Download Chemical Rock Organic Rock

Chemical Rock
The water in seas, lakes, swamps, and underground reservoirs often
contains dissolved minerals. Chemical sediments form when these
minerals precipitate, or fall out of solution. Precipitation can occur through
evaporation or through chemical action—as, for example, when dissolved
ions combine to form new minerals. Common chemical sedimentary rocks
are rock salt, rock gypsum, and some limestones.
Rock salt, or halite, occurs in thick layers in many parts of the world.
Rock gypsum occurs in layers or as nearly pure veins of the mineral
gypsum. Although they are uncommon, limestones of chemical origin may
form when tiny grains of calcite are deposited on the bottoms of seas or
lakes. These limestones are often gray or tan, compact and dense, and
smooth to the touch.
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image in the textbook
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ROCK SALT FLAT at Bonneville, Utah
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Please refer to the
image in the textbook
or in the eEdition CD.
ORGANIC LIMESTONE CLIFFS in Dover, England
Organic Rock
An organic sedimentary rock forms from sediments consisting of the
remains of plants and animals. Common organic sedimentary rocks are
limestone and coal. Although coal consists primarily of carbon, it is formed
from the fossilized remains of plants. The formation of coal is discussed in
more detail in Chapter 7.
Organically formed limestones contain the mineral calcite. Limestone
formation begins when water dissolves calcite out of rocks on land and
carries it in the form of calcium ions to an ocean or lake. There, certain
organisms use the ions to produce calcium carbonate shells or other support
structures. Clams, corals, and some algae are just a few of these organisms,
many of which live in shallow water near ocean shores. When the organisms
die, their calcium-rich remains pile up on the ocean floor. The type of
sedimentary rock formed from these sediments depends on the type of
structures and on the environment in which it forms. For example, the rock
coquina develops when masses of whole or mostly whole shells are
cemented together by minerals in ocean water. More often, waves break the
shells into fragments. In time these fragments may become cemented into
limestones. Limestones that form near shore may contain large amounts of
clay. Those that form farther from shore may be almost pure calcite.
Chapter 6 Rocks
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