The non-clastic sedimentary rocks are
made of sediments of two basic types,
chemical precipitates and organically derived
sediments. Chemical precipitates are solid
mineral matters precipitated from an aqueous
solution in which the component ions have
been transported.
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The rock salt and gypsum
are its examples. The organically derived
sediments consist of remains of plants or
animals as well as mineral matters produced
by the activities of plants and animals. Coal
and limestone are very good examples of this
kind. Important non-clastic sediments are the
carbonate minerals, compounds of calcium or
magnesium ion or both. Sulphate compounds
are also minerals of the chemical sediments,
anhydrite (calcium sulphate) and gypsum
(hydrous calcium sulphate) are important
minerals.
Metamorphic Rocks
Metamorphic Rocks
These types of rocks are formed when a rock
is subjected to heat and/or pressure.
The heat and/or pressure alter the
characteristics of the original rock by forming
new minerals. Clay when subjected to heat and/
or pressure gets metamorphosed to slate.
Similarly, limestones give rise to marble.
The metamorphic rocks can be broadly
grouped into two major classes: cataclastic
rocks and recrystallised rocks. Cataclastic
rocks are formed by mechanical disruption
(breaking and crushing) of the original
minerals without appreciable chemical
change. The process is described as dynamic
metamorphism. The recrystallised rocks are
formed by the recrystallisation of the original
minerals. Minerals with different chemical
formulae and crystal lattices are produced due
to recrystallisation.
The recrystallised rocks
are further divided into two subclasses:
contact and regional metamorphic rocks. The
contact metamorphic rocks are formed by
recrystallisation under high temperature
caused by the intruding magma. The rocks
are not subjected to bending and breaking but
new mineral/s, emanating from magma are
added to metamorphosed rocks. The regional
metamorphic rocks undergo recrystallisation
during the process of being deformed by
shearing, often under conditions of high
pressure or high temperature or both.
As metamorphism continues a large
percentage of the minerals assume plate-like
shape and are assembled in parallel
orientation in the rock, a structure known as
foliation. Strongly developed foliation is known
as schists. In schists, crystals of certain
minerals grow in size. These large crystals are
referred to as porphyroblasts. Another
structure found in metamorphic rocks is
lineation. In this, mineral grains are drawn out
into long, thin, pencil-like objects, all in
parallel alignment. In another form of
structure, known as banding, minerals of
different varieties or groups are segregated
into alternate layers. These layers are usually
of light or dark shades, making the banding
conspicuous. Gneiss and schist have well
marked banded structure.