Environmental Engineering Reference
In-Depth Information
TABLE 5.10
Characteristics of Nondetrital Sedimentary Rocks
Rock Type
Characteristics
Plate
Calcareous Precipitates
Limestone
a
Contains more than 50% calcium carbonate (calcite); the remaining percentages
General
consist of impurities such as clay, quartz, iron oxide, and other minerals. The
calcite can be precipitated chemically or organically, or it may be detrital in
origin. There are many varieties; all effervesce in HCl
Crystalline
Relatively pure, coarse to medium texture, hard
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limestone
a
Micrite
Microcrystalline form, conchoidal fracture, pure, hard
Oolitic
Composed of pea-size spheres (oolites), usually containing a sand grain as a
limestone
nucleus around which coats of carbonate are deposited
Fossiliferous
Parts of invertebrate organisms such as mollusks, crinoids, and corals cemented
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limestone
with calcium carbonate
On Barbados, dead coral reefs reach 30 m thickness, and although very porous,
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are often so hard as to require drilling and blasting for excavation
Coquina
Weak porous rock consisting of lightly cemented shells and shell fragments.
Currently forming along the U.S. south Atlantic coast and in the Bahama Islands
Chalk
Soft, porous, and fine-textured: composed of shells of microscopic organisms;
normally white color. Best known are of Cretaceous Age
Dolomite
Harder and heavier than limestone (bulk density about 179 pcf compared with
169 pcf for limestone). Forms either from direct precipitation from seawater or
from the alteration of limestone by “dolomitization.” Effervesces in HCl only
when powdered. Hardness
5
Gypsum
a
An evaporite, commonly massive in form, white-colored and soft
Anhydrite
a
An evaporite, soft but harder than gypsum, composed of grains of anhydrite.
Ranges from microcrystalline to phanerocrystalline. Normally a splintery
fracture, pearly luster, and white color
Halite
a
An evaporite: a crystalline aggregate of salt grains, commonly called rock salt.
Soft, tends to flow under relatively low pressures and temperatures, and forms
soil domes. Since the salt is of substantially lower specific gravity than the
surrounding rocks it rises toward the surface as the overlying rocks are eroded
away, causing a dome-shaped crustal warping of the land surface. The
surrounding beds are warped and fractured by the upward thrust of the salt
plug, forming traps in which oil pools are found
Organic Origin (Composed of Carbonaceous Matter)
Coal
Composed of highly altered plant remains and varying amounts of clay, varying
in color from brown to black. Coalification results from the burial of peat and is
classified according to the degree of change that occurs under heat and pressure.
Lignite
(brown coal) changes to
bituminous coal
(soft coal) which changes to
anthracite
(hard coal)
Biogenic and Chemical Origin (Siliceous Rocks)
Chert
Formed of silica deposited from solution in water both by evaporation and the
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activity of living organism, and possibly by chemical reactions. Can occur as
small nodules or as relatively thick beds of wide extent and is common to many
limestone and chalk formations. Hardness is 7 and as the limestone is removed
by weathering, the chert beds remain prominent and unchanged, often covering
the surface with numerous rock fragments.
Flint
is a variety of chert;
jasper
is a
red or reddish-brown chert
Diatomite
Soft, while, chalklike, very light rock composed of microscopic shells of diatoms
(one-celled aquatic organisms which secrete a siliceous shell): porous
Other Materials Often Included by Geologists
Duricrusts
Discussed in
Chapter 7
Caliche
Laterite
Ferrocrete
Silcrete
Loess
Marl
