Geology Reference
In-Depth Information
Inorganic:
Inorganic sediment grains are any minerals that form a chemical sedi-
mentary rock that are not related to a living organism.
Chemical sedimentary rocks are named according to their mineral composition, as I
show in Table 7-3.
Table 7-3 Chemical Sedimentary Rocks
Composition
Rock Name
Calcite (shells)
Biogenic limestone (coquina)
Silica (diatom shells)
Biogenic chert
Carbon (plants)
Coal
Calcite (CaCO
3
)
Inorganic limestone (travertine)
Dolomite (CaMg(CO
3
)
2
)
Dolostone
Silica (SiO2)
Inorganic chert
Halite (NaCl)
Salt evaporite (rock salt)
Gypsum (CaSO
4
* 2H
2
O)
Gypsum evaporite
Here's a bit of information about each type:
Biogenic limestone (coquina):
These rocks are shells glued together by the miner-
al calcite as it
precipitates,
or solidifies, out of warm sea water. The shells may be
microscopic, visible, or broken into bits. The cementing mineral calcite is easily
dissolved by water, so limestone landscapes often form caves or karst (see
Chapter 12) such as Mexico's Yucatan Peninsula.
Biogenic and inorganic chert:
This type of rock is created by the crystallization of
silica. The crystals are so tiny they can be seen only with a microscope. Most
cherts are inorganic, but some are created by microscopic silica-shelled organ-
isms called
diatoms.
When the organisms die, their shells settle to the seafloor and
eventually create
biogenic chert.
Coal:
Coal forms when massive amounts of plant materials and other living matter
are compressed and lithified. Most of the coal that exists today is the remains of
swamps from many millions of years ago. As the swampy vegetation dies,
peat
forms and is compacted into soft
lignite
and eventually hard
bituminous
coal. The
carbon from the organisms is released into the atmosphere when you burn coal
for fuel.
