor, and some are associated

with black smoker types of submarine hydrothermal vents.

Others are located in shallow water environments. Ironstones

and banded iron formations are iron-rich sedimentary rocks

that are especially common in Precambrian rocks and may

reflect different levels of atmospheric oxygen and iron redox

state in the early oceans. These unique formations contain the

world’s largest reserves of iron ore, but the lack of a modern

analog environment makes understanding their origin somewhat

enigmatic.

See also BANDED IRON FORMATION; BLACK SMOKER

CHIMNEYS; CARBONATE; CHERT.

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar