Clastic
sedimentary
rocks are formed from material that has been eroded
from other
rocks
by physical
weathering. They contain clasts, which have been eroded,
transported and finally cemented. They can be classified by grain size
such as (from largest to smaller) conglomerate,
breccia,
sandstone,
shale,
siltstone
and mudstone.
Others can be
classified by their
mineralogy, such as flint, ironstone, arkose, marl, greywacke and
greensand.
Chemical
Sedimentary Rocks
Chemical
sedimentary
rocks are
also formed from rock
material, but the type produced by chemical
weathering. That means the previous rocks have been partly
dissolved in
chemical precipitation. Sometimes they can be directly precipitated as
solid rock (e. g. BIF), other times they are transported. These rocks
include rock
salt,
rock
gypsum,
anhydrite, travertine,
dolostone and tufa.
Biochemical
Sedimentary Rocks
Biochemical
sedimentaries are formed from bones
and shells of dead organisms. These are made
of carbonates, less often calcite, and sometimes aragonite and
silica. Some examples include limestone
and chert. While chert
contains silica, limestone contains carbonates. As other rocks that
contain carbonate minerals, it is sensitive to acids and even water,
forming impressive stalactities and stalagmites in limestone caves.
Organic
Sedimentary Rocks
Organic
sedimentary rocks contain organic chemicals and carbon
derived from
organic chemicals. These rocks are partly formed from
materials left behind by dead plants, plankton, algae and
bacteria. These rocks
include coal,
and chalk. Coal contains a lot of
carbon.
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