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CHAPTER – I I
CRYSTALS,MINERALS AND ROCKS
2.1 Crystals and their characteristic
features
❑ Crystal means simply it is a word came to
be applied to any solid body that grown with
planar surface.
❑ Crystalline structure is arrangement of
atoms in regular, repeating patterns of
minerals. These patterns may not be
apparent to the naked eye (they are
microscopic) and their structures are
studied using X-rays. Glass and plastic are
non crystalline solids, hence they are not
minerals.
Figure (a and b) The orderly arrangement of sodium and
chlorine ions in halite.(c) Halite crystals.
2.2 Minerals and physical properties
in mineral identification
The make-up of solid matter on Earth:
Atoms → Elements → Compounds → Minerals → Rocks
(smallest)
What are minerals?
(largest)
❖ There are approximately 3,500 minerals,
but only a few (about 30) make up about
99 per cent of the volume of the Earth’s
crust. Silicate minerals are the most
abundant rock-forming minerals in the
Earth’s crust, and oxide minerals are the
second most abundant. Other common
mineral families are the carbonates,
phosphates, sulfates and sulfides.
Mineral Properties
• Minerals are analyzed for these common properties
– Color
– Crystal Form
- reaction with acid
– Luster
- magnetism
– Hardness
- striations
– Streak
- specific gravity
– Cleavage
- Density
– Fracture
• Be able to identify minerals in hand sample on
the basis of their properties
Rock Forming Minerals
All the minerals of the earths crust occur in
and among the rocks, they are an integral part
of the rock.
• Silicate minerals are the most common rock
forming minerals; they constitute about 95% of
the earths crust.
•The important silicate minerals are;
•
Feldspars, Feldspathoids, Quartz,Pyroxenes,
Amphiboles, Micas, and Olvines
2.3 ROCKS
Rocks are the essential building materials of which
the earth is constructed.
• The geologist’s definition of rock;
- Any mass of mineral matter, whether
consolidated or not, which is forming part of the
earth’s crust.
• Rocks may have only one mineral species, in which
case they are known as monomineralic but they
usually have an aggregate of mineral species.
The rocks are broadly classified
into three groups;
1. Igneous Rocks
2. Sedimentary Rocks
3. Metamorphic Rocks
Rock cycle
Rocks are always
changing in a pattern
called the rock cycle.
Over
time,
they
break down. The
minerals in them get
recycled to form new
rocks. These
processes can take
millions of years.
1. Igneous Rocks
❖ Igneous rocks are formed by cooling
and solidification of magma.
❖ Magma is a hot viscous, siliceous melt
containing water vapor and gasses.
-It is composed mainly of O, Si, Al, Fe,
Ca, Mg, Na, and K
When a magma comes out upon the
earths surface it looses its gasses such a
magma is called lava
Origin of Magma
❖ At certain distance below the surface ,the
temperature is such that all rocks would be
molten
❖ A number of possible sources of heat creating
the magma are described below;
I. Original
heat
II. Compaction and contraction
III. Radioactivity
Original Heat - rocks are very conductors of heat, and
the loss of heat from the earth’s is extremely small.
- If the earth was one time an incandescent (glowing)
ball of fire, a molten sphere, or even a hot solid sphere,
some of the original heat may still remain .
Compaction and Contraction- If the earth would
develop considerable internal heat through the slow
compaction of the material (under the pressure of
gravity) and contraction of the cooling planet,
- Also contraction associated with the folding of
layers of rocks would generate large quantities of
heat.
Radioactivity- Certain unstable chemical elements
break down spontaneously, providing energy that is a
likely source of all the heat necessary to explain the
phenomena of volcanism
Occurrence of igneous rocks
Magma is produced deep in the
earths crust where temperature is
900-1600oC
-Based on consolidation or cooling
process there are
two major types of igneous rocks;
-
I. Extrusive
II. Intrusive
I. Extrusive - The rocks formed
due to solidification of lava are
called extrusive rocks.
- When magma
reaches the earth’s
surface, it causes a
volcanic eruption.
• As the lava tend to cool
and crystallize rapidly, the
texture of the volcanic rocks
are generally fine grained .
Pyroclastic material- volcanic ejected rock
fragments.
fine dust-sized particles erupted during
volcanism referred as volcanic ash.
II. Intrusive Rocks – are formed when magma crystallizes beneath
the earths surface.
Depending on the depth of formation, intrusive rocks are divided in to two
groups;
I. Plutonic rocks
II. Hypabyssal rocks
Sedimentary Rocks
The total amount of sedimentary rocks that exists in
the upper 16 km of the earths crust is estimated to be
only about 5%.
❑ sedimentary rock are found chiefly as an extensive
cover over the continents
❑ Sedimentary rocks are formed by consolidation and
cementation of sediments deposited under water.
❑ Sedimentary rocks also include the rocks formed by
accumulation of chemically precipitated or organically
derived material .
❑ sedimentary rocks occur in layers and frequently
contain fossils
❑
Sedimentary rock
The formation of sedimentary rocks takes
place in three ways:
I. Weathering and erosion
- weathering and erosion
sediments are produced
transported
(water or wind) deposited
II. Sedimentation
- The process of accumulation of sediments at a
site of deposition is called sedimentation. The material
carried in solution precipitates and accumulates
III.
Lithification and diagenesis
- It is a process by which soft and loose
sediments are converted in to hard and firm rocks.
During this process many physical and chemical
processes take place with in the sediment, such
changes are called the diagenetic changes and the
process is described as diagenesis.
The three diagenesis processes are;
• Compaction- in which the rock or mineral
particles are brought closer together by the pressure of
overlying materials.
• Cementation- in which porous materials are bound
together by minerals precipitated from water
solution such as silicon dioxide (quartz), calcium
carbonate (calcite), and the iron oxides(limonite and
hematite)
• Recrystallization- in which a rock with
an interlocking crystalline fabric or
grain, such as crystalline limestone, is
developed by the continued growth of
the mineral grains in a sediment.
• The sediments from which sedimentary
rocks are formed may be divided into
two major groups;
-Clastic sediments
-Nonclastic sediments
-Clastic sediments - are broken fragments of
preexisting rocks ranging in size from minute
clay particles to very large boulders
Conglomerate
-Non-clastic Sediments- include those sedimentary
rocks which are formed by chemical precipitation of
minerals from water or by accumulation of remains of
animals and plants.
Kaolinite clay
Metamorphic Rocks
❖ Metamorphic rocks are formed from older rocks
when they are subjected to increased
temperature, pressure and shearing stress at
considerable depth in the earth’s crust.
❖ During metamorphism recrystalization takes
place essentially in the solid state and new
minerals and new textures are produced
Agents of Metamorphism
a) Heat
b) Uniform pressure
c) Directed pressure or stress
d) Chemically active fluids and gases
a) Heat - when the earth temperature increases with
depth, as a result appreciably high temperature
exists at great depths.
b) Uniform Pressure- The pressure is due to the
weight of the overlying rocks
c) Directed Pressure - Directed pressure or stress
operates during folding movements that accompany
mountain building.
d) Chemically Active Fluids and Gases - chemically
active fluids and gases when pass through the
pores of the rocks, they bring changes in their
original composition.
Process of Metamorphism
a. Granulation
b. Plastic deformation
c. Recrystalization
d. Metamorphism
Granulation - pressure shatters the rocks and the
pressure is so great that the rocks are partially melted.
This process where crushing of the rocks takes place
with out loss of coherence is called granulation.
Plastic Deformation - when a solid is subjected to
stresses, it shape changes. on the removal of stresses if
the solid does not regain its original shape, it is said to
be plastically deformed
Recrystalization
- Recrystalization the process of
formation of either new minerals or the formation of new crystals
of the preexisting minerals Recrystalization causes mineralogical
and textural changes in rocks during metamorphism